Effect of Nanofluids on Boiling Heat Transfer Performance

Abstract

At present, there are many applications of nanofluids whose research results are fruitful. Nanofluids can enhance the critical heat flux, but the effect on boiling heat transfer performance still has disagreement. Base liquids with higher viscosity improve the boiling heat transfer performance of nanofluids. When the base liquid is a multicomponent solution, the relative movement between the different solutions enhances the microscopic movement of the nanoparticles due to the different evaporation order during the boiling process, so that the boiling heat transfer performance is enhanced. Compared with the thermal conductivity of the heated surface, the deposition of the low thermal conductivity nanoparticles reduces the heat dissipation rate of the heated surface and improves the wall superheat. Then the enhancement of the boiling heat transfer coefficient should be attributed to the thermal conductivity improvement of base fluid and the bubble disturbance resulted from the nanoparticle’s microscopic motion.

1. Introduction

In 1756, Leidenfrost [1] dropped droplets on two aluminum plates of different temperatures, and it was found that the water droplets evaporate faster on the low temperature plates. This phenomenon has laid a milestone in the development of boiling heat transfer. In 1931, Jakob used a high-speed camera to study the generation and development of bubbles [2,3]. In 1936, the roughness effect on boiling heat transfer was investigated [4]. However, Kutateladze [5,6] used fluid dynamics to study the boiling critical heat flux in 1950, which promoted the theoretically research process of boiling heat transfer. In the early 1970s, the rapid development of industry further promoted the study of two-phase flow boiling heat transfer.

In 1995, the Argonne National Laboratory proposed to disperse metal or nonmetal particles in water or organic solvents to form a stable suspension solution, which can improve the thermal conductivity of the base liquid. It has been verified by many scholars such as Akhgar [7], Kakavandi [8], Aparna [9], Hamid [10], and so on. The nanofluids stability is affected by the nanoparticles, base fluid, and dispersant. In order to improve the dispersibility of nanoparticles, the dispersant with appropriate concentration and type was added in nanofluids and then oscillated in the oscillator [11,12].

At present, the development of electronic devices in the direction of high integration and high computing speed has brought about the “thermal barrier” and “thermal management” problems that have become a hidden danger for the safe operation of equipment. Therefore, the search for high-efficiency heat conduction material and heat-dissipating methods has become the hot focus in industry [13,14,15,16]. During the boiling heat transfer process, bubbles are formed at the vaporization core point, and continuously absorb heat to disengage, float, and escape the water surface. The bubble detachment cause severe disturbance to the liquid of the heating surface, so that the boiling heat transfer is stronger than the convective heat transfer of a single-phase fluid. Therefore, boiling heat transfer is widely used in heat dissipation and cooling of many industrial fields, such as electronic equipment, because of its small temperature difference and high heat transfer capacity [17,18,19].

Kim [20] et al. carried out the bubble operation of the classic pool boiling curve at different boiling stages from the Figure 1. There is no bubble generation in the natural convection stage. The nuclear boiling phase can be divided into partial development stages and full development stages. There are relatively few bubbles in partial development stages and significant bubble coalescence has not yet occurred. During the full development stage, the bubbles appear to slip on the heated surface and merge into a mushroom-like and then escape from the water surface [21,22]. During the transitional boiling phase and the film boiling phase, the bubbles gradually form a gas film on the heating surface, and the heat transfer is carried out by means of heat conduction and radiation, which is likely to cause the device to burn out.

Nikkhah [23] et al. investigated experimentally the convective boiling heat transfer coefficient of spherical CuO(II) nanoparticles dispersed in water inside the vertical heat exchanger. Results show that by increasing heat and mass fluxes, the heat transfer coefficient considerably increases for both heat transfer regions, while by increasing the nanoparticle weight concentration the heat transfer coefficient increases in convective heat transfer (~35% at the maximum concentration) and deteriorates the heat transfer coefficient (~9% at the maximum concentration) in nucleate boiling region due to the formation of nanoparticle deposition of heating surface.

Sarafraz [24] et al. experimentally performed the flow boiling heat transfer characteristics of MgO/therminol 66 nanofluid at wt.% = 0.1 and wt.% = 0.3 as a potential coolant on a copper-made disc. Results revealed that bubble formation induces a pressure drop within the test section. Heat flux had no influence on the pressure, while an increase in the fluid flow rate caused an increase in the pressure drop. It was also found that the heat transfer coefficient decreased with operating time due to the presence of nanoparticles on the boiling surface resulting in the creation of thermal resistance on the surface. Also, an asymptotic behavior for the fouling thermal resistance over the time was registered. The maximum enhancement for the heat transfer coefficient was 23.7% at wt.% = 0.1. For wt.% = 0.2 and wt.% = 0.3, the maximum enhancement of 16.2% and 13.3%, were achieved, respectively. At the same time, Sarafraz [22] et al. found that by increasing the applied heat flux, the flow boiling heat transfer coefficient increases for both test fluids at both heat transfer regions. In addition, by increasing the flow rate of fluid, the heat transfer coefficient dramatically increases at both regions. Furthermore, higher heat transfer coefficient can be obtained due to interactions of bubbles and local agitations. In addition, Author experimentally measured the nucleate pool boiling heat transfer coefficients of Al₂O₃–water and Ti₂O–water nanofluids on three horizontal tubes with different materials and similar roughness [25]. Results revealed that presence of nanoparticles in the base fluid leads to an increase in pool boiling heat transfer coefficients on stainless steel and brass tubes in contrast to copper tube. Presence of nanoparticles had no effect on the pool boiling heat transfer coefficient for the copper tube. Variations of surface excess temperature for the copper tube were higher in comparison with that of the other tubes tested.

Salari [26] et al. experimentally studied the pool boiling heat transfer characteristics of gamma Fe₃O₄ aqueous nanofluids on a flat disc heater. The nanoparticle mass concentration was 0.1–0.3% and the heat flux was 0–1546 kW/m². Results indicated that the pool boiling heat transfer coefficient increases with increasing the mass concentration and applied heat flux. In addition, the rate of bubble formation is significantly intensified at higher heat fluxes, and subsequently larger bubbles detached from the surface due to the intensification of bubble coalescence. In terms of fouling formation, it can be stated that fouling of nanofluids is a strong function of time and rate of deposition is increased over the extended time while the pool boiling heat transfer coefficient was not decreased over the time, as the porous deposited layer on the surface detached from the surface by bubble interactions. In terms of critical heat flux, capillary action of the deposited layer was found to be the main reason responsible for increasing the critical heat flux as liquid is stored inside the porous deposited layer, which enhances the surface toleration against the critical heat flux crisis.

The nanoparticles can increase the thermal conductivity of the base fluid. In addition, the deposition of nanoparticles can reshape the heated surface, thereby affecting the boiling heat transfer performance. So the summary of this article were shown below.

• Application of nanofluids.
• Effect of nanoparticle type on boiling heat transfer.
• Effect of base fluid type on boiling heat transfer.
• Outline the effect of nanofluids on boiling critical heat flux.
• Outline the effect of nanofluids on boiling heat transfer coefficient.

This paper summarizes the existing literature and understands the research and value of nanofluids in different application fields, providing readers with a relatively macroscopic understanding. The disagreement and differences in the effects of nanofluids on the boiling heat transfer coefficient [27,28] are explained by the heated surfaces, boiling bubbles, and the microscopic motion of nanofluids. It also provides some ideas for future research for nanofluids.

2. Nanofluid Application in Engineering

Nanofluids have great potential applications in the fields of energy, chemical, automotive, construction, microelectronics, and information, and thus have become a research hotspot in materials, physics, chemistry, heat transfer, and other fields.

2.1. Nanofluid Application in Heat Pipes

Heat pipes operate under a number of physical constraints including the capillary, boiling, sonic and entrainment limits that fundamentally affect their performance [29]. Temperature gradients near the heated end may be high enough to generate significant Marangoni forces that oppose the return flow of liquid from the cold end in heat pipes. In the presence of significant Marangoni forces, dry out is not the initial mechanism limiting performance, but that the physical cause is exactly the opposite behavior: flooding of the hot end with liquid. The observed effect is a consequence of the competition between capillary and Marangoni-induced forces.

Maximum heat transfer ability was estimated as the minimal value from the heat transfer abilities specified by the hydrodynamic, the sonic and the boiling crisis limitations in the HPs. These limitations were calculated as [30] follows.

The hydrodynamic limit,

$${\mathrm{Q}}_{\max }^{\mathrm{h}\mathrm{y}\mathrm{d}}=2.19\frac{\mathrm{N}·\mathrm{S}·{\mathrm{F}}_{\mathrm{W}}}{{\mathrm{L}}_{\mathrm{e}\mathrm{f}\mathrm{f}}}$$

(1)

The sonic limit,

$${\mathrm{Q}}_{\max }^{\mathrm{s}\mathrm{n}}={\mathrm{F}}_{\mathrm{v}\mathrm{c}}·{\mathsf{\rho }}_{\mathrm{v}}·\mathrm{r}·\sqrt{\frac{{\mathsf{\gamma }}_{\mathrm{v}}{\mathrm{R}}_{\mathsf{\mu }}^{\mathrm{v}}{\mathrm{T}}_{\mathrm{s}}}{2\mathsf{\mu }({\mathsf{\gamma }}_{\mathrm{v}}+1)}}$$

(2)

The boiling crisis limit,

$${\mathrm{Q}}_{\max }^{\mathrm{b}}={\mathrm{L}}_{\mathrm{e}\mathrm{v}}\frac{2\mathsf{\pi }{\mathsf{\lambda }}_{\mathrm{e}\mathrm{f}\mathrm{f}}^{\mathrm{w}}{\mathrm{T}}_{\mathrm{s}}}{\mathrm{r}{\mathsf{\rho }}_{\mathrm{v}}\ln (\frac{{\mathrm{d}}_{\mathrm{i}}^{\mathrm{s}\mathrm{h}}}{{\mathrm{d}}_{\mathrm{v}\mathrm{c}}})}(\frac{2\mathsf{\beta }}{{\mathrm{r}}_{\mathrm{c}\mathrm{r}}^{\mathrm{b}}}-{\mathrm{P}}_{\mathrm{c}})$$

(3)

Figure 2 depicts the choosing of the optimal wick parameters. It can be seen from Figure 2 that the heat transfer ability at saturation temperatures below −20 °C is extremely low and in the temperature range of −60 °C~0 °C is specified by the sonic limit; at 0 °C~+40 °C, the hydrodynamic limit; and at +40 °C~+60 °C, the boiling crisis limit.

Heat pipe that is a compact and effective heat exchanger includes microchannel heat pipe, mesh wick heat pipe, sintered core heat pipe, oscillating heat pipe and thermosyphon heat pipe, and other heat pipe forms [31]. The researchers conducted experimental studies on different types of nanofluids such as Al₂O₃ [32,33], CuO [34,35], SiO₂ [36,37], Fe₂O₃ [38,39], and graphene nanosheets [40,41,42] in heat pipes. The results indicated that nanofluids can enhance heat transfer of the heat pipes. Figure 3 shows the working principle of the heat pipe.

Kim [18] et al. studied the heat transfer performance of heat pipes and mesh cores filled with water-based graphene oxide nanofluid (GON) with volume concentrations of 0.01% and 0.03%. The experimental results show that the nanofluid makes the wall temperature lower than the water. In addition, heat pipes containing graphene oxide exhibit lower evaporation heat resistance. The 0.01% GON exhibits better heat transfer due to the different nanoparticle layers deposited on the core structure. The nanoparticle deposition layer changes the capillary force, resulting in an increase in the maximum fluid velocity through the core structure.

Zhou et al. [43] studied the heat transfer characteristics of graphene nanosheets (GNs) nanofluids in an oscillating heat pipe. The volume concentration of graphene was 1.2–16.7% and the filling rate was 45–90%. The results showed that compared with deionized water, GNs nanofluids improve the heat transfer characteristics of the oscillating heat pipe, and the optimum concentration is 2–13.8%, mainly because the addition of GNs reduces the drying phenomenon of the heated surface.

In addition, Li [44], Nazaria [45], Hosseinian [46], and others also studied the effect of nanofluids on the heat transfer of heat pipes, and found that the addition of nanofluids can enhance the heat transfer characteristics of heat pipes. Mainly reason is that the deposition of nanoparticles changes the wettability of the evaporation surface, which enhances the capillary action and reduces the drying of the heated surface.

2.2. Nanofluid Application in Automobiles

The car radiator consists of three parts which are including the inlet chamber, the outlet chamber and the radiator core. The coolant flows in the radiator core and the air passes outside the radiator. The hot coolant cools due to heat dissipation from the air and the cold air warms up by absorbing the heat dissipated by the coolant. Nanofluids can make the car radiators smaller and lighter, and develop in an efficient direction, which reduce the cost of the car [47,48].

Subhedar [47] et al. used a mixed liquid whose volume ratio is 50:50 with ethylene glycol and deionized water (50EG/50DW) Al₂O₃-based nanofluid as a coolant for automotive radiators to investigate its heat transfer performance. The nanoparticle volume fraction is 0.2–0.8%, the coolant flow is 4–9 L/min, and the inlet temperature is 65–85 °C. Nanofluids can improve the heat transfer performance of heat sinks compared to conventional coolants. When the nanoparticle concentration is 0.2%, the heat transfer increased by 30%. Contreras [48] and others introduced the 50EG/50DW liquid-based graphene and silver nanofluids with the volume concentrations of 0.01%, 0.05%, and 0.1% in the automotive radiator. The coolant flow rate is between 0.08 kg/s and 0.11 kg/s, the coolant inlet temperature is between 55 °C and 85 °C, and the air flow rate is maintained at 2.1 m/s. The experiment found that the silver nanofluid increased the heat transfer rate by 4.4%, while the graphene nanofluid decreased compared with the base fluid. In the same ratio mixture of ethylene glycol and water, Tijani [49] evaluated the heat transfer characteristics of CuO and Al₂O₃ nanofluids, which include the heat transfer coefficient, thermal conductivity, Nusselt number, and heat transfer rate. The CuO nanofluid exhibited the highest heat transfer performance, whose heat transfer coefficient was recorded as 36,384.41 W/(m²·K), the thermal conductivity was 1.241 W/(m²·K), the Nusselt number was 208.71, and the heat transfer rate was 28.45 W. The Al₂O₃ nanofluid has a heat transfer coefficient of 31,005.9 W/(m²·K), a thermal conductivity of 1.287 W/(m²·K), a Nusselt number of 173.19, and a heat transfer rate of 28.25 W.

Chaurasla [50] and Ahmed [51] studied the heat transfer performance of automotive radiators using water-based Al₂O₃ and water-based TiO₂ nanofluids, respectively. The fluid poisoning and the increase in the flow rate of the air improve the heat transfer performance. Adding the 0.2% volume of Al₂O₃ nanoparticles, the heat transfer efficiency of the radiator is increased by up to 44.28%. While adding TiO₂ nanoparticles with concentration of 0.1% and 0.3% can improve the efficiency of the car radiator by 47%. The average heat transfer coefficient is directly affected by the increase in Reynolds number and volume concentration of the nanofluid.

2.3. Nanofluid in the Collector

Farhana et al. [52] reviewed the importance of nanofluids for thermal performance for six types solar collectors. In addition, an improved quantification of the performance of solar collectors by different types of nanofluids is compiled. Bellos et al. [53] studied in detail the use of nanofluids in concentrating solar collectors, including heating, cooling, power, and triads. Emphasis is placed on the enhancement of the thermal efficiency of the collector using nanofluids. Figure 4 shows the types of nanoparticles in solar collectors and the range of applications of solar collectors. As can be seen from the figure, there are many types of nanofluids used in solar collectors. The types of solar collectors include Flat plate, evacuated tube, direct absorber, parabolic trough, solar dish, and thermal photovoltaic. Solar collector is a special kind of heat exchanger which convert solar radiation into two types of energy such as thermal energy in solar thermal applications and electrical energy directly in photovoltaic applications. In the case of thermal application, solar devices absorb the solar radiant energy incident on its surface; transform it into heat and transfer the heat to the working fluid flowing through them.

Nanoparticle shape and type have an effect on the heat transfer of direct absorption solar collectors [54]. At room temperature, the thermal conductivity of nanofluids is 0.4% higher than that of pure water. At the surface of the photothermal conversion test of simulated solar irradiation with an intensity of 1 kW/m², after irradiation for 3000 s, the temperature of all spherical metal nanofluids was increased by 5 K compared with pure water, and the efficiency was improved by 35%. Nonspherical silver and graphene/silver are more efficient and increase by 35%. Water-based single-walled carbon nanotube nanofluids with a volume concentration of 0.2% can increase the efficiency of vacuum tube solar collectors by 66% [55]. Water-based CuO and Al₂O₃ nanofluids can improve the thermal efficiency of parabolic trough collectors [56]. Verma [57] evaluated the performance of water-based CuO/MWCNT mixture nanofluids and water-based MgO/MWCNT mixture nanofluids for flat plate collectors. The results show that the optimal operating conditions of the plate collector occur at a mass flow rate of 0.025–0.03 kg/s, and the nanoparticle concentration range is 0.75 to 1.0%. Compared with the base fluid, the energy efficiency of the MgO/MWCNT mixture nanofluid is improved, 25.1%, and compared with water-based MgO nanofluid energy efficiency increased by 16.28%. The performance of the MgO/MWCNT mixture nanofluid is closer to that of the water-based MWCNT nanofluid than the CuO/MWCNT mixture nanofluid.

It can be seen that the application of nanofluids in solar collectors can effectively increase the temperature of the base fluid and have a positive impact on the efficiency of the collector.

2.4. Nanofluids in Other Applications

At the same time, the application and research of nanofluids in batteries [58], energy [59], geothermal energy utilization [60], and chemical aspects [61,62] have gradually increased, and some positive research results have been achieved.

It can be seen from the existing literature that nanofluids are applied to the research of enhanced heat transfer in various fields. However, there are many shortcomings and differences on the boiling heat transfer of nanofluids. Summarize the consensus results of nanofluids currently in the field of boiling heat transfer, and it is necessary to sort out some existing differences. The following is mainly to analyze the effects of nanoparticle type, concentration, base type, and structure of heated surface on boiling critical heat flux and boiling heat transfer coefficient. We put forward ourselves views on the divergence of the nanofluid on the boiling heat transfer coefficient and give ideas and insights for future research.

3. Effects of Nanofluid Physical Properties and Heated Surface on Boiling Heat Transfer

The nanofluid will form a sedimentary layer on the heated surface during the boiling process. The deposited layer will change the number of vaporization core points and the surface wettability, resulting in changes in the growth and development and escape of the bubble. Therefore, the boiling heat transfer performance of nanofluids is mainly affected by the size of nanoparticles, base fluids and heating surfaces. In addition, it affected by the microscopic motion of nanoparticles and different types of base fluids, including the Brownian motion of nanoparticles [63], and the Marangoni effect between different base liquids [64]. The applied electric field [65], magnetic field [66,67], photothermal boiling [68], and pressure [69] also have a certain influence on the boiling heat transfer of the nanofluid. The physical properties of nanofluids are influenced by the type of nanoparticles and base fluids. At present, the research parameters of nanoparticles include the type, particle size and concentration of nanoparticles. The base liquid includes water, alcohol, oil, and a refrigerant. The heated surface contains the effects of material, surface shape and surface roughness. The current research results of nanofluids are summarized in

Table 1. Nanofluid boiling heat transfer.

Nanoparticles	Base Liquid	Concentration	Heated Surface	CHF/HTC	Reasons
Al2O3	DW	0.5, 1, 2, 4 wt.%	copper surface	CHF enhancement HTC deterioration	CHF enhancement is due to surface characteristics of nanoparticle deposition	[70]
		0.1, 0.2, 0.3 wt.%	stainless steel rod	CHF enhancement HTC enhancement	Nanoparticle deposition causes changes in surface roughness and vaporized core points.	[71]
		0.0007, 0.007 Vol.%	copper block	HTC enhanced 15–75% (0.0007%)	HTC is affected by surface roughness.	[72]
		0.005, 0.05, 0.01, 0.1 wt.%	steel plate	CHF enhancement	The nanoparticles accelerate the liquid perturbation, increasing the frequency of bubble detachment.	[73]
		0.001, 0.01, 0.05, 0.1 wt.%	copper surface	CHF enhancement HTC deterioration	It subject to surface roughness and concentration.	[74]
		0.01, 0.1, 0.5 vol.%	copper surface	HTC enhancement (0.01%)	The effect of the thermal conductivity of the nanofluid is more pronounced than the effect of nanoparticle deposition on the surface.	[75]
		0.02, 0.1 vol.%	copper surface	CHF enhanced 26–37% HTC deterioration	CHF enhancement is due to the complete wetting behavior exhibited by the nanoparticles. HTC deterioration is due to the increase in wettability and thickness of the porous layer structure.	[76]
		0.001, 0.01, 0.1 Vol.%	copper surface		HTC depends not only on cavity size and surface wettability, but also on the heat flux range.	[77]
		0.001, 0.002, 0.02, 0.05, 0.1 Vol.%	flat plate heater	CHF enhancement HTC deterioration	The deposition of nanofluids on the surface causes changes in wettability and thermal resistance.	[78]
		0.04, 0.4, 1 kg/m3	copper surface	CHF enhanced 2.5–3 times HTC enhancement	Wall superheat is reduced quickly.	[79]
		0.01, 0.05 wt.%	horizontal rod heater	HTC deterioration	Nanoparticle deposition causes a reduction in surface roughness.	[80]
		0.05–1 vol.%	cylindrical heater	CHF enhancement	Nanoparticle deposition causes an increase in surface roughness.	[81]
		0.025, 0.05, 0.075, 0.1 vol.%	Theoretical analysis	CHF enhancement	The bubble detachment frequency is lowered, the detachment diameter is increased, and the wettability is enhanced.	[82]
	EG	0.1, 0.2, 0.3 wt.%	stainless steel cylinder	HTC deterioration	The surface average roughness is reduced and the number of bubble nucleation is reduced.	[83]
	EG/DW	0.05, 0.1, 0.25, 0.5, 0.75, 1 vol.%	copper cylinder	HTC enhancement, when concentration is 0.75%, HTC enhance 64%	In addition to the increased properties of the base fluid, changes in the state of the heated surface are the main reasons.	[84]
	R141b	0.001, 0.01 vol.%	Microchannel surfaces	HTC enhancement	The low pressure causes an increase in the heat transfer coefficient.	[85]
		0.001, 0.01, 0.1 vol.%	copper surface	HTC enhancement	The addition of SDBS resulted in less deposition of nanoparticles.	[86]
CuO	Water	0.1, 0.2, 0.3 wt.%	stainless steel rod	CHF enhancement HTC enhancement	Nanoparticle deposition causes changes in surface roughness and vaporization core points.	[71]
		0.01, 0.02 wt.%	surface of rod heater	HTC enhancement	SDS significantly reduces surface tension.	[87]
		0.1, 0.2, 0.3, 0.4 wt.%	stainless steel cylinder	HTC enhancement	Surfactants increase surface wettability and the contact angle of the bubbles with the surface is reduced.	[88]
		0.001, 0.01, 0.05, 0.075, 0.2 wt.%	flat heater plate	HTC enhancement	The deposited nanoparticles have a positive effect on heat transfer.	[89]
	pentane	0.005, 0.01 vol.%	circular brass surfaces	HTC enhancement	Surface trenches are the dominant condition for increasing nuclear boiling.	[90]
	EG/DW	0.1, 0.2, 0.3 wt.%	cylindrical cartridge heater	HTC enhancement. When concentration is 0.5%, HTC enhanced 55%	Nanoparticle deposits alter surface roughness and wettability, which changes bubble shape and behavior.	[91]
Fe2O3	Water	0.02, 0.1 vol.% copper surface	copper surface	CHF enhance 26–37% HTC deterioration	CHF enhancement is due to the complete wetting behavior exhibited by the nanoparticles. HTC deterioration is due to the increase in wettability and thickness of the porous layer structure.	[76]
		0.05–1 vol.%	cylindrical heater	CHF enhancement	Nanoparticle deposition causes an increase in surface roughness.	[81]
Fe3O4	Water	0.01, 0.05, 0.075, 0.1, 0.2, 0.4 vol.%	cylindrical copper surface	When concentration is 0.1%, the HTC enhanced 43%	Surface characteristics have a significant effect on boiling heat transfer.	[67]
	EG/DW	0.01, 0.05, 0.1 vol.%	horizontal Ni–Cr wire	CHF enhancement HTC deterioration	The deposited layer increases surface wettability, resulting in significant CHF enhancement.	[92]
GNs	EG/DW	0.005, 0.01, 0.02, 0.05, 0.1 wt.%	heater wire	CHF enhancement HTC enhancement	The surface wettability caused by the deposition of nanoparticles is enhanced.	[93]
GONs	Water	0.0001, 0.0002, 0.0005, 0.001 wt.%	Copper spheres	CHF non-monotonic change	Non-monotonic changes consistent with changes in surface wettability caused by GON	[94]
CNT	Water	0.1, 0.2, 0.3 wt.%	stainless steel rod	CHF enhancement HTC enhancement	Nanoparticle deposition causes changes in surface roughness and vaporization core points.	[71]
		0.02, 0.1 vol.%	copper surface	CHF enhanced 26–37% HTC deterioration	CHF enhancement is due to the complete wetting behavior exhibited by the nanoparticles. HTC deterioration is due to the increase in wettability and thickness of the porous layer structure.	[76]
		0.01, 0.05 wt.%	horizontal rod heater	HTC enhancement	The larger size of the nanoparticles results in an increase in surface roughness.	[80]
		0.1, 0.3, 0.5, 0.7, 1 wt.%	cooper surface	HTC enhancement	The covalent modification of the nanofluid can avoid the deposition of nanoparticles and is beneficial to HTC enhancement.	[95]
		0.5 wt.%	stainless steel spheres	CHF enhancement	Longer and thicker CNTs tend to form microporous layers, increasing surface roughness.	[96]
		0.01, 0.05, 0.1 wt.%	heating element	CHF enhancement HTC enhancement	Using the covalent functionalization of cysteine and silver nanoparticles; the smaller the size, the increased surface area.	[97]
SiO2	Water	0.04, 0.4, 1 kg/m3	copper surface	CHF enhanced 2.5–3 times HTC deterioration	The separation of the nanoparticle layer may significantly deteriorate the heat transfer coefficient of boiling.	[79]
		0.05–1 vol.%	cylindrical heater	CHF enhancement	Nanoparticle deposition causes an increase in surface roughness.	[81]
		0.001, 0.01, 0.05, 0.1 wt.%	vertical cylinder	HTC deterioration	The change in surface properties caused by the deposition of nanoparticles on the surface has a major influence on the quenching process.	[98]
	EG/DW	0.1, 0.25, 0.5, 0.75 vol.%	hot-wire	HTC enhancement with concentration of 0.25%	Boiling heat transfer changes caused by nanoparticle coatings.	[99]
ZnO	Water	0.01, 0.05 wt.%	horizontal rod heater	HTC deterioration	Nanoparticle deposition causes a reduction in surface roughness.	[80]
		0.01, 0.02 wt.%	surface of rod heater	HTC enhancement	SDS significantly reduces surface tension.	[87]
	EG	0.35, 0.62, 0.93, 1.6, 2.6 vol.%	cylindrical copper block	HTC firstly increase and then decrease with the concentration	It is affected by the surface roughness of the nanoparticle deposition.	[100]
	EG/DW	5.25, 7.25, 8.25 wt.%	Ni–Cr wire	CHF enhancement HTC enhancement	Heating the coating reduces surface wettability.	[101]
TiO2	Water	0.04, 0.4, 1 kg/m3	copper surface	CHF enhanced 2.5–3 times	It depends on the concentration of the nanoparticles.	[79]
		0.001, 0.01, 0.05, 0.075, 0.2 wt.%	flat heater plate	HTC deterioration	The bubbles have a more spherical shape and their size is reduced.	[89]
		12, 15 wt.%	copper surface	HTC enhanced 124–138%	Verified by Pioro correlation [102]	[103]
		0.00005, 0.0001, 0.0005, 0.005, 0.01 wt.%	Horizontal circular plates	HTC firstly increase and then decrease with concentration, which 0.0001% is best.	Subject to surface roughness.	[104]
	EG	0.01, 0.025, 0.05, 0.075 vol.%	cylindrical brass surface	HTC deterioration	The bubble size decreases with increasing concentration.	[105]
	R141b	0.01, 0.025, 0.05, 0.075 vol.%	cylindrical brass surface	HTC deterioration	The bubble size decreases with increasing concentration.	[105]

.

3.1. Effect of Nanoparticles on Boiling Heat Transfer

3.1.1. Effect of Nanoparticle Types on Boiling Heat Transfer

The nanoparticles have a large specific surface area and the thermal conductivity is larger than that of the base liquid. However, the thermal conductivity, specific surface area of the different nanoparticles [106,107], the difference in density between the nanoparticles and the base [85], and the affinity for the base liquid are different [107,108]. At present, the study of boiling heat transfer of nanoparticles includes metal oxides Al₂O₃ [70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86], Fe₂O₃ [76,81], Fe₃O₄ [67,92], CuO [71,87,88,89,90,91], ZnO [80,87,100,101], TiO₂ [79,89,103,104,105], nonmetal oxide SiO₂ [79,81,98,99], carbon nanotubes [71,76,80,95,96,97], graphene [93,94] etc.

Sarafraz et al. [71] studied the flow boiling heat transfer performance of water-based Al₂O₃, CuO and functionalized multiwalled carbon nanotube (MWCNT) nanofluids in annulus heat transfer. In terms of boiling heat transfer, the functionalized multiwalled carbon nanotube nanofluid has better and lower deposition resistance than Al₂O₃ and CuO nanofluids. The density of Al₂O₃ is much lower than that of CuO, so the difference in density between Al₂O₃ nanoparticles and water is much smaller than the difference in density between CuO nanoparticles and water. The stability of the water-based Al₂O₃ nanofluid is better, thereby reducing the scaling thermal resistance of the heated surface, and the thermal conductivity is better than that of the water-based CuO nanofluid. Since the deposition of the nanoparticles on the boiling surface reduces the surface roughness and the number of nucleation sites, so that the bubble transport phenomenon is weakened and the heat transferred by the surface transfer is reduced, which make the heat transfer deteriorated. The boiling heat transfer coefficient curves of water-based Al₂O₃, CuO, and CNT nanofluids are shown in Figure 5. It can be seen from the figure that adding the nanoparticles to deionized water can enhances the boiling heat transfer performance. CNT nanofluids have the best boiling heat transfer performance, followed by CuO nanofluids, and finally Al₂O₃ nanofluids.

In short, the starting point temperature of the nucleate boiling, the average bubble diameter, the heat transfer coefficient and the heat flux at the nucleate boiling start point depend on the rate of mass flow of the nanofluid.

Shoghl et al. [80] studied the effect of adding ZnO, Al₂O₃, and CNT nanoparticles with mass concentration of 0.01% and 0.05% to deionized water on boiling heat transfer performance. The results show that ZnO and Al₂O₃ nanoparticles deteriorated the boiling heat transfer performance and the CNT enhanced boiling heat transfer performance compared with the deionized water. The reason is that the ZnO and Al₂O₃ nanoparticles reduce the roughness of the heated surface, while CNT improves the roughness of the heated surface, but at low concentrations the surface deteriorates. Figure 6 shows the different kinds of nanometers deposition surface after the experiment.

Neto [76] et al. studied the boiling heat transfer performance of water-based Fe₂O₃, Al₂O₃, and CNTs nanofluids on the copper surface at a volume concentration of 0.02% and 0.1%, and the deposition surface of Fe₂O₃, Al₂O₃, and CNTs after experiment on the deionized water. The CHF and Zuber correlation curves [109] and Kindlikar correlation curves [110] of different nanoparticle types were compared. The Zuber correlation curve and the Kindlikar correlation curve are as shown in Equations (4) and (5).

$${\mathrm{q}}_{\max }={\mathrm{K}}_{\mathrm{Z}}{\mathrm{h}}_{\mathrm{l}\mathrm{v}}{\mathsf{\rho }}_{\mathrm{v}}{[\frac{\mathsf{\sigma }\mathrm{g}({\mathsf{\rho }}_{\mathrm{l}}-{\mathsf{\rho }}_{\mathrm{v}})}{{\mathsf{\rho }}_{\mathrm{v}}^2}]}^{0.25}{(\frac{{\mathsf{\rho }}_{\mathrm{l}}}{{\mathsf{\rho }}_{\mathrm{l}}-{\mathsf{\rho }}_{\mathrm{v}}})}^{0.5}$$

(4)

$${\mathrm{q}}_{\max }={\mathrm{h}}_{\mathrm{l}\mathrm{v}}{\mathsf{\rho }}_{\mathrm{v}}^{0.5}[\frac{1+\cos \mathsf{\beta }}{16}]{[\frac{2}{\mathsf{\pi }}+\frac{\mathsf{\pi }}{4}(1+\cos \mathsf{\beta })\cos \mathsf{\varphi }]}^{0.5}{[\mathsf{\sigma }\mathrm{g}({\mathsf{\rho }}_{\mathrm{l}}-{\mathsf{\rho }}_{\mathrm{v}})]}^{0.25}$$

(5)

where ${\mathrm{q}}_{\max }$ is the maximum heat flux, ${\mathrm{h}}_{\mathrm{l}\mathrm{v}}$ is the latent heat of vaporization, $\mathsf{\rho }$ is the density, $\mathsf{\sigma }$ is the surface tension, $\mathrm{g}$ is the acceleration of gravity, and the subscripts $\mathrm{l}$ and $\mathrm{v}$ represent liquid and vapor, respectively; ${\mathrm{K}}_{\mathrm{Z}}$ is the empirical constant ($0.12\le {\mathrm{K}}_{\mathrm{Z}}\le 0.16$). Kandilkar [110] predicted the CHF of pure liquid saturated pool boiling, taking into account fluid dynamics and nonfluid dynamic effects. $\mathsf{\beta }$ is the power receding contact angle and $\mathsf{\varphi }$ is the heating surface inclination angle. The experimentally obtained CHF value, Zuber predicted value, Kandlikar predicted value, and the deviation between them are shown in

Table 2. Predictions of CHF obtained using Zuber [109] and Kandlikar [110] correlations [76].

Experimental Number	Working Fluid	CHF (kW/m2)	Zuber (Kw/m2)	Kandlikar (kW/m2)
1	Distilled water	1200	1108(−8%)	1205(+0.4%)
2	Fe2O3–Water	1514	1108(−36%)	1507(−0.4%)
3	Al2O3–Water	1542	1108(−39%)	1507(−2%)
4	CNTs–Water (0.02%)	1552	1108(−40%)	1507(−3%)
5	CNTs–Water (0.1%)	1496	1108(−35%)	1507(+0.7%)
6	Distilled water/layers of Fe2O3	1526	1108(−37%)	1507(−1%)
7	Distilled water/layers of Al2O3	1652	1108(−49%)	1507(−9%)
8	Distilled water/layers of CNTs	1612	1108(−45%)	1507(−7%)

.

As can be seen from Table 2, the CHF of the coating formed on the heated surface after boiling on the deionized water is higher than the CHF of the nanofluid on the smooth surface, and because the separation of the nanoparticle layer is enhanced, the critical heat flux is increased [79].

Kim [108] and others studied the boiling heat transfer of water-based Al₂O₃ and RGO (reduced graphene oxide) nanofluids. The volume concentration ratio of alumina nanoparticles to reduced graphene oxide was 1:1 and the solution volume concentration was 0.001%. The experimental results show that the enhancement of the critical heat flux for mixture nanoparticles reaches 473%, which is 9 times that of alumina nanoparticles and 12–47 times the graphene enhancement. The reason for this is because the deposition of alumina nanoparticles on the surface of the reduced graphene oxide greatly enhances the capillary force of the porous deposition surface.

Park [111] and others studied the boiling heat transfer performance of water-based Al₂O₃, GNs, and GONs nanofluids on the surface of the heating wire at a volume concentration of 0.001%. The results show that the CHF of Al₂O₃, GNs, and GONs nanofluids increased by 152%, 84%, and 179%, respectively, compared to deionized water. The authors point out that the CHF enhancement cannot be explained simply by surface wettability and capillary action of nanoparticle deposits. Therefore, it is proposed that the ordered porous structure surface structure of nanoparticle deposition changes the critical unstable wavelength [112], resulting in the critical heat flux increases.

Liter [112] et al. proposed a formula for the influence of modulation wavelength on the critical heat flux of a porous surface, as shown in Equation (6).

$${\mathrm{q}}_{\mathrm{p}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{u}\mathrm{s}}^{\prime \prime }=\frac{\mathsf{\pi }}{8}{\mathrm{h}}_{\mathrm{f}\mathrm{g}}{(\frac{\mathsf{\sigma }{\mathsf{\rho }}_{\mathrm{g}}}{{\mathsf{\lambda }}_{\mathrm{m}}})}^{1/2}$$

(6)

Figure 7a,b shows the contact angle and the wavelength of the porous surface for the critical heat flux of different types of nanofluids, respectively.

The boiling heat transfer of nanofluids is not only affected by nanoparticle type, but also the thermal conductivity of the base liquid [106]. Because of the density [89] and specific surface area [107], the roughness of deposited topography on heated surface is different during the boiling heat transfer process. The wettability and contact angle of the deposited surface have a great influence on the formation, growth, and detachment of boiling bubbles [82,103]. The type of nanoparticles and the size of the nanoparticles have a significant effect on the boiling heat transfer. Compared with the above literature, the thermal conductivity of nanoparticles is higher and the heat transfer performance is better [71,80]. That is, the better the thermal conductivity of the prepared nanofluids, the better the heat transfer performance. However, in view of the long-term static or high temperature conditions, the stability of the nanofluid is affected, so that the thermal conductivity of the nanofluid is reduced while changing the morphology of the heated surface. The morphology of the heated surface changes the number of boiling vaporization core points and bubble formation and detachment, thereby further affecting the boiling heat transfer performance. It can be concluded that the stronger stability and thermal conductivity of the prepared nanofluid, the stronger the heat transfer. However, when the nanofluid cannot guarantee the long-term stability under the test conditions, the deposition of the nanoparticles on the heated surface causes the influence about the change of the physical properties of the heated surface. Firstly, the different nanoparticles type causes the different morphology and thickness of the deposited layers at the same concentrations. Secondly, the different nanoparticle types cause different wettability of the heating surface, thereby affecting the formation and detachment of bubbles, which include the shape and the diameter of the bubbles. Finally, the different kinds of nanoparticles make the different running speed in base liquid. During the boiling process, the difference in the speed caused by the microscopic movement of the nanoparticles causes the frequency of the disturbance and the separation of the bubbles to be different, thus affecting the heat transfer performance.

3.1.2. Effect of Nanoparticle Concentration on Boiling Heat Transfer

The concentration of the nanoparticles largely determines the deposition morphology of the heated surface. Appropriate nanoparticle concentration can increase the number of vaporization core sites and increase the wettability of the heated surface. If the concentration is too high, the surface roughness of the deposited surface will be reduced or the deposited layer will be too thick, resulting in a decrease in the boiling heat transfer coefficient.

Ham [74], Shahmoradi [78], and others studied the boiling critical heat flux curve and boiling heat transfer coefficient of water-based Al₂O₃ nanofluids with different concentrations. It can be seen from Figure 8a that the addition of Al₂O₃ nanoparticles makes the boiling curve move to the right and the heat transfer performance is weakened. As the concentration of nanoparticles increases, the heat transfer performance of Al₂O₃ nanofluids first decreases, and then increases. It can be seen from Figure 8b that the addition of Al₂O₃ nanoparticles to deionized water, in addition to moving to the left in boiling curve of the Al₂O₃ nanofluid with a volume concentration of 0.001%, right with the concentration. However, it can be seen from the literature that the addition of Al₂O₃ nanoparticles increases the boiling critical heat flux of the base liquid. The literature also points out that the increase of the critical heat flux of nanoparticles is due to the enhanced wettability of the porous surface formed by the deposition of Al₂O₃ nanoparticles on the heated surface, and the increase of the wettability causes the bubble contact angle to decrease. The decrease of the boiling heat transfer coefficient is due to the porous deposition layer formed by the deposition of the nanoparticles, which increases the thermal resistance of the heat conduction, as shown in Figure 9.

Bang [70], Ham [74], Neto [76], and Shahmoradi [78] et al. experimentally studied the boiling heat transfer performance of water-based Al₂O₃ nanofluids with different concentrations. The literature [76] also studied the boiling heat transfer of water-based CNTs and water-based Fe₂O₃ nanofluids. Sulaiman [79] et al. studied the boiling heat transfer of water-based SiO₂ nanofluids and finally reached the same conclusion. That is, the deposition of the nanoparticles causes an increase in wettability, which in turn increases the critical heat flux. However, the porous deposition layer causes an increase in the thermal resistance of the heat conduction, thereby increasing the wall superheat and reducing the boiling heat transfer coefficient.

Sheikhbahai [92] et al. studied the boiling heat transfer characteristics of ethylene glycol and deionized water mixed with a liquid-based Fe₃O₄ nanofluid on the Ni–Cr wire surface. The volume concentration was 0.01%, 0.05%, and 0.1%. The volume ratio of ethylene glycol to deionized water is 50:50. The results show that as the concentration increases, the deposition of nanoparticles increases and the critical heat flux increases. At a concentration of 0.1%, the critical heat flux is 100% higher than that of the mixed base liquid. Firstly, the concentration of the nanoparticles increases, resulting in enhanced wettability of the heated surface, as shown in Figure 9. Secondly, the enhanced surface wettability results in an increase in the detachment diameter of the bubble, and the bubble detachment frequency is lowered, as shown in Figure 10.

The study on boiling heat transfer of nanofluids, including Al₂O₃ nanofluids from Sarafraz [71], Manetti [72], Ahmed [75], Sulaiman [79], Raveshi [84], Diao [85] and others; CuO nanofluids from Shoghl [87], Sarafraz [88], Karimzadehkhouei [89], Umesh [90], Heris [91], and others; as well as GNs [92], Fe₃O₄ [67], ZnO [101], SiO₂ [99], and CNT [71,80,95,97] nanofluids, indicated that nanofluids enhance the boiling heat transfer coefficient. The current explanation for this phenomenon is that nanoparticles increase the number of vaporization core points on the heated surface [67,71,72,84,92] and increase the thermal conductivity of the solution whose stability of the solution enhanced [75,84,86,95]. Decreased surface tension [87,91] leads to a decrease in the diameter of the bubble, which is conducive to detachment of the bubble. While a decrease in the wettability of the heated surface is also advantageous for the detachment of the bubble [101]. The boiling heat transfer coefficient is increased under the same heat flux by reducing the wall superheat. However, Ham [74] pointed out that the boiling heat transfer performance of water-based nanofluids is weaker than that of deionized water. However, because the critical heat flux is faster than the wall superheat, the maximum boiling heat transfer coefficient is enhanced.

It can be seen from the above influence of the nanoparticles concentration on the boiling heat transfer that there have an optimum value of the nanoparticles concentration making the maximum critical heat flux or the boiling heat transfer coefficient. A low concentration of nanoparticles can increase the thermal conductivity of the base liquid, and can increase the number of gasification core points, so that the generation of bubbles increases and the heat transfer is enhanced under the premise of the same bubble overflow frequency. However, high nanoparticle concentration causes a decrease in the operating frequency of the bubbles, which increases liquid viscosity and increases the thickness of the deposited layer. Under the premise of increased roughness of the surface deposited layer, the critical heat flux increases with the increase of the nanoparticles concentration. As the surface roughness begins to decrease as the concentration increases, the critical heat flux begins to decrease. The presence of a certain cavity in the deposited layer formed by the nanoparticles on the heated surface causes the heat to be not dissipated in time, resulting in an increase of wall superheat, so that the critical heat flux increases ratio less or less than the wall superheat, which causes a decrease in the boiling heat transfer coefficient. Therefore, it can be concluded that there is an optimum nanoparticles concentration dispersed in the base liquid, which can increase the number of boiling vaporization core points and the microscopic motion disturbance of the bubbles. At the same time, the formation of the nanoparticle deposition cavity does not cause the wall superheat to rise too fast. Thereby the maximum boiling heat transfer coefficient is improved.

3.2. Effect of Base Fluid Type on Boiling Heat Transfer

When the nanoparticles are dispersed in different base fluids with different physical properties, the boiling heat transfer performance of nanofluids is also affected by the type of base fluid in addition to the influence of nanoparticles. Different base fluids exhibit different thermal conductivity [113,114] and different viscosities [115,116] (as shown in Figure 11), resulting in difference in convective heat transfer performance of the nanoparticles, and the bubble generation and escape rates as well as the surface tension. At present, the base fluids include deionized water, organic solvents, and refrigerants. The thermal conductivity of deionized water is higher than that of organic solvents and refrigerants, but some of the high specific surface area nanoparticles have very poor dispersibility in deionized water, so there are many studies on dispersing in organic solvents or refrigerants [93,117], otherwise the nanoparticles are functionalized. Therefore, the influence of the physical parameters of the base liquid on the heat transfer has also received extensive attention and research.

The viscosity increased with MWCNT concentration in the ethylene glycol and deionized water mixed liquid, but the viscosity decreased with increasing temperature [118]. Xu [119], Sarafraz [83], Raveshi [84], and others studied the physical properties and heat transfer properties of ethylene glycol and deionized water mixed liquid-based Al₂O₃ nanofluids. The viscosity of the base liquid and the charge on the nanoparticles surface determine the degree of aggregation. Moderate aggregation of nanoparticles can make nanofluids more thermally conductive. In addition, it was found that the ethylene glycol and deionized water mixture has higher thermal conductivity than the single ethylene glycol or deionized water-based Al₂O₃ nanofluid. As the heat flux increases, the boiling heat transfer coefficient increases remarkably. However, as the concentration increases, the surface roughness and heat transfer coefficient deteriorate significantly. There is an optimum concentration to enhance the boiling heat transfer performance. When the optimum volume concentration of the nanoparticles is 0.75%, the boiling heat transfer coefficient is increased by 64%. Kole [100], He [101], and others studied the boiling heat transfer of ethylene glycol-based ZnO nanofluids, ethylene glycol and water mixed liquid-based ZnO nanofluids respectively. The thermal conductivity increased by 40% when the volume concentration of nanoparticles is 3.75% dispersed in ethylene glycol. As can be seen from Figure 12, the volume fraction of 1.6% ZnO can increase the boiling heat transfer coefficient by 22%, and 2.6% ZnO can increase the critical heat flux by 117%. When ZnO nanoparticles with a mass fraction of less than 7.25% are dispersed in the 80EG/20DW (the volume ratio of ethylene glycol and deionized water is 80:20) mixed liquid, the critical heat flux and boiling heat transfer coefficient are enhanced due to the decrease in surface wettability. Hu [93] then studied the boiling heat transfer performance of the graphene nanosheet in the 60EG/40DW mixed liquid. When the graphene concentration is less than 0.02%, critical heat flux increases with the increase of graphene concentration, because the deposition of graphene on the heated surface causes the wettability to increase. When the mass concentration is greater than 0.02%, the critical heat flux remains basically unchanged with the increase of concentration. In addition, SiO₂ [120], SiC [116], Fe₃O₄ [92], CuO [91,121], and TiO₂ [122] were also studied with regard to the heat transfer characteristics of nanoparticles in a mixture of ethylene glycol and deionized water. In the same mixture of ethylene glycol and water, as the concentration of nanoparticles increases, the thermal conductivity of the nanofluid increases. But under the same nanoparticle type and concentration, the thermal conductivity of the nanofluid is weakened with the ethylene glycol ratio. In addition, as the nanoparticles concentration and the proportion of ethylene glycol increases, the viscosity of the nanofluid increases. However, as the temperature increases, the viscosity of the nanofluid decreases. The different types of nanoparticles lead to the different thermal conductivity, but the effect on the boiling heat transfer performance depends largely on the deposition morphology of the nanoparticles on the heated surface. A small amount of deposition of nanoparticles increases the number of vaporization core points, and the boiling heat transfer performance is enhanced. The deposition of the nanoparticles increases the roughness of the heated surface, which increases the wettability, resulting in an increase in critical heat flux. The formation of a closed porous deposit of the nanoparticles on the heated surface causes an increase in the heat transfer resistance and a decrease in the boiling heat transfer coefficient.

In view of the divergence of nanofluids in boiling heat transfer, Nikulin [123] found that the addition of Al₂O₃ nanoparticles to isopropanol resulted in an increase in boiling heat transfer coefficient at low heat flux and a decrease at high heat flux. The effect of nanofluids on the boiling heat transfer coefficient depends on the concentration of the nanoparticles and the boiling temperature. Trisaksri [124] and Naphon [105] investigated the effect of refrigerant R141b-TiO₂ nanofluid on the boiling heat transfer performance. The addition of nanoparticles deteriorated the boiling heat transfer coefficient and deteriorated severely with the increase of nanoparticle concentration. It can be seen that the influence of nanoparticles on the boiling heat transfer performance still differs, but it is certain that the deposition of nanoparticles on the heated surface changes the wettability, resulting in an increase in the critical heat flux.

At present, the effect of nanofluids on boiling heat transfer is mainly aimed at the increase of bubble activation points caused by the deposition of nanoparticles. Therefore, it is necessary to investigate the effect of the nanoparticle deposition surface on the boiling heat transfer performance. Nanoparticle deposition on a smooth surface increases the density of vaporized core points and increases the boiling heat transfer [110]. The deposited layer of nanoparticles on the heated surface will still have partial peeling during the boiling process. As the mass of the nanoparticles in the exfoliated layer increases, the boiling heat transfer coefficient decreases [125].

The difference in the base fluid makes a large difference in the physical properties of the nanofluid. The viscosity of the base fluid is greater, while the viscosity of the nanofluid is greater. The intense disturbance caused by the generation and detachment of bubbles is the main reason for the strong boiling heat transfer in the nucleus. Therefore, the difference in the nanofluid viscosity caused by the difference in the base fluid is crucial for the influence of boiling bubbles. It is easy to see from the above literature that the addition of nanoparticles to a base liquid having a relatively high viscosity greatly enhances the boiling heat transfer performance. Mainly reasons are that, first, the nanoparticles are dispersed in a solution with a high viscosity, and the stability is well guaranteed. Second, the addition of nanoparticles has little effect on the viscosity of the solution, resulting in a small change in bubble detachment and operation. Finally, the stability of the nanofluid is better, the nanoparticle deposition on the heating surface is less, the number of gasification core points on the heating surface is easily increased, and the wettability of the heating surface is enhanced, so that the boiling heat transfer is strengthened.

At present, the boiling heat transfer performance of nanoparticles dispersed in a multicomponent solution has also received great attention. However, the literature has more to attribute the effect on boiling heat transfer performance to the deposition of nanoparticles on the heated surface, resulting in enhanced wettability and increased surface roughness. It can be seen that the analysis is mostly from the macroscopic point of view, but no one has studied the effect of the change of physical properties and interaction of binary solution on boiling heat transfer. The current boiling heat transfer devices basically have a condensing tube located inside the evaporation chamber and disposed above the solution. However, the difference in physical properties of the binary solution makes the boiling evaporation temperature different, and the binary solution in the evaporation chamber will be unevenly distributed during the boiling process. At this time, the interaction between the binary solutions becomes an important factor affecting the boiling heat transfer performance. As a carrier of nanoparticles, the binary solution accelerates the movement of the nanoparticles and causes a substantial change in heat transfer.

3.3. Effect of Heated Surface on Boiling Heat Transfer of Nanofluids

The effect of the roughness of the heated surface on the boiling heat transfer performance is to increase the heat transfer surface area, thereby increasing the influence of the vaporization core on the boiling heat transfer. However, the different heating surfaces and the surface roughness are also different for the heat transfer performance of the nanofluid. In addition, in order to further explore the influence of nanofluids on boiling heat transfer, it is necessary to study the boiling heat transfer of the pure base liquid on the deposition surface formed by the nanofluid boiling heat transfer test.

Compared with deionized water, Al₂O₃ nanoparticles with a volume concentration of 0.001% can enhance the boiling heat transfer performance of a smooth surface with an average roughness of 25 nm, but the effect on the heat transfer of a rough surface with an average roughness of 420 nm is not obvious [52]. Ham [53] et al. further promoted the boiling heat transfer of water-based Al₂O₃ nanofluids on different roughness heating surfaces. The volume concentration was 0–0.05% and the heating surface roughness was 177.5 nm and 292.8 nm, respectively. The results show that when the concentration is increased from 0% to 0.05%, the critical heat flux increases by 224.8% and 138.5% on the surfaces of roughness 177.5 nm and 292.8 nm, respectively. The boiling heat transfer performance of Al₂O₃ nanofluid with a concentration of 0.05% is lower than that of deionized water at Ra = 177.5 nm, but the maximum boiling heat transfer coefficient increases due to the increase of critical heat flux. The weakening of the boiling heat transfer coefficient is mainly caused by the deposition of nanoparticles on the heated surface, which increases the heat transfer resistance, but the wettability of the deposited surface is enhanced and the critical heat flux is increased.

By welding the foam metal on a smooth surface, the boiling heat transfer surface area can be increased. In addition, there is an influence on bubble growth and detachment during boiling process. In order to further enhance the boiling heat transfer of the foam metal surface, Xu et al. [126] investigated the effect of nanoparticles concentration and nanoparticles size on the boiling heat transfer of copper foam. The nanoparticles were Al₂O₃ and SiC. The foam copper thickness was 7 mm; the pore density was 5 PPI, 60 PPI, 100 PPI; and porosity was 0.9, 0.95, and 0.98. Preliminary studies have found that the addition of nanoparticles can enhance the boiling heat transfer performance of low porosity foamed copper, and the deposition of nanoparticles on the surface of foamed copper can increase the capillary force of copper foam, in addition to increasing the number of vaporization core points. In addition, the boiling heat transfer performance of nanofluids on the gradient hole surface was also studied, because the addition of nanoparticles blocked the voids of high density foam copper (100 PPI), and the bubble escape resistance increased, so that the heat transfer performance decreased [127]. Karthikeyan et al. [128] found that the ethanol-based MWCNT nanofluids increased the boiling heat transfer rate by 59% on the surface of the disk with laser inscribed square column texture.

It can be seen that the boiling heat transfer performance of nanofluids is affected not only by the nanofluids themselves, but also by the heating surface. Besides the roughness of the heated surface, the ratio of the average particle size to the roughness of the heated surface is also related to the boiling heat transfer performance. Using larger ratios, the deposition of nanoparticles can further increase the roughness of the heated surface, and, on the contrary, reduce the roughness. However, the boiling heat transfer performance will be significantly deteriorated by the formation of porous deposits on the surface of nanoparticles with high concentration.

Since the boiling heat transfer strength of the nanofluid is related to the number of boiling vaporization core points. So the boiling heat transfer performance of the nanoparticle deposition surface becomes critical after the boiling experiment in the pure base liquid. Dadjoo et al. [129] examined the boiling heat transfer of water-based SiO₂ nanofluids and nanoparticle deposition surfaces in deionized water, and also investigated the effect of heating surface inclination on boiling heat transfer, as shown in Figure 13. It can be seen that when the inclination angle is 0 degrees, the boiling heat transfer curve of the deionized water on the smooth surface, the water-based SiO₂ nanofluid on the smooth surface and the deionized water on the deposition surface is sequentially moved to the left. Nanofluids have the highest critical heat flux, followed by the deposition surface, and finally the smooth surface.

It can be seen that the boiling heat transfer of the nanofluid is strong, and besides the physical properties of the nanoparticle itself, it is also affected by the heated surface. The most intuitive effect of the modification of the heated surface is to increase the heat transfer area, thereby increasing the number of vaporization core points. From the indirect point of view, it is because the modification of the heating surface changes the number of boiling bubbles generated and the operating state in addition to increasing the number of gasification core points. Therefore, the nanoparticles have a larger deposition surface, and a higher concentration of the nanofluid solution can be used, so that the thermal conductivity of the base liquid increases greatly and the nanoparticle does not rapidly deteriorate due to the porous deposition layer formed on the heated surface. In addition, the bubbles generated by the irregular surface change the movement form of the bubble due to other forces such as capillary force, and are not singly subjected to upward buoyancy.

4. Effect of Nanofluid on Critical Heat Transfer and Boiling Heat Transfer Coefficient

This paper discusses the boiling heat transfer performance from the four aspects including nanoparticle types, concentrations, base fluids, and heating surfaces. Considering the divergence of nanofluids on boiling heat transfer performance, the differences between the enhancement of critical heat flux and the boiling heat transfer coefficient of nanofluids are considered. The addition of nanoparticles can enhance the critical heat flux, and this conclusion is well supported. The strengthening factor is mainly due to the following two reasons.

(1)

The addition of nanoparticles increases the thermal conductivity of the base fluid.

(2)

The deposition on the heated surface during boiling process reduces the contact angle and changes the wettability, so that the liquid is quickly replenished to the dry spots formed after the bubbles are detached.

When the concentration of the nanoparticles is too large and the heating surface roughness begins to decrease, the critical heat flux no longer increases and begins to deteriorate gradually. In addition, the deposition morphology of different nanoparticles increases the capillary force and enhances the liquid replenishing ability after the bubbles are detached.

From the current research literature, it is concluded that the influence of nanofluids on the boiling heat transfer coefficient is mainly affected by the following factors.

(1)

The influence of the deposition morphology of the nanoparticles on the heated surface.

(2)

The size of the bubble diameter.

(3)

Liquid convection effect on the heated surface.

It is certain that the higher concentration nanoparticles deteriorates the boiling heat transfer coefficient. Since there are many cavities in the deposited layer formed on the heated surface and the contact thermal resistance increases. It can be known from the boiling theory that the amount of bubble detachment from the heated surface determines the boiling heat transfer coefficient in a certain period of time. The number of gasification core points on nano-heated surface plays an important role, so the boiling heat transfer coefficient can be enhanced by grooving [130,131,132] and polishing [129]. In addition, the diameter and detachment state of the bubble are changed by changing the pressure of the boiling environment, and the addition of magnetic fields, electric fields, and gravitational fields enhances the microscopic motion of the nanoparticles. The frequency of the bubble’s detachment determines how quickly the heat on the heated surface is carried away. To some extent, the external field changes the perturbation effect of the microscopic motion of the nanoparticles on the boiling bubbles, thereby enhancing the detachment of the bubbles and enhancing the heat transfer. The low pressure effect is beneficial to the detachment of bubbles [85]. In addition to making the nanofluid more stable, the surfactant can reduce the surface tension, so that the bubble detachment diameter is reduced, the heated surface cannot easily form a film, and the boiling heat transfer performance is enhanced [87,88].

The reasons for the current boiling heat transfer coefficient are presumed to include the following factors.

(1)

The thermal conductivity of nanoparticles compared to heated surfaces.

(2)

Bubble detachment.

(3)

Microscopic motion between multiple solutions.

Copper blocks are used as electronic components with high thermal conductivity, and there are relatively many studies on heating surfaces. However, the thermal conductivity of metal oxides such as Al₂O₃, CuO, and Fe₂O₃ is much lower than that of copper, resulting in an increase in thermal resistance at the deposition point of the heated surface and the heat cannot be taken away in time, which increased the wall superheat. However, since the specific surface area of the nanoparticles is large, although the thermal resistance at the deposition point with the heating surface is increased, it is advantageous for the nanoparticles to be in contact with the heating surface and the foaming of the nanoparticles themselves. This greatly increases the number of vaporization core points and also changes the shape of the bubble generation and detachment [91]. During bubble growth, most of the heat is absorbed from the heated surface, and a small portion of the heat is absorbed from the nanoparticles deposited on the heated surface, while the nanoparticles absorb heat at a slower rate than the heated surface to slow the growth of the bubbles. When the thermal conductivity of the nanoparticles is higher than that of the heated surface, the rate of bubble growth is increased because the bubbles absorb heat from the deposited nanoparticles at a higher rate than the heated surface. Therefore, compared to conductivity of the heated surface, the boiling heat transfer of the nanoparticle with the high thermal conductivity becomes critical. However, current high thermal conductivity nanomaterials, including graphene (500–5000 W/(m²·K)), multiwalled carbon nanotubes (6000 W/(m²·K)), and other materials, but they all have a commonality: the density is small, resulting in a specific surface area that is too large. In the boiling process, graphene nanosheets are likely to cover the heated surface in sheet shape, resulting in a cavity between the nanosheets and the covered surface. In addition, because the specific surface base is too large, the boiling speed is relatively small, which restrains the escape frequency of bubbles to a certain extent, thus seriously deteriorating the boiling heat transfer. Therefore, the stability study of graphene nanofluids will not be neglected. At the same concentration, the deposition area of the MWCNT on the heated surface is small, and the boiling heat transfer performance is easily improved [72,80,95,97]. However, it can be seen from the literature [96] that the boiling heat transfer performance of the ethylene glycol and deionized water mixed liquid-based graphene nanosheet nanofluid is improved compared with the pure base liquid because the surface roughness caused by the deposition of the nanoparticles is increased.

In addition, we conclude that the microscopic motion of the multisolution nanofluid base fluid has a significant effect on heat transfer. Firstly, the Marangoni effect [64] and the thermophoresis [133] effect of nanoparticles are enhanced due to the enhancement of the density difference between the base liquids. Secondly, the micromotion of the multisolution during the boiling process causes the disturbance of bubbles on the heated surface to increase. Finally, the micromotion of the nanoparticles makes the flow of the heated surface liquids quicker, which is conducive to the heat being taken away, heat transfer performance is enhanced.

Therefore, it can be seen that when the thermal conductivity and the fluidity of the base liquid are strong, the strength of the boiling heat transfer of the nanofluid is mainly influenced by the thermal conductivity and concentration of the nanoparticle and the shape of the heated surface. When the base liquid is an alcohol with weak fluidity, the viscosity of the base is small, and the influence of the addition of the nanoparticles on the viscosity of the base liquid is large. In addition, the dispersibility and the stability of the nanoparticles in the high viscosity solution are better than lower viscosity base liquid. Therefore, the deposition of the nanoparticles on the heated surface is small, so that the enhancement of the thermal conductivity of the base liquid is stronger than the microscopic movement of the heated surface nanoparticles, and the boiling heat transfer performance is enhanced. When the base liquid is a multicomponent solution, the relative movement between the solutions enhances the microscopic motion of the nanoparticles due to the different evaporation order during the boiling process. Therefore, the disturbance of the bubble on the heating surface is enhanced, and the boiling heat transfer performance is enhanced.

5. The Direction of Nanofluid Research

At present, there are still many scholars who have conducted in-depth research on the boiling heat transfer performance of nanofluids. The innovative ideas include selecting nanoparticles with high thermal conductivity and increasing their properties by surface functionalization or dispersion in a base liquid with a high viscosity to improve boiling heat transfer performance. In terms of analytical methods, the following research methods are included.

(1)

Scanning electron microscopy is used to analyze the sedimentary morphology and atomic force microscopy is used to analyze the roughness of the deposited surface.

(2)

Analysis of the generation and detachment of boiling bubbles by high-speed cameras.

(3)

Analysis of the wettability of the heated surface.

However, through the analysis in this paper, in terms of the thermal conductivity of the heated surface, the deposition of the low thermal conductivity nanoparticles on the heated surface reduces the heat dissipation rate of the heated surface and improves the wall superheat of the heated surface [70,78]. Then, the enhancement of the boiling heat transfer coefficient of the nanofluid should be attributed to the thermal conductivity of the nanoparticle to the base fluid and the influence of the microscopic motion of the nanoparticle on the disturbance of the heated surface bubble. The nanometer particles can be appropriately labeled, and the compositional analyzer can be used to observe the motion path during the boiling process. The influence of the microscopic motion of the nanoparticles on the boiling heat transfer is deeply studied and analyzed.

In addition, because the surface of the nanoparticle is deposited on the heating surface to change the bubble generation state, it is necessary to use a high-speed camera to observe and analyze the bubble generation state around the deposition of a single nanoparticle. Raza [134] first experimentally studied the surface wettability-independent critical heat flux during boiling crisis with foaming solutions. Foaming solution refers to those aqueous surfactant solutions that are very effective in avoiding bubble coalescence and form vapor foam. The slowly rising small bubbles in the foam crowd the heater surface to inhibit rewetting and trigger boiling crisis. Similar to the typical relation between the terminal velocity and the bubble size, we observe a power law exponent of half between the CHF and the bubble size. Such a behavior suggests that the ability of buoyancy to remove bubble swarm away from the heater surface dictates the CHF during boiling with foaming solutions. Resulting premature dry-out not only reduces CHF significantly in comparison to boiling with pure water, but also renders the effect of wettability improvements from micro-/nanotexturing inconsequential to CHF enhancement. Terminal velocity of the rising bubbles is estimated to model the maximum vapor-removal capacity and successfully predict the CHF over a wide range of concentrations. Weakly foaming surfactants, or the strongly foaming surfactants but at lower concentrations, behave similar to water wherein they form large bubbles and wettability improvements via surface modifications exhibit CHF enhancements. The physical insights gained in this study can now be used to devise strategies for CHF enhancement with aqueous surfactant solutions. High-speed visualization of the nucleate boiling process illustrated a fundamental difference in bubble dynamics (Figure 14). It can be seen from the Figure 14, comparison to water and Tween 80, bubbles relatively large in number and small in size were formed with aqueous SDS and Triton X-100 solutions. Boiling videos further suggested that the size and the number of bubbles correlated with the bubble coalescence properties of these solutions. For example, pure water allows the formation of relatively large bubbles, but a few in numbers, due to significant coalescence. Similarly, coalescence also prevails in Tween-80 solution and hence the formation of large bubbles. On the contrary, nucleating bubbles in SDS and Triton X-100 solutions rarely merged/coalesced. Accordingly, high-density small sized bubbles formed with these foaming solutions.

Bottom view images of vapor bubbles at different subcooling and heat fluxes, for aqueous solutions and SDS at critical micelle concentration, during pool boiling on an inverted heater as shown in Figure 15. From the Figure 15, it can be suggested that the effective bubble size was found to decrease with the increase in subcooling due to enhanced condensation. Subsequently, vapor crowding was decreased and the CHF was found to increase with subcooling for ionic liquid and surfactant.

Kumar [135] et al. found that unlike water the aqueous solution of surface active ionic liquid avoids coalescence to form multiple small bubbles with significantly large wet area on the heater surface. Resulting rewetting of the heater surface increases the critical heat flux to ~950 kW/m², which is an enhancement of $\approx 4:5\times$ in comparison to pure water. It can be seen from the above literature that the formation, size and detachment state of boiling bubbles have a significant influence on the boiling heat transfer performance. So the influence of nanofluids on boiling heat transfer can be studied by the following means.

(1)

By disposing the nanoparticles with different thermal conductivity on the heated surface and not escaping due to the severity boiling during the boiling process. The effects on the critical heat flux and boiling heat transfer coefficient in the pure solution are investigated and the bubbles around a single nanoparticle are observed.

(2)

The current boiling heat transfer devices are all boiling in a large container saturated pool boiling, so the influence of the multicomponent solution on the boiling heat transfer can be observed by externally condensing the device, and the relative movement between the multicomponent solutions can be clearly observed. The motion and the disturbance of the bubble were observed and analyzed, and the effects on the critical heat flux and boiling heat transfer coefficient were investigated.

(3)

By investigating the boiling heat transfer performance of mixed nanoparticles in multisolution, the relative motion between base liquids and the influence of micromotion of different nanoparticles due to their density difference and specific surface difference on the critical heat flux and boiling heat transfer coefficient were analyzed.

(4)

To classify the heat transfer between different nanoparticles and different base liquid nanofluids and to correlate the effects of nanofluids on boiling heat transfer to a general purpose test of boiling critical heat flux and boiling heat transfer coefficient. Subsequent further in-depth research laid the foundation.

The excellent physical properties of nanofluids make it gradually applied to many fields of enhanced heat transfer, but the research and conclusions on its boiling heat transfer performance still lack a lot of experimental verification and convincing mechanism explanation. Therefore, it is necessary to examine the microscopic motion of nanoparticles and the operating state of bubbles from deeper and more diverse studies to reveal the mechanism of strengthening or suppressing heat transfer.

6. Conclusions

Nanofluids are widely used in the future, technology, and military industries. Compared with other applications, nanoparticles have relatively more research and application in heat transfer, and the results are better. However, there are still differences in the effects of the boiling heat transfer performance of nanofluids. This paper analyzes and summarizes the current research literature, and obtains the following conclusions and ideas for the future research of nanofluids in the field of boiling heat transfer.

(1)

The boiling heat transfer performance of nanofluids is affected by the type, concentration of nanoparticles, base type, and heating surface. Nanoparticle deposition causes the wettability and the number of gasification core points on the heated surface to change, which has an effect on the boiling heat transfer performance. In addition, the nanoparticle deposition surface changes the generation state of the boiling bubble and the escape frequency per unit time.

(2)

The nanofluid has a strong strengthening effect on the critical heat flux. It is mainly manifested that when the concentration of the nanoparticles is too large and the heating surface roughness begins to decrease, the critical heat flux no longer increases and begins to deteriorate. In addition, the deposition morphology of different nanoparticles increases the capillary force, thereby enhancing the liquid replenishing ability after the bubble is detached. This in turn increases the critical heat flux.

(3)

The influence of nanofluids on the boiling heat transfer coefficient is still different. The main explanation and analysis is attributed to the formation of a porous deposit on the heated surface of the nanoparticles, which increases the heat transfer resistance of the heated surface and reduces the boiling heat transfer coefficient. In addition, the deposition of nanoparticles on the heated surface causes the critical heat flux to increase faster than superheat, resulting in an increase in the maximum boiling heat transfer coefficient. There is also a large increase in the thermal conductivity of the base liquid to the base liquid, so that the boiling heat transfer coefficient is enhanced. The perturbation of the boiling surface by the nanoparticles increases the thinning of the liquid microlayer on the heated surface, the bubble disturbance is enhanced, and the boiling heat transfer coefficient is enhanced.

(4)

In view of the divergence of the nanofluids on the boiling heat transfer coefficient, it is found that the dispersion of nanoparticles in the base liquid with higher viscosity has a positive effect on the boiling heat transfer performance because the nanoparticles have a great effect on the thermal conductivity of the base liquid, and the stability is good. Therefore, the deposition of the nanoparticles on the heated surface is small, and thus the enhancement of the thermal conductivity of the base liquid is stronger than the microscopic movement of nanoparticles on the heated surface and the boiling heat transfer performance is enhanced. When the base liquid is a multicomponent solution, the relative movement between the solutions enhances the microscopic motion of the nanoparticles due to the different evaporation order during the boiling process. Therefore, the disturbance of the bubble on the heating surface is enhanced, and the boiling heat transfer performance is enhanced.

(5)

Compared with the thermal conductivity of the heated surface, the deposition of the low thermal conductivity nanoparticle on the heated surface reduces the heat dissipation rate and improves the wall superheat of the heated surface. Then the enhancement of the boiling heat transfer coefficient of the nanofluid should be attributed to the thermal conductivity of the nanoparticle to the base fluid and the influence of the microscopic motion of the nanoparticle on the disturbance of the heated surface bubble. The movement path in the boiling process can be observed by using a component analyzer by appropriately marking the nanoparticles. In-depth research and analysis on the effects of microscopic motion of nanoparticles on boiling heat transfer. In addition, because the surface of the nanoparticle is deposited on the heating surface to change the bubble generation state, it is necessary to use a high-speed camera to observe and analyze the bubble generation state around the deposition of a single nanoparticle.