*2.3. Fundamental Time-Dependent Features*

#### *2.3. Fundamental Time-Dependent Features*  2.3.1. Fouling Resistance

2.3.1. Fouling Resistance The fouling related with nanofluids is a type of particulate fouling phenomenon. The suspended nanoparticles lose their stability over time and adhere to the heating surface, mainly due to the interactions between the dispersed nanoparticles and the fluid The fouling related with nanofluids is a type of particulate fouling phenomenon. The suspended nanoparticles lose their stability over time and adhere to the heating surface, mainly due to the interactions between the dispersed nanoparticles and the fluid and between the nanoparticles and the heating surface along with temperature gradients within the base fluid [48]. Figure 4 shows the typical time-dependent stages of the particulate fouling.

and between the nanoparticles and the heating surface along with temperature gradients within the base fluid [48]. Figure 4 shows the typical time-dependent stages of the par-

**Figure 4.** Thermal resistance vs. time and main stages of the particulate fouling. Adapted from [49]. **Figure 4.** Thermal resistance vs. time and main stages of the particulate fouling. Adapted from [49].

Moreover, other parameters can be referred that determine the fouling occurrence when using nanofluids such as chemical composition, homogeneity, viscosity, diffusivity, density, interfacial properties, and compatibility of the nanoparticles with each other, the base fluid, and boiling surfaces. Regarding the nanoparticle concentration, an excessive amount of nanoparticles entails sedimentation concerns and, consequently, tends to promote particulate fouling over the surfaces that usually leads to a considerable decrement in the heat transfer performance. It should be stated that another vital concern of the particulate fouling over time is the clogging caused by the clustering of the nanoparticles. The fouling onto a boiling process surface has a considerable negative impact on the working of thermal management units, entailing negative consequences [50,51]. The latter are present in the operational damages caused by the shutdowns provoked by fouling, and in the maintenance costs that arise from cleaning the surfaces and equipment replacement [52]. The heat transfer performance decrement is the most observed effect of fouling [53,54]. This reduction is closely linked with the poor thermal conductivity of the fouling layer, pressure drop increase, clogged equipment, erosion and corrosion of the surfaces, and friction augmentation. To summarize briefly, the boiling Moreover, other parameters can be referred that determine the fouling occurrence when using nanofluids such as chemical composition, homogeneity, viscosity, diffusivity, density, interfacial properties, and compatibility of the nanoparticles with each other, the base fluid, and boiling surfaces. Regarding the nanoparticle concentration, an excessive amount of nanoparticles entails sedimentation concerns and, consequently, tends to promote particulate fouling over the surfaces that usually leads to a considerable decrement in the heat transfer performance. It should be stated that another vital concern of the particulate fouling over time is the clogging caused by the clustering of the nanoparticles. The fouling onto a boiling process surface has a considerable negative impact on the working of thermal management units, entailing negative consequences [50,51]. The latter are present in the operational damages caused by the shutdowns provoked by fouling, and in the maintenance costs that arise from cleaning the surfaces and equipment replacement [52]. The heat transfer performance decrement is the most observed effect of fouling [53,54]. This reduction is closely linked with the poor thermal conductivity of the fouling layer, pressure drop increase, clogged equipment, erosion and corrosion of the surfaces, and friction augmentation. To summarize briefly, the boiling fouling by nanoparticle deposition can be addressed through the interplay of the following phenomena [52]:


tions between them. In the cases where the adhesion forces have a lower magnitude than the hydrodynamic counterparts, the break-up of aggregates of the nanoparticles may occur. The hydrodynamic transport of the nanoparticles and attachment onto the heating surface of the nanoparticles are the involved phenomena in the two-step process of the structure of fouling [55,56]. In particulate fouling, the main thermophysical features involved are the motion of the nanoparticles by inertia, diffusion, and thermophoretic The hydrodynamic transport of the nanoparticles and attachment onto the heating surface of the nanoparticles are the involved phenomena in the two-step process of the structure of fouling [55,56]. In particulate fouling, the main thermophysical features involved are the motion of the nanoparticles by inertia, diffusion, and thermophoretic forces, linkage of the nanoparticles by the acting Van der Waals forces and superficial charges, and erosion. In pool-boiling scenarios, the fouling of nanoparticles resistance depends on [57]:

forces, linkage of the nanoparticles by the acting Van der Waals forces and superficial


The fouling of the copper oxide nanoparticles onto a heat exchanger surface was investigated with a variety of experimental parameters and a correlation accounting with the flow of the nanoparticles was introduced by the authors Nikkhah et al. [58]. The researchers reported that with a larger amount of nanoparticles and increased heat flux, a more considerable surface fouling occurred. Moreover, a reduction in the fouling resistance was found with increasing surface temperature. In particular, the consecutive heating and cooling cycles or the high temperature of the nanofluids tended to promote the impact between the nanoparticles and, consequently, their aggregation trend.
