*3.2. Critical Heat Flux*

The CHF occurs when a layer of vapor is formed between the thermal fluid and the heat transfer surface. The continuous replacement of this vapor layer by the liquid keeps the heating surface temperature among a safe range. Moreover, the contributing mechanisms for the vapor layer removal from the surface or for the enhancement of the rewetting increased the CHF. Furthermore, the enhancement of the wettability caused by the deposited nanoparticle layer was considered to be a likely reason behind the enhancement of the nanofluid CHF. The effect of the wettability of the surface on the CHF was taken into account in the macrolayer dryout model proposed by Haramura and Katto [82] and in the hot/dry spot theory introduced by the authors Theofanous and Dinh [83]. Nevertheless, further experimental data revealed that all the hydrophobic, hydrophilic, and super hydrophilic surfaces improved the pool-boiling CHF [41]. Considering this fact, it is logical to assume that the wettability might not be the only possible characteristic for the CHF improvement. With the boiling-induced nanoparticle deposition phenomenon, the receding and advancing contact angle and consequent contact angle hysteresis, and the equilibrium contact angle are all subject to change [2]. In the experimental work performed

by the researchers Forrest et al. [18], it was confirmed that heating surfaces having small receding contact angles enhanced the CHF. The values encountered for the equilibrium, receding, and advancing contact angles of the hydrophobic heating surface were found to be 140◦ , 20◦ , and 160◦ , respectively. The authors stated that this surface was found to, at the same time, improve the boiling HTC and CHF. Moreover, it should be noted that the lack of homogeneity of the fluids and surface are factors directly influencing the advancing and receding contact angles, and correspondent hysteresis. Furthermore, it is critical to infer on the conditions that increase the nucleate boiling HTC and the CHF of nanofluids simultaneously. Additionally, there are several theories for explaining the boiling mechanisms for the departure from nucleate boiling, which causes a sudden rise in the surface temperature and in the CHF. Such theories include, among others, the hydrodynamic instability, vapor bubble interaction, and the already mentioned macrolayer dryout and hot/dry spot theories. The hydrodynamic instability approach confirms that the hydrodynamic effect associated with the counter current flow of vapor and fluid in the nearby surface region is the fundamental reason for the departure from nucleate boiling. This theory also reclaims that the departure from nucleate boiling happens when the down flow of the fresh fluid to the heating surface is averted by the rising vapor [71]. The macrolayer approach defends that the bubbles are separated from the heating surface by the fluid macrolayer and the departure from nucleate boiling occurs when this macrolayer dries out [84]. The hot/dry spot theory fundamentally deals with the reversibility and irreversibility of hot/dry spots and the departure from nucleate boiling occurs in irreversible hotspots where the rewetting is no longer permitted [85]. In the bubble interaction theory, the departure from nucleate boiling is conducted when the density of the bubbles on the surface is high enough to achieve a complete covering of the surface with a vapor layer, preventing the access of the fluid to the surface [86].
