# Effect of Nanofluids on Boiling Heat Transfer Performance
## 4 Effect of Nanofluid on Critical Heat Transfer and Boiling Heat Transfer Coefficient
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/(m\({}^{2}\cdot\)K)), multiwalled carbon nanotubes (600 W/(m\({}^{2}\cdot\)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.