Thermophoresis and Brownian motion effects on heat transfer enhancement at film boiling of nanofluids over a vertical cylinder

Thermophoresis and Brownian motion are two important sources of nanoparticle migration in nanofluids, which have considerable effects on the thermophysical properties of nanofluids. In the present study, a theoretical investigation on the impact of nanoparticle migration on the heat transfer enhancement at film boiling of nanofluids over a vertical cylinder has been conducted. Alumina-water and titania-water nanofluids have been considered to examine the impacts of different nanoparticle types and the modified Buongiorno model is employed for modeling the nanoparticle migration in nanofluids. The results are obtained for different parameters, including the Brownian motion to thermophoretic diffusion N_BT, saturation nanoparticle concentration ø_sat, ratio of film thickness to cylinder radius ε, and normal temperature difference γ = (T_w - T_sat) / T_w. A closed form solution for the nanoparticle distribution is obtained and it has been indicated that nanoparticle migration considerably affects the flow fields and heat transfer rate. It is shown that the smaller nanoparticles are able to accumulate at the heated wall and enhance the heat transfer rate. For larger nanoparticles, however, nanoparticle depletion at the heated walls prevents considerable enhancement in the heat transfer rate. Furthermore, inclusion of alumina nanoparticles signifies a better cooling performance compared to titania nanoparticles.