Effects of temperature-dependent thermophysical properties on nanoparticle migration at mixed convection of nanofluids in vertical microchannels

The current paper investigates the effects of temperature-dependent thermophysical properties on nanoparticle migration at the mixed convective heat transfer of nanofluids inside microchannels in the presence of heat generation/absorption. The chief motivations of this study are to examine different modes of nanoparticle migration induced by asymmetric heating and to evaluate how temperature dependency influences the thermophysical properties of nanofluids. To those ends, the modified, two-component heterogeneous Buongiorno's model is employed for nanoparticle-fluid suspension, which takes into account nanoparticle slip velocity relative to the base fluid originating from thermophoretic diffusion—that is, a force driven by the temperature gradient—and Brownian diffusion, or a force driven by the nanoparticle concentration gradient. Due to surface roughness within the solid–fluid interface in microchannels, a slip condition is employed at the wall surfaces to assess the non-equilibrium region near the interface. Once the fluid flow is fully developed, governing equations including continuity, momentum, energy, and nanoparticle volume fraction are simplified to ordinary differential equations and solved numerically. The results are obtained with and without consideration of the dependency of thermophysical properties upon temperature. Results indicate that neglecting the temperature dependency of thermophysical properties does not significantly influence the flow fields and heat transfer behavior of nanofluids, but changes the relative magnitudes. Moreover, unlike temperature-dependent buoyancy force, concentration-dependent one exerts considerable effects upon flow fields and nanoparticle migration. Furthermore, one-sided heating enhances the heat transfer rate anomalously, especially for larger nanoparticles.