Role of nanoparticles on critical heat flux in convective boiling of nanofluids: Nanoparticle sedimentation and Brownian motion

Abstract

This study presents simulation of heat and mass transfer in two-phase flows of various nanofluids at various volume fractions and mass flow velocities. In contrary to most of research in the available literature, nanoparticles sedimentation along with Brownian motion of nanoparticles are comprehensively modeled and effects of these two important parameters on heat transfer and critical heat flux are evaluated. The aim of this modeling is to investigate the effect of nanoparticles and their sedimentation on the critical heat flux and mass transfer phenomena. All four mechanisms of mass transfer are simulated. The studied nanofluids are water-silver, water-copper, water-nickel and water-alumina with volume fractions of 1 to 5% and their results are compared against saturated water. Simulation of sedimentation together with Brownian motion of nanoparticles considerably improves the model precision. Nanofluids are seen to result in increased critical heat flux, heat transfer coefficient and net mass transfer. Moreover, the heat transfer coefficient and the critical heat flux are increased with the volume fraction of nanoparticles. Increase in mass flow velocity leads to rise of critical heat flux. In the event of nanoparticle sedimentation during the boiling process, the critical heat flux and heat transfer coefficient are seen to decrease.