Numerical simulation of nanorefrigerant flow boiling: Mechanism of particle behavior on heat transfer enhancement

This study used the Computational Fluid Dynamics-Discrete Phase Model (CFD-DPM) method to investigate the flow boiling of R141b-Al2O3 nanorefrigerant in a horizontal tube and clarify the enhancement mechanism of particles on heat transfer. The Volume of Fluid (VOF) method was employed to solve the evolution of gas-liquid interfaces, whilst the motion and heat transfer of individual particles were studied using the DPM method. The predicted results are in good agreement with the analytical solution. The results show that the migration of nanoparticles absorbing heat is important reason for enhancement of heat transfer in nanorefrigerant flow boiling. The nanoparticles near the wall reduce local drying of the heated wall by absorbing heat, and thus enhance the heat transfer efficiency. The heat transfer enhancement by nanoparticles is closely related to the boiling two-phase flow patterns (including bubbly, slug and annular flow). The larger the vapor volume fraction is, the better heat transfer enhancement by nanoparticles is. The increase of particles enhances the heat transfer efficiency between the particle and fluid phases, leading to that the enhanced heat transfer rate of nanoparticles increases with the increasing particle mass fraction.