Numerical and experimental investigations of boiling enhancement in buoyancy-driven microchannels

In this study, confinement-driven boiling enhancement trends and experimental data from narrow parallel plate channels are presented and analyzed via comparison with numerical simulations of buoyancy-driven boiling and two phase flow using the commercially-available Fluent CFD software package. An Euler-Euler multiphase approach, known as the volume of fluid (VOF) method, is employed, as bubbles sizes are on the order of the channel dimensions. Numerical results suggest that enhanced natural convection already accounts for a large portion of the unconfined pool boiling heat flux. While the increased buoyancy from large vapor fractions in narrow channels may lead to an order of magnitude increase in channel mass flux, confinement-driven convective enhancement is found to increase the unconfined boiling heat flux by less than 10%. Further, simulated convective enhancement is found to be a maximum for intermediate size channels, in direct contrast to experimental data which show maximum enhancement (500%) for the smallest channels investigated. Experimental results for different channel wall materials suggest an enhancement mechanism highly dependent on boiling surface characteristics.