CFD Simulation of Annular Flow Boiling in Microchannels

Abstract

Flow boiling in microchannels has received enormous interest over the past few decades because of its importance in the thermal management of micro-structured devices. Few of previously published studies focus specifically on microchannel annular flow boiling which is very important due to its prevalence in this system. This thesis provides understanding of the heat and mass transfer in microchannel annular flow boiling via the use of a computational fluid dynamics (CFD) approach. The commercial software ANSYS Fluent was chosen to perform the CFD simulations. A modified height function (HF) method was implemented into the default CSF model to improve the performance of surface tension modelling. Simulation of an inviscid parallel flow demonstrated successful prediction of the onset of Kelvin-Helmholtz (K-H) instability in close accord with the analytical criterion. Studies of imposed numerical perturbations in laminar annular air-water flow showed that viscosity does not affect the stability of interfacial waves but has large impacts on the growth rates. A phase change model was formulated using a kinetic-based model to calculate the interphase mass flux. An established numerical smoothing procedure was used to improve numerical stability. A detailed study of a laminar annular flow boiling was performed using water at 160 kPa in a 0.5 mm diameter channel with constant fluid mass flux G = 60 kg m−2 s−1 and inlet vapour quality x = 0.1. Interfacial waves were observed and flow recirculation and a localised increase of heat transfer coefficient occurred at the interfacial wave troughs, where the liquid film was very thin. A parametric analysis showed that boiling heat transfer coefficient increases with increasing mass flux, system pressure, vapour quality and heat flux but decreases with increasing tube diameter.