Numerical Simulations of Flow Boiling in Constricted Microchannels: Bubble Dynamics and Heat Transfer Characteristics

Flow boiling in microchannels as a method for cooling electronic systems is constrained by inherent boiling instabilities. These instabilities are largely caused by the sudden expansion of bubbles inside the channels resulting in reversed flow and flow maldistribution. One approach which has been suggested to mitigate flow reversal is to add a restriction at the channel inlet. However, the impact of this inlet restriction on bubble growth dynamics remains largely unexplored. This article presents a numerical study of the effect of restrictions at microchannel inlets on bubble growth dynamics and associated heat transfer characteristics. We consider a refrigerant, R245fa, as the fluid flowing inside a microchannel of hydraulic diameter 100 μm. The mass flux range considered is 500 − 1900 kg/m2s (Reynolds number, Rel = 130 − 505) and a constant heat flux of 150 kW/m2 is applied at the base of the evaporator. We carry out transient, three-dimensional numerical simulations using boilingFOAM, a custom version of OpenFOAM which utilises the volume-of-fluid method to capture the dynamics of the liquid-vapour interface accounting for heat transfer, phase change and also incorporates conjugate heat transfer through the evaporator wall. Our results demonstrate that the bubble nucleation location exerts a significant influence over the bubble growth rate, heat transfer, and pressure fluctuations. Further, with an increase in Reynolds number, we observe a breakdown in the symmetry of the fluid flow giving rise to asymmetric bubble growth inside the channel. Finally, we demonstrate that reducing the restriction ratio from 0.5 to 0.2 leads to an increase in inlet pressure, and a marked rise in the Nusselt number.