Impact of Flow Boiling Instabilities on Heat Transfer Coefficient and Critical Heat Flux in a Microchannel

Microchannel flow boiling heat sinks that leverage the highly efficient heat transfer mechanisms associated with phase change are a primary candidate for cooling next-generation electronics in electric vehicles. In order to design flow boiling heat sinks for such practical applications, one key obstacle is an understanding of the conditions for occurrence of dynamic two-phase flow instabilities, to which microscale flow boiling is particularly susceptible, as well as their impact on heat transfer performance. While mapping the operational regimes of these instabilities has been well-studied, with numerous stability criteria available, their impact on the heat transfer performance of heat sinks in practical applications is not understood. This work seeks to assess the impact of pressure drop oscillations and parallel channel instabilities on the surface temperature and critical heat flux in parallel microchannel heat sinks. This is achieved through measurement of time-averaged steady-state temperatures and pressures, combined with high-frequency pressure signals and high-speed flow visualization. These data are compared across three controlled flow configurations that comprise a condition of stable flow boiling, a condition where only parallel channel instabilities can occur, and a third where both pressure drop oscillations and parallel channel instabilities can occur. Experiments are performed using the dielectric refrigerant HFE-7100 in 2 cm long parallel microchannel heat sinks with square-cross-section channels (0.25, 0.5, 0.75, and 1 mm widths) at three mass fluxes (100, 400, and 1600 kg/m2s). Across this range of conditions, the time-averaged surface temperature and critical heat flux were remarkably insensitive to the occurrence of these instabilities despite the significant hydrodynamic events and transient flow patterns observed.