Repercussions of maldistribution on subcooled fluid boiling in microchannel heat sink

Abstract

Flow boiling in microchannels still attracts enormous interest among the heat transfer community worldwide due to its exceptionally brilliant heat dissipation capability and the unfolded mysteries revolving around the flow reversal phenomenon. Finding ways of delaying/disrupting the flow reversal phenomenon acts as a counteractive strategy against the flow boiling instabilities in microchannel heat sink (MCHS). The presence of uneven fluid flow distribution leads to the early occurrence of flow reversal and associated flow boiling instabilities. In that context, this work addresses flow maldistribution and its intrinsic connection with flow boiling instabilities at the microscale level. Two configurations of MCHS, conventional design MCHS (CD-MCHS) and a design evolved by flow maldistribution mitigation: variable height design MCHS (VH-MCHS), are tested experimentally. It is noticed that under the strong flow maldistribution, boiling inception occurs in the side microchannels, while the flow remains single-phase in the central microchannels in the CD-MCHS design. On the other hand, uniform fluid flow distribution in the VH-MCHS design helped in removing the flow boiling phenomenon lag between the side and central microchannels, as observed in the CD-MCHS design. Flow uniformity across the parallel channels uplifts the supplied heat flux corresponding to the inception of the boiling process; a 7.7 - 17.3 % improvement is observed for the studied mass flow range of at ${\dot{m}}_{in}$ = 0.0008 - 0.0032 kg/s. The proposed design also brings down wall superheat at the onset of nucleate boiling from 107.5°C for CD-MHCS to 106.3°C for VH-MCHS design at ${\dot{m}}_{in}$ = 0.0024 kg/s and T_in = 30°C. Furthermore, the VH-MCHS design provided better surface temperature uniformity and lower vapor backflow intensity and low fluctuations in the pressure signals than the CD-MCHS design. A correlation is also proposed to predict a two-phase pressure drop ratio during subcooled flow boiling.