Effect of inlet subcooling on flow boiling in microchannels

•Increasing subcooling reduced the range for active nucleation sites in flow boiling.•Microchannel flow patterns and two-phase instability affected by subcooling.•Heat transfer rates affected by inlet degree of subcooling.•Two-phase pressure drop decreases with increasing subcooling at a given heat flux.•Influence of subcooling may explain discrepancies in microchannel flow boiling data. The paper describes experimental work to assess the effect of inlet subcooling on the flow boiling characteristics of HFE-7200 in a multi-microchannel heat sink. The experiments were performed at a mass flux of 200 kg/m2 s and system pressure of 1 bar for inlet subcooling conditions of 5 K, 10 K and 20 K and wall heat flux between 25.9 and 180.7 kW/m2. The equivalent base heat flux was between 99.1 and 605.3 kW/m2. The microchannel evaporator had 44 channels which were 0.36 mm in width and 0.7 mm in depth. The thickness of the fin between the channels was 0.1 mm and the base area of the microchannels was 20 mm × 20 mm. Increasing inlet subcooling generally decreased two-phase pressure drop in the channels and delayed flow regime transitions in the heat sink. Flow instability was also found to be dependent on the degree of inlet subcooling. The extended subcooled region of the channels at higher degrees of subcooling resulted in lower average heat transfer coefficients in the heat sink, especially at low heat fluxes. The effect of inlet subcooling on local heat transfer coefficients was less pronounced in the saturated boiling region. The study verified the importance of the degree of subcooling when comparing heat transfer and pressure drop characteristics in microchannel heat sinks and when optimising the design of integrated thermal management systems for high heat flux electronic devices.