Effect of Inlet Subcooling on Flow Boiling Behaviour of HFE-7200 in a Microchannel Heat Sink

Abstract

Miniaturised electronics pose challenging thermal demands, not only at chip-level power dissipation but also at the complete system-level heat rejection in modern electronic packages. Chip-level power densities are projected to be as high as 4.5 MW/m2 in computer systems by 2026 [1] and have been reported to exceed 10 MW/m2 in power modules for defence applications [2]. For instance, microwave power modules used in critical applications such as radar systems and satellites operate at high frequencies to improve dynamic response and reduce component size, albeit at the cost of device efficiency. For a typical efficiency of 20 % and input power of 850 W, almost 700 W of waste heat must be rejected from the system into the immediate environment, proving to be particularly challenging in aerospace applications where air-cooling is preferred. Flow boiling in microchannels is regarded as a promising cooling solution [1] for microelectronic systems where surface temperatures are limited to between 85 – 125 °C. The effect of various operational parameters on the performance of the cooling system must be well understood to facilitate successful integration of the developed technology into real-life systems. One of the important parameters in microchannel flow boiling is the degree of subcooling condition at the inlet of the heat sink as it affects the conditions for bubble nucleation, subsequent bubble growth, saturation conditions as well as flow reversal and instabilities in the channel. Deng et al. [3] studied the effect of inlet subcooling at 10 and 40 K of ethanol and found larger flow boiling heat transfer coefficients at higher degree of subcooling but concluded that subcooling had a negligible effect on pressure drop. On the contrary, in the subcooled boiling study of HFE-7100 in [4], heat transfer coefficient and pressure drop both experienced a slight decrease at higher degree of subcooling. Although considerable effort has been dedicated to the development of microchannel evaporator modules for high heat flux cooling, little attention has been paid to the study of integrated thermal management loops complete with a suitably-sized condenser. The degree of subcooling used will influence the design and size of both the evaporator and the condenser. In this study, the effect of inlet subcooling on the flow boiling characteristics of a microchannel evaporator, developed for the cooling of high heat flux devices using dielectric refrigerant HFE-7200 was investigated.