Comparative Assessment of Micro-Scale Heat Exchangers Operating in Flow Boiling

The increasing demand to dissipate high heat fluxes from small areas in electronic equipment necessitated the move from air-cooled to liquid-cooled heat sinks and now to heat sinks operating in flow boiling. Researchers have intensified their efforts over the last few years and have examined and recommended various designs that fall into two main categories, namely rectangular, multi-micro-channel and micro pin fin heat exchangers. The research presented here describes experimental flow boiling work in these two geometries. A base benchmark design of a single micro-gap heat exchanger is also included. The base area of all three designs is the same to allow direct comparison of the heat dissipation capabilities for similar flow conditions. A dielectric HFE-7100 was used as the working fluid entering at 1 bar inlet pressure. The inlet subcooling was 5 K, the mass flux was varied between 100‒300 kg/m 2 s and the wall heat flux up to 0.324 MW/m 2 . In all experiments, the input heat was increased gradually until the exit vapour quality approached one or until near critical heat flux conditions were observed. A high-speed and high-resolution camera was used to visualise the flow boiling patterns. The comparison of the heat transfer results indicated that the micro pin fin heat sink provided better thermal performance than the other geometries. In both the micro-gap and the multi-channels, the dryout region, which precedes the critical heat flux, was reached at lower wall heat flux values than in the pin heat sink. In addition, flow reversal and system instability were found to be suppressed in this design. The pressure drop in the micro pin fin heat exchanger was higher, but it is worth noting that this is only a fraction of the total pressure drop and hence the pumping power of the thermal management system. Therefore, it does not constitute a restrictive parameter in design when, the designer needs to maximise the heat dissipation rate.