Relationship between heat flux and bubble population density in heat transfer enhancement of flow boiling using microstructured surfaces

Recently, the thermal management of electronic devices has become paramount because the power density of semiconductor elements is rapidly increasing with the rapid miniaturisation of electronic devices. Boiling heat transfer (BHT) is an effective technique to achieve high cooling systems, and micro-structures are expected to enhance the BHT. In this study, copper, a hydrophobic surface, was used as a heat transfer surface, and microstructure was applied to the surface to investigate the effect of hydrophobic microstructured surfaces on the enhancement of BHT, and the effects of changing flow velocity from laminar (Reynolds number = 1,220) to the turbulent (Reynolds number = 6,640) were investigated. Comparing the boiling curves of smooth and microstructured surfaces, we confirmed that the heat flux on the microstructured surface was higher than that on the smooth surface at the same superheating, attributable to the increase in the number of bubble points caused by the cavities of the microstructured surface, which facilitated heat transfer. For the effect of changing the flow velocity, the critical heat flux (CHF) increased with the flow velocity for the smooth surface, but no significant improvement was observed for the microstructured surface. The prediction of the relationship between heat flux and bubble population density was found to be reasonable because the difference between the experimental values and the theoretical values obtained from a well-known prediction equation was small. Using this equation to predict the bubble population density on the microstructured surfaces, it was estimated that a small fraction of the number of cavities was activated, even near the CHF condition. In the future, it will be necessary to investigate the cause of the small number of activated cavities of the microstructured surfaces in detail.