Pressure drop across micro-pin heat sinks under unstable boiling conditions

Flow boiling was investigated under unstable boiling conditions in three different micro-pin fin heat sinks using water and R-123 as working fluids. Once boiling was initiated severe temperature fluctuations were recorded for all the tested (three) micro-pin fin heat sinks. Flow images and fast-Fourier transform (FFT) of pressure signals during flow boiling were used to explain experimental results. The boiling instability mechanisms behind unstable boiling were discussed for both water and R-123. Accordingly, no significant pressure fluctuations with respect to time averaged pressure drop were evident for the tested micro-pin fin heat sinks before and after flow boiling instability initiates. However, a step change in the pressure signals were recorded with the inception of unstable boiling, and a sharp increase in the magnitude peaks of the FFT profiles was observed in the device operated with R-123, while there was no significant change in the FFT profiles in the devices operated with water. According to complementary flow visualization studies, the oscillation frequency of the periodic flow patterns for the device operated with R-123 was higher (f∼80 Hz) than that of the devices operated with water (f∼20 Hz).