Experimental study on the attenuation of pressure waves in a cavity induced by flow boiling

The experimental work presented in this article deals with the study of low-frequency high-amplitude pressure wave attenuation in a cavity when boiling takes place on the inside. The working fluid was HFE7100. Regarding the flow regime, the study considered two different situations: pool boiling and flow boiling in the low to moderate Reynolds number regime. Regarding the operating parameters, the study considered: (a) the type of evaporator, (b) the temperature of the evaporator, and (c) the frequency of the pressure waves. Three different types of evaporators were manufactured and tested: a flat one and another two based on a micro pin fin structure. Pin fin sizes for these two evaporators were 500 μm × 500 μm × 500 μm and 1000 μm × 1000 μm × 1000 μm respectively. The temperature of the evaporators was varied between 40 °C and 80 °C. Frequency of the pressure waves changed between 35 Hz and 75 Hz. An additional isothermal reference case (no boiling) was used for comparison purposes. The typical order of magnitude of the peak-to-peak applied pressure amplitudes was 0.2 bar. Regarding the results, it was found that boiling causes a significant attenuation of the peak-to-peak amplitude of the pressure waves in the cavity. The main parameter acting on this attenuation was the temperature of the evaporator (related to the input electrical power), while the actual micro-structure of the evaporator played a nearly negligible role. Also, it was found that while a small electrical power input of about 5 W achieves an attenuation factor of about 0.5, four times as much power (20 W) is needed to halve again the attenuation factor to 0.25. This suggests a scaling law relating attenuation and power which could be used for engineering design purposes.