Experimental and theoretical studies on oscillation frequencies of liquid slugs in micro pulsating heat pipes

Abstract

The purpose of this study is to experimentally examine the oscillation frequencies of liquid slugs in multi-turn micro pulsating heat pipes (MPHPs) and to perform a theoretical study for better understanding of the experimental results. A series of experiments on silicon-based MPHPs with different overall lengths of 40, 50, and 60 mm are performed at different heat inputs in a bottom-heating mode. Ethanol is used as a working fluid at a filling ratio of 55%. From a spectral analysis on flow visualization data, dominant frequencies of the MPHPs are identified for each experimental condition. To theoretically estimate the dominant frequencies, a 'non-adiabatic' vapor spring liquid mass model is proposed: The spring action of a vapor plug is linked not only to a volume variation but also to a mass variation of a non-adiabatic vapor plug via phase change processes. A distinguishing feature of this model is that it is capable of handling the mass variation of a vapor plug due to phase change processes. Based on the model, a set of parameters related to the oscillation frequency is explicitly determined: the number of turns, channel length, filling ratio, liquid density, vapor pressure and specific heat ratio. A closed-form correlation of the oscillation frequency is proposed and found to be accurate in predicting the experimental data to within 15%. It is also shown that the oscillation frequency is over-predicted by more than 100% of the experimental data when the spring-mass model is used without including the mass variation due to phase change processes.