Wavelet Analysis of the Pressure Signal in a Pulsating Heat Pipe

Thermally induced oscillations in two phase slug flow may largely affect the design heat transfer in microchannel heat exchangers and in passive wickless two-phase systems, i.e. Pulsating Heat Pipes (PHPs). The occurrence of flow instabilities and how and when possible dominant frequencies appear during the device operation are still open issues in the scientific community and, most important, are not known a priori, neither can be derived only from physical and analytical considerations. In the literature, studies about different types of time-frequency analyses on the fluid pressure, the fluid and wall temperature, the liquid slug velocity and the vapor plugs displacement signals can be found. The results are incomplete because the link between frequencies and device performance is still not clear. In this work the time-frequency analysis has been applied to the evaporator and condenser pressure signals of a Pulsating Heat Pipe and individually analyzed to investigate the existence of dominant frequencies. Data recording is performed varying the heat power input at the evaporator zone, ranging from 68 W to 146 W. To characterize the signal in the frequency domain and identify the time interval in which the dominant frequency occurs the selected tool is the Wavelet Transform, a good compromise between resolution and complexity of implementation. During the slug-plug flow regime, the results show that the dominant frequencies always fall in the range 0.6 - 0.9 Hz, with an increasing trend with the heat input level. Moreover, the two signals at the evaporator and at the condenser were compared through the Wavelet Cross-Correlation, identifying the dominant frequency common to both signals and the phase angle 〖10〗^o 〖-20〗^o. The understanding of the complex phenomena related to the thermally induced oscillations is essential for the development of reliable heat transfer models and robust design tools for Pulsating heat pipes.