Time-Frequency Analysis of a Pulsating Heat Pipe in Microgravity Environment

The thermofluidic operation of Pulsating Heat Pipe is affected by thermally induced fluid oscillations of unknown frequency and amplitude. In line with previous studies, the time-frequency analysis of experimental signals is performed to investigate the existence of local characteristic frequencies. The spectral analysis of PHPs by means of Fast Fourier Transforms (FFT) performed so far in the literature, shows contradictory results. Since it is expected that the flow signal parameters may vary in time, the time frequency analysis seems a more suitable tool for catching the local dominant frequencies. This work applies the wavelet transform to the evaporator and condenser fluid pressure signal of a passive two-phase heat transfer which can work as a Thermosyphon or as a Pulsating Heat Pipe, depending on the gravity acceleration. The results, obtained by means of a parabolic flight campaign, shows that the local characteristic frequencies are present only during the microgravity phase and in a frequency range from 0.8 to 2 Hz. Understanding the complex phenomena related to thermally induced oscillation is essential for the development of reliable heat transfer models and robust design tools for Pulsating Heat Pipes.