Wavelet analysis of temperature oscillation signals in a global glass pulsating heat pipe

To effectively control the complex vapor–liquid two-phase oscillatory flow in a pulsating heat pipe, it is essential to investigate its thermo-hydrodynamic behaviors and understand the underlying heat transfer mechanisms. However, the direct observation of the two-phase flow pattern within the pulsating heat pipe has been a long-standing challenge. To overcome this obstacle, a global glass pulsating heat pipe was constructed to visually observe the flow pattern, while the temperature oscillation signals were monitored and analyzed by continuous wavelet transform. Using such a method, this work connected flow patterns with measured temperature signals, and analyzed their variations with input heat fluxes. Results showed that the thermal inertia of glass material is not negligible, which leads to signal distortion of wall temperature. For the fluid temperature, its fluctuation amplitude decreases while frequency increases as the heat flux of inner wall surface rises (0.35 W/cm² to 3.18 W/cm²), meanwhile, its dominant frequency increases from 0.02 Hz to 3.88 Hz. During this process, it was observed that the flow patterns within the pipe gradually changed from the slug flow to the annular flow, and eventually to the general circulation at higher heat flux. Based on these results, we established a relationship between dominant frequencies of fluid temperatures and flow patterns, which can be extended to other metal pulsating heat pipes and assist the future design of pulsating heat pipes.