Measurements and identification of supercritical pseudo-boiling heat transfer modes based on fiber optic probes and multiscale entropy

Abstract

Subcritical boiling includes interfacial evaporation and heat transfer induced by bubble dynamics. However, for supercritical heat transfer, direct experimental evidence of the existence of pseudo-evaporation and pseudo-boiling heat transfer, as well as the conversion between them is lacking. In this work, the experimental study of supercritical carbon dioxide pool heat transfer is conducted. The pressure and temperature of the cell are 8–10 MPa and 15 ℃, respectively. As heating element and temperature sensing element, a nickel-chromium alloy wire with a length of 22 mm and diameter of 70 μm is placed horizontally in the high-pressure cell. The fiber optic probe is placed vertically, with its tip 200 μm above the wire. Four heat transfer modes, i.e. natural convection, pseudo-evaporation, transition of evaporation and boiling, and pseudo-boiling, are found to occur sequentially with the increase of heat flux density or wall superheat. Natural convection occurs when the wall temperature is below the pseudo-critical temperature. This work focuses on pseudo-evaporation and pseudo-boiling heat transfer and the transition between them. In the pseudo-evaporation mode, the heat transfer coefficient decreases slightly with wall superheat increasing. The fiber outputs a high frequency signal with small amplitude, and there is no dominant frequency. The multiscale entropy is large, characterizing random signal fluctuations. In the transition of evaporation and boiling mode, the fiber outputs a large-amplitude/low-frequency periodic signal with a significant dominant frequency and small multiscale entropy, representing an ordered periodic pulsating heat transfer. In the pseudo-boiling mode characterized by bubble-like structure, the fiber signal fluctuation amplitude and multiscale entropy are between the counterparts of the first two modes i.e. natural convection mode and pseudo-evaporation mode. The dominant frequency is not obvious. The multiscale entropies in the specific case are calculated under different key parameters, such as dimensionality, time scale factor, and length of origin data. Optimal parameters are selected based on the best separation of heat transfer modes. Finally, pseudo-boiling can be distinguished from pseudo-evaporation by multiscale entropy of 0.9, and from transition of evaporation and boiling by multiscale entropy of 0.5. In this work, direct experimental evidence of supercritical-like boiling is obtained, which deepens the understanding of the supercritical heat transfer mechanism and provides a basis for theoretical studies and engineering applications in future.