Explosive breakup and evolution of the thermal boundary layer around a pulse-heated microwire in sub- and supercritical CO2

This paper reports our experimental findings aimed to understand the importance of compressibility in fluid flow and heat transfer. A platinum microwire of diameter 50 μm was immersed in a pressure vessel filled with CO2 at different thermodynamic states around the critical point. The microwire was heated by an electric pulse resulting in a temperature rise of about 667 K during 0.35 ms. The snapshots of CO2 and the temporal profiles of mean temperature of the microwire were recorded. An explosive breakup of the thermal boundary layer is identified, manifested by a radial spreading fluid layer with a “fluffy” boundary. Since buoyancy can only drive upward motions, such a phenomenon is closely related to compressibility, as a result of complex interactions between thermoacoustic waves and large-density-gradient interfaces. This phenomenon is also responsible for the efficient cooling observed in the first 10 ms because expansion is a cooling process and can also help to evacuate high-temperature fluid. Afterward, the flow exhibits various buoyancy-driven patterns depending on the existence and intensity of surface tension: garland-like cluster, unstable gas column, or normal bubble, followed by a continuously thinning thermal boundary layer. Both the classic and the newly revised thermodynamic phase diagrams are employed and compared in this paper, suggesting the latter is proper and informative.