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

Abstract

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 [Formula: see text] was immersed in a pressure vessel filled with [Formula: see text] 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 [Formula: see text] 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.