Measurement and comparison of transient thermal boundary layer of a Local-Heated Mini-Channel under sub- and super-critical conditions

•Quantitative measurement of the turbulent thermal boundary under sudden heat addition.•Localized heat accumulation and a slowly thermal response have been observed in subcritical state.•Steeper temperature gradient seen at larger heat fluxes, resulting in enhanced heat transfer.•Relatively high density and low temperature changes generated for shorter distances to critical point. This study is focused on experimentally measuring the transient thermal boundary layer following a sudden heat addition to turbulent flow boundary in supercritical mini-channels. Interferometric diagnostics were performed using a visualization channel to capture the time-evolution boundary layer. The measured pressure drop shows good agreement with numerical iterations according to the friction laws. Transient scenarios have been extracted from the raw interferograms to facilitate quantitative comparison of boundary conditions, such as bulk temperature and heat flux: (1) Localized heat accumulation and a slowly thermal response have been observed in subcritical state; (2) Relatively high density of 1.2 kg/m3 and low temperature changes of 0.008 K are observed in cases with relatively shorter critical distances to the critical point, attributed to the sharp increase in heat capacity and thermal expansion coefficients. (3) A relatively fast thermal convective field is established at larger heat fluxes due to the rapid density shifts of 1.2 kg/m3 and associated flow acceleration in the boundary layer.