Study on the coupling characteristics of flow and heat transfer in transcritical channel flows at low Reynolds numbers

In micro-scale cooling passages such as microchips, turbulent heat transfer must be achieved at low Reynolds numbers, and transcritical flows are particularly attractive because their drastic thermodynamic properties variations near the pseudo-critical point can trigger and sustain turbulence at these conditions. To quantify the pronounced sensitivity of thermodynamic properties to temperature variations, direct numerical simulation of supercritical channel flows at low Reynolds numbers was conducted to identify the dominant mechanisms. A comprehensive analysis has shown that turbulent kinetic energy (TKE) transport is governed by production, dissipation, and diffusion, with mean properties (density, viscosity) playing a dominant role in the production, dissipation term, and property fluctuations significantly influencing the diffusion term. Similarly, second-order momentum can be accurately scaled without accounting for fluctuating properties, an insight that drastically simplifies turbulence modeling for supercritical fluids where such properties fluctuations (density) are large. Crucially, density fluctuations are identified as the universal source of Reynolds stress generation, thereby pinpointing why the Morkovin hypothesis fails in transcritical flows. Budget analyses further reveal that pressure–strain redistribution dominates Reynolds-stress transport, while baroclinic torque governs vorticity dynamics, jointly dictating the turbulence anisotropy sustained by intense density gradients. Leveraging this finding, new scaling laws are derived for streamwise velocity, Reynolds stresses, and vorticity (the dominant component of TKE dissipation). The revised laws collapse the data across a wide range of wall-temperature differences, confirming their superior universality and predictive accuracy. These revised correlations improve the predictive accuracy for thermal performance in energy systems operating at transcritical conditions, providing critical insights for optimizing high-pressure fluid engineering applications.