Large eddy simulations of high-heat-flux supercritical CO₂ convection in microchannels: Mixed convection and non-uniform heating

Near-critical-point supercritical fluid convection is a promising solution for emerging high-flux thermal management needs because of the high fluid thermal conductivities and specific heats. Supercritical convection has been extensively studied for large diameter channels with uniform heating (4.1 < D < 22.7 mm, 0.05 < q^'' < 330 W cm⁻²). However, limited information is available on transport processes to guide engineering of high-flux compact supercritical heat transfer equipment, which often have non-uniform heating distributions. To address this need, large eddy simulations (LES) are employed to study supercritical CO₂ convection in microchannels (750μm×737μm cross-section). First, the simulation approach is validated with published experimental data. Studies are then conducted for horizontal isothermal heated channels at reduced pressure P_r=1.1, mass fluxes G=100-1000kgm^-2s^-1 (Re=3000-35,700), average wall heat fluxes q^''=24-62Wcm^-2, and bulk flow temperatures T_bulk=291-317K (inside and outside pseudocritical range). Results are used to assess the applicability of published supercritical convection correlations for microchannel heat exchangers. All available supercritical correlations are found to under-predict heat transfer coefficients at high heat fluxes (q^''=58-62Wcm^-2). The bottom-to-top wall heat transfer coefficient (HTC) ratios from these cases are used to determine a new criterion for the onset of significant mixed convection effects. At low mass fluxes (G=100kgm^-2s^-1), this HTC ratio is found to exceed 2.5×. Simulations indicate that, at these conditions, increased heat fluxes lead to reduced HTCs for low and high mass fluxes, but increased HTCs at intermediate mass flux values. Finally, an illustrative case is evaluated to assess the impact of conjugate heat transfer effects in microscale supercritical convection, and highlight limitations of conventional modeling approaches. This study quantifies the accuracy of convection correlations for high-heat-flux microchannel pseudocritical conditions, provides new criteria for predicting the onset of mixed convection in microchannel supercritical flows, and demonstrates the impact of conjugate heat transfer effects in microchannel supercritical heat exchangers.