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In the present investigation, high resolution large eddy simulations (LES) of sCO2 vertical upward flows in microchannels are performed at high mass fluxes (G = 1000 kg/m^2-s) and moderate heat fluxes (q'' = 1.6 − 8.7 W/cm^2), and flow inlet temperature in the range of T = 20 − 100 ℃ to predict sCO2 heat transfer coefficients inside and outside pseudocritical region. Results are compared with our prior computational study of horizontal microchannels at similar thermophysical conditions to determine the effect of channel orientation on possible enhancement or deterioration of heat transfer. Results are also compared with available empirical supercritical heat transfer correlations to assess their applicability at these working conditions. 

Although supercritical CO2 (sCO2) heat transfer has been employed in industrial process since the 1960s, the underlying transport phenomenon in high-flux microscale geometries, as could be employed in concentrating solar receivers, is poorly understood. To date, nearly all experimental studies and simulations of supercritical convective heat transfer have focused on large diameter vertical channel and tube bundle flows, which may differ dramatically from microscale supercritical convection. Computational studies have primarily employed Reynolds averaged (RANS) turbulence modeling approaches, which may not capture effects from the sharply varying property trends of supercritical fluids. In this study, large eddy simulation (LES) turbulence modeling techniques are employed to study heat transfer characteristics of sCO2 in microscale heat exchangers. The simulation geometry consists of a microchannel of 750×737 μm cross-section and 5 mm length, heated from all four sides. Simulation cases are evaluated at reduced pressure P_r = 1.1, mass flux G = 1000 kg/m^2-s, heat flux q'' = 1.7 − 8.9 W/cm^2 , and varying inlet temperature: 20 − 100℃. Computational results reveal thermal transport mechanisms specific to microscale sCO2 flows. Results have been compared with available supercritical convection correlations to identify the most applicable heat transfer models for engineering of microchannel sCO2 heat exchangers.