Numerical Analysis of Heat Transfer Characteristics to Supercritical CO2 in a Vertical Mini-Channel: Transition and Pseudo-Boiling

Large eddy simulations have been conducted to study upward transitional flows and heat transfer characteristics of supercritical CO2 in a vertical mini-channel. The numerical simulation was carried out on a modified buoyantPimpleFOAM solver in OpenFOAM 7, and was verified using experiment data. Numerical results indicate that increasing the Grashof numbers can reduce the flow stability and make the flow transition earlier. There are four stages of heat transfer in the transition process, i.e., weakened, improved, recovered and normal heat transfer. These heat transfer phenomena in the transition process were explained from three perspectives: thermal boundary layer theory, turbulent transport and pseudo-boiling theory. Heat transfer enhancement during transition is related to the transport of supercritical molecular clusters, and these molecular clusters are regarded as pseudo-bubbles in pseudo-boiling theory. The flow pattern of the pseudo-phases in the dia-Widom process contains single-phase flow, steady pseudo-film flow, unsteady pseudo-film flow, partial pseudo-bubbles flow and flocculent pseudo-film flow. Pseudo-bubbles have similar behaviors to subcritical bubbles, i.e., break-up, deformation, condensation and coalescence. Relevant researches in this work are favorable for understanding the heat transfer mechanism of supercritical fluids during flow transition.