Flow regimes and heat transfer mechanisms affecting supercritical transition in microchannels

Supercritical carbon dioxide (sCO₂) exhibits unique thermophysical and transport properties, which have the potential to enhance a wide range of thermal systems. Significant property variations accompanying the pseudocritical transition preclude accurate and generalized predictions of heat transfer, particularly at the microscale. A novel method for investigating fundamental fluid flow and heat transfer mechanisms through heat transfer coefficient measurements and side-view high-speed (8000 fps) schlieren imaging was developed. Experiments are conducted in a square microchannel ($D_h=500\mu \text{m}$) at reduced pressure near unity ($P_R=1.05$) over a range of heat and mass fluxes ($q_c^{″}=1.3-82.6\text{ W}{\text{cm}}^{-2}$; $G=280-1380\text{ kg}{\text{m}}^{-2}{\text{s}}^{-1}$). Non-uniform density profiles within the boundary layer and distinct, freely mixed liquid-like and gas-like packets at various stages of pseudocritical transition were observed. Three flow regimes were identified as a function of heat flux with unique convection boundary layer characteristics. Transport of liquid-like and the production of gas-like sCO₂ at the wall were found to be the primary mechanisms affecting heat transfer and were quantified using a modified form of the Richardson number. The experimental approach and mechanistic insight developed herein provide a basis for high-fidelity heat transfer models for the design of supercritical fluid systems.