Heat transfer and various convection structures of near-critical CO2 flow in microchannels

Supercritical/near-critical fluid is very dense and much expandable, and with its preferable flow and heat transfer properties, it has been proposed in various kinds of energy conversion systems. The fluid critical transitions and diverges are very important for both hydrodynamic study and heat transfer applications. The current study deals with the near-critical CO2 horizontal flow and heat transfer performance in microchannels. Careful numerical procedures are carried out with Navier-Stokes equations, energy and state equations, which are treated with special care for micro-scale investigations. Due to the thermal-mechanical effects of critical fluid, abnormal thermal convection structure and transient micro-scale vortex mixing evolution mode are found in microchannels, which is identified to be governed by basic Kelvin-Helmholtz instability. New sources of near-critical fluid thermal-mechanical perturbations, instead of gravity waves, are found to be responsible for the microscale instabilities for the transient vortex flow development. It is also found that while in closed systems the Piston Effect (PE) is dominant, the thermal-mechanical expansion characteristics yield new convective structures and evolutions in open system (microchannels). At the same time the microchannel heat transfer is greatly enhanced due to the vortex evolutions. Possible model extensions are also discussed, and the near-critical fluid convection problem is then characterized from a more general viewpoint in this study.