Numerical simulation of stability behaviors and heat transfer characteristics for near-critical fluid microchannel flows

• Microchannel vortex flow can be achieved by near-critical CO2 flow boundary heating.
• Multi-time scale analysis of microchannel vortex mixing flow is conducted.
• Critical boundary thermal–mechanical effect is found responsible for the near-critical instability.
• Self-accelerating/expanding nature and enhanced heat transfer of microchannel vortex flow are identified.

This paper deals with the CO2 near-critical convective flow inside channels of micro-scale. Under near critical conditions, the CO2 fluid is very much expandable/compressible, the density and thermal conductivity change to be near one order of magnitude lower when temperature goes near the critical point. At the same time the Prandtl number and specific heat also form high peaks. In microchannels the effect of natural convection becomes much smaller and the highly thermal expansive fluid with very small thermal diffusivity will act like a periodic thermal plume structure evolution mode. The current study, transient stability and heat transfer characteristics of near-critical microchannel flow are analyzed by solving conservative equations of Mass, Momentum and Energy together with non-Boussinesq incorporation of thermal physical properties. The numerical study is conducted under the ranges of εT=0.00023-0.06533εT=0.00023-0.06533 and εP=0.01626-0.21951εP=0.01626-0.21951 (for critical distance parameters) with boundary heat flux (from several hundreds to 50,000 W/m2). It is found that in microchannels vortex flow is generated by applied boundary heat flux. The thin hot boundary perturbation and thermal–mechanical process of near-critical fluids are major factors. The local span-wise and horizontal parameter changes are also analyzed for the unique near-critical fluid flow. The heat transfer characteristics, especially horizontal acceleration and expanding features are also discussed in this study.