Numerical investigation of near-critical fluid convective flow mixing in microchannels

• Microchannel mixing can be achieved by near-critical CO2 flow boundary heating.
• Near-critical boundary thermal-mechanical effects will lead to strong thermal disturbance in microchannels.
• New kind Kelvin–Helmholtz instability is identified in the near-critical instability evolution.
• Based on the current findings, potential applications are proposed in micro-mixing field.

Supercritical CO2 fluid has been widely used in chemical extraction, chemical synthesis, micro-manufacturing and heat transfer apparatus, and so forth. The current study deals with near-critical CO2 microchannel mixing flow and its basic characteristics. Careful numerical investigations are carried out by solving the coupled computational fluid dynamic equations. The results show that strong near-critical vortex flows can be achieved in a relatively wide range of initial and controlling conditions in microchannels. Basic, isothermally developed flows are simulated and then used as the initial state for a heat convective simulation. After the wall heat flux is applied, the vortex mixing flow originates from the hot boundaries in microchannels with height D=100 μm to 200 μm, while natural convection will gradually become dominant for microchannels with D=300 μm to 500 μm. The current micro-mixing evolution can be ascribed to a novel type of Kelvin–Helmholtz instability. Well-correlated characteristic numbers are identified for the effective near-critical microchannel mixing cases. The vortex growth and evolution mode in microchannels are found to differ greatly from previous micro-mixing methods. Possible applications in micro-engineering/chemical process are also discussed in this study.