Mixing in microchannels is an important problem because the flow is always in laminar mode even though the velocity could be high. This present paper is trying to expose the inherent effect factors on micro gases mixing by using the direct simulation Monte Carlo (DSMC) method at high Knudsen numbers. Before simulations the discretization errors of the DSMC method were discussed to ensure the numerical veracity. The simulation results show that the wall characteristics have little effects on the mixing length when the main gas flow velocities for different wall characteristics were at a same value. The gas temperature increase decreases the mixing length at a nearly inverse proportional rate. After defining a new dimensionless mixing coefficient by the mixing length to the channel height ratio, it was found that the mixing coefficient was proportional to the Mach number, and inversely proportional to the Knudsen number. This conclusion was validated in gas flows at microscale, while may be expected to expand to all laminar gas flows at all scales.

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