The Direct-simulation Monte Carlo (DSMC) method has been employed to analyze the rationality of the 2-D simplification for a 3-D straight rectangular cross-sectional channel, a 3-D straight rectangular cross-sectional channel with microstructures and a 3-D straight rectangular cross-sectional channel with a backward-facing step. An implicit treatment for low-speed inflow and outflow boundaries for the DSMC of the microchannel flow is employed. The 3-D microchannel flows are simulated with the various cross-aspect ratios ranging between 1 and 5. The calculated heat and flow properties in the 3-D cases are compared with the results of the 2-D case. It shows that when the aspect ratio is less than 3, the two extra side walls in the 3-D case have significant effects on the heat transfer and flow properties. When the aspect ratio increases, the flow properties and heat transfer characteristics of the 3-D simulations tend to approach that of the 2-D results. This paper also provides the information of the approaching level for different geometry of the microchannels. It is found that the approaching level becomes slower when the geometry of the channel becomes more complicated. In the present study, when the cross-section aspect ratio is 5, the approaching level of the above mentioned three different geometry of microchannels are found to be about 99, 97.8 and 97 respectively.
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