Nitrogen gas flow in two-dimensional microchannels with a sudden contraction or expansion is simulated by direct simulation Monte Carlo (DSMC) method in both slip and transition regimes. The resistance coefficient is calculated and compared with that for conventional-scale channels. It is found that the resistance coefficient for microchannels differs from that for conventional-scale channels in two aspects. One is the resistance coefficient for microchannels is much smaller and the divergence between microchannels and conventional-scale channels increases with the increase of Knudsen number. For the cases considered in this paper, the coefficients for microchannels are about 5% to 61% of those for conventional-scale channels. On the other hand, the resistance coefficient for microchannels is more sensitive to the expansion ratio of channels, especially when the expansion ratio is less than 3.

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