Gaseous Flows and Heat Transfer through Micro- and Nano-channels

The overall object of this paper is a systematic study of gaseous flows and thermal transport in twodimensional microand nano-channels using direct simulation Monte Carlo (DSMC) method. In the flow study, a validation of DSMC code was conducted by simulating a continuum flow in microchannel and the results show that the discrepancy of friction coefficient from theoretical prediction is well below 5%. Then, the effects of compressibility and rarefaction on the flows were investigated through simulating flows with (a) same outlet Knudsen number (Kn) but different pressure drop ratios (=1.3 and 4.5) and (b) low pressure drop ratio (=1.9) but different Kn numbers (=0.043 and 0.083), respectively. For the situation (a), it was found that the high pressure drop flow (pressure ratio: 4.5) show a 15% higher friction coefficient than that of a fully developed flow while the low pressure drop flow (pressure ratio: 1.5) is consistent with incompressible flow prediction. The inspection for the velocity profile development shows that when pressures drop increase along the channel, the center-line velocity become flatten and the velocity gradients near the wall are higher compared with parabolic velocity profile. However, for the situation (b), the rarefactions actually reduce the friction coefficients by 22% (Kn: 0.083) and 36% (Kn: 0.043). An apparent velocity slips along the channel wall exist for both flows. We also studied gaseous flows in microchannels with different surface roughness. The DSMC results show that both relative surface roughness and roughness distribution play very important roles in microchannel flows. High magnitude and densely distributed surface roughness induce higher friction coefficient than that of smooth channels. In the thermal transport study, we simulated gaseous flows in micro/nano channels under uniform wall temperature (500K) boundary condition. Both temperature distribution and the effects of rarefaction on Nusselt number (Nu) are discussed by comparing with those of fully developed flows.