The effects of outlet boundary conditions on simulating supersonic microchannel flows using DSMC

High speed gas flows through two-dimensional microchannels have been investigated using the Direct Simulation Monte Carlo (DSMC) method, where the pressure boundary condition has been implemented using the theory of characteristics as an alternative to the vacuum boundary. Two species, nitrogen and helium, have been used to conduct the flow simulations. It was found that the pressure boundary condition cannot only predict the flow with exit-plane pressure equal to the back pressure, which the vacuum boundary condition fails to do, but can also simulate the flow with expansion waves outside the channel. Therefore, it is considered to be more appropriate. Two inlet Mach numbers, 4.15 and 3.39, have been employed for the nitrogen flow cases with an inlet Knudsen number (Kn) of 0.062. It has been shown that for cases with an inlet Mach number equal to 4.15, the back pressure only has an effect on flow in the latter half of the microchannel, where the wall heat flux can be enhanced by increasing the back pressure. At an inlet Mach number of 3.39, the wall heat flux has the same trend as that in the higher Mach number case, though its magnitude is considerably lower. In addition, no significant effect of a step change in wall temperature distribution on the total heat exchange between the wall and the bulk flow was detected for the same inlet Mach number and back pressure.