The Study Of The Subsonic Flow Field At The Outlet Of Micro-channel Using DSMC Method

The flow field at the outlet of a beam source decides the beam's working efficiency,the flow field in micro channel is key to the performance of some MEMS devices,and the flowfield in vaccum chamber is closely related to the supressing of contamination gases in Dynamic Gas Lock,so the study of the flow field of micro-structure or under vacuum condition has been very important.Due to the charactistics of rarefied gas dynamics,the flow field cannot be solved by traditional CFD software,and the analytical solution of Boltzmann equation is very difficult.So the Direct Simulation Monte Carlo(DSMC)method is used here to study the flow field of micro channel under rarefied gas condition,and new treatments of the subsonic boundary conditions are also proposed.They are used to study the angular distribution of the flow field at the outlet of the capillary,and a diverging cone end is proposed to decrease the width of the beam formed by the capillary.Since the flow in the capillary is subsonic,the subsonic boundary conditions in DSMC are studied first,and new treatments of the constant flow rate inlet and the pump outlet are proposed.In the proposed constant flow rate inlet,new treatment of the total number of inserted molecules and new treatment of the distribution of molecules over the inlet surface are proposed.The proposed pump outlet is achieved by the porous outlet,and a new method is proposed to calculate the deleting probability of the molecules that impinge on the outlet surface.Then the proposed constant flow rate inlet and pump outlet boundary,the common used pressure inlet and outlet boundary,and the outgassing wall boundary are implemented in the open source solver dsmcFoam.The DSMC method and the implemented subsonic boundary conditions are used to study the flow field of the two-dimensional rectangular channel.The simulation results are compared with that of DS2 V and the analytical solution,it shows that the proposed constant flow inlet,the pump outlet and the pressure inlet/outlet that implemented are correct.The outgassing wall boundary is also verified by comparing with the result of DS2 V.Then the flow field of two-dimensional T shape channel with more complex boundary settings is studied to verify the subsonic boundary conditions further.Last,the flow through the rectangular channel of different length and with same inlet flow rate is evaluated.The simulation results not only verify the proposed boundary conditions,but also show that the constant flow rate inlet can achieve a more accurate setting flow rate and reduce the computing region,and the proposed pump outlet can achieve the desired outlet pressure when only setting the pump speed.The angular distribution of velocity of molecules that come out of the capillary in a wide rarefaction range is studied.First,the test particle Monte Carlo method is used to calculate the angular distribution of the velocity at the outlet of cylindrical capillary in free molecular flow regime.Based on the results,a diverging cone end is proposed to decrease the divergence of the beam,and the influence of the geometry parameter of the capillary and the CL accommodation coefficients on the angular distribution is also evaluated.Then the DSMC method and pressure inlet boundary are used to calculate the angular distribution at the outlet of capillary in a wider rarefaction region,the effect of the rarefaction coefficient,CL accommodation coefficients and VSS parameter on the distribution is evaluated.The results show that the cone end can decrease the divergence of the angular distribution of velocity at the outlet of capillary under different wall conditions and with different VSS parameter in a wide rarefaction region.DSMC method and the constant flow rate inlet are used to study the angular distribution of number density out of the capillary.The effect of flow rate and measurement position on the angular distribution and the FWHM of the distribution out of cylindrical capillary is studied,and the results are compared with the available experiment data,the deviation of the simulation results of FWHM is within the angular resolution of experiment data when Knudsen number is small,and a bit larger than the resolution with large Knudsen number.The simulation data and experiment data show the similar relationship of the distribution with flow rate.Then,the effect of the cone end on the distribution is evaluated,the results show that the cone end can't decrease the FWHM of the distribution of number density,but it can increase the number density where the polar angle is small,while decreasing the density where the polar angle is large,thus decreasing the divergence of the beam.