The angular distribution at the outlet of a capillary in a wide range of rarefaction

• The angular distribution is evaluated in a wide range of rarefaction using DSMC.
• The distribution is in good agreement with data available in free molecular regime.
• The distribution is based on CL reflection and VSS collision model.
• Conical end can decrease the divergence of the beam in a wide rarefaction range.

The angular distribution at the outlet of a capillary is an important topic in rarefied gas dynamics. Here the procedure to calculate the near field angular distribution based on velocity distribution function or DSMC (Direct Simulation Monte Carlo) method was proposed. Then, the angular distribution at the outlet of cylindrical capillary was evaluated with different rarefaction parameter δ using DSMC method with open source solver dsmcFoam. The results showed that the normalized flux intensity at small polar angle decreased with δ when δ was smaller than 10, and the distribution did not approximate the cosine distribution when δ at the inlet was large enough. Besides, capillary with conical end was also evaluated. The results showed that the conical end could decrease the divergence of the beam even δ at the inlet increased to 50 when the outlet was set as vacuum. Last, the influence of CL (Cercignani Lampis) accommodation coefficients and VSS (Variable Soft Sphere) parameters on the angular distribution was studied.