Rarefied gas flow into vacuum through a pipe composed of two circular sections of different radii

•Rarefied gas flow into vacuum through a finite-length composite pipe is studied.•The pipe is made of two cylindrical sections of different diameters.•Results are obtained by solving numerically the S-model kinetic equation.•Mass flow rate is computed and flow structures such as Mach disk are studied.

The paper is devoted to the study of a rarefied gas flow through a composite circular pipe into vacuum. The pipe is made of two cylindrical sections of different diameters. Two cases are studied: wider pipe followed by narrower pipe (converging configuration) and the narrower pipe followed by the wider (diverging configuration). The analysis is based on the direct numerical solution of the Boltzmann kinetic equation with the S-model collision integral. The results are presented for two length to radius ratios and a large range of Knudsen numbers. The main computed characteristic is the mass flow rate through the pipe. The dependence of the flow field on pipe's geometry and Knudsen number is established. Formation of special features of the flow, such as recirculation zones and a Mach disk, is studied.