Simulating high velocity flow into a tube

Hello, I am new to Molflow and need some guidance on how to simulate high-velocity, parallel flow into a chamber.

I have a body, similar to a tube with an inlet and an outlet. Gas is flowing into the inlet of the body at around 1 km/sec and each incoming particle should be mostly parallel. The body of the tube is cooled down to about 20 K and I’m trying to figure out how many particles will reach the exit.

Thank you for any help and guidance you can give!