I'm designing a nozzle for space with Flow Simulation. I had some problems with vacuum until i detected that SW dosen't run properly in this conditions.

I'm trying to simulate a nozzle with 20bar in input and 1atm  at output. When i plot the results using cut plots tool i obtain that results.

All this results are completely consistence according my conditions. High temperature, high pressure low Mach number at the entrance  and low temperature, low pressure and high Mach number at the exit. However when I use Surface Parameters tool something happens. Using Surface Parameters tool the average output temperatures and pressure are bigger than input temperature and pressure.

Can anybody tell me what is happening?