Learning Forum

[sar123]

How to prevent the backflow in inlet and outlet of a turbo expander when simulating using Ansys CFX?

For my final year project, I am trying to reproduce thesis results to get familiar with the simulation procedure and to validate my method. I modeled the rotor blade and stator vane using BladeGen and after meshing, imported those to Ansys CFX. My working fluid is NOVAC 649. Since I couldn't get the RGP files, I manually added the material to the CFX setup. After that, I introduced the inlet total temperature and pressure and for the outlet, the static pressure is provided as the boundary condition. But after a few loops, 100%area of both the inlet and the outlet has been covered with the wall and the solution gives an error. When I try the open boundary condition, I cannot see any pressure, temperature variations in the results. Hence I tried introducing the mass flow rate for the outlet and ended up with different results. Can anyone help me with this problem?

[rfblumen]

If you get 100% walls at an inlet total pressure boundary, it means the total pressure just downstream is higher (maybe only slightly) than the boundary. Similarly, for the outlet static pressure boundary, the pressure just upstream is slightly lower if you get 100% walls. Assuming you're running at design point conditions, this would seem to indicate the boundary condition values are not appropriate.
Using total pressure at the inlet and static pressure at the outlet is an appropriate set of boundary conditions for a turbine, but using a mass flow outlet should work as long as the stator/rotor are not in a choked condition. If you specify a mass flow outlet with an appropriate mass flow, determine in CFD-Post (or include a solution monitor) what the outlet state pressure is. Try running with that value of static pressure with an Average Static Pressure outlet boundary.