Question about the outlet boundary condition.
 

Hello! I am simulating the impeller passsage flow in centrifugal compressor. As far as I konw, impeller passage is short, thus I think it is difficult to apply outlet boundary condition. Neumann condition (gradient of variable is zero) is not appropriate. Would you like to inform me a best way. Thank you.

Re: Question about the outlet boundary condition.
 

Hi, Have you tried using a pressure boundary condition? Regards

Re: Question about the outlet boundary condition.
 

Hi, a colleague of mine is working in a similar area (looking at inlet guide vanes). Knowing the mass flow rate, he estimated the pressure at the exit, and then ran the simulation to find the new mass flow rate. The exit pressure was then revised accordingly. This procedure was iterated until the simulated mass flow rate (obtained with the estimated exit pressure) coincided with the actual mass flow rate.

Hope this helps.

rich

Re: Question about the outlet boundary condition.
 

But usually the mass flow rate is already given at the inlet. Which inlet condition was choiced by him ?

Re: Question about the outlet boundary condition.
 

(1). For turbomachinery computations, especially the compressible flow cases, the total pressure and total temperature are specified at the inlet, with velocity, static pressure and density updated from each iteration. The outlet condition is the specified static pressure. (2). One can use the specified conditions to obtain one converged solution. Or one can zoom in the outlet static pressure to obtain a solution with the target flow rate. (3). Given enough distance from the blade trailing edge, the parallel downstream condition can be used with the specified inlet velocity. There will be some up-stream effect of downstream condition, but that's life. It is problem dependent.

Re: Question about the outlet boundary condition.
 

My fault; I should have been more specific. A total pressure is applied at the inlet (based on atmospheric pressure and inlet bulk velocity) and the static pressure (estimated) is applied at the outlet. The static pressure is continuously updated until the predicted mass flow rate coincides with the actual (known) value.

rich