3 Replies Latest reply on Dec 12, 2015 8:47 AM by Vincent Garvey

    Compressor - Steam result does not correlate with Air

    Vincent Garvey

      A simulation of a micro centrifugal compressor using Air, P-P gives a reasonable result compared to test data, but trying M-P or P-M does converge, but gives a difference in inlet/outlet mass flow. The project is actually for a Steam compressor, but running with steam never converges - using any combination of I/O boundary condition (P.envir-P.envir, Static P in and out either equal or based on expected dP, Pin and MassFlow out etc.).

      Screen grab of both Air and Steam under identical conditions attached. Intended to use saturates steam at about 100C at inlet, in both simulations, the boundary conditions have been set to 120C to ensure clearance form the saturation curve. I have tried rotating region, averaging and sliding - in all cases the  Steam model runs nominally for a short while, but usually the temperature increases rapidly to >800C and / or I get mass flow inward from both openings. Any guidance most welcome.

      Air P-P.PNGSteam P-P.PNGI

        • Re: Compressor - Steam result does not correlate with Air
          Amit Katz

          Unfortunately I don't have useful advice for you, but I just wanted to chime in and say that I also have the most luck doing pressure-pressure boundary conditions for this type of analysis.

           

          BTW, have you tried creating a custom gas that mimics saturated steam and running that? That way you don't have to worry about the phase change.

            • Re: Compressor - Steam result does not correlate with Air
              Vincent Garvey

              Hi

              Thanks for the replies. I have just now run the model using water (liquid). The system does indeed converge - although at crazy levels of performance - 120,000rpm may be just a bit high for liquid water :-) torque is a few orders of magnitude above what can be provided, BUT it seems to prove that the physical model etc. is valid.

              It does therefore suggest that the modelling of Steam (real gas) has some discontinuities.

              The plot thickens .. see this extract from the help file:-

              Edit Steam.PNG

              This is how the system responded to liquid water:-

              Water P-P.PNG

            • Re: Compressor - Steam result does not correlate with Air
              David Paulson

              Because your model seems to run somewhat OK on air, I would have to conclude that there may be an error in your steam table in your database.  You can also try running with water as a real gas (the Flow database lsits water as a real gas, not sure how this should be different than steam),  or R-123 as a real gas as R-123 is commonly used in centrifugal compressors.  If your program is still not converging, you need to contact your reseller to obtain SW opinion of issue as the solver does not seem to like steam.