CFX – working medium parameters verification

    • MRMR


      I am calculating nozzle flow in ANSYS CFX and I have met some issues with regard to the enthalpy/entropy values in CFX-Post. I would really appreciate any feedback so thank you in advance. 

      Simulation details: steady state, material: air ideal gas, domain reference pressure: 0 Pa, non buoyant model, stationary domain, total energy heat transfer option incl. Viscous Work Term, SST Turbulence with Automatic Wall Function, none combustion and none thermal radiation. 

      Inlet parameters: subsonic flow, total pressure (stable) is one of the variables but we can assume the values in the range of 0.13-0.18 MPa, medium turbulence option and static temperature in the range of 300-900 deg. C. 

      Outlet parameters: subsonic, average static pressure of 0.11 MPa, pres. Profile Blend 0.05 and Average Pressure Over Whole Outlet.

      The geometry of the nozzle is also one of the variable but we can also assume that for that discussion it is a truncated cone. 

      The problem is that I get the convergence during the simulation and values like pressure, velocity, density, cp seem ok but i.e. enthalpy and entropy differs from the values expected based i.e. on NIST Refprop database.

      In one of the flow channels in the Inlet I get the results for 573 K and 0.18 MPa and the enthalpy based on Refprop should be equal to approximately 579 kJ/kg but in CFX Post when I calculate mass flow average static enthalpy for the Inlet I get the results of 276.21 kJ/kg so the difference is huge. Same for entropy: Refprop 7.36 kJ/kgK vs 0.492 kJ/kgK.

      What can cause such huge differences?
      Thank you again for any tips.

      With kind regards


    • rfblumen
      Ansys Employee

      When using an ideal gas in CFX, the enthalpy and entropy values are related to the reference values based on a reference pressure and temperature.  By default, the values are taken as zero at standard pressure and temperature.  You can modify these reference values under the Material Properties tab of the material if you wish to see enthalpy and entropy values closer to the Refprop values.  Changing these reference values won't have any impact on the solution in terms of the resulting pressure and velocity fields.

    • MRMR

      Dear rfblumen,

      thank you very much for the explanation. 

      With best regards

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