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Old   January 28, 2010, 13:26
Default CFX overpredicting turbine massflow
Sherif Kadry
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I have been modeling a 2 stage axial turbine, and basing my Total Pressure inlet boundary condition and static pressure exit condition based on my experimental results. The turbulence model used is SST, and its a steady state case that's being run. I notice that at all shaft speeds the massflow is overpredicted by 5%. However, total-to-static efficiencies are within 0.5%. What could be the reason for the overprediction? Here is an image of my

Any expert opinions would be helpful thanks.
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Old   January 31, 2010, 16:40
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5% is not too much I think. It can caused by the inaccuracy of the measurement too. You can find it out by measuring an other quantity. Or doing some mesh sensitivity check can be helpful..
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Old   January 31, 2010, 17:04
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Glenn Horrocks
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In general, to get the simulation accurate within that last 5% will require very careful and detailed refinement. You will need to do mesh, convergence and timestep sensitivity studies to a much tighter tolerance than you usually would. You will also have to consider the accuracy of your physics to additional precision - for instance does surface roughness contribute? Do you know your gas composition accurately? Are you using real gas models? Is SST suitable for this level of detail? You obviously have a good handle on most of these things if you are within 5% but to get even more accurate you will need to reconsider.
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Old   February 1, 2010, 04:15
Sherif Kadry
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attesz, ghorrocks, thanks for your replies. Ghorrocks you banged on an important point here. When I ran the cfd simulations I ignored the effect of humidity, being that I ran my experiments in Texas (very humid), I think humidity plays an important role, now, I need to figure out how to incorporate humidity, which reduces the density and could explain the 5% difference. Anyone have any ideas on how best to incorporate humidity into CFX, should I run a water-vapour+air ideal gas mixture?
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