# Discussion on the computation of the multi-stage axial compressor

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 January 3, 2011, 09:28 Discussion on the computation of the multi-stage axial compressor #1 Member   Join Date: Sep 2010 Posts: 45 Rep Power: 8 I am totally out of practice to the CFX computation of turbomachinery. But the computation a very important part in my PhD thesis. I will appreciate you very much for your suggestion. My present object is a six rows compressor, with two rows of IGV, rotor, stator, and two rows of OGV. The first row of IGV is 8 passages, the second IGV 24 passages, rotor 22 passages, stator 24 passages, the first OGV 24 passages, second OGV 8 passages. To save the CPU, i only simulate one passage of the first IGV and the second OGV, three passages of second IGV, rotor, stator, and the first OGV. The boundary condition is given by total pressure inlet and static pressure outlet. The outlet pressure is guessed by a Numeca computation, which results in a total massflow rate of 12kg/s. But the present computation results in a total massflow rate only about 6kg/s. The reason may be the nonconvergence, but it seems very difficult to converge in the compressor computation. Does it? Thanks first, BalanceChen

January 3, 2011, 21:14
Mass flow rate and RMS in compressor computation
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To be continued, I found that the mass flow rate in the compressor computation is fairly hard to conserve, while the RMS seems be normal. Attached is my present RMS residual curve and Mass flow rate curve at the inlet and outlet. Anyone knows what should I do to help the mass flow rate conserve?

Thank you very much,

BalanceChen
Attached Images
 RMS.jpg (40.2 KB, 31 views) Massflowrate.jpg (25.5 KB, 27 views)

 January 5, 2011, 00:16 #3 Member   D Join Date: Nov 2010 Posts: 50 Rep Power: 8 Hi, I am very new to CFX and CFD as whole. By looking at your graphs, it seems that your simulation is tending towards a mass conservation at the end, as the mass imbalance curves are beginning to coincide with their amplitude reducing, it already seems to be in the range of +-0.2 % Am I missing something here ? or the simulation is heading towards convergence ? Also could some one help me to know why is the mass imbalance even when residuals seem to be converged. Should one take a look at max residual curves in such cases ? Thanks in Advance, Dinesh

January 5, 2011, 02:51
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You are right, the result does converge at the end, but the problem is that, the converged mass flow rate is not the right one, as compared to the NUMECA result. Because this is a test computation, so the NUMECA is applied here as a verificating simulation.

Attached is the Max Residual curve, it seems that the Max residual is a bit to large, is it the cause to the wrong mass flow rate? What should I do then?

BalanceChen
Attached Images
 MaxResidual.jpg (32.9 KB, 19 views)

 January 5, 2011, 03:00 #5 Member   D Join Date: Nov 2010 Posts: 50 Rep Power: 8 I hope that you had defined a monitor for pressure rise across the domain and mass flow through say exit. if you have done, that might help to understand what is going on. What are the total and static pressure rises across the domain from your simulation ? How different are they when compared to Numeca results ? Same for mass flow, I mean is it higher, lower ?? Generally I define monitors for pressure rise across domain and maybe in your case across rotors, stators, IGV's etc.. They generally help to find out a problem if at all it occurs during a simulation. -Dinesh

 January 5, 2011, 03:23 #6 Member   Join Date: Sep 2010 Posts: 45 Rep Power: 8 Thank you very much, Dinesh, what you suggest should be a good method to find the problem, I will take it a try, thank you again BalanceChen

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