# Mass Flow Rate is not converging

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 December 7, 2010, 03:08 Mass Flow Rate is not converging #1 New Member   Ghulam Destgeer Join Date: Nov 2010 Posts: 4 Rep Power: 15 I'm analyzing heat transfer and pressure drop in a heat exchanger. All the residuals are converging up till (at least) 10^-3. But some residuals' lines do not go below 10^-3 mark. After completion of solution, when I check Mass Flow Rate at the Inlet which I initially assigned as Inlet Boundary Condition, do not remain same. In fact, Mass Flow Rate becomes significantly low. In addition, if I apply velocity as the Inlet condition, velocity remains same at the inlet, but again when I calculate mass flow rate, it gives very low value. I need to know, how can I make Mass Flow rate to converge. At least mass flow rate should remain same at the inlet as I assign as a boundary condition. Your help would be highly appreciated. Thanks, Ghulam

 December 7, 2010, 17:48 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,695 Rep Power: 143

 December 9, 2010, 07:55 #3 New Member   Ghulam Destgeer Join Date: Nov 2010 Posts: 4 Rep Power: 15 Thanks a lot ghorrocks... It was really helpful...

 December 10, 2010, 07:18 #4 New Member   Ghulam Destgeer Join Date: Nov 2010 Posts: 4 Rep Power: 15 I have followed the instructions and the solution converged to 1e-4 in 50 iterations. But the mass flow rate which I applied as the inlet boundary condition (i.e. value 0.02 kg/s) do not remain same after the solution. As Mass Flow rate is a boundary condition, it should remain same, but the mass flow rate turns out to be 0.0001 kg/s when I calculate it in post processing (Mass Flow over area average ~0.0001kg/s). what should I do to make it remain same as I specify it in the beginning. Regards, ghulam Destgeer Last edited by destgir448; December 10, 2010 at 08:01.

 December 11, 2010, 05:13 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,695 Rep Power: 143 Something is not right in your simulation setup when that happens. Do you have warnings about reverse flow? Also, how are you calculating mass flow rate? Use the mass flow rate calculator in CFD-Post. Mass Flow over area average is not the correct way of calculating it.

 December 11, 2010, 05:55 #6 New Member   Ghulam Destgeer Join Date: Nov 2010 Posts: 4 Rep Power: 15 Thank you so much for your reply. You are right, i was not calculating mass flow rate in the right way... now it is OK. I got my results. Thanks, Ghulam Destgeer

 Tags cfx, convergance, mass flow