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October 3, 2004, 07:44 
Mass imbalance

#1 
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Hello every body
i am new in this cfd field .......... i am trying to simulate two phase airwater flows in pipe, main inlet for water and side inlet for air both have velocity_inlet conditions and single outlet defined by pressure_outlet condition. the lenght of pipe is 11m by 254 mm, using EulerEuler model. I have got some solutions which are converged (i think) bcz the residuals line have gone below 10^8 but when i see the outlet mass flow rates from report/surface integrals they dont not telly Reort/fluxes (may be i dont kno whow interpret the data) ............. is there somehting missing/wrong ....... plz answer the following: (1)as water inlet mass flow rates match but air inlet do not (0.00039 kg/s and to 0.00196 kg/s? (2)plus my outlet mass flow is 0.1320 kg/s which should be 0.18 kg/s ????????? (3) my hand calculated values of air flow rate gives me 0.00196 kg/s but it deos not give me the water flow rate as 0.1854 kg/s report/surface integrals my answers are: At inlet m(air)= 0.0003934 kg/s and m (water) = 0.1854 kg/s & net = 0.1858 kg/s and At outlet = 0.1320 kg/s while for Reort/fluxes: At inlet net mass flow (air) = 0.00196 kg/s at inlet net mass flow (water) = 0.1854 kg/s at outlet net mass flow (mixture) = 0.1874 kg/s Thanks in advance Aly 

October 3, 2004, 08:35 
Re: Mass imbalance

#2 
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1) The problem with the water inlet mass flow rate is probably due to a mistake in specifing the boundary condition at the inlet. Check the volume fraction of the secondary phase.
2) It's not unusual there is some imbalance. Usually it's possible to solve this problem by reducing the value of residual reduction required for convergence. Also try to use a higer order dicretization scheme for convective terms in the solver settings (Solve > Control > Solution). Hi ap 

October 3, 2004, 14:11 
Re: Mass imbalance

#3 
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Thanks i did it
Aly 

October 4, 2004, 02:47 
Re: Mass imbalance

#4 
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I have the same problem as yours,Also you can try the same problem by changing the bubble size, I think it will not converged.


October 4, 2004, 02:53 
Re: Mass imbalance

#5 
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I suggest you to see your velocity and pressure profiles, if you see a very high velocity or pressure gradiants at inlet or outlet your problem cnverges to a faults answer and you must change your inlet profile form constant conditions,I think it will help you to good solution.


October 4, 2004, 22:08 
Re: Mass imbalance

#6 
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hi,Aly: I have the same problem as yours. How did you do?Thanks!


October 5, 2004, 05:09 
Re: Mass imbalance

#7 
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The ratio of the revior's profile area vs inlet's(outlet's) area is very high. It is about 880. So the velocity gradiants at the inlet(outlet) is very high. And can I consider it is not converged? Thanks Best regards!


October 6, 2004, 21:07 
Re: Mass imbalance

#8 
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hi,Aly: How did you solve the problem of mass imbalance? I have the same problem and don't know how to do. Thank you very much. Best regards


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