Learning Forum

[SEANGHAI]

Hello! I am modeling a big building and let the air go inside the building by creating many inlets and release the air from the building by creating many outlets, after running the calculation, I go to Report Fluxes and check the mass flow rate, then I realized that the mass flow rate of all the inlets are the same to what I applied before running the calculation, but all the mass flow outlets are not the same as what I applied, so it it correct or not? If it is correct, what is the reason? ( For my thought, the reason why the mass flow outlet are not the same is because of the the changing of air density in the domain, but because I am unconfident with my idea, so can you give me your advice?) You can see my figures below

These the boundaries' names:

[YasserSelima]

How much is the difference %?n

[SEANGHAI]

Thank you, for quick reply. Because there are many outlets, so the differences depend on the location of each outlet, but for the minimum difference is 57% and the maximum difference is 92%. The results of mass flow outlet after running calculation is smaller than the amount of mass flow outlet which I applied before running calculation from 57% to 92% according to the location of each outlet. Thank you! I will appreciate for all your advices!n

[DrAmine]

I generally does not like this kind of weak over-constraining the system by providing mass flow at the outlets. At least you should leave a couple of them as pressure boundaries. Is the air compressible? Is the case converging?n

[SEANGHAI]

! Hello! Sir! This is the assignment provided by my professor, so all the outlets are provided in m?/hr, then I converted them to kg/s. I do not know how to convert them to Pressure. I think the air is incompressible in my case, because the air density decreased. And Yes! the solution is converged at 1760 iterations. Is there any method which I can convert volume rate (m?/hr) to Pressure (Pa)? Thank You!n

[Karthik R]

Hello,nWhat boundary conditions are you applying on your inlets and outlets? nKarthikn

[YasserSelima]

I remember from earlier post that your case was steady. In steady state, the ntotal mass flow rate in = total mass flow rate out. nThe difference should be minor, Less than 1-2% ... But you have to apply this to the total. i.e add all the mass flow in and all the out. nThe volue flow rate can change due to the density as you mentioned. This could be larger than the previous estimate .. like it can reach 5-10%.he is applying mass flow inlet, we discussed this in earlier post. Not sure about the outlet, but I am assuming it is pressure outlet.n

[YasserSelima]

How do you calculate the mass flow at outlet? Are you using report?n

[SEANGHAI]

!Thank you again! and I am using mass flow inlet for the all the inlets and mass flow outlet for all the outlets, because they are all the given data which I get from my professor. And regarding to how to check the mass flow outlets, I used report fluxes after running the calculation, but they are different from the amount that I applied before running the calculation. For mass flow inlets, all are correct. Just only the mass flow outlets are wrong, so in this case, as I think and as you mentioned as well, maybe it is because of the changing of the air density in the domain of the building that can change the volume rate at the outlet. nAnother one thing, there is no balance between total mass flow rate in and total mass flow rate out. Total mass flow rate out is bigger than total mass flow rate in around 7.5%. So it is bigger than what you told me that it should be less than 1%-2%.n

[SEANGHAI]

!Hello! Sir! I am using mass flow inlet for all the inlets and mass flow outlet for all the outlets in the building. I want to change some outlets to pressure outlet as advice me, but I could not, because all the data were given in volume rate (m?/hr), thus I converted all the data to mass flow rate, I do not know how to convert volume rate to pressure. Thank you!n

[YasserSelima]

Use outlet pressure. Give it a try for one simulation and check the mass-balance. If you get better results, your professor will be happy ... believe me! n

[DrAmine]

As I said: try to avoid the prescription of mass flow at outlets. Use pressure outlet: I assume outside we have ambient pressures. You can set that on a couple of outlets and set the rest the the prescribed mass flow rate.n

[SEANGHAI]

!Yes! Sir! Thank you! I will try it! n