Mass flow rate in a duct with fixed inlet and outlet pressure conditions

[vijayay]

Hi,

I am trying to compare the flow in different ducts. The duct inlet is smaller in area than the duct outlet. The way I want to compare the duct is that I am applying fixed pressure at inlet and outlet in all variants of the duct and comparing the mass flow rate in each duct for same pressure boundary condition. 

The problem is my duct is smaller at inlet and bigger at outlet, So I can't apply a higher pressure at inlet and lower pressure at outlet. So I applied lower pressure at inlet and higher at outlet. Please see attached results and geometry images.

 Due to higher pressure at inlet and lower at outlet, my streamlines return back immediately to inlet after getting out from the inlet. My velocity and pressure plots looks reasonable but it seams there is some problem with flow direction. Can anyone explain this.  Also my mass flux at inlet is positive, which should be negative if the flow is entering in to the duct at inlet. Please see attached images.

Duct geometryDuct mass flux at inlet (circular cross section)Duct pressureDuct streamlinesDuct velocity

[cfdgeek]

Hi,

Pressure is not pressure force.  You can always apply high pressure at the inlet and low pressure at the outlet, no matter how big of the area of your inlet/outlet are. What you set up in your project just drives your flow from outlet to inlet. That is due to the pressure gradient is wrong. That's why you have streamlines go backward. That's also the reason you have a positive flow rate at the inlet (outward flow from inlet). just apply the pressure boundary condition correctly should fix your problems.

Thanks.