Negative pressure but Positive flow?

ashrawage · February 18, 2012, 21:28

Guys...
I have a tank in which water comes in at 6.33kg/sec from the top centre at a pressure which is unknown,but around 218 ATM. The outlet pressure is fixed at round 217 ATM. So you see there is not a big pressure drop across but there is some! Ie inlet pressure is greater than outlet. Now when I use mass flow inlet and pressure outlet I do get the flow and there is some back flow in the system on one or two faces or sometimes 8 faces of the pressure outlet but the overall vectors show a positive flow from inlet to outlet. But when I look into the values of pressure at inlet...it is around 0.5-1 ATM lower than the outlet!! What the hell is going on here?
Please give me some pointers regarding this anomaly.
Thanks guys.

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I have been getting a lot of help form all you cfd pros for the past month or so and I wanna thank you all for this. Hopefully I will be able to contribute to this forum in the near future as well rather than just ask questions.

Abhijeet

Zigainer · February 20, 2012, 06:31

Actually you should not have any backflow. So try to decrease the under-relaxation factors at the beginning and run the simulation until there is no backflow.
Another point would be to check the operating pressure. What have you set? Just do not forget that the pressure at the "pressure-outlet" is a gauge pressure!
Another point would be the hydraulic pressure, if I see your draft correctly (you have a vertical distance from inlet to outlet)

ashrawage · February 20, 2012, 07:56

@ZIGAINER
I am not sure what you meant exactly by hydraulic pressure and how to set it?
Also my operating pressure is 1 ATM: i.e. it is default. But as I understand I have to subtract 1 ATM from my outlet pressure which is 217. So I set my outlet pressure at 216 ATM.and sorry for the bad sketch but the inlet is at the centre(0.15m) and the inlet pipe goes 2.5 m deep in the tank which is 4.5 m deep. And the outlets are on either side of the inlet 0.1m diameter.as for the elevation between inlet and outlet, there is no difference.
Thanks a lot
Abhijeet

February 18, 2012, 21:28	Negative pressure but Positive flow?	#1
ashrawage Member Abhijeet Shrawage Join Date: Feb 2012 Posts: 31 Rep Power: 14	Guys... I have a tank in which water comes in at 6.33kg/sec from the top centre at a pressure which is unknown,but around 218 ATM. The outlet pressure is fixed at round 217 ATM. So you see there is not a big pressure drop across but there is some! Ie inlet pressure is greater than outlet. Now when I use mass flow inlet and pressure outlet I do get the flow and there is some back flow in the system on one or two faces or sometimes 8 faces of the pressure outlet but the overall vectors show a positive flow from inlet to outlet. But when I look into the values of pressure at inlet...it is around 0.5-1 ATM lower than the outlet!! What the hell is going on here? Please give me some pointers regarding this anomaly. Thanks guys. \| \|\| \| \| \|\| \| \|. \| \|. \| \|. \| \|. \| \|. \| \|. \| \|. \| \|___________\| I have been getting a lot of help form all you cfd pros for the past month or so and I wanna thank you all for this. Hopefully I will be able to contribute to this forum in the near future as well rather than just ask questions. Abhijeet

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February 20, 2012, 06:31		#2
Zigainer Senior Member Join Date: May 2011 Location: Germany Posts: 130 Rep Power: 15	Actually you should not have any backflow. So try to decrease the under-relaxation factors at the beginning and run the simulation until there is no backflow. Another point would be to check the operating pressure. What have you set? Just do not forget that the pressure at the "pressure-outlet" is a gauge pressure! Another point would be the hydraulic pressure, if I see your draft correctly (you have a vertical distance from inlet to outlet)

February 20, 2012, 07:56		#3
ashrawage Member Abhijeet Shrawage Join Date: Feb 2012 Posts: 31 Rep Power: 14	@ZIGAINER I am not sure what you meant exactly by hydraulic pressure and how to set it? Also my operating pressure is 1 ATM: i.e. it is default. But as I understand I have to subtract 1 ATM from my outlet pressure which is 217. So I set my outlet pressure at 216 ATM.and sorry for the bad sketch but the inlet is at the centre(0.15m) and the inlet pipe goes 2.5 m deep in the tank which is 4.5 m deep. And the outlets are on either side of the inlet 0.1m diameter.as for the elevation between inlet and outlet, there is no difference. Thanks a lot Abhijeet