Air flows from lower total pressure to higher total pressure

kevinpwp · July 2, 2022, 23:50

Hello,

I'm simulating a natural exhaust hood.

The air near the hot surface has a lower density so it will go up due to buoyancy.

I have checked the outlet flow rate and it has an outlet volumetric flow rate of 1000 m3/hr without any reverse flow.

The problem is that the total pressure at the outlet is much higher than the duct entry. So, how the air can exit the outlet while the fluid flows from higher total pressure (TP) to lower pressure?

You can see the pressure contours below:

Thank you!

LuckyTran · July 3, 2022, 00:11

You have an external force, gravity aka your buoyancy force!

Not sure if it's relevant but you should also be careful what is the definition of pressure, static pressure, and total pressure when working with gravity driven flows because it may or mot not include the hydrostatic component.

kevinpwp · July 3, 2022, 00:29

Quote:

Originally Posted by LuckyTran

You have an external force, gravity aka your buoyancy force!

Not sure if it's relevant but you should also be careful what is the definition of pressure, static pressure, and total pressure when working with gravity driven flows because it may or mot not include the hydrostatic component.

Thank you for your reply. I use ANSYS Fluent and I think the total pressure also includes the buoyancy force effect which results a pressure difference of "dP=g*h*(rho_1-rho_2)".

You mean I have to define a function as: FULL_PRESSURE= Total Pressure -rho*g*y?

I'm confused.

LuckyTran · July 3, 2022, 01:06

There is a lot going on in this picture that is not easily explained away by a few words especially when you are confused what is pressure.

To make it even more confusing, there's also heat transfer and you're putting energy into the flow.

July 2, 2022, 23:50	Air flows from lower total pressure to higher total pressure	#1
kevinpwp New Member Kevin Join Date: Jul 2022 Posts: 9 Rep Power: 3	Hello, I'm simulating a natural exhaust hood. The air near the hot surface has a lower density so it will go up due to buoyancy. I have checked the outlet flow rate and it has an outlet volumetric flow rate of 1000 m3/hr without any reverse flow. The problem is that the total pressure at the outlet is much higher than the duct entry. So, how the air can exit the outlet while the fluid flows from higher total pressure (TP) to lower pressure? You can see the pressure contours below: Thank you!

July 3, 2022, 00:11		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,668 Rep Power: 65	You have an external force, gravity aka your buoyancy force! Not sure if it's relevant but you should also be careful what is the definition of pressure, static pressure, and total pressure when working with gravity driven flows because it may or mot not include the hydrostatic component. kevinpwp likes this.

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July 3, 2022, 01:06		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,668 Rep Power: 65	There is a lot going on in this picture that is not easily explained away by a few words especially when you are confused what is pressure. To make it even more confusing, there's also heat transfer and you're putting energy into the flow.