Heat flux direction

CFD10111 · September 21, 2022, 07:38

Hi,

I'm quite confused on what my heat flux direction should be. I have a geometry like in the image attached in this email. The circle is generating 6W of heat. I defined the circle as solid & the rest as fluid. ANSYS says "A positive value of heat flux implies that heat enters into the domain"; so is this the direction of the red arrows or the green?

Thank you

FMDenaro · September 21, 2022, 12:02

Quote:

Originally Posted by CFD10111

Hi,

I'm quite confused on what my heat flux direction should be. I have a geometry like in the image attached in this email. The circle is generating 6W of heat. I defined the circle as solid & the rest as fluid. ANSYS says "A positive value of heat flux implies that heat enters into the domain"; so is this the direction of the red arrows or the green?

Thank you

The normal unit to the boundary must be outward oriented. The vector heat flux in a fluid is q=-k*Grad T

LuckyTran · September 21, 2022, 13:30

If the black circular line is a wall then the red arrows is positive heat flux. Problem is you have an interface which means you'll have a wall and shadow-wall pair and you really should be careful because positive means along the surface normal of whichever is the wall and the wall could be facing either the fluid side or the solid side. A best practice is to put the wall facing the fluid side but it is entirely up to you and how you click buttons in Ansys which side is which.

CFD10111 · September 23, 2022, 10:08

I see, thank you!

CFD10111 · September 23, 2022, 10:12

That's very useful, thank you! I haven't used Fluent much so I wasn't aware of the shadowing at interfaces. Can I ask if you know what the wall thickness should be for a solid cylinder generating heat? I know you'd need to input the layer thickness for composite cylinders, but I'm not sure if, for a solid cylinder (i.e. the battery cells), I should type in the diameter or 0 for the wall thickness? Thanks! (I'm on about the wall thickness in the thermal tab btw)

LuckyTran · September 23, 2022, 10:16

The wall thickness is for applying a shell conduction model. If you know, you know. If there is no shell conduction, then leave the thickness at 0.

CFD10111 · September 23, 2022, 10:59

Oh, I see. I'm not modelling two fluids separated by an interface - moreso a fluid flowing around solid cylinders, so I don't think I'm supposed to use a shell conduction model. Sorry last question lool: can I double check if I'm correct in thinking that if the wall thickness is 0, the heat generation rate (in W/m^3) should also be zero? I've made the heat flux non-zero though, by using Q"=Q/A, where A=2*pi*R*L (for the solid cylinders). It is a 2D simulation btw. Thank you so much!

September 21, 2022, 13:30		#3
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,747 Rep Power: 66	If the black circular line is a wall then the red arrows is positive heat flux. Problem is you have an interface which means you'll have a wall and shadow-wall pair and you really should be careful because positive means along the surface normal of whichever is the wall and the wall could be facing either the fluid side or the solid side. A best practice is to put the wall facing the fluid side but it is entirely up to you and how you click buttons in Ansys which side is which. CFD10111 likes this.

September 23, 2022, 10:16		#6
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,747 Rep Power: 66	The wall thickness is for applying a shell conduction model. If you know, you know. If there is no shell conduction, then leave the thickness at 0. CFD10111 likes this.

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September 23, 2022, 10:08		#4
CFD10111 New Member Avon Join Date: Aug 2022 Posts: 26 Rep Power: 4	I see, thank you!

September 23, 2022, 10:12		#5
CFD10111 New Member Avon Join Date: Aug 2022 Posts: 26 Rep Power: 4	That's very useful, thank you! I haven't used Fluent much so I wasn't aware of the shadowing at interfaces. Can I ask if you know what the wall thickness should be for a solid cylinder generating heat? I know you'd need to input the layer thickness for composite cylinders, but I'm not sure if, for a solid cylinder (i.e. the battery cells), I should type in the diameter or 0 for the wall thickness? Thanks! (I'm on about the wall thickness in the thermal tab btw)

September 23, 2022, 10:59		#7
CFD10111 New Member Avon Join Date: Aug 2022 Posts: 26 Rep Power: 4	Oh, I see. I'm not modelling two fluids separated by an interface - moreso a fluid flowing around solid cylinders, so I don't think I'm supposed to use a shell conduction model. Sorry last question lool: can I double check if I'm correct in thinking that if the wall thickness is 0, the heat generation rate (in W/m^3) should also be zero? I've made the heat flux non-zero though, by using Q"=Q/A, where A=2piR*L (for the solid cylinders). It is a 2D simulation btw. Thank you so much!