# Heat flux

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 April 25, 2012, 10:27 Heat flux #1 Member     Felice Join Date: Nov 2010 Posts: 43 Rep Power: 6 Hi all, I have a gearbox filled with oil. The motion of the internal gears heat the oil. I want to calculate the temperature distribution in the oil bath. Which BC should I set on the external boundaries (the case of the gearbox)? I do not know the temperature of the walls but only the temperature of the ambient (air). I know that the heat transfer coeff. h is depending on the delta T between the walls and the external air. If I want define that coefficient according to a known law, what should I do? Can someone help me writing an UDF? Regards F air | wall | oil | gears Last edited by felicemastronzo; April 25, 2012 at 10:50.

 April 25, 2012, 11:03 #2 Senior Member   Join Date: May 2011 Location: Germany Posts: 130 Rep Power: 6 Hi, I haven't done conjugated heat transfer with fluent, but I did it with CFX .... so I might be able help you. First question is, what are you interested about? Are you interested in a temperature distribution after a specific time? -> Transient run or "just" in a steady state case? As boundary condition: for the wall | air interaction use a heat flux. This heat flux is, as you mentioned, not constant it is rather dependent on the surface temperature. So you have to create a "heat-flux-function"(Temperature) .... I haven't done this in fluent, but in CFX this is quite simple. Hope some other guys here can help you out with a UDF. And how do you define your "heat-source"? Are you simulating the gears as well, I guess the gears would lead to some motion of the nearby oil ....

April 25, 2012, 12:11
#3
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Felice
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Quote:
 Originally Posted by Zigainer Hi, I haven't done conjugated heat transfer with fluent, but I did it with CFX .... so I might be able help you. First question is, what are you interested about? Are you interested in a temperature distribution after a specific time? -> Transient run or "just" in a steady state case? As boundary condition: for the wall | air interaction use a heat flux. This heat flux is, as you mentioned, not constant it is rather dependent on the surface temperature. So you have to create a "heat-flux-function"(Temperature) .... I haven't done this in fluent, but in CFX this is quite simple. Hope some other guys here can help you out with a UDF. And how do you define your "heat-source"? Are you simulating the gears as well, I guess the gears would lead to some motion of the nearby oil ....
In my case I have a single gear that is rotating in the oil bath, so the heat is directly generated by the motion of the gear. I have already performed transient simulations without the energy equations, so I already know the power losses (--> heat source). In order to calculate them a transient simulation was mandatory, but now I think it is possible to distribuite this power on the whole gear surfaces so to be able to solve only a steady thermal analysis... I hope it is clear what I mean.

Franco

 April 25, 2012, 12:32 #4 Senior Member   Join Date: May 2011 Location: Germany Posts: 130 Rep Power: 6 Okay, so steady state should be fine. But as I mentioned, you need to define a UDF for the heat flux .... sorry that I canīt hekp you with that.

April 25, 2012, 12:57
#5
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Felice
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Quote:
 Originally Posted by Zigainer Okay, so steady state should be fine. But as I mentioned, you need to define a UDF for the heat flux .... sorry that I canīt hekp you with that.
No problem and thanks for the help.
Tscüß!

April 25, 2012, 13:37
#6
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Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
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Quote:
 Originally Posted by felicemastronzo Hi all, I have a gearbox filled with oil. The motion of the internal gears heat the oil. I want to calculate the temperature distribution in the oil bath. Which BC should I set on the external boundaries (the case of the gearbox)? I do not know the temperature of the walls but only the temperature of the ambient (air). I know that the heat transfer coeff. h is depending on the delta T between the walls and the external air. If I want define that coefficient according to a known law, what should I do? Can someone help me writing an UDF? Regards F air | wall | oil | gears
I just wanted to double check that a UDF is necessary.

What is your known law for heat transfer coefficient?

April 26, 2012, 02:51
#7
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Felice
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Quote:
 Originally Posted by LuckyTran I just wanted to double check that a UDF is necessary. What is your known law for heat transfer coefficient?
I have a linear trend for h like h = 0.1 DeltaT + 10. (In which DeltaT is the temperature difference between the external side of the wall and the Tinfinte of the air)

Regards
F

April 26, 2012, 03:12
#8
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Lucky Tran
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Quote:
 Originally Posted by felicemastronzo I have a linear trend for h like h = 0.1 DeltaT + 10. (In which DeltaT is the temperature difference between the external side of the wall and the Tinfinte of the air) Regards F
Where is this relation coming from? I can find little physical justification for treating HTC in this manner when it is a simple convective flow of air against a wall.

Ignoring units you have:
q" = h(DeltaT) and
h = 0.1 DeltaT + 10
q" = (0.1 DeltaT^2 + 10DeltaT)

This type of heat transfer coefficient will lead to nonlinear results. I imagine the solution would also be highly unstable since the heat flux is a quadratic in temperature difference, with the potential to lead to runaway solutions and diverge. Am I overthinking this?

Last edited by LuckyTran; April 26, 2012 at 03:30.

April 26, 2012, 03:37
#9
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Felice
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Quote:
 Originally Posted by LuckyTran Where is this relation coming from? I can find little physical justification for treating HTC in this manner when it is a simple convective flow of air against a wall. Ignoring units you have: q" = h(DeltaT) and h = 0.1 DeltaT + 10 so that leads to: q" = (0.1 DeltaT^2 + 10DeltaT) This type of heat transfer coefficient will lead to nonlinear results. I imagine the solution would also be highly unstable since the heat flux is a quadratic in temperature difference, with the potential to lead to runaway solutions and diverge.
Ok, and what is your suggestion? Which can I than describe the convection phenomena? My idea was to define h and then to define the thermal BC with the "convection option" (that means defining the heat transfer coefficient h, the free stream temperature and the eventual heat generation rate). I understand your point of view but my boss want that I try to solve the problem with this empiric equation for h. If there are not other solutions, can you help me to write the UFD to define h?

Thanks
F

 April 26, 2012, 07:30 #10 Member     Felice Join Date: Nov 2010 Posts: 43 Rep Power: 6 Nobody can help me in writing the UDF? F

 April 27, 2012, 04:27 #11 Member     Felice Join Date: Nov 2010 Posts: 43 Rep Power: 6 Really nobody can help me in writing the UDF? At least some tips.... F

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