Applying heat transfer boundary conditions

pkladisios · August 10, 2015, 06:01

Greetings!

I have a solid containing a fluid (water tank, 100% filled). My question pertains to the heat transfer between the tank and its environment. Is there a way to apply boundary conditions like convection and solar radiation?

Steffen595 · August 11, 2015, 01:13

heat flux from where to where?

pkladisios · August 11, 2015, 02:57

For instance, on the top surface of the tank we have heat gain from solar radiation (heat flux) and heat loss to the environment via convection. From what i understand, we can't apply two boundary conditions on the same surface. How should i approach this? Haven't found any example or tutorial where we have both convection and heat flux on the same surface...

Edit: I just realized that you cannot use a surface as a boundary condition and as an interface at the same time, something that perplexes things. I really need a comprehensive example.

ghorrocks · August 11, 2015, 05:11

Quote:

both convection and heat flux on the same surface

It is pretty simple maths to derive it and use CEL expressions to apply it.

Quote:

I just realized that you cannot use a surface as a boundary condition and as an interface at the same time

Do you mean the free surface? That is inside the domain, not on a boundary - that is why you cannot apply a boundary condition on it

pkladisios · August 11, 2015, 08:56

Thank you for your answer!
Lets us ignore the first part of your answer for the time being. I'll figure out the specifics about CEL expressions later.
What i've done so far is define a solid and a fluid domain, as well as their automatically generated solid-fluid interface. If i go ahead and insert a boundary condition in a surface, that surface will be subtracted from the solid-fluid interface. Naturally, if i do this (add boundary conditions) for all the surfaces within the interface, i'll end up without interface. Weird huh?
On top of that, i have NO heat transfer between the domains. I've been "tinkering" for quite some time without results...

ghorrocks · August 11, 2015, 19:02

Quote:

Weird huh?

Not really. You are replacing the interface condition with a boundary condition. So the interface disappears. It does not seem weird to me

You can add heat fluxes to the interface boundary condition. This allows you to have an interface with additional heat input. I think this is what you want. Have a look at the interface boundaries on the domain level (not the interface condition by itself).

pkladisios · August 12, 2015, 02:59

I see....Each domain's interface provides a heat source option. How do i make the domains convect heat? I don't want just a steady heat flux.

Steffen595 · August 12, 2015, 05:17

you have heat flow and heat flux option.
You can set one domain as heat source or select some faces and set them as heat source. Then you need buoyancy setting enabled for the fluid.
For domain you can set heat as kW per m^3 and for a surface kW per m^2.
Or you can specify a temperature.
It is a good idea to create named objets in the mesher. I.e. name the domain or the surfaces in question something descriptive like heat source. Then they are easier to identify in the Preprocessor

pkladisios · August 20, 2015, 03:47

I apologize for taking a long time to answer. I' d like to thank both of you, for pointing me to the right direction. I have successfully run simple simulations (at least i think so), both steady and transient. Now my problem lies in simulating a partially filled water tank. Including a second fluid certainly perplexes things...

shadnia · November 17, 2015, 11:52

Quote:

Originally Posted by ghorrocks

It is pretty simple maths to derive it and use CEL expressions to apply it.

Do you mean the free surface? That is inside the domain, not on a boundary - that is why you cannot apply a boundary condition on it

Hi, I have the same problem. when I combine heat flux and convection into one equation, FLUENT does not accept that equation, because the derived equation is related to time (due to heat flux) and temperature (due to convection). I do not know how to solve the problem.
Any help will be appreciated!

ghorrocks · November 17, 2015, 16:35

Try the fluent forum.

shadnia · November 18, 2015, 11:51

Quote:

Originally Posted by ghorrocks

Try the fluent forum.

Thanks a lot for your quick respond. I could not find anything to solve my problem. Since you did kindly suggest up in this threat that "It is pretty simple maths to derive it and use CEL expressions to apply it", I derived the equation, but does not work. Therefore I would appreciate if you or any body else kindly have any idea to solve the problem. Thanks in advance

ghorrocks · November 18, 2015, 20:05

Convection is Flux = h(Twall - Tambient)

So if you have a heat flux you wish to apply on top of that the Flux = h(Twall-Tambient)+A where A is your additional heat flux.

shadnia · November 18, 2015, 21:32

Quote:

Originally Posted by ghorrocks

Convection is Flux = h(Twall - Tambient)

So if you have a heat flux you wish to apply on top of that the Flux = h(Twall-Tambient)+A where A is your additional heat flux.

Thanks again for your kindly respond. I did so, but as I already mentioned, since I am applying the solar heat flux for a whole day (24 hours) which is based on time, the final derived equation is related to time (due to solar heat flux) and temperature (due to convection). Therefore when I use "DEFINE_PROFILE" to write the corresponding UDF, FLUENT does not accept it. I think here is the reason from the FLUENT manual "Note that DEFINE_PROFILE allows you to modify only a single value for wall heat flux". I was wondering if there is any other way to approach to this problem. Thanks again

ghorrocks · November 19, 2015, 05:42

Switch to CFX and it will work fine. Otherwise refer to post #11.

shadnia · November 23, 2015, 14:08

Thanks again. For solving my problem, after studying much more, I came up with the following conclusion. If I define the environment around the top box for FLUENT, I will have two separate surfaces for applying the boundary conditions. Therefore I will have the heat flux for one of two surfaces and convection and radiation for the other one. Please see the attached, figure 1 shows the geometry I have already worked with and Figure 2 is the new one. The problem I have now is that the surfaces I want to apply boundary conditions for are predefined by FLUENT as interface, so I have no access to assign boundary conditions for them. so the question will be: Is there any way to convert interface to wall? If yes so, I will be able to follow my hypothesis. ThanksGeometry.pdf

shadnia · November 24, 2015, 12:28

This is what I do ready need, but how can I add heat fluxes to the interface boundary condition? Thanks in advance

ghorrocks · November 24, 2015, 18:28

Let me quote post #11 for you:

Quote:

Try the fluent forum.

This is the CFX forum. For details on Fluent try the Fluent forum.

Shomaz ul Haq · November 26, 2015, 05:43

Quote:

Originally Posted by Steffen595

you have heat flow and heat flux option.
You can set one domain as heat source or select some faces and set them as heat source. Then you need buoyancy setting enabled for the fluid.
For domain you can set heat as kW per m^3 and for a surface kW per m^2.
Or you can specify a temperature.
It is a good idea to create named objets in the mesher. I.e. name the domain or the surfaces in question something descriptive like heat source. Then they are easier to identify in the Preprocessor

Hi all. Hey Steffen I have made double pipe heat exchanger with hot water fluid domain inside, an inner pipe and then an outer cold water fluid. There is no heat transfered along the length of the pipe when I opened CFD-Post the same inlet temperatures run through all contours for both pipes. If I define heat as kW/m3 would ay heat transfer take place then. Also I wanted to ask how can I define heat as kW/m3 for a a domain as I don't see any option for that when I select Thermal Energy In Option in Heat Transfer in Fluid Models in Domain:Hot water (or Cold water). I have searched a lot to define a heat in kW/m3 but couldn't find anything. Would be grateful for reply. Thanks.

ghorrocks · November 26, 2015, 05:54

Either the no heat transfer result is correct or you set the simulation up wrong.

If you are in a flow regime when very little heat transfer occurs then it is correct that you don't see much heat transfer. This could be the case if the intermediate material has high thermal resistance or specific heat or many other factors.

If the simulation is wrong then you set it up wrong. The most obvious thing is whether the interfaces between the fluids and solids are correct. If they are wrong it will stop heat transfer. Have a look at the non-overlap % reported in the output file.

Also what do you mean by heat as kW/m3? Do you mean there is a volumetric heat source in there somewhere? Or is that the heat content of the incoming fluids?

August 11, 2015, 02:57		#3
pkladisios Member pkladisios Join Date: Jul 2015 Posts: 39 Rep Power: 10	For instance, on the top surface of the tank we have heat gain from solar radiation (heat flux) and heat loss to the environment via convection. From what i understand, we can't apply two boundary conditions on the same surface. How should i approach this? Haven't found any example or tutorial where we have both convection and heat flux on the same surface... Edit: I just realized that you cannot use a surface as a boundary condition and as an interface at the same time, something that perplexes things. I really need a comprehensive example. Last edited by pkladisios; August 11, 2015 at 03:53. Reason: omission

August 20, 2015, 03:47		#9
pkladisios Member pkladisios Join Date: Jul 2015 Posts: 39 Rep Power: 10	I apologize for taking a long time to answer. I' d like to thank both of you, for pointing me to the right direction. I have successfully run simple simulations (at least i think so), both steady and transient. Now my problem lies in simulating a partially filled water tank. Including a second fluid certainly perplexes things... Last edited by pkladisios; August 20, 2015 at 04:55. Reason: correction

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August 10, 2015, 06:01	Applying heat transfer boundary conditions	#1
pkladisios Member pkladisios Join Date: Jul 2015 Posts: 39 Rep Power: 10	Greetings! I have a solid containing a fluid (water tank, 100% filled). My question pertains to the heat transfer between the tank and its environment. Is there a way to apply boundary conditions like convection and solar radiation?

August 11, 2015, 01:13		#2
Steffen595 Senior Member Join Date: May 2012 Location: Melbourne Posts: 161 Rep Power: 13	heat flux from where to where?

August 11, 2015, 08:56		#5
pkladisios Member pkladisios Join Date: Jul 2015 Posts: 39 Rep Power: 10	Thank you for your answer! Lets us ignore the first part of your answer for the time being. I'll figure out the specifics about CEL expressions later. What i've done so far is define a solid and a fluid domain, as well as their automatically generated solid-fluid interface. If i go ahead and insert a boundary condition in a surface, that surface will be subtracted from the solid-fluid interface. Naturally, if i do this (add boundary conditions) for all the surfaces within the interface, i'll end up without interface. Weird huh? On top of that, i have NO heat transfer between the domains. I've been "tinkering" for quite some time without results...

August 12, 2015, 02:59		#7
pkladisios Member pkladisios Join Date: Jul 2015 Posts: 39 Rep Power: 10	I see....Each domain's interface provides a heat source option. How do i make the domains convect heat? I don't want just a steady heat flux.

August 12, 2015, 05:17		#8
Steffen595 Senior Member Join Date: May 2012 Location: Melbourne Posts: 161 Rep Power: 13	you have heat flow and heat flux option. You can set one domain as heat source or select some faces and set them as heat source. Then you need buoyancy setting enabled for the fluid. For domain you can set heat as kW per m^3 and for a surface kW per m^2. Or you can specify a temperature. It is a good idea to create named objets in the mesher. I.e. name the domain or the surfaces in question something descriptive like heat source. Then they are easier to identify in the Preprocessor

November 17, 2015, 16:35		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Try the fluent forum.

November 18, 2015, 20:05		#13
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Convection is Flux = h(Twall - Tambient) So if you have a heat flux you wish to apply on top of that the Flux = h(Twall-Tambient)+A where A is your additional heat flux.

November 19, 2015, 05:42		#15
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Switch to CFX and it will work fine. Otherwise refer to post #11.

November 23, 2015, 14:08		#16
shadnia New Member R-Sh Join Date: Oct 2014 Location: USA Posts: 23 Rep Power: 11	Thanks again. For solving my problem, after studying much more, I came up with the following conclusion. If I define the environment around the top box for FLUENT, I will have two separate surfaces for applying the boundary conditions. Therefore I will have the heat flux for one of two surfaces and convection and radiation for the other one. Please see the attached, figure 1 shows the geometry I have already worked with and Figure 2 is the new one. The problem I have now is that the surfaces I want to apply boundary conditions for are predefined by FLUENT as interface, so I have no access to assign boundary conditions for them. so the question will be: Is there any way to convert interface to wall? If yes so, I will be able to follow my hypothesis. ThanksGeometry.pdf

November 24, 2015, 12:28		#17
shadnia New Member R-Sh Join Date: Oct 2014 Location: USA Posts: 23 Rep Power: 11	This is what I do ready need, but how can I add heat fluxes to the interface boundary condition? Thanks in advance

November 26, 2015, 05:54		#20
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Either the no heat transfer result is correct or you set the simulation up wrong. If you are in a flow regime when very little heat transfer occurs then it is correct that you don't see much heat transfer. This could be the case if the intermediate material has high thermal resistance or specific heat or many other factors. If the simulation is wrong then you set it up wrong. The most obvious thing is whether the interfaces between the fluids and solids are correct. If they are wrong it will stop heat transfer. Have a look at the non-overlap % reported in the output file. Also what do you mean by heat as kW/m3? Do you mean there is a volumetric heat source in there somewhere? Or is that the heat content of the incoming fluids?