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Forced Convection over a plane plate

Posted Oct 2, 2013, 10:19 a.m. EDT Fluid & Heat, Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD) Version 4.3a 7 Replies

Matthias Flatscher
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Hi guys,

I calculated the cooling of a plate by a turbulent forced convection analytically. Now I want to check my results with COMSOL but I have some troubles, maybe someone would be so kind and help me.

My plate is 0.075 m long, to keep the plate at +4 °C i need a power of ~130 W. The cooling fluid is air and has a temperature of -30 °C with a "inflow" - velocity of 45 m/s.

I tried to model my plate in COMSOL with a rectangle ( 0.075 m x 0.005 m ), this plate is in the upper right corner of an "larger" rectangle. The inlet is on the left. The material of my plate is copper.

My first problem is how to fit the power, because for my analytic approach i have just a surface (~0.017 m^2) but no volume, which is necessary for a heat source with a unit of W/m^3. When I try to run the simulation with "Total Power" equal my required 130 W, the result is far away from my analytical one.

I'm thankful for any advice, I'm pretty new at COMSOL and an electrical engineer so I'm not really familiar with themal stuff ;-).

BR
7 Replies Last Post Oct 17, 2013, 6:54 a.m. EDT
Matthias Flatscher
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Posted: 1 decade ago Oct 3, 2013, 12:14 p.m. EDT
Hi,

I just simplified my structure to test the stuff with a constant temperature of +4°C. It's really strange because I get a maximum temperature of -12°C, so not even my boundary with my constant temperature has the +4°C. I already used a very fine mesh.

I atteched the file, maybe someone can tell me what I make wrong!?!?!?

thx
Hi, I just simplified my structure to test the stuff with a constant temperature of +4°C. It's really strange because I get a maximum temperature of -12°C, so not even my boundary with my constant temperature has the +4°C. I already used a very fine mesh. I atteched the file, maybe someone can tell me what I make wrong!?!?!? thx

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Matthias Flatscher
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Posted: 1 decade ago Oct 4, 2013, 9:29 a.m. EDT
Hi,

I just checked it with an older version of COMSOL (3.5). When I set there a constant temperature at a boundary this temperature also appears in the result (direct on the boundary, of course ;-) )!!!

So PLEASE can anyone tell me what I make wrong with version 4.3 ?!?!?!?!?

thx
Hi, I just checked it with an older version of COMSOL (3.5). When I set there a constant temperature at a boundary this temperature also appears in the result (direct on the boundary, of course ;-) )!!! So PLEASE can anyone tell me what I make wrong with version 4.3 ?!?!?!?!? thx
Tero Hietanen
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Posted: 1 decade ago Oct 7, 2013, 1:44 a.m. EDT
Hi,

I tested your model and noticed same problem. This has maybe something to do how Comsol handles wall functions. I don't know is this right or wrong but if you want to get rid of this use " Low-Reynolds k-epsilon" turbulence flow. This will give you more accurate results.

Best regards

Tero Hietanen
Hi, I tested your model and noticed same problem. This has maybe something to do how Comsol handles wall functions. I don't know is this right or wrong but if you want to get rid of this use " Low-Reynolds k-epsilon" turbulence flow. This will give you more accurate results. Best regards Tero Hietanen
Matthias Flatscher
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Posted: 1 decade ago Oct 10, 2013, 11:04 a.m. EDT
@ Tero:

it works with Low-Reynolds k-epsilon. But when i investigate my real problem with a boundary heat source my results are a little bit strange.

@ all:

To be sure I can use the turbulent model I increased the length of my plate to 1m, thus I get a Reynolds number of 3.2491 * 10^6. For that length and an area of 1 m^2 I need a power of 4349 W to keep my plate at +4°C (inlet @ -30°C and 45m/s).

When I simulate it the end of my plate is at a temperature of ~ +240°C and the front is at ~ -20°C. So my question is, is it possible to validate my calculations. Because it is a logical result that the end of the plate is the warmest spot and the end the hottest. Maybe it is juast a meshing problem!?!?

One problem that I have is I can not use the COMSOL implementet material for AIR. When i try to use it I get different error massages or the error did not convergate. Currently I use constant (mean) values for air in the region of -30°C to +10°C.

I atteched the file, maybe someone would be so kind and have a look at it. I'm trying to verify my results scince almost one week :-(.

thx
@ Tero: it works with Low-Reynolds k-epsilon. But when i investigate my real problem with a boundary heat source my results are a little bit strange. @ all: To be sure I can use the turbulent model I increased the length of my plate to 1m, thus I get a Reynolds number of 3.2491 * 10^6. For that length and an area of 1 m^2 I need a power of 4349 W to keep my plate at +4°C (inlet @ -30°C and 45m/s). When I simulate it the end of my plate is at a temperature of ~ +240°C and the front is at ~ -20°C. So my question is, is it possible to validate my calculations. Because it is a logical result that the end of the plate is the warmest spot and the end the hottest. Maybe it is juast a meshing problem!?!? One problem that I have is I can not use the COMSOL implementet material for AIR. When i try to use it I get different error massages or the error did not convergate. Currently I use constant (mean) values for air in the region of -30°C to +10°C. I atteched the file, maybe someone would be so kind and have a look at it. I'm trying to verify my results scince almost one week :-(. thx

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Tero Hietanen
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Posted: 1 decade ago Oct 11, 2013, 5:17 a.m. EDT
Hi Matthias,

You have used Turbulent flow and Heat transfer by separated physic. Instead of this, use Isothermal Flow where turbulent flow and heat transfer are connect with each other. Also check what values you get to "dimensionless distance to cell center". This should be low as 0.5.

This may help you:

www.comsol.com/blogs/which-turbulence-model-should-choose-cfd-application/

Best regards

Tero
Hi Matthias, You have used Turbulent flow and Heat transfer by separated physic. Instead of this, use Isothermal Flow where turbulent flow and heat transfer are connect with each other. Also check what values you get to "dimensionless distance to cell center". This should be low as 0.5. This may help you: http://www.comsol.com/blogs/which-turbulence-model-should-choose-cfd-application/ Best regards Tero
Matthias Flatscher
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Posted: 1 decade ago Oct 11, 2013, 10:57 a.m. EDT
Hi Tero,

thank you for the link. I already read it but i jumped over the last paragraph. Thank you very much!!!!!!

Can someone tell me what the "dimensionless distance to cell center" is, and how is it calculated? I couldn't find anything on the web or even at the COMSOL website. And is there a possebility to set this value before I start the simulation?

I still have the problem that I can't use the COMSOL implemented air - material. I get error massages like "Attempt to evaluate non-integral power of negative number". How can I fix that??

BR

Hi Tero, thank you for the link. I already read it but i jumped over the last paragraph. Thank you very much!!!!!! Can someone tell me what the "dimensionless distance to cell center" is, and how is it calculated? I couldn't find anything on the web or even at the COMSOL website. And is there a possebility to set this value before I start the simulation? I still have the problem that I can't use the COMSOL implemented air - material. I get error massages like "Attempt to evaluate non-integral power of negative number". How can I fix that?? BR
Matthias Flatscher
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Posted: 1 decade ago Oct 17, 2013, 6:54 a.m. EDT
Hi guys,

I'm still searching for a definition of the dimensionless distance to cell center. Can someone tell me how it is calculated!?!?!?

thx
Hi guys, I'm still searching for a definition of the dimensionless distance to cell center. Can someone tell me how it is calculated!?!?!? thx

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