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December 5, 2007, 05:10 
Natural Convection

#1 
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Hi FLUENT users.
I'm working on a natural convection problem, ie a solar chimney. air will be drawn from the bottom of my system upwards. Due to temperature difference, air will flow through the chimney. I have problems with setting up the parameters in FLUENT. the minimum temperature involved is about 25C whereas teh highest temperature is 80C for this system. For operating conditions: I have set gravity to be 9.81m/s2, correct? For Boussinesq parameters, what is the operating temperature that I have to set or shall I keep the default value? Do I need to specify variabledensity parameters? For Material (air): I use Boussinesq method to specify density, but do I need to key in a value (by default FLUENT changed to 0)? If so, what is the value? what other parameters that I have to set in the material panel? I have used PRESTO! for discretization method, is that good? What else do I have to consider? Sorry for such a long question, hope to see your reply, thanks! 

December 5, 2007, 13:07 
Re: Natural Convection

#2 
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Hi Racheal If you are using Boussinesq approximation, specify density at one ref temp which might be room temp or 25C in your case.
Specify the volumetric coefficient of expansion for air. Specify density in material panel box (No need to specify variable density parameters) Please refer the fluent tutorial on natural convection for more details. Vaibhav 

December 5, 2007, 20:17 
Re: Natural Convection

#3 
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hi!
I have actually went through the tutorial 'modeling radiation and natural convection', but I'm not sure why those values were selected, eg the '1000 kg/m3' for density and '1e5 K1' for thermal expansion coefficient. I wonder how these values are calculated. Hope to see your reply, thanks! 

December 17, 2007, 15:07 
Re: Natural Convection

#4 
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sent to me your file" cas and data" i try to help you


December 18, 2007, 03:08 
Re: Natural Convection

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Racheal,
Since you deal with air at nearly standard conditions, the ideal gas model may be a good approximation for estimating the thermal expansion coefficient. With this model, the volumetric thermal expansion coefficient =1/T (T in absolute units, e.g., K). I hope this helps, Rami 

September 26, 2009, 23:42 
hello,please help me

#6 
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zhangmin
Join Date: Sep 2009
Posts: 9
Rep Power: 8 
I read your post in the forum,and I have the same questions with you.since the post is a long time ,I think you must have solve the problems,please please help me!
For operating conditions: how can I set the gravity? For Boussinesq parameters, what is the operating temperature that I have to set or shall I keep the default value? Do I need to specify variabledensity arameters? For Material (air): I use Boussinesq method to specify density, but do I need to key in a value (by default FLUENT changed to 0)? If so, what is the value? what other parameters that I have to set in the material panel? I have used PRESTO! for discretization method, is that good? Where can I find the air's thermal expansion coefficient and absorption coefficient? What else do I have to consider? I really hope to see your reply, thanks a lot! please help me ! 

September 26, 2009, 23:52 

#7  
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zhangmin
Join Date: Sep 2009
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Quote:
For operating conditions: how can I set the gravity? For Boussinesq parameters, what is the operating temperature that I have to set or shall I keep the default value? Do I need to specify variabledensity arameters? For Material (air): I use Boussinesq method to specify density, but do I need to key in a value (by default FLUENT changed to 0)? If so, what is the value? what other parameters that I have to set in the material panel? I have used PRESTO! for discretization method, is that good? Where can I find the air's thermal expansion coefficient and absorption coefficient? What else do I have to consider? I really hope to see your reply, thanks a lot! please help me ! 

September 27, 2009, 06:17 

#8 
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Akour
Join Date: May 2009
Posts: 79
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hello,
a lot of people have been asking about what parameters to use in a natural convection problem. This is exactly why in cfd you should always try to work using nondimensional numbers, that way you can characterise your problem with any physical properties you use. For natural convection the Rayleigh number is what you are interested in, depending on what sort of convection you want to see you can choose this number accordingly, along with th Reynolds number which will define the level of turbulence. so to answer the question, type rayleigh number in to google and look at how it is calculated. Also, remember that the the boussinesq approximation is NOT always the way to go, i recommend you look up the limitations of using the boussinesq approximation, e.g. for large temperature gradients and specific thermal expansion coefficients the boussinesq approximation is not good. Instead of the boussinesq approximation you could calculate density on each iteration using a 1st order polynomial = rho(T)=rho(0) + beta*DT sort of thing hope that helped a bit. good luck
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akour 

September 28, 2009, 08:41 
re: natural convection

#9 
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Hi, About the problem of the parameter that you set in fluent in boussinnesq appox. I think that everything depends on the reference temperature that you want to use; that means, density change with temperature and so the thermal expansion.... once u decide the bulk/refernce temperature, this will be the the operating temperature in the operating condition pannel....
for this temperature check in the web the density, and the thermal expansion parameter and put the value in the material pannel. using densityboussinsq model, means using a density constant value but you have to indicate how... that means that is you reference temperature value is the default Fluent reference temperature, density will have the default density reference value. For the other parameter try to use the default parameter of the fluid that you are using. however on the Udf manual there is a strategy for natural convenction and I have find very very helpful; they suggest both PRESTO! and also body force weighted. hope it helps 

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