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Temperature dependent wall heat flux

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Old   December 6, 2018, 19:22
Default Temperature dependent wall heat flux
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Hello All,

I am trying to model a temperature dependent wall heat flux for a forced convection problem, and I cannot find any simple examples or help on the UDF. From my understanding I will be using DEFINE_HEAT_FLUX, but I am lost when it comes to writing the code.

The wall heat flux is a simple expression where it is directly proportional to the temperature q=aT where "a" is a constant.

Thanks in advance.
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Old   December 6, 2018, 23:07
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All information about this topic could be found in Ansys Fluent Customization manual.

Direction for you:
from manual

DEFINE_HEAT_FLUX Description

You can use DEFINE_HEAT_FLUX to modify the heat flux at a wall. Despite the name, a DEFINE_HEAT_FLUX UDF is not the means to specify the actual heat flux entering a domain from the outside. To specify this type of heat flux, you would simply use a DEFINE_PROFILE function in conjunction with a heat flux thermal boundary condition. In contrast, a DEFINE_HEAT_FLUX UDF allows you to modify the way in which the dependence between the flux entering the domain and the wall and cell temperatures is modeled.

DEFINE_PROFILE Description

You can use DEFINE_PROFILE to define a custom boundary profile or cell zone condition that varies as a function of spatial coordinates or time.
Note that DEFINE_PROFILE allows you to modify only a single value for wall heat flux. Single values are used in the explicit source term which ANSYS Fluent does not linearize.

Simply saying for your case DEFINE_PROFILE is appropriate macro
code example from manual
Code:
/***********************************************************************
UDF for specifying steady-state parabolic pressure profile boundary
profile for a turbine vane
************************************************************************/
#include "udf.h"
DEFINE_PROFILE(pressure_profile,t,i)
{
real x[ND_ND]; /* this will hold the position vector */
real y;
face_t f;
begin_f_loop(f,t)
{
F_CENTROID(x,f,t);
y = x[1];
F_PROFILE(f,t,i) = 1.1e5 - y*y/(.0745*.0745)*0.1e5;
}
end_f_loop(f,t)
}
Once again, I recommend to you read Ansys Fluent Customization manual first

best regards
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Old   December 9, 2018, 15:42
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Hi Alexander,

Thanks for the response. I did go over the manual briefly before posting this and it seems as if the Define_Heat_Flux UDF is more fitting for a temperature dependent wall or cell heat flux.

The manual states, Define_Profile is used to "to specify the actual heat flux entering a domain from the outside." Whereas define_heat_flux UDF is used to "modify the way in which the dependence between the flux entering the domain and the wall and cell temperatures is modeled"

Attached is a picture of the geometry for clarification.

thanks,
Justin
Attached Images
File Type: png Geometry Schematic.PNG (119.0 KB, 24 views)
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Old   January 25, 2020, 03:36
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Hi there,


Could you find the answer?
I have faced the same problem.
All I know is that we should use Define_heat_flux for temperature dependent heat flux.


https://www.afs.enea.it/project/neptunius/docs/fluent/html/udf/node247.htm
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