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Old   November 4, 2013, 07:22
Default Radiation exchange
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Jens Wiese
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Hello everybody,

i have got some problems to exchange radiation energy between Fluent and an external solver. At first some facts to my simulation case:

- i am simulating a small wood burner for domestic heating
- using DOM, because p1 model doesn't work for my case.
- coupling Fluent with an external solver for the fixed bed. I am exchange mass and heat transfer between the external solver and Fluent.

I want to couple the radiation exchange between fixed bed and gas phase by a surface. This surface should have the external radiation temperature of the fixed bed surface.

I have the following problems:

- i find no options to exchange radiation energy over an interior surface

- i tried to seperate the mesh above the fixed bed. Then i had an outlet at the lower mesh zone and an inlet at the mesh zone above. I wrote an udf to interpolate between the outlet and the inlet to "merge" these both zones. Except that recirculating zones can't be realised in this separated part, this way works well. But for an inlet i can just consider a global black body temperature for the inlet. However, i prefer to use a temperature profile.

- I tried also to use a wall at the upper mesh part instead of an inlet. Then i tried to use this wall as an inlet by means of an UDF to merge the both mesh parts. That doesn't work, because Fluent does not respect modified cell data. Anybody tried to use a wall as an inlet?

- The last chance i see is to implement the DOM model per UDF in Fluent myself.

Has anybody any suggestions or maybe tried to implement DOM by means of an UDF? Maybe there is an alternative and i can't see it!

Thanks in advance for your help

Jens
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Old   November 30, 2013, 22:26
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Tomas Jurena
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Hi Jens,

I´ve been dealing with the same problem for some time. There are a lot of scientific articles on coupled simulations of e.g. solid fuel combustion in grate furnaces, where the in-bed and freeabord processes are solved by separate models and coupled through boundary conditions. However, the critical part of the modelling methodology, i.e. the coupling technique including radiation heat transfer, is not documented well (at least I have not found any article discussing these problems in detail).

The only exception seems to be an up-to-date work by Gómez at al. (2014), which provides for the most complete description of the radiation heat tansfer between the fixed bed and freeboard regions. However, they do not use an external code for simulation of fixed-bed combustion. Instead, the use is made of porous zone, where processes and all additional equations for the solid phase are included in the model by means of User Defined Scalars and Functions.

Speaking about coupling of two separate codes, beside UDF, which unfortunatelly do not seem to be fully applicable in your (neither my) case, profiles of various variables can be defined through Define-Profiles... Profiles are written in a simple data file, that can be easily generated by an external code. The loaded profile is then accessible in the drop-down list on Radiation tab of Inlet Boundary Conditions. However, Internal Emissivity seems to be the only quantity that can be defined by the profile, not the Black Body Temperature, which is actually needed.

I don't think inlet boundary conditions such as mass flow or velocity can be defined at walls by any means (except injections for DPM).

I wonder if anybody else on the forum managed to solve this radiation heat transfer problem.

Tomas

References:
Gómez M.A., Porteiro J., Patiňo D., Míguez J.L., 2014, CFD modelling of thermal conversion and packed bed compaction in biomass combustion, Fuel 117, 716-732.
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