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Temperature & Heat Flux Boundary

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Old   May 21, 2009, 00:18
Default Temperature & Heat Flux Boundary
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James Macphee
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Hi,

I am currently working on a project of modelling a rotary lime kiln.

At this stage I am trying to model purely the flow domain yet at the bed boundary face I need to define a set temperature AND set heat flux. Due to the chemical reaction that is occuring in the bed a high amount of energy is absorbed. So essentially what I need to do is have a fixed temperature and heatflux at the boundary, something which many people are telling me is mathematically impossible given that one is a function of the other.

However, on reading past forums it appears that someone may have written a UDF to do exactly what I am after, I cannot figure how to define both, only how to modify the heat flux equation.

The other idea is to include the solid bed and give it a fixed temperature with a sink term.....something which Fluent doesnt directly allow so I guess I would have to write a UDF to do this also. I would like to simply model the domain if I can.

If anyone has any experience with modelling lime kilns or a similar situation and can give me some advice as to where to go it would be greatly appreciated.

Thanks
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Old   May 22, 2009, 15:55
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Daniel Tanner
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Have a look at the thin-wall and shell conduction models in Fluent. These allow you to model heat conduction through a virtual wall boundary (of thickness X, you specify the solid wall properties) adjacent to the flow. This would allow you to specify a constant temperature and a sink/generation term in the "virtual" wall.
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Old   May 22, 2009, 23:19
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James Macphee
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Daniel, thanks for your reply.

However, I have been playing around with these models. The problem I encounter however is that the temperature that you define is that on the outside surface of the wall, not at the interface of the solid fluid. When I set there to be a negative heat generation term (ie a sink) the temperature of the wall adjacent to the fluid is reduced to the temperature needed to achieve the required heat sink. I dont see a way to set the temperature of the fluid/solid interface and have a heat sink in the wall.

If you have any other suggestions for me they would be greatly appreciated.

Thanks for your help

James
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Old   May 23, 2009, 09:36
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Daniel Tanner
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You could set idealised wall properties and a very small delta x (wall thickness) so the temperature at both inner and outer wall are essentially the same. However, as you say, if you include a negative generation term the temperature at the fluid-wall interface would be affected.

Is your problem sensibly posed? How about you only set a fixed temperature at the wall and include a energy source term in the fluid to account for the heat released during the reaction. If the temperature in the fluid is higher you get a resultant heat flux to the wall which you can monitor.
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Old   May 25, 2009, 21:47
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James Macphee
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Thanks Daniel. I will look into your latest suggestion and see what happens.

Thanks for all your help

James
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