# Radiation and Conjugate Heat Transfer Method

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 November 7, 2014, 07:48 Radiation and Conjugate Heat Transfer Method #1 New Member   ahmad bakri Join Date: May 2010 Posts: 20 Rep Power: 7 Hi All, I have a problem with simulating a complex problem that involves combustion inside a boiler and then heat transfer from flames to metal tubes inside the boiler, the tubes also have gas flowing inside ( Combustion -->Metal-->Process Gas). The problem I have is this: 1-Which radiation model to use and which settings, so far weighted some of gray gases seemed more accurate, but the problem is that I can't activate it in the metal side and don't know if it is the right way to go with such a combination of CHT and radiation. 2-Which mesh topology to use at the interface of metal and gas or metal and flame side. I noticed that If I have the same mesh density on both interfaces, higher temperatures are created in the metal, gas and flame side exit temp. Also shall I use an inflation layer in the metal, gas or flame side or not. finally is there a way to quantify how much energy was transferred to the process gas side and how much energy lost in the combustion side thank you

 November 8, 2014, 05:18 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,929 Rep Power: 85 Read the modelling guide on radiation models. All the radiation models have important implications you should be aware of. You have not provided enough information for me advise you which model to use - and I would rather you understood the models yourself and made the choice yourself from a position of knowledge anyway. The mesh topology should be matched to the flow conditions present. You probably have a flow and thermal boundary layer, so inflation layers are probably required. If this is a steady state model the solid mesh can be quite coarse in comparison. If this is a transient model you may require a fine mesh in the solid as well.

November 8, 2014, 05:29
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 Originally Posted by ghorrocks Read the modelling guide on radiation models. All the radiation models have important implications you should be aware of. You have not provided enough information for me advise you which model to use - and I would rather you understood the models yourself and made the choice yourself from a position of knowledge anyway. The mesh topology should be matched to the flow conditions present. You probably have a flow and thermal boundary layer, so inflation layers are probably required. If this is a steady state model the solid mesh can be quite coarse in comparison. If this is a transient model you may require a fine mesh in the solid as well.

Thank you for the reply, in fact I read the guide but couldn't understand it a lot or get straight answer to which of them fits more my model.

as for the details of the problem I will include more details:

I am modeling combustion of Methane with a peak flame temperature of roughly 2300 K, the furnace domain has interface with thin metal tube of 10 mm of steel, and then the steel has an interface with process gas that has certain composition and certain flow rate. The flow velocity inside the furnace is quiet low and the furnace elevation from burners side to outlet is more 20 m, so buoyancy is also activated. This is normally the radiative part which runs on radiation.

 November 8, 2014, 05:39 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,929 Rep Power: 85 The first question is how optically thick the fluid is. Is the fluid in the burner reasonably transparent? Or will radiation only go a short distance before being absorbed?

November 8, 2014, 05:48
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 Originally Posted by ghorrocks The first question is how optically thick the fluid is. Is the fluid in the burner reasonably transparent? Or will radiation only go a short distance before being absorbed?
Could you please guide me of how to quantify the fluid optical thickness, in my case, the flames are more 1-2 m long at max, then I have tubes starting from 1 m and then extends until the full height of the furnace.

thank you for your time

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