# How define Internal Emissivity for flue gas INLET?

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 March 28, 2001, 06:48 How define Internal Emissivity for flue gas INLET? #1 Harry Qiu Guest   Posts: n/a Dear Mr. and Mrs.: Can you tell me (1)how to define the REFRACTIVE COEFFICIENT for non-penetrated solid and for flue gas(CO2 and H2O)? (2)Is it ok to define "internal emissivity = 1 "for INLET of the flue gas? Thank you very much. Sinserely, Harry Qiu

 April 2, 2001, 04:18 Re: How define Internal Emissivity for flue gas IN #4 Volker Pawlik Guest   Posts: n/a Hi Harry, if I understand you correctly you want to model radiation in a combustion chamber (in your case it is that porous ceramic) without modelling the flame but letting enter hot flue gases into your computational domain? Following this kind of problem you'll have the difficulty to define radiation boundaries at your inlet (for your flue gases). Now the inlet is no solid but can be considered as one with the properties of the gas. The "only" thing you have to do is to calculate the emissivity of that gas body based on the mole-fractions of H2O and CO2 (and CH4 in case of fat combustion: equivilance ration > 1) which for a methane combustion are the radiating components. The emissivity depends on the geometrical situation (==> layer thickness, correction factors, etc.), the mole-fractions in order to calculate the partial pressure, and the temperature. (e.g. see VDI Wäremeatlas Chapter Kc, unfortunalety for you it is in German, but I am sure there is a lot of literature in English too) E.g for a equiv. ratio of 0.74, methane flame, 1923 K I got an emissivity of 0.0555. Volker

 April 2, 2001, 06:18 Re: How define Internal Emissivity for flue gas IN #5 Harry Qiu Guest   Posts: n/a Hi,Volker: Thank you for your answer. What is equivelence ratio? You know, this concept seldom emerges in Chinese literature. Can you introduce it in brief. Did you use steady equation or unsteady equation when you modeling the laminar methane combustion? Thank you. Sincerely yours, Harry.

 April 2, 2001, 07:43 Re: How define Internal Emissivity for flue gas IN #6 Volker Pawlik Guest   Posts: n/a Hi Harry, equivialance ratio is the invert of the "air number" (that is just a direct translation from German to English I don't know whether it or s.th. simlar exists). The "air number" is the ratio between mass-flow of air which is supplied to mass-flow needed for a stoichometric combustion. "air number"=1 /equi. ratio = 1 stoichometric combustion "air number" > 1 / equi. ratio <1 : lean combustion (more air than needed) " air number" < 1 / equi. ratio <1 :fat combustion (less air than needed) P.S: I used the steady state approach P.P.S: Please do not mix up your questions. Other participants of the forum maybe confused.

 April 2, 2001, 09:48 Re: How define Internal Emissivity for flue gas IN #7 Harry Qiu Guest   Posts: n/a Hi, Mr. Volker: you are right, I should not mix the questions which are of different type.I do like that only because I know you are familar with the gasous combustion and I guess you can answer me. you know, sometime we post the message, but no one answers at all. Sorry! and thank you for your good explanation. Sincerely, Harry.

 April 2, 2001, 21:14 Re: How define Internal Emissivity for flue gas IN #8 Jin-Wook LEE Guest   Posts: n/a Dear Volker Pawlik Your answer is perfect for numerical situation. Fluent's default value(inlet emissivity = 1.0) might be good for some sense. It might be sorry but I have one question. In real system, do you think, where does the energy(transfered to the inlet from the combustor) go to ? Does it increase the temperature of swirler, supporting frame or ...... ? Or the energy can be treated as heat loss, in real system ? So, I think that your approach, which is very good for numerical aspect, might be not good for the modeling of real system, especially when inlet is very wide. The above is my opinion only. So, I would like to hear your opinion. Sincerely, Jinwook

 April 5, 2001, 08:50 Re: How define Internal Emissivity for flue gas IN #9 Volker Pawlik Guest   Posts: n/a I think that we are talking of different things. 1.)Imagine the following situation: You have a burning chamber but you cannot / want not model the whole chamber but just a part far away from flame. Hence hot gas is entering your domain. In case of really hot gases radiation becomes important. Referring to this case I made my suggestion of setting the (gas-) emissivity for the inlet. 2.) In your case you want to consider radiation losses to the burner (air supply) e.g. through the air supplying holes. 2.1) I think that in this case setting the emissivity to 0 is not the right way from the point of the burning chamber. Then the losses are negelected. Paying just attention to the contribution of the losses without considering what is happening with them outside the chamber (as you said they may heat the air), setting the emissivity of the inlet to the emissivity of the walls outside the burning chamber in combination with the black body temperature of the outside walls should be the right way. 2.2) If the preheating of the supplied air by the radiative losses shall be taken into account then maybe two or three iterative loops should help: a.) calculate the radiative loss once and simulate or calculate the new temperature of the wall and the suppied air (assume constant heat transfer coeff., complete transfer of the radiative loss to the supplied air) b) Take the new external wall temperature as the new black body temperature and carry out a new simulation with the new inlet temperature you got from the step above c) continue with a) and new values This is certainly laborious but should lead to the aim. I think that is what you did/do with your UDF? Volker

 April 8, 2001, 20:32 Re: How define Internal Emissivity for flue gas IN #10 Jin-Wook LEE Guest   Posts: n/a Dear Volker Pawlik I entirely agree ypur suggestion of 2.2. Actually I did. But it required more than two or three iteration loop and the result was not so different from that of inlet emmisivity=0.0. That's why I use the value(inlet emmisivity=1.0) for engineering approach. Anyway, I think that your approach is theoretically perfect, so that it should help the accuracy of the simulation result. Sincerely, Jinwook

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