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Harry Qiu March 28, 2001 06:48

How define Internal Emissivity for flue gas INLET?
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

Jin-Wook LEE March 28, 2001 21:02

Re: How define Internal Emissivity for flue gas IN
Dear Harry Qiu

I would like to hear other specialist's opinion, too. So I am attaching the post.

RE : Internal emissivity for INLET = 1.0

I think that Fluent's default value(1.0) is correct for surrounding boundary condition. Energy balance of combustor should be perfect if you have converged solution. But, it causes energy unbalnce of the combustor for the real(physical) situation. Consider the real situation. I think that radiation energy is transfered to the inlet and the same amount of energy is transfered into the combustor by the advection of the inflow gas(usually air). Otherwise, the temperature of air supplying system(swirler or supporting frame ...) would be increased continuously. That's why I think that some amount of energy is transfered to the inlet by radiation and the same amount of energy is transfered into the combustor by the advection of inflow gas. The best way to model this situation is, I think, to add sensible energy of inflow gas(air) by the same amount of radiation energy. However, it is impossible by the default setting of Fluent(maybe other commercial code, too) because we do not know the amount of energy before calculation. So, I have considered the use of UDF, but it would require another big-loop-calculation, may result in terrible computational time.

Fortunately, because the area of inlet for the combustor is fairly small for most cases, it has not been severe problem. However, when I was trying to simulate incinerator(assummed inlet area was very large) using inlet emissivity of 1.0, the temperature of combustion gas in the incinerator is very low because much energy(20~30%) is transfered to the inlet by radiation. At that time, I used NUMERICAL TRICK, which is, to set the emissivity of inlet as 0.0. The concept of TRICK is to change the real system to the virtual system.

Real system : Energy is transfered to the inlet by RADIATION and the same energy is transfered into the combustor by the ADVECTION.

Virtual system : Energy is transfered to the inlet by RADIATION and the same energy is transfered into the combustor by the RADIATION. This can be done easily by setting inlet emissivity as 0.0. This is not the same as the real situation. But the result was, I think, fairly reasonable. Most of all, it satisfies energy balance of the real system and numerical system simultaneously.

This trick(emissivity at inlet is 0.0) is, I think, not good for research purpose but fairly good for engineering purpose.

Frankly speaking, I would like to hear other specialist's approach for inlet emissivity when they use Fluent and to hear opinion or criticism for my numerical trick.

Sincerely, Jinwook

Harry Qiu March 29, 2001 09:12

Re: How define Internal Emissivity for flue gas IN
Dear Jinwook:

How are you. Your answer is always the best for me. Do you mind my asking you so many questions? Actually I try to answer some others' question. I think, I get much help from here, especally from you. I do must repay.

I'm still modeling laminar premixed gasous combustion.

There are two zones in my geometry model, solid and fluid. The fluid zone is surrouded by the solid zone. the fluid is flue gas, and solid is ceramic(a type of non-penetratable material).

I want to model the radiatin of both ceramic wall and flame(flue gas).

After I define DO(discrete ordinat) model, I meet two difficulties :

(1)I don't know how to define absorption coefficient for solid(ceramic) in "Material Panel". As we all know, FLUENT set solid absorption coefficient as zero by default. Can I keep this default value "zero" as non-penetratable solid absortion coefficient?

(2)When I define Boundary condition, I have to set solid zone "Participate in radiation", otherwise wall radiation will not be calculated when iteration is started. Will radiation in the interior of the solid zone be computed as in the flue gas since I set solid absorption coefficient as zero and specify solid zone "Participate in radiation"?

One more question:(3)Can I calculate wall radiation(no fluid radiation) without define any radiation model?

looking forward to your answer!

Tank you for your help.

Sincerely yours, Harry Qiu.

Volker Pawlik April 2, 2001 04:18

Re: How define Internal Emissivity for flue gas IN
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.


Harry Qiu April 2, 2001 06:18

Re: How define Internal Emissivity for flue gas IN

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.

Volker Pawlik April 2, 2001 07:43

Re: How define Internal Emissivity for flue gas IN
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 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.

Harry Qiu April 2, 2001 09:48

Re: How define Internal Emissivity for flue gas IN
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.

Jin-Wook LEE April 2, 2001 21:14

Re: How define Internal Emissivity for flue gas IN
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

Volker Pawlik April 5, 2001 08:50

Re: How define Internal Emissivity for flue gas IN
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?


Jin-Wook LEE April 8, 2001 20:32

Re: How define Internal Emissivity for flue gas IN
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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