Why no emissivity in mixture parameters?
 

Hi,Mr. or Mrs:

I want to consider the radiation of the flue gas(co2 and H2o) in a space with GRY-DO-MODEL. When I define the parameters of mixture(methane-air) in material PANEL, I find there are only absorption coefficient but no emissivity parameter for methane-air. Can you tell me why?

Thank you very much. Sincerely yours, Harry.

Re: Why no emissivity in mixture parameters?
 

It's fairly difficult question because it is very basic rule. Let me see, to explain.

Most solid radiation is energy transfer FROM or TO the soild surface. Energy transfer by radiation IN the solid is, in general, not considered because radiation energy can not penetrate(transmit) into the solid for most solid. So, for the (numerical) analysis, meaningful phenomena are reflection and absorbtion. We should know the relative amount of reflection energy and absorbtion energy. Parameters which denote the ratio are REFLECTIVITY and ABSORPTIVITY(not absorbtion coefficient). The parameters are NON-DIMENSIONAL, as you might guess now, because they represent only RATIO. For the calculation of emissive power, one rule(Kirchhoff's Identity) verified that ABSORPTIVITY is the same as EMISSIVITY. Now, we use EMISSIVITY for the radiation calculation at the solid SURFACE.

Consider gas radiation. Radiation energy penerates(transmits) the gas. Now, the important parameter is how much radiation energy is absorbed in the unit depth of gas. Consider the situation that radiation energy is decreased as it passes in the gas. We can simply set the decreasing energy as,

dI = - alpha * I * dx (I : radiation intensity).

alpha is a kind of coefficient and we can call it as 'ABSORBTION COEFFICIENT. It's a definition. That's why the unit of absorbtion coefficient is inverse of length, i.e., 1/m, whereas absorptivity is non-dimensional. Of course, absorbtion coefficient is deeply related to the emissivity of gas.

It may or may not be enough explanation. Simply speaking, solid radiation is surface related and gas radiation is volume related. We use the parameters which are convenient to represent the phenomena as our convenience for the mathmatical(or numerical) form.

Sincerely, Jinwook

Re: Why no emissivity in mixture parameters?
 

Hi,Jinwook:

Tank you for your such long perfect answer.I got it. 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

Re: Why no emissivity in mixture parameters?
 

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 for 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 sub-iteration, may result in terrible computational time.

Fortunately, the area of inlet for the combustor is fairly small for most case, it has not been severe problem. However, when I was trying to simulate incinerator(assummed inlet area is 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. It is impossible to model this situation by Fluent's default setting. Or terrible effort would be required by using UDF.

Virtual system : Energy is transfered to the inlet by RADIATION and the same energy is transfered into the combustor by the RADIATION. This is possible by setting inlet emissivity as 1.0. This is not the same as the real situation. But the result is, 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 1.0) 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

Re: Why no emissivity in mixture parameters?
 

Hi, Jinwook: your answer is so long. I really appreciate you very much. Your answer gave me much help. I posted a new message for this problem. Let's see if some people would talk about this topic and give us more useful information. Thank you! Sincerely yours, Harry Qiu.