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Old   February 14, 2002, 07:30
Default Decreasing temperature of combustion
  #1
Jacek
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Hello everybody,

I would like to ask you is it possible to decrease temperature of combustion durning simulation in Fluent. I have changed emissivities and temperatures of walls in combustion chamber but temperature is still to high a bit.

Best regards and thank you for help.

Jacek
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Old   February 15, 2002, 02:29
Default Re: Decreasing temperature of combustion
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Jin-Wook LEE
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I do not fully understand your meaning because the meaning of decreasing temperature in your system. I can only mention couple of numerical aspects.

1. Reaction Model : Peak temperature can be reduced by using PDF combustion model.

2. Radiation model : If you include radiation model, heat flux to the cold walls becomes much more, so that the temperature decreases.

3. Absorption coefficient for radiation model : If you increase absortion coefficient of radiation model, temperature is reduced for most cases(not for every case). This is valid for gray gas assumption.

4. Specific heats : Do you use constant specific heat ? Increasing specific heat means decreasing temperature.

Sometimes, I have used combination of above mentioned 'tricks' if temperature is too high or too low because of uncertainties of operating condition and/or input condition etc, for 'calibration purpose'.

Sincerely, Jinwook

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Old   February 15, 2002, 05:13
Default Re: Decreasing temperature of combustion
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Jacek
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Thank you for your comments,

By subject decreasing temperature of combustion I have tough, how can I decrease temperature of hot flue gases durning combustion process. I have used P1 radiation model and not constant specific heat - precise polynomal, but when DPM model was used temperature increased and I haven't idea why. Now I try to change absorption coefficient and hope it will work ok.

Thank you again.

Jacek

P.S. Have U got some comments about P1 radiation model or which should I used in calculations.
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Old   February 15, 2002, 07:00
Default Re: Decreasing temperature of combustion
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Jin-Wook LEE
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1. What kind of fuel(droplet or combusting particle) do you use ? If heating value is high, it is natural that the temperature increases. I believe that you have enough knowledge about combustion. But many engineers have mistake for the calculation of 'standars state enthalpy' of vaporized fuel. Please check that value.

2. P1 model is known to be fairly poor radiation model. But it is very good for convergence and it requires short CPU time.

Sincerely, Jinwook

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Old   February 15, 2002, 08:15
Default Re: Decreasing temperature of combustion
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Jacek
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Hello,

thanks for help and conversation. In my case of simulation I use particles to combustion. The heating value is about 31000 kJ/kg and but about standards state entalphy is set 0. Is it ok? I know that there are several models to calculate radiation and I set P1 because when I wanted to set Rosseland calculations stacked. There was some problem. What you suggest? Which model of radiation should I use? The combustion chamber is not so big 7x6 m. Also I read that for quite large optical length P1 and Rosseland models are good and for small dimensions of optical length DTRM and other. Do my thinking is correct?

Best regards.

Jacek
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Old   February 18, 2002, 01:45
Default Re: Decreasing temperature of combustion
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Jin-Wook LEE
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1. Radiation Model : I often use P1 model, because it is easy to converge, not because it is accurate model. It is your decesion to use 'cheaf and stable, but simple model(P1)' or 'expensive and not stable, but more accurate model(DOM or DTRM...)'. But this is, I think, not related to your problem of very high temperaure.

2. Standard state enthalpy : This value is determined by the fairly simple calculation of coefficients of chemical reaction model, considering mole of reactant(oxidant and fuel) and mole of product(for combustion, mole of CO2 and H2O). If you use default value of standard state enthalpy for CO2 and H2O, and you set the value of vaporizing fuel as zero, it is definitely incorrect. Please refer 'thermodynamyics textbook' to calculate standard state enthalpy of species for which that value is unknown. Now I believe that your problem(too high temperature) is caused by your wrong input for standard state enthalpy of vaporizing fuel.

Sincerely, Jinwook

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