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September 12, 2007, 07:28 
rate exponent for combustion modeling..

#1 
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hi.. i am working on boundary layer combustion inside a cylindrical chamber..oxidiser (O2  gas) comes thru the port and pyrolysed fuel (butadiene  gas) comes from the wall..a boundary layer develops on the wall along the axis.. fuel and oxidiser mix here and combust..the reactions tht i use are
C4H6 + 3.5 O2 > 4 CO + 3 H2O CO + 0.5 O2 <> CO2 can i use laminar flamelet model under "species transport and finite rate chemistry"..?? if yes, how to set the rate exponents..?? i dint find much abt its setting in fluent's user guide..Stephen R Turns in "An intro to combustion" says that they are determined experimentally..Kenneth K Kuo in his book "Principles of combusition" uses examples having stoichiometric coefficients as rate exponents.. if no, then what model can i use..??with "eddy dissipation" model i got unbelievable wrong results.. i let them combust in fuel rich condition (equivalence ratio 2.33)..so can i use the actual number of moles reacting as rate exponents..?? For ex. i have 1 mole of C4H6 reacting with 1.5 moles of O2..So can i set the stoichiometric coefficients as 1 and 3.5 and the rate exponents as 1 and 1.5 for C4H6 and O2 respectively..??? thanks in advance.. Palani Kumar. 

September 12, 2007, 08:52 
Re: rate exponent for combustion modeling..

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Hi Palani,
If you are using the pressure based solver the best approach to model hybrid rocket combustion is to create a prePDF look up table for use with the nonpremixed combustion model(non adiabatic beta shape PDF). To my knowledge it is the most accurate model for this type of problem and makes things alot easier when setting the model up as you can define the equivalence ratio through the relative mole and species fractions. You are able to generate either single or multiple flamelets although I think a single stretched laminar flamelet is most suitable for modelling the diffusion flame. Although you can use equilibrium chemistry instead which does not require flamelet generation. If your modelling compressible flow I'd go for the finite rate/eddydissipation or EDC (use the ISAT tables as direct integration of chemistry is CPU intensive). I'd recommend looking at the following AIAA papers: Investigation of Metallized and Nonmetallized Hydroxyl Terminated Polybutadiene/Hydrogen Peroxide Hybrid Rockets Simulation of the ColdWall Swirl Driven Combustion Chamber Of course this is only my view on the subject but the papers mentioned should help you develop an understanding how to model combustion. Hope this helps you and would be interested to hear how your simulations go. Neil 

September 13, 2007, 05:56 
Re: rate exponent for combustion modeling..

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yes..i am modelling compressible flow..so i use density based solver..
even in finiterate/eddy dissipation model i need to give stoichiometric coefficients and rate exponents for the reactions that i am considering..now huv do i decide upon the rate exponents..??? i will look at usage of ISAT tables..as of now, i am not yet into PDF models.. thanks.. palani. 

September 13, 2007, 11:08 
Re: rate exponent for combustion modeling..

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This is something I'm also currently having problems with but I'd suggest typing 'chemical equilibrium' into wikipedia and follow the various links from the article. From what I understand it is not a particularly straight forward process and requires estimates for chamber pressure and combustion temperature. As well as the preexponential factor, temperature exponent and activation energy to find the rate constant of the reaction. For the CO + 0.5O2 <> CO2 reaction I calculated a forward rate constant of around 1.6e+13 for for a chamber pressure of 20.41 bar and assuming a reaction temperature of 3000K. But am unsure if this is correct as I have nothing else to compare this value to or whether the I used the correct units in the calculation. Hopefully someone else might be able to provide more specific details or figures but hope this helps a bit for now.
Neil 

April 26, 2015, 16:18 

#5 
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sanjeet Limbu
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Hi
I am trying to do auto ignition for nheptane/Ar/O2/N2 mixture... with adiabatic wall Can anyone suggest how to do .. 1. I tried the laminer  species transport: It shows no rise in temp / pressure (I did the cold flow calculation + patch temp 1400K and species reactant), as there is no combustion even after patch temp... just increase in temperature as in attached snapshot 2. I tried the flame let model in Partially premixed but that show error in the unsteady initialisation that the mixture fraction limit is grater than the largest domain fraction even as I entered 0 in it 

May 17, 2016, 03:27 

#6 
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Latha
Join Date: Apr 2013
Location: Chennai
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Hello Palani,
Did you get the exact rate exponent value? how did you calculate those? please explain, Im also working on that 

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