three body reaction rate constants

[Sam]

Hi,

Excuse me if this seems basic. I'm new to Cantera.
I ran h2adiabatic script along with my input file which includes regular elementary and three body reactions.
For regular elementary reactions everything is fine. The constant returned by cantera is equal to A(T^b) exp{-E/RT}
But in the case of three-body reactions, the constant returned by cantera seems to have the form (1e-3) . [M] . AT^b exp{-E/RT} where [M] is the concentration of the third body. I assumed that Cantera includes [M] in forward_rate_constants but I still do not know the origin of that (1e-3) factor.

Attached is the python script and the input file. More specifically, I considered reaction 6:

# Reaction 6
three_body_reaction('O + H + M <=> OH + M', [6.750000e+18, -1.0, 0.0],
                    efficiencies='H2O:5.0')

and calculated the value: 1e-3* (6.75e15 T.^(-1) .* M ) and plot it against location (z the first column of the .csv output file) and in the same figure plotted Forward_Rate_Constants returned by cantera. As you can see they perfectly match. (For the value of M I put: H + H2 + O + O2 + 5*H2O + HO + HO2 + H2O2 + N2 + AR where each variable denotes the concentration of that species returned by f.concentrations)

Thank you.
Sam

[Steven C. Decaluwe]

Hi Sam,

This is a shot in the dark, but is this perhaps just a units issue?  Cantera outputs generally have units in terms of kmol and m; if you are expecting [M] to be in mol/m3, this could explain the discrepancy.

Cheers,

Steven

----------------------------------------------------------

Steven DeCaluwe, Ph.D.

Assistant Professor of Mechanical Engineering

Colorado School of Mines

G. Brown Hall, W410 B

Golden, CO, 80401

deca...@mines.edu

(303) 273-3666

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<Konnov.cti><fwd_cmpr.pdf><h2adiabatic.py>

[Sam]

Dear Steven,

Yes, you are right. I just realized that after posting the topic. that 1e-3 originates from unit conversion. 

However, I would appreciate a confirmatory comment on my guess that Rate Constants returned by Cantera have [M] inside of them. 

Thank you.

Sam 

[Nick Curtis]

Sam,
You are correct, you can see it in the code here.
More generally speaking, if you consider a forward (for simplicity) reaction rate of progress as:

R_fwd = kf * PI[Ci^(nu_i'), i = 1...N_species]

Where PI is the multiplicative sum operation

Then the pressure modification term (whether it be a third body, or falloff/chemically activated reaction) should be embedded in the kf term

Nick