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Old   December 14, 2011, 10:18
Unhappy UDF for VR Rate calculation
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A. N.
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Hello All,
I am fairly new to using UDF along with Fluent. I am trying to model reduction of CO2 with coal or Carbon. The reation rate is defined using Langmuir-Henselwood formulation and I am trying to do that with the UDF because most of the Species Transport models use the Arrhenius rate equation.

My question is, how do I use concentrations of the species' as my variable in the code.

Here is the code I have so far (fairly simple and premature):

#include "udf.h"

#define K 1000
#define K2 0.508
#define Mtot 84.

DEFINE_VR_RATE(lang_hens_reac_rate,c,t,r,mole_weig ht,species_mf,rate,rr_t, yi)
real s1 = species_mf[0];
real s2 = specief_mf[1];
real mw1 = mole_weight[0];
real mw2 = mole_weight[1];

if (FLUID_THREAD_P(t) && THREAD_VAR(t).fluid.porous)
*rate = K2*mw1/(1+K*(mw1+mw2));
*rate = 0.;

*rr_t = *rate;

However, all the mw variables need to be concentrations. Please help me as soon as possible as I am running out of ideas and options

I found this piece of code that people are using to obtain concentration:

ci = yi[dindex][sp_id]*C_R(c,t_reactant)/mw[dindex][sp_id];
ci = MAX(ci,SMALL_S);

But i have no idea what all those arguements and variables and syntax are. Please help

The basic equation for calculating the reaction rate is:
rate = K2*[species 2]/(1+K*([species1]+[species2]))

where K and K2 are constants given and the objects in the square brackets are obviously concentrations that I need.

Last edited by ADI; December 14, 2011 at 10:47. Reason: More info found after further research.
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Old   May 20, 2014, 08:19
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Kostis Chatzi
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hi, i am experiencing a rather similar issue, have you found any luck with yours?
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Old   May 27, 2014, 23:11
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Jackson Sim
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I am also dealing with reaction rate equation in my CFD.

This is just an extract out from my UDF for concentration calculation:

#define mw_0 18.;

/* mass fraction of species based on species index on fluent panel */
real H2O;
H2O = C_YI(c,tp,0); /* H2) is the first species in my list thus id is 0 */

real density; /* rho is total density */
rho = C_R(c,t);

/* concentration of individual species */
/* mass fraction x density / molecular weight */
real conc_H2O;
conc_H2O = H2O * rho / mw_0;

Hope this helps
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Old   June 23, 2014, 17:31
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Kostis Chatzi
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thanks for your response
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