Defining reaction rate with multiple surface reaction

Fred Marias · September 9, 2013, 06:30

Dear Ansys Fluent user,
I am trying to model coal gasification combustion in Fluent.
I use finite rate/eddy dissipation model for volumetric reactions occuring with the volatiles released by the coal, and DPM for heterogeneous reaction of the remaining char.
My questions deal with the units that should be use in the reaction panels for both the volumetric and surface reactions.
For volumetric reactions the reaction rate seems to be in kmol/(m3.s) with the concentrations in mol/m3.
This means that, if the reaction order is n with respect to concentration and 0 with respect to temperature the reaction rate is :
Rkin=Aexp(-Ea/RT)C^N (where C is the molar concentration of the reacting species) So A should be in kmol^(1-N)/(m3^(1-N).s)
Am I correct?
Dealing now with the particle surface reaction, it is less clear for me.
In section 14.3.1 of Fluent's User Guide, it is said that the unit for Rj,r is kg/(m2.s) and that Rj,r=Rkin,r*(Pn-Rj,r/D0)^N
So this means that Rkin,r should be in kg/(m2/s.Pa^N) and hence A (Rkin,r=Aexp(-Ea/RT)) should be in the same unit. Am I right?
Is it this Way that Fluent computes the reaction rate for surface reaction?
Also in expression Rj,r=Rkin,r*(Pn-Rj,r/D0)^N, a units analysis says that D0 (the diffusion rate coefficient) should be in m-1. I do not quite well understand the physical significance of this parameter and Is it that one that should be entered in the reaction panel under the topic Diffusion rate constant (with a default value of 5e-12)
Thank you very much for your answers

Fred Marias · September 12, 2013, 05:56

Does anybody have an answer to my problem?
Thanks!

September 9, 2013, 06:30	Defining reaction rate with multiple surface reaction	#1
Fred Marias New Member Fred Marias Join Date: Nov 2009 Location: Pau (France) Posts: 3 Rep Power: 17	Dear Ansys Fluent user, I am trying to model coal gasification combustion in Fluent. I use finite rate/eddy dissipation model for volumetric reactions occuring with the volatiles released by the coal, and DPM for heterogeneous reaction of the remaining char. My questions deal with the units that should be use in the reaction panels for both the volumetric and surface reactions. For volumetric reactions the reaction rate seems to be in kmol/(m3.s) with the concentrations in mol/m3. This means that, if the reaction order is n with respect to concentration and 0 with respect to temperature the reaction rate is : Rkin=Aexp(-Ea/RT)C^N (where C is the molar concentration of the reacting species) So A should be in kmol^(1-N)/(m3^(1-N).s) Am I correct? Dealing now with the particle surface reaction, it is less clear for me. In section 14.3.1 of Fluent's User Guide, it is said that the unit for Rj,r is kg/(m2.s) and that Rj,r=Rkin,r(Pn-Rj,r/D0)^N So this means that Rkin,r should be in kg/(m2/s.Pa^N) and hence A (Rkin,r=Aexp(-Ea/RT)) should be in the same unit. Am I right? Is it this Way that Fluent computes the reaction rate for surface reaction? Also in expression Rj,r=Rkin,r(Pn-Rj,r/D0)^N, a units analysis says that D0 (the diffusion rate coefficient) should be in m-1. I do not quite well understand the physical significance of this parameter and Is it that one that should be entered in the reaction panel under the topic Diffusion rate constant (with a default value of 5e-12) Thank you very much for your answers

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September 12, 2013, 05:56		#2
Fred Marias New Member Fred Marias Join Date: Nov 2009 Location: Pau (France) Posts: 3 Rep Power: 17	Does anybody have an answer to my problem? Thanks!