Doubt in fluent species transport equation formualtion ?

  • Last Post 08 March 2019
sooraj546 posted this 05 March 2019

 \frac{\partial}{\partial t} (\rho Y_i) + \nabla \cdot (\rho \vec{v} Y_i) = -\nabla \cdot \vec{J}_{i} + R_{i} + S_{i}

The laminar finite-rate model computes the chemical source terms using Arrhenius expressions, and ignores the effects of turbulent fluctuations. The model is exact for laminar flames, but is generally inaccurate for turbulent flames due to highly non-linear Arrhenius chemical kinetics. The laminar model may, however, be acceptable for combustion with relatively slow chemistry and small turbulence-chemistry interaction, such as supersonic flames.

The net source of chemical species $i$ due to reaction is computed as the sum of the Arrhenius reaction sources over the $N_R$ reactions that the species participate in:  R_{i} = M_{w,i} \sum_{r=1}^{N_R} \; \hat{R}_{i,r}

where $M_{w,i}$ is the molecular weight of species $i$ and $\hat{R}_{i,r}$ is the Arrhenius molar rate of creation/destruction of species $i$ in reaction $r$

So if i am define overall rate of equation through UDF will this Ri be computed ?  

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abenhadj posted this 05 March 2019

Check the Manual for Define Net Reaction Rate.

Best regards,


sooraj546 posted this 05 March 2019

I have gone through the fluent...................but they are not explaining about it much 

My eq forms

gas species

solid species


What i meant to ask is if fluent will calculate the summation term at the end of gas and solid species eq shown above automatically as Ri or i need to add it as a source term ?


abenhadj posted this 05 March 2019

Fluent take care automatically about any reactions involved in your mixture. If you want to override it then you need to use DEFINE_NET_REACTION_RATE

Best regards,


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sooraj546 posted this 08 March 2019

DEFINE_SPECIFIC_HEAT(Magnetite_cp, T, Tref, h, yi)


real Cp;


           Cp= -72.613+(1289.43*T/1000)+(-2250.183*T*T/1000000)+(-2250.183*T*T*T/1000000000);


*h = Cp*(T-Tref);

return Cp;


So my doubt ................what is this T for multi phase problem............... is it the average temperature ?

rwoolhou posted this 08 March 2019

In multiphase you have the mixture and phase threads depending on which model you're using. For properties it'll take the cell temperature, but I'm not sure whether that will be mixture or phase:  if you report the cell temperature what cp value do you get?

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