# which model should i use..........?

INDIAMember

I am facing a difficulty in selecting which model TO should for this KIND OF PROBLEM etc.......................

[U]Model description[/U]

The mixture of iron ore and coal particle make up a composite pellet which is considered as a porous medium. Both solid and gas phases are assumed to be continuum. The pellet shape is assumed to be spherical. these are the reactions which are happening inside the pellet.

3Fe2O3  + CO  =  2Fe3O4  +  CO2

Fe3O4  + CO  =  3FeO  +  CO2

FeO + CO  =  Fe  +  CO2

CO2  + C  =  2CO

eq solved include - continuity, mass balance , momentum eq that's all

rate of the first 3 reactions are given by

Ri= Si*ni*(4phi*(r^2))*kio*exp(-Ei/RT)*(Cco-(Cco2/keq))

Ri = Reaction rate Equation (mol/m3.s)

S = Shape factor of particles

r = Average particle radius (m)

ko = pre-exponential constant (m/s)

Ea = Activation Energy (J/mol)

R = Universal Gas constant. (J/ mol.K)

Cco , Cco2 = concentration of Cco , Cco2 (mol/ m3)

n = number of particles of solid reactants for reaction i in the mixture (m^-3) etc

In reality there are 2 different type of solid particle of different sizes are there which interact with gas as surface reactions,

• GermanyForum Coordinator
edited November 2018

Please update about the morphology of the phases and how many phases are involved? What is the goal of your project?

• INDIAMember
edited November 2018

except co and co2 all others are solid , My intention is just to find out the concentration and temperature evolution with time inside the pellet. So initially a porous-spherical pellet is there which contain carbon(C) and iron ore particle (Fe3O4) only,  it reacts with CO giving the following feo fe co2 etc as shown in the reactions above. In mass balance the diffusion of the gases to solid particle should be considered. All reaction are happening in the surface of the particle .

I have seen surface reaction in CVD and particle surface reaction etc.  But the problem here is the average radius of the particle is considered in the reaction rate of each reaction which is dependent of (particle radius * reactant left to be converted) i.e. as fe3o4 is completely converted to fe or feo reaction rate will become zdero etc i have to input the rate of reaction as UDF only

• GermanyForum Coordinator
edited November 2018

My question is whether one can consider the pellet as as porous zone with a certain porosity and loss functions and all of the parameters will be steered via UDF where the kinetics are implemented.

It sounds similar to the combustion of a wood chip

• INDIAMember
edited November 2018

ya is possible, so the solid part of the pellet in porous medium will be this particles rt, then we can active surface reaction and do it . Is this what u meant . Sorry sir i am new in this area

• GermanyForum Coordinator
edited November 2018

The pellet will be a porous medium with initial porosity where you can account for porosity increase and all reactions. You require UDF knowledge.

Perhaps other community members can chime in to suggest something different!

• INDIAMember
edited November 2018

Sir u haven't suggested me anything, should i go for heterogeneous reaction eluerian mixture or something etc or dispersed phase model or just take a porous pellet and put everything into surface/wall reaction rate UDF and do it.

I have already worked in fluent combustion etc i known UDF to some extend sir.

• UKForum Coordinator
edited November 2018

This comes down to what you want to achieve.

If you want to combust a dispersed solids phase use DPM. You may need to adjust reaction rates etc to account for diffusion within the particles (combustion modelling isn't a field I cover). If it's a much denser particle laden flow then Eulerian may be a better approach.

If you want to study the diffusion through the particle you'll need to model the geometry including pores etc. or come up with a porous media approach.

• GermanyForum Coordinator
edited November 2018

That was my suggestion is to use a porous media approach without any multiphases. The other suggestions have been well introduced by my colleague rwoolhou. But you said you have a single big particle / pellet that is why I suggested the approach.

• INDIAMember
edited November 2018
Thanks a lot for all your replies .... ok so I will start with a single porous pellet having some porosity but on udf ..... what type should I use particle surface or wall surface or just volumetric ...... in reality reactions happen in particle surface only
• GermanyForum Coordinator
edited November 2018

1/There are no particles in the porous solution I am talking about. All reactions are described in the UDF. Only gas phase reaction are given through GUI. That is the approach one will use to burn a wood pellet.

2/Using particles: DPM or DDPM rwoolhou provided a good summary. Here you also need to adjust the reactions to what you have.

3/You can use the packed bed approach for the pellet using the Eulerian-Granular model define your heterogeneous reactions and use a UDF for the diameter of the granular phase.

• INDIAMember
edited November 2018
Oops sorry I forgot that u r correct sir , thanks a lot for all ur replies let me start and get back to u soon with quires
Once again thanks a lot amine