Hi,

I am working on sawdust combustion and currently using non-premixed combustion model. I basically just consider the fuel devolatilisation and char burnout in this case. From the results I got, I think that the 'drying' process is not correctly represented because the sawdust I used has high moisture content. I wonder if I should use wet combustion. If wet combustion should be used, why can't I click on the 'wet combustion' tab? The evaporating species, devolatilizing species, product species are all gray and I can't make any selection. Can I know what is the problem?

Please help!! Thanks a lot!!

Jacklyn

In your particle set-up, you would first choose "Combusting" for the particle type (rather than inert or droplet). This should let you choose the "Wet combustion" panel. In the wet combustion panel, check the box for the "wet combustion model". This should allow you to enter the materials for evaporation, volatilization, and char combustion.

Thanks Allan.

Thanks for your reply. I'm using 'combusting' for particle type, but yet the 'wet combustion' and 'multiple reactions' tabs are gray. the 'evaporating species', 'devolatilizing species' and 'product species' are gray as well. I really couldn't figure out what happen.

Actually, when I'm dealing with high moisture content biomass, what is the best way to model the 'drying'process before devolatilization?

Really hope to get some help! Thanks a lot!!!

Jacklyn

I guess I have always used the species-transport model. What species do you have set-up? What will be the evaporated species?

For modeling sawdust (or hog fuel) combustion, we started using our own drying and combustion models based on UDFs without relying in Fluent built-in models. One problem with the built-in model is that if fuel particles land on a boundary (and are trapped) char carbon will be lost.

One trick that is mentioned in the Fluent tutorials, and has worked good for us, is to have the char carbon go to a psuedo gas species, carbon, which can then react with other gases. We also do the same for water that is evaporated. It goes to a psuedo species, liquid water, which then goes to gasesous water without kinetic or turbulent limits. This makes the energy accounting easier.

Thanks Allan.

I'm wondering if it is alright to use Non-premixed combustion model, as in Tutorial 13, Fluent 6.2, for sawdust combustion. I only have the information from proximate and ultimate analysis for the sawdust, so I don't know what are the individual species components of the fuel, instead, I just define the atomic fractions of C, H, O, N, S using the non-premixed combustion model. Is this method not good as compared with species-transport model?

May I know the 'trick' that you mentioned if obtained from which tutorial and which version of FLUENT is that?

Hope to see your reply, thanks!

You can look at Tutorial 12 Non-premixed combustion which is for pulverized coal for use of solid carbon species "trick".

For various reasons, perhaps not all of them valid, I have always choosen the species transport model for combustion.

The ultimate composition of sawdust, at least C, H, O, doesn't vary too much and can found in several databases on the internet.

Hi,

This is a question about modeling species transport and finite-rate chemistry. Taking the example in Fluent 6.2 User guide chapter 14 modelling species transport and finite-rate chemistry, pg 28, with this chemical reaction C4.17H6O2.75 + 4.295O2 --> 4.17O2 + 3H2O, when I'm defining rections under the 'Reactions' panel, what is the value that I should set to 'rate exponent' for the two reactant species and the two product species? Please help!

Jacklyn

I'm sure you meant C4.17H6O2.75 + 4.295O2 --> 4.17CO2 + 3H2O. For the rate exponents, a good assumption would be that the rate is first order or a function of the concentration of the reactants. So, the rate exponent for the "volatiles" and oxygen would both be one. If there is no backward reaction, which you wouldn't use for this case, the concentration of the products wouldn't come into play so the rate exponents would be 0.

Dear Allan,

It seems that the Fluent does not provide the friendly interfaces to incorporate the own-developed models except for the char combustion( through particle surface rate model).

I wonder how you could embed the drying, combustion, and especially the devolatilization model into Fluent and replace the built-in ones? Could you give me some hints?

Hello Mikeron,

Just wonder, did you use non-premixed model or species transport model or something else?

I would also like to know the method to include own codes into FLUENT built-in functions.

Thank you.