I would like to get in touch with people modelling coal combustion in utility boilers with CFX 4.3 I have several questions regarding particle treatment, radiation... Thank you for your help

What is your specific problem/question? You can post your question here, and you might get help from people with knowledge about coal combustion in CFX. I think you should post your questions here, they, and the answers, might be interesting for other people as well.

Regards Jan

I am using CFX4.3. When I model coal combustion , I get results for: TOTAL MASS OF VOLATILE GASES TRANSFERRED TO CONTINUOUS PHASE and TOTAL MASS OF CHAR PRODUCTS TRANSFERRED TO CONTINUOUS PHASE for each particle iteration. I think both values should follow the same trend, I mean, they should start increasing up to a maximum value and then stay there. In my simulations, they reach the maximum and then start decreasing again. Is this possible? What am I doing wrong?

The total amount of volatile gases and char products may vary from particle iteration to iteration. The amount released depends on the history for all the trajectories. The source terms from your particles changes the flow field etc. - which results in new properties for the particles. the total amount released may therofore go up or down depending on the solution. I would expect that they increase from the beginning and then after a overshoot decreasing and the increasing again after a while until a constant level is reached, after many particle iterations! After a converged solution have been reached then should the total amount of volatile gases and char products be constant from iteration to iteration.

Hope it helps, Regards Jan

Thank you for your help. Therefore, If I want a converged solution , how many particle iterations do you recommend me? I am tracking about 5000 particles, and I usually set 10-50 particle iterations and 50-100 flow iterations. RGDS ISABEL

If your calculations is to get an initial idea about the flow field and doing coal dust calculations in CFX - then you could save a lot of time by reducing the number of particle trajectories- to say 50 or 100 (not for the final). In that case you should not include turbulent dispersion of the particles - it slows down convergence, especially with few particle trajectories. About the flow iterations - you should monitor your screen to see if the residuals are going down to a reasonable steady level.

For a final calculation then it looks like your setting are absolute enough, either you have a very huge computer or you grid is very coarse! -Be careful with that.

The settings (iterations) might be case specific, many cells or highly swirled flows would probably require more iteration on the flow field. If turbulent dispersion of the particles has an impact on the results, many particle trajectories are necessary. The size distribution of the coal also have an impact on the required amount of trajectories. Different users have different philosophy about doing the calculations, so it is difficult to give some general rules and again it is case specific.

Regards Jan

Mr Jan Rus has told many thing about the particles and I think it's very useful for you. I only want to say some thing in the particle combustion. 1, in fact ,the number of particles depend on the particle position and the particle diameter distribution. and the accuration of the calculation is dependent on the particles position and the particle diameter distrubution 2,In order to make the your model accurate,It's necessary to set enough particles when you in the last culculation.but it's not necessary when you just begin your calculation. 3,you said " and I usually set 10-50 particle iterations and 50-100 flow iterations. " I think you set it good enough, but you must change you flow iterations according to the convergence of program,becaut the flow iterations have great relation with the fluid field. If you can't get the good fluid field,the particles combustion is not correct too. 4,In the particle calculation, you must pay enouth attention to the grid. d(x+1)<2*dx<2*d(x-1) is very important. Otherwise you will found many particles calculation can't be convergency. 5, If your boiler is a symmetry system, It's useful to set a Symmetry plane and only calcuate part of boiler. it can simplify the calculation, and if you use the whole boiler,sometimes the fluid field is not symmetry because the particle calculation. I hope this can give you some help.

Thank you for your help, but could you clarify what do you mean with: "... pay attention to the grid. d(x+1)<2*dx<2*d(x-1)..." What does 'x' mean? and 'd'?

d(x+1)<2*dx<2*d(x-1) there are three directions:x, y ,z. in any direction , the grid length must be larger than the 0.5 (n+1),and less than 2*(n-1).It's good for the convergence of particle. Is it clear enough?

Sorry, but I don't understand it very well. Now you talk about 'n', what does it mean?

for example,we have some grid 5*5*5 in x direction, we have grid1,2,3,4,5 the number is 1,2,3,4,5 and the length of grid in the x is d1,d2,d3,d4,d5 So we set the 2d1>d2>0.5d3>0.25d4>0.125d5 , y dirction and z is like that too. Is it clear? Best regards Sincerely yours Li Xinfeng

Hi Isabel

I recommend that you never use turbulent particle dispersion in this case. 5000 particles should be enough to give a good statistical distribution of particles throughout the furnace provided you have a distribution of sizes and starting positions. Turbulent dispersion only ever gave me comnvergence difficulties because every iteration the particles will end up in a different spot entirely

James

Hi James,

Careful when you recommend, If turbulent dispersion is importanf for the e.g. flame position, burnout, emmisions etc. then it must be included. In some cases then it is important, reported in the litterature. It is true that including dispersion can lead to convergence problems, but it is typically restricted to solutions with few particle trajectories, where the change for the trajectorie from iteration to iteration will become important due to the poor statistical representation.

Regards Jan Rusås

I am modelling 5760 coal particle trajectories to simulte a coal mass flow of 2.33 kg/s entering 12 burners on the front wall of a furnace. From your expert point of view, do you recommend me to include particle dispersion? Thank you

Dear Isabel

It has been concluded by several authors that turbulent dispersion has an influence on the overall combustion process, Smoot, 1984, Truelove 1984, Smith 1993, Costa 1994, (I do not have the exact references at this point-do hope you recognize the names at least) . I did investigate the influence of turbulent dispersion, (even also anisotropic and time correlated dispersion) when I was doing my PhD. I found that turbulent dispersion did not have any impact ! as many other authors also have found. So what is the reason for this, I believe it is simple- There exist numerous different coal types burned in many different burners with different settings for swirlnumber- local velocities air to fuel ratio etc. So no one can conlude that dispersion is important or not, in general, because it is very case specific. I concluded in my thesis that the impact of dispersion is highly related to coal type- for high volatile coals, turbulent dispersion is not so important, but for medium volatile coals turbulent dispersion can have a substantial effect. This can also be understood physically.

You have to be careful when modelling coal dust combustion, that you focus on the important factors. Maybe in your case is a correct modelling of the devolatilization rate the most important- or char combustion rate or.. so on. You have to find out what are the important parameters for your case!!!!! and careful when you read other people statement about what is important or not.

That is a bit of a task when you are trying to model 12 burners at the same time- you should consider simplifying your model maybe to one burners and investigate what is important.

Hope this helps

Regards Jan

Thank you for your prompt answer. I think that your phD thesis could be interesting for me. Could you tell me something more about your phD (brief index)? Best