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September 2, 2005, 03:30 
Energy balance?

#1 
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Hi users! All the equations converged in the simulation I run (pf combustion in boiler) except for energy (scaled residual 8e6). I check the total heat transfer rate from all the boundaries and I get 3.8e8(W). Then I check the DPM enthalpy source and I get 8.3e8(W). Due to tutorials the sum of the two should be zero or a very small number. Is my solution totally wrong?
Thanks in advance for your prompt response! 

September 2, 2005, 13:13 
Re: Energy balance?

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Yes, apparently it is. In my experience, the residuals are not very useful. Instead, monitor the temperature at the furnace outlet. Has it stabilized? If so, do you have other energy sinks that might not be accounted for in your balance? Superheaters, etc? Are you using underrelaxation for enthalpy? Radiation? How much?


September 5, 2005, 03:25 
Re: Energy balance?

#3 
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Dear Allan,
First of all in the domain superheaters are not included (it is cut before the SH). The underelaxation factors are (at the current time of the solution) 1 for all equations except for pres=0.5 mom=0.5 k=e=0.8 volatiles=0.9 (EBU model) and dpm=0.25 . I calculate the dpm sinks every 25 iterations where I inject 88200 particles (coupled heat mass transfer on). I don't think that there is other energy sink. At the current moment energy res is at 2e6. Total heat flux from all boundaries is 4.5e8 W and from dpm is 8.25e8 W. Are there 375 MW missing? I am generally monitoring (after you adviced me in the past) mean temperatures at the furnace exit and at vapour burners level. I could send the plots. The mean temperature at the exit decreases continuously but not steeply. At the last 300 iterations the mean temperature decreased about 10K. What do you think about it? I am looking forward to your advice. Best regards, Ilias 

September 6, 2005, 12:56 
Re: Energy balance?

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You should be able to set the underrelaxation factors for the particles (DPM) to 1 if you are reasonably converged (or at least close to 1). What is the underrelaxation for the energy equation? And do you have radition turned on? What is UR for radiation?
Do the gas specific heats look ok? How are you calculating them? What about the temperatures? Reasonable? These are the factors that you would want to look at  and since it is your problem, you are the best person to review them. 

September 7, 2005, 04:41 
Re: Energy balance?

#5 
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UR for DPM=0.25 (80,000 particles are injected every 25 iterations). UR for energy=1 T=1 and P1 =1. For DPM UR=1 I will check.
Specific heats look ok (peacewisepolynomial). Mean temperature at furnace exit and at burners level almost stabilized (+1K every 100 iterations). Temperatures look reasonable. I calculated the mean temperature profile with height and compared it with another solution (of another cfd code) of the same boiler. The pattern was the same but the Fluent temperature profile was increased by 100K. Dear expert, could you please explain how is it possible to have almost 380MW energy imbalance(!) and the solution to look like converged (judging by energy residual=2e6 and stable mean temperature at exit)? 

September 7, 2005, 12:24 
Re: Energy balance?

#6 
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Hard to say why your case doesn't balance up. I have modeled about 50 industrial boilers with a variety of fuels  only half a dozen in Fluent 6.x  but the balance has always come out within a few percent.
The things I recommended are the first things I'd check. Then I'd look at comparing the DPM enthalpy source from Fluent with a hand calculation. And look at the DPM sensible enthalpy source term as well. What about the radiation to the walls? Magnitude looks okay? Comparable with other CFD simulation? As you probably well know, with CFD details are important, and one error or misrepresentation will give the wrong result. I'm sure if you keep looking, you will find the error or the reason why apparently it doesn't balance. 

September 8, 2005, 06:16 
Re: Energy balance?

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Dear Dr. Walsh,
Thank you for your help. It have been precious to me since not many people are interested in large scale boiler simulation with cfd techniques. I have done hand calculations for the fuel mass and energy contribution. Could you please explain to me how can I calculate the initial heat content of the fuel as it is given by the dpm summary report? I mean that (finalinitial) must be equal to the dpm total enthalpy contribution to the continuoum. But with what should the initial enthalpy content be equal? (e.g. initial enth. content= char enth. + vols enth.  latent heat  fuel heating +...). I think that I have to be able to calculate this initial heat content of the dpm by hand and that this way I will be finally able to isolate the problem! Now about the parameters I have used for the simulation; I also have solved the boiler with Fluent but with chem. equill. chemistry instead of ebu (I kept all parameters same e.g. wall temperatures, emissivities etc.). The energy imbalance of the solution was about 5% of the dpm enthalpy contribution (is 5% small enough?). Then I tried to compare the temperature profiles of the pdf and ebu solution; totally different! And the most weird is that pdf resulted in higher temperatures than the ebu approach (I think that the opposite is more usual). I am not quite sure that it would be worth while to compare the radiation intensity with such temperature differences between the two approaches. Thank you again for your time. I am looking forward to your response at your earliest convenience. Best regards, Ilias 

September 8, 2005, 22:35 
Re: Energy balance?

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Yes, the dpm enthalpy source is the heat of formation of each of the components released from the particles multiplied by their mass. For coal, this will depend on how you define the products of combustion. For example, if you use carbon as a product of char combustion, its heat of formation is negligible.
Perhaps you need to look at your pdf formulation. I don't use this approach so I'm not sure if or why this could be the source of your energy imbalance. Do you have any moisture in the coal? But, since it is just accounting, the balance has to balance, at least to some reasonable level. Sometimes it is easier to setup a simplier subset of your problem for tracking down errors. For example, you could do an excercise where the only component released from the burning particle is methane. The dpm energy balance would be straight forward and you would have a representation of combustion. Or setup a much smaller box that can be solved quicly. Good luck! 

September 9, 2005, 03:30 
Re: Energy balance?

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Dear expert, thank you very much for your advices. I am certainly going to follow them!
Just by curiosity, have you ever used the edc approach for large scale boiler? 

September 9, 2005, 06:58 
Re: Energy balance?

#10 
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During the solution beyond the total energy balance, I used to look at the wall fluxes as well. But just now I noticed that the heat fluxes are:
walls: 406,437,500 W carrier gas and air inlets: 1,166,526,700 W (negative????) exit: 1,313,942,000 W (possitive?????) suction ducts (outlet): 706,446,500 W (possitive?????) Can you tell me what is going on? How can the energy fluxes be so wrong (negative values instead of possitive!)? The solution seems to be reasonable (velocities, temperatures, etc.). I am really looking forward to your answer! PS. The above data concern my ebu solution. Similar fluxes I obtain by the pdf approach as well! 

September 9, 2005, 16:29 
Re: Energy balance?

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First, I'm not familiar with the ebu approach. For combustion, I use the finite rate species transport model with about 15 species (H2, H2O, CO, CO2, CH4, NH3, NO, O2, N2, H2S, etc) and a dozen or so equations. Twenty years ago, when we did our first boiler modeling in Fluent, the pdf approach was not available. Using species transport with several species seems to give more control and doesn't seem to require an inordinate amount of cpu time.
Second, on the energy balances, look at the way these are defined in Fluent. There is the sensibile part and the chemical part. It is not unusual to have negative quanties for the inputs and positive for the outlets. Inlet fluxes are positive but heats of formation of inlet species may be negative, yielding negative quantities, and vice versa for outlets. It is not necessarily intuitive, but it works. Double check these by hand for your case. 

September 10, 2005, 07:32 
Re: Energy balance?

#12 
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Thank you once again expert. It seems that I have to calculate by hand these enthalpies. It won't be a problem, in the contrary I am now convinvced that it will solve mine.
Please allow me one question about the finite rate model you use. I use this model but only with 'fast' species (not NO for example) and coupled with ebu to take into consideration turbulence effect on combustion. How do you consider turbulence effects if you use finite rate (laminar) model? And doesn't from the fast chemistry coupled with the slow come up an extremely stiff matrix (where coupled solver is essential)? Thank you for your time. 

September 12, 2005, 13:28 
Re: Energy balance?

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I'm using finite rate with eddydissipation. At times, I use the Fluent builtin formulation and constants for the kepsilon model and at times I have used "bestfit" parameters with a customized UDF.
I assumed you meant that you tried the EDC model in Fluent 6.2, which I'm not familiar with, rather than the old eddy breakup model from the 80's. Solution of equations for species does not seem to be a problem. I would say solution of the energy equation takes longer (but of course the temperatures influence the kinetics of the reactions, which in turn effects heat release). Hope this answers your questions. 

September 12, 2005, 14:22 
Re: Energy balance?

#14 
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Yes it answers many of my questions (I'll stop here because my questions don't end!). Could you please also suggest me any book or writing about state of the art cfd modeling of boiler? I have some, but I'd like your opinion (again!).
Thanks again for your help, Ilias 

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