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Old   February 14, 2007, 05:48
Default CONVERGENCE PROBLEM - oil boiler
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I have convergence problem and I am asking kindly for help! I am using Fluent to simulate the combustion in oil boiler (25 kW). My domain consists of two separated models (when using non-premixed combustion it is not possible to define any new fluid in my case water) Model 1 â€" non-premixed combustion (model-1.cas, model-1.dat) Model 2 â€" water (model-2.cas, model-2.dat) that I have connected writing commands in Solve®Execute commands…

Each model has around 2 millions cells. First I run model-1 at the moment 800 iterations In model-1 under Solve®Execute commands… I wrote following three commands:

1. file/write-profile yes surface, inv-flux

2. !fluent 3d â€"t2 â€"path/usr/local/Fluent.Inc â€"g â€"i /home/marina/marina-02/water.jou

3. file/read-profile

that are executed every 800 iterations.

So model-1 runs 800 iterations, than the profile is written on the connected areas between two models (command 1). With the second command I am starting to run journal file water.jou that opens model-2, reads the, runs 200 iterations, saves case/date file and at the end exports the temperature distribution temp.flux on the connected areas between two models. Than the model-1 import the temp.flux and so on…. im Kreis.

My problem is that I can't obtain convergence.I have first obtained a non-reacting flow solution... My model 1 is defined as follow:

k-e model,Realizable, Enhanced Wall Treatment

Number of continuous phase iterations per DPM iteration: 100

Tracking parameters Max. number of steps: 25000

Step length factor: 20

Injection typesressure-swirl-atomizer

Number of particle streams: 10

Particle type: Droplet

Material: fuel-oil-liquid

Evaporating species: CH4

Stohastic tracking - Discrete random walk model

Number of tries: 8

Pressure-velocity coupling: SIMPLE


Pressure: PRESTO!

All other at the moment First order upwind

Solution controls under relaxation factor:

Pressure: 0.5

Density: 1

Body forces: 1

Momentum: 0.5

Turbulence kinetic energy: 0.8

Turbulence dissipation rate: 0.8

Turbulent viscosity: 1

Energy: 1

Temperature: 1

Discrete ordinates: 1 0.975

Mean mixture fraction: 1

Mixture fraction variance: 0.9

Discrete phase sources: 0.25

Please, can someone who is dealing with the non-premixed combustion suggest me what to do in order to obtain convergence ?? In 14000 iterations i wasn't able to obtain the convergence. I observe the temperature and velocity at the exit and those value are still varing a lot (temperature lies between 700 and 200°C and should be around 200°C). also the total heat transfer rate in kW exchanged between two models vary between 30 and 45 kW (it should be 25 kW) There is no problem with the water side...

I hope someone can help me.

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Old   February 15, 2007, 05:24
Default Re: CONVERGENCE PROBLEM - oil boiler
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Please try to change:

Define->models->solver->gradient option to Green-Gauss node based Or Least Squares cell based maybe it would work, regards, nicoslaw
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