[Muneeb]

I am working on simulation of non premixed combustion of a gas turbine engine afterburner in FLUENT. when I run the simulation for ADIABATIC CASE in non premixed combustion (energy eqn 'OFF' , compressibility effects disabled) the solution is easily converged and reasonable results are obtained.

While I enable the compressibility effects in non premixed combustion window followed by recalculating the pdf table, the solution does not proceed beyond 05 iterations with error 'INCORRECT SPECIFIC HEAT VALUE' displaying for almost 100 times followed by divergence in AMG solver. (Note: I am dealing with a fuel; hydrocarbon named JP-8; chemical formula C31H48; molecular weight 420 KJ/kgmol: specific heat value 2200 J/KgK. This hydrocarbon is not available in the FLUENT database (thermodb file) therefore I am defining the fuel empirically by MANUALLY defining these values. I have also tried using various hypothetical low values of specific heat but no success. I tried with using a fuel from FLUENT database but in this case the problem of incorrect specific heat value is not encountered however convergence is still not achieved {maximum pdf table enthalpy exceeded in ------ cells}).

I tried to run the solution with species model OFF (combustion OFF), taking Air as ideal gas (compressible + energy Eqn ON), solution is converged and reasonable results are achieved. Problem is encountered only with enabling the compressibility effects while modeling non premixed combustion.

Further details are as follow:

Mesh size: 6 Million elements (hexahedral + tetrahedral)
steady state
standard k-epsilon turbulence model
radiation: OFF
periodic boundary conditions (axisymmetric 30 deg sector) with mass flow air and fuel inlets and pressure outlet
solver: Coupled (Courant number 0.3)

kindly advice how to proceed with compressibility effects enabled in modeling non premixed combustion in order to avoid the aforesaid errors. please also advice am I correctly defining the empirical fuel. Thanks in anticipation for any advice please.

[racing_student]

Hi,
I had a similar problem with the non-premixed-combustion. The solution diverged with enabeling the compressibility effects. After checking the user manual I found out the compressibility effects function is only for transient flows. With out the compressibility effects I hadn't any problems after this

[Muneeb]

Originally Posted by racing_student

Hi,
I had a similar problem with the non-premixed-combustion. The solution diverged with enabeling the compressibility effects. After checking the user manual I found out the compressibility effects function is only for transient flows. With out the compressibility effects I hadn't any problems after this

thankyou Marius for your input.
Can you plz share any reference where it is mentioned that 'compressibility effects function is only for transient flows'. I tried to find it out in user manual but couldn't reach it.

[racing_student]

Oh sorry, my fault. I remembered wrong.
I searched for it again and it was not transient flow. It was Adiabatic and Non-Adiabatic. For adiabatic I had the convergence problem with compressibility.
Sorry for wrong information.

[Muneeb]

Originally Posted by racing_student

Oh sorry, my fault. I remembered wrong.
I searched for it again and it was not transient flow. It was Adiabatic and Non-Adiabatic. For adiabatic I had the convergence problem with compressibility.
Sorry for wrong information.

no problem Marius..

but compressibility effect option is only enabled for non adiabatic case. (you cannot turn on the compressibility effects for adiabatic case as said option is not available). I am having the convergence issue for non adiabatic case (with compressibility effects enabled)

Marius did you finally able to overcome your convergence problem??

[racing_student]

I did a lot of mesh study, I had a 2-D axissymmetric mesh. At my case, I had to simulate liquid oxidizer with gaseous fuel. This always causes problems with the non premixed combustion. Finally I changed to the Eddy- Dissipation- Model. It is less accurate but I found good convergence