I have a relatively simple axisymmetric model in Star-CD (actually a 5 deg. sector since Star doesn't handle pure axisymmetric) with methane combustion. The predicted temperatures are much higher (about 300K) than measured values. In order to explain this difference, I have tried to include radiation heat transfer effects in the model, but all my efforts only seem to reduce the temperatures by about 20 deg. over those predicted with adiabatic walls. I have tried increasing the beams/patch, changing the wall emissivity, wall temperature, reflectivity, etc. The only thing which seems to have a significant impact is to increase the absorption coeff. to an unrealistic value of 20 or 30 (I calculate the "true" value of this factor to be about 0.05/m).

Am I missing something fundamental?

Thanks for any advice you can give.

Ken

What combustion model are you using? MIB will over predict temperatures.

Has the radiation calculation converged, if it hasn't then it will tell you in the info file.

allan

Thanks for your response. I am using a Local source - Diffusion flame with an EDBR rate model. Also, my radiation calculations seem to be fully converged - no messages to the contrary in the .info file. However, the Total Heat Balance does seem to be slightly higher with the radiation model: 3.9 W vs. 0.6 W.

are you sure that the mass flows of gas and air are correct? Do you have enough cells? have you tried increasing the amount of cells and comparing the result with the previous mesh?

allan

I have double checked the fuel and air flows in the model, and they seem to be correct (the scalar concentrations provided in the inlet boundary conditions are mass fractions, right?).

In the past I have refined the mesh and seen some decrease in predicted temperatures, though nothing dramatic. But I will try this again and see what happens.

Would you expect this to make a difference of several hundred degrees if my mesh were too coarse?

Thanks.

tes your correct scalers are mass fractions. With regards to your mesh I don't really think increasing the size will make several hundred degrees difference but it's another parameter that can be ruled out. Just out of interest, are you using mars differencing on UVW, and I take it your running in double precision?

allan

Yes, I am using double precision. Additionally, I have been using upwind differencing, although I tried MARS just the other day to see what would happen. The max temperature actually went up about 50K.

I know it sounds worse but your mars solution is more accurate. I would suggest that the model is not converged sufficiently or perhaps your boundary conditions are not quite right.

allan

Thanks for your help. I will check out several things based on your input.

Ken