INTERNAL GAS ANALYSIS AND CFD PREDICTIONS OF DOUBLE CONTRA-ROTATING AXIAL SWIRLERS WITH FUEL INJECTION FROM BETWEEN THE TWO OUTLETS

A double contra-rotating axial swirler was investigated at 0.38 equivalence ratio and 600K inlet temperature at 1 bar, both experimentally and using CH.). Natural gas fuel was injected from between the two contra-rotating flows. The experimental results showed that whilst the NOx emissions were relatively high, the system has excellent flame stability characteristics meaning the flame could be maintained at very low fuel-air ratios, down to an equivalence ratio of around 0.28. CFD predictions were carried out using the equilibrium pdf combustion model alongside the partially premixed combustion model using default model constants, and the NOx model was applied as a post-processing application. The results showed that whilst the location and shape of the flame and the combustor fuel-air distribution could not be precisely captured, the peak temperature; CO, UHC and NOx levels were correctly predicted. The influence of the turbulent Schmidt number, Sc-t, was therefore investigated in an attempt to improve the predictions. Lower values offered an improvement in the fuel-air mixing predictions, but not in the combustion predictions, where the peak flame temperature could only be predicted correctly when using the default value of Sc-t = 0.85. This indicates that the use of a universal turbulent Schmidt number for both the fuel-air mixing and combustion models may not be suitable for the present swirler and combustor configuration, therefore future work will look to use a separate value of Sc-t for the mixture fraction than for the pdf combustion models.