CFD Prediction and Design of Low NOx Radial Swirler Systems

Abstract

A radial swirler with vane passage fuel injection using a radial fuel spoke with one fuel hole per passage was investigated using CFD at 0.5 equivalence ratio and 600K inlet temperature at 1 bar. Experimental measurements of the internal flame composition from water cooled gas sample probes were the experimental results used for comparison. Three combustion models were compared: flamelet with two difference kinetic schemes; PDF transport with two step chemistry and finite rate eddy dissipation model. Both models consistently underpredicted the turbulent flame thickness to 90% heat release by a factor of about 2. The PDF model with postprocessing NOx predictions over estimated the NOx emissions considerably and the best model was the flamelet model with full chemistry. The under prediction of the turbulent reaction zone thickness was concluded to be due to inadequate modelling of strained flame quenching for very lean flames with large laminar flame thickness and very low burning velocities. This flamelet model was applied to predict the influence of the radial swirler outlet geometry on the flame development, fuel and air mixing and NOx emissions. A dump expansion from the radial swirler outlet was compared with the addition of a shroud at the outlet and with the addition of a 60mm long outlet throat. The shroud was shown to increase the peak turbulence and confine it very close to the shroud lip. This improved the fuel and air mixing and lowered the predicted NOx from 2.7ppm to 1.2ppm with the shrouded swirler and 0.3ppm with the 60mm outlet throat and mixing length.