Effect of exhaust nozzle geometry on combustor flow field and combustion characteristics

Abstract

A combustion system of multiple swirlers coupled with distributed fuel injection is studied as a new concept for reducing NO x emission of gas turbine engines. The present paper investigates the significant effect of the downstream nozzle contraction ratio on the combustor flow field, flame structure, temperature distribution, and emission characteristics. A small contraction ratio yields a typical bubble-shaped recirculation zone where the flame is stabilized by vortex breakdown. The larger contraction ratio transforms the recirculation zone into a conical “V”-shaped configuration with the stagnation point of the vortex breakdown moving upstream and the flow recovering to a downstream flow direction following this point. The recirculation zone area is reduced, forming strong swirl, higher strain rate, and turbulent fluctuation in the recovery flow region around the centerline. The downstream contraction effects are due to the fact that the flow remains subcritical after the transition from super- to subcritical state through the vortex breakdown. The flow field is not influenced by the downstream boundary conditions if the flow recovers to supercritical state following vortex breakdown. The effects of the downstream exhaust nozzle on the combustion structure, temperature distribution, and emissions were shown to be closely related to changes in the central recirculation zone: higher contraction ratio forms higher NO x and lower CO emissions due to the elevated temperature field and increased reaction rate. For gaseous fuel with higher inlet air temperature, the flame structure and NO x emissions were less sensitive to the exhaust nozzle contraction ratio, indicating that the combustion flow recovered to the supercritical state downstream of the vortex breakdown region before the exit region due to a more homogeneous temperature field.