59286 - Numerical Investigation of the Low-Swirl Flow in an Aeronautical Combustor With Angular Air Supply 

Civil air traffic is predicted to further grow in the near future. Hence, the development of aircraft propulsion systems will face major challenges to meet future stringent environmental regulations. Since the formation of pollutants is strongly related to the combustion process, the design of combustion chambers is a key element for reducing pollutant emissions. Lean burn concepts feature a great potential to significantly reduce combustion emissions. However, the application of lean burn concepts in aircraft jet engines will require a breakthrough change of fundamental combustion technologies in particular with respect to stable operation.

In the present study, an innovative gas turbine combustor called Short Helical Combustor (SHC) is investigated. The main feature of this concept is the tilting of the burners in circumferential direction relative to the rotational axis of the engine. This particular arrangement may lead to an increased combustion stability since flame piloting is achieved through interaction of adjacent burners. Therefore, safe and stable operation of the combustor can be ensured. By exploiting the angular momentum of the flow coming from the compressor, the axial combustor length can be reduced and, consequently, weight may be saved. Aiming at the implementation of a lean burn configuration, a low swirl lifted flame will be adopted. This flame is lifted off and not anchored to the injector which opens the potential of low NOx emissions due to a high degree of premixing.

In this work, isothermal flow characteristics of such a generic SHC combustor are studied by use of RANS predictions with special emphasis on the interaction of adjacent swirling flows. To this purpose, tangential velocity profiles of Rankine vortices are imposed at the inlet modeled as a coaxial two-stage swirl injector. For evaluating the influence of injector parameters on the flow field, a parametric study based on single sector simulations is performed. Furthermore, the confinement ratio between the generic injector and the flame tube is varied. It is shown that the asymmetrically confined swirling jet flow is strongly deflected towards the sidewall of the staggered SHC dome. The deflection of the flow is associated with an asymmetric pressure field in the vicinity of the burner which is generally known as the Coandă effect. As a result, the angular momentum flux at combustor outlet is increased. By evaluating the mean outflow angle measured relative to the machine axis at the combustor exit, a noticeable increase of the flow angle is found which is even higher than the burner tilting angle. For stabilization of this specific type of lifted flame, outer recirculation zones are of primary importance. Hence, the interaction of the low swirl jet and the SHC sidewall is investigated with regards to backflow momentum. It is concluded that by modifying the momentum of the injector flow entering the combustor, the amount of recirculating air flowing back along combustor walls is strongly affected. The present work establishes an understanding of the underlying aerodynamics of the SHC concept which is essential for matching the requirements of lean lifted flames. This knowledge gives the opportunity of tailoring the flow for example with focus on a desired outflow angle.