Aerodynamics of multi-passage swirling flows for gas turbine fuel injectors

Growing concern over the impact of man on the environment, especially through the phenomenon of global warming, has over the past decades led to heightened levels of social and political pressure, bent on curbing the anthropogenic emissions responsible. This drive has been accompanied by the adoption of increasingly stringent legislation, thus forcing the industry across all sectors, including aviation, to seek new technologies capable of delivering such emissions reductions, whilst still meeting society’s needs. In the field of gas turbine engineering, lean burn combustion is one such technology, showing great potential for reduction of pollutant emissions. There are, however, challenges in its implementation, which lean direct injection (LDI) offers the best chances of solving.A numerical study has been conducted to understand the aerodynamic flow field generated by the Mains section of a typical LDI injector, centring on the interaction between the flows through different passages. In a literature review, parameters (“goodness factors”) have been identified to link injector performance (in terms of the ability to atomise and mix the fuel and air) to properties of the aerodynamic flow field. This enables the impact on performance to be assessed in a broad parametric study performed using only single-phase, steady, incompressible conditions.Analysis of a range of models of varying complexity has allowed identification of the underlying mechanisms governing the flow field, including the influence of the far field (outlet conditions) on the injector near field, the interaction (through the pressure field and shear) between different jets issuing from the injector, and their trajectory (governed by momentum and relative pressures in the recirculation regions). The tendency of neighbouring jets to coalesce has been identified as a key factor for performance due to the impact it has on the local flow velocity. Similarly, the relative swirl between the jets is the main factor governing shear and turbulence production in the region of direct fuel propagation.The knowledge obtained in the fundamental analysis has been used to investigate the impact on performance (through the goodness factors) of modifying the geometry and boundary conditions in a model broadly representative of the Mains section of a typical LDI injector. This includes investigation of different passage relative bulk swirls and swirl profiles, passage relative meridional angles, contraction ratios and relative cross-sectional areas, and the use of curved and straight passage geometry. Similarly, the impact of flow field confinement and of either blocking or generating an imbalance in the pressure feed to one injector passage has been assessed.Guidelines for the design of the Mains section of an LDI injector, focused on improving performance, have been written according to the findings made. Results obtained in the analysis of a modified injector design proposal, generated following these guidelines, promise significant improvement in performance. Key design benefits include an increase in the velocity (up to 7.9%) and shear stress (up to 78%) over the pre-filming surface, a reduction in boundary layer thickness (50%) and an increase in the velocity difference across the shear layer (up to 70.8%); goodness factors all linked to improved atomisation and mixing. This is achieved with the same pressure drop and injector effective area.An experimental investigation, undertaken to validate some of the observations of the numerical study, has shown good agreement in the results obtained. Factors investigated include boundary conditions (confinement and outlet geometry, blocked and imbalanced pressure feed to each passage) and also different injector geometries (varying passage relative meridional angles).