It has been known for a sometime that the compressor exit profile can have a significant effect on the overall performance of the fuel injector. This effect has been increased recently with the advent of larger leaner injection systems. With a modern gas turbine combustion system the fuel injectors are presented with a non-uniform feed generated by the upstream compressor and OGV/pre-diffuser assembly. For generic lean burn combustion systems previous experimental and numerical work highlighted a complex interaction between the compressor efflux and the upstream diffuser. Circumferential non-uniformities in the flow presented to the fuel injector can amount up to ±10% of the mean velocity. Previous investigations examined only the isothermal flow field and the effects of this level of non-uniformity on reacting performance are not known. There are potential impacts on local fuel atomisation, air/fuel mixing and hence emissions performance.
The main aim of this paper is to observe and assess the effect of these upstream conditions using a reacting flow test facility. In the initial design phases reacting flow experiments are generally conducted in simple, single sector plenum fed test facilities. Since this does not capture the effects of non-uniformities modifications were made to the facility to produce an aerodynamically representative feed. CFD was used in the design process to ensure that the aerodynamic features present in engine geometries would be faithfully reproduced by the test rig modifications. The CFD also highlighted changes in the downstream isothermal flow field. This included differences in the overall effective area of the fuel injector and, importantly, a redistribution of mass flow between the various fuel injector passages. Additionally, the cone angle, and the flow structure downstream was observed to change.
Back-to-back tests were then conducted in a reacting flow test facility for various pilot-mains fuel flow splits and air-to-fuel ratios. Visualisation of the flame showed notable qualitative differences in the structure and stability of the flame. Quantitative measurements indicated that, compared to a plenum feed, a representative feed produced changes in the production of carbon monoxide, unburned hydrocarbons and oxides of nitrogen. Emission results were used to calculate the extent of the mass flow redistribution between passages. From this a correction was applied to the estimated AFRs. This correction did not fully collapse the emissions data, suggesting that while the mass flow redistribution contributes to the change in emissions it is not fully responsible. Covered in this paper are initial observations. However, further work is required to fully understand how the changes to the aerodynamic flow field alter the emissions performance, but it is clear that having a representative fuel injector feed is important in low TRL testing.