The Influence of Dump Gap on External Combustor Aerodynamics at High Fuel Injector Flow Rates

The increasing demand to reduce fuel burn, hence CO₂ emissions, from the gas turbine requires efficient diffusion to reduce the system pressure loss in the combustor. However, interactions between pre-diffuser and combustor can have a significant effect on diffuser performance. For example, the consequence of increased fuel injector flow at a dump gap set using conventional design guidelines has been shown [2] to introduce a destabilising interaction between fuel injector and upstream components. The present paper concentrates on examining the effects of increased dump gap. Dump gap ratios of 0.8, 1.2 and 1.6 were employed, with each test utilising the same IGV, compressor rotor, integrated OGV/pre-diffuser and dump geometry. The flow fraction of compressor efflux entering the combustor cowl was set to be representative of lean combustors (50% – 70%). Measurements were made on a fully annular rig using a generic flametube with metered cowl and inner/outer annulus flows. The results demonstrate that, with fixed cowl flow, as the dump gap increases component interactions decrease. At a dump gap ratio of 0.8, the proximity of the flametube influences the pre-diffuser providing a beneficial blockage effect. However, if increased to 1.2, this beneficial effect is weakened and the pre-diffuser flow deteriorates. With further increase to 1.6 the pre-diffuser shows strong evidence of separation. Hence, at the dump gaps probably required for lean module injectors it is unlikely the pre-diffuser will be influenced beneficially by the flametube blockage; this must be taken into account in the design. Further, with small dump gaps and high cowl flow fraction, the circumferential variation in cowl flow can feed upstream and cause OGV/rotor forcing. At larger dump gaps the circumferential variation does not penetrate upstream to the OGV and the rotor is unaffected. The optimum dump gap and pre-diffuser design for best overall aerodynamic system performance from rotor through to feed annuli is a compromise between taking maximum advantage of upstream blockage effects, whilst minimizing any 3D upstream forcing.