Study of Hybrid Diffusers for Use in Gas Turbine Combustors 2011-01-2496

Significant development in the gas turbine technology has brought about an increase in the performance requirements for modern engines. This has generated a significant interest in researching into implementation of novel technologies for various engine components, which will allow for the design of engines to match the new performance requirements. One such technology is the use of hybrid diffusers in gas turbine combustors against conventional combustors like dump diffusers. The hybrid diffuser concept has been around for a while and has the potential of giving a greater performance than conventional diffusers. However, due to limited information available in the public domain, not much has been fully understood about the mechanism of the hybrid diffuser concept. Much of the previous work done on hybrid diffusers are done on designs having a vortex chamber bleed, based on the belief that vortex chambers helps to stabilize the flow separation. However, this paper takes looks into the proposition that the primary mechanism of a hybrid diffuser is the air bleed rather than the vortex chamber itself. This paper looks at a comparative study between a hybrid diffuser with a vortex chamber and that of a new hybrid diffuser design where the vortex chamber is replaced with a duct bleed. The Diffuser Pressure Loss, Bleed Pressure Loss and Pressure Recovery Coefficients of each were analyzed at different bleed rates. The results yield that the duct bleed hybrid diffuser has a similar performance to that of a vortex chamber hybrid diffuser. However, it was observed that a duct bleed needed even less bleed air to achieve a good performance and thus suggesting Walker's proposition to be true that a vortex chamber is not a necessary configuration in hybrid diffuser.

Also included is the analysis of performance characteristics of various configurations of duct bleed hybrid diffusers. These include changes in post-diffuser divergence angles, bleed angles and changes in the dump gap. Changes in Diffuser angles do give a better performance but at the expense of larger amount of air bleed from the mainstream. Reducing the bleed angle made no significant gain to the diffuser performance but improved the quality of bleed air which can be used for other purposed like turbine cooling. The paper also discusses as to why there hasn't been any real practical application of this novel technology until now and also suggests specific area where hybrid diffuser could provide immediate benefits.