Experimental investigation of the Effect of Air Diffusive injection on premixing swirl flames

In recent decades there has been a strong trend towards the use of lean premixed combustion in order to produce gas turbine systems that are compliant with air quality regulations. Additional growing interest in using alternative fuels has drawn many problems in terms of operational stability, thus there have been extensive investigations to achieve more stable and reliable combustion systems. Flame flashback has been one of the major instability problems that have the potential of causing considerable damage to the combustion system hardware in addition to significant increase of emissions such as CO and NOx. Swirl combustors are proven as effective flame stabilisers over wide range of operation conditions due to swirl structures which provide a low velocity region that enables flame anchoring. However the interaction between swirl structures and swirl burner geometries can considerably alter the stability regime downstream the burner exit plane. Using central injectors either as a central bluff body or to inject fuel diffusively have been used successfully to achieve wide stability limits and prevent upstream flame propagation. However, central injectors in swirl combustors can be subject to large amounts of heat, which can cause flashback, subsequently increasing maintenance cost and reducing the predicted operating life of the system. This paper present a series of experiments using a well-characterised tangential swirl burner to investigate the effect of using diffusive air injection on flow field characteristics and how it can affect the lower instability limits through altering the flashback mechanism by Combustion Induced Vortex Breakdown (CIVB). Results show that using diffusive air stream injection affects flashback trends significantly by providing a wider range of stability limits both in terms of equivalence ratio and mass flow rate which implies the possibility of working at higher power outputs compared to the case when using a central body injector. This technique can be considered a promising technology in terms of flexibility of operation because it enables switching to another air stream diameter while maintaining full load operation as opposed to the use of central body injectors.