Effects of different nozzle configurations on swirl flow topology in tangential swirl burners

Flame flashback is one of the central combustion instabilities, especially when it appears in the form of boundary layer flashback (BLF) or combustion induced vortex breakdown (CIVB) flashback, some of the most common instabilities in swirl combustors. This paper focuses on mitigating the phenomenon of CIVB and BLF flashback mechanisms using different nozzle configurations while using central air injection. Studies were conducted on a 150-kW tangential swirl burner manufactured and previously characterised at Cardiff University. The effects of different nozzle heights (hn) with and without microstructure on the swirl flow characteristics were investigated experimentally by utilising an LDA system. Different strip heights (hm) of a wire woven mesh have been employed as a liner on the smooth nozzle to change its surface roughness. It was found that longer smooth nozzles (hn/Ro=2.3) led to promotion of stability in the swirl burner by minimising the axial velocity defect while decreasing turbulence downstream the dump plane. Moreover, the average measurements show that the burner nozzles with microstructured surfaces enable improvements in controlling the BLF flashback and hence reduces outflow drag. It was found that the microstructured mesh alters the flow structure near the wall by increasing the velocity adjacent to this region delivering further resistance to BLF. On the other hand, using both central air injection and the nozzle with and without the microstructured surface can affect the operability of the gas turbine combustors.