Experimental analysis of a swirl burner for propulsion applications: influence of thermal and fluid dynamic field on pollutant emissions

Non-premixed swirl burners are widely used in technical appliances (such as propulsion, gas turbines, boilers) by virtue of high flame stability, mainly due to the generation of a central recirculation region characterized by efficient mixing between the reactants and rapid homogenisation of the combustible mixture. Swirl motion imparted to the air flow presents a strong influence upon combustion features (i.e.: flame morphology, thermal and fluid dynamic field) and pollutant emissions. In spite of the wide use, many aspects of swirling reacting flows still have to be thoroughly investigated: experimental measurements are difficult owing to high turbulence levels and possible onset of instability phenomena and, consequently, numerical simulation of this flow typology is far to provide reliable results, especially at high Reynolds number of the reacting flow. This paper presents the experimental results obtained comparing different natural gas injection typologies in a swirl burner. Particularly, both co-axial and radial (i.e.: transverse) injection, with respect to the rotating air stream, have been characterised through different techniques: particle image velocimetry and laser Doppler anemometry for flow field analysis, temperature measurements by thin thermocouple and pollutant emissions measurement at the exhaust. The results put into evidence that, although the global mixing process is mainly governed by the swirling air stream, in the region close to the reactants efflux the fuel injection procedure plays an important role for flame stabilization and development in the primary mixing zone of the device. Moreover, the general behaviour of the two different injectors (mainly as for pollutant emissions) seems to reflect the generation of two different flame typologies: a partially premixed one for the radial injector and a purely diffusive flame for the axial one.