Recirculation phenomena in a natural gas swirl combustor

This paper presents the experimental results obtained in a natural gas swirl combustor (input thermal power=17 kW) through different techniques (laser Doppler Anemometry for flow field characterisation, temperature measurements by thin thermocouples, emission spectroscopy of the flame front and pollutant emissions analysis at the exhaust). The main aim of the performed research was to investigate the recirculation phenomena induced by the swirl motion imparted to the air stream (swirl number S=0.82) inside the combustor: in fact, different recirculating regions (central and corner) have been observed and, by integration of the velocity profile measured by LDV, the corresponding flow rate has been estimated. Particularly, it has been found that flame confinement in the presence of intense swirl generates a wide central recirculation zone and a large corner vortex. The hot reverse stream propagating on the burner axis prevents penetration of the fuel jet, induces a rapid mixing and burning and provides flame stabilisation. The corner recirculation of hot burned gases favours entrainment in the outflowing reactants mixture contributing to their progressive pre-heating and leaning, thus influencing combustion process development and pollutants formation.