Experimental study of turbulent Bluff-Body flames stability by simultaneous high speed flame imaging and Particle Image Velocimetry

Development of innovative combustion regimes improving energy efficiency and limiting pollutant emissions requires better knowledge of the limits of turbulent flame stability and of the physical phenomena controlling them. The present work is focused on the description and the quantification of intermittent phenomena occurring in non-premixed turbulent flames generated by a Bluff-Body burner. The central jet of methane and the annular co-axial air flow are separated by a cylindrical Bluff-Body with a large blockage ratio. The aim is to generate a recirculation zone in the wake of the Bluff-Body in order to induce a partial mixing of reactants with trapped combustion products close to the burner exit. In such configuration, the modes of stabilization of turbulent flames are often controlled by intermittent unsteady phenomena in the recirculation zone which are complex and still poorly understood. Their study requires the ability to make time-resolved measurements by the use of high-speed optical and laser diagnostics. For this purpose, high speed flame imaging (1 kHz) is performed for three operating conditions. For a constant central methane jet velocity (i.e. constant thermal power), the variation of annular air velocity allows us to cover different flames modes from a fully stabilized turbulent flame to the limit of flame blow-out. The typical structure of the turbulent flame consists in two adjacent parts: a torus flame stabilized in the recirculation zone followed by a jet-like flame developing from the end of the recirculation zone. Time-resolved flame imaging allows to reveal the occurrence of intermittent ejections of reactive pockets from the recirculation zone to the base of the main jet-like flame, ensuring its stabilization. Image processing of time-series is performed to evaluate the characteristic frequency of this periodic ejection and its variation with the annular air flow velocity. Time-series of instantaneous velocity fields in the flames are obtained by 5 kHz Particle Image Velocimetry (PIV). Their analyses show the original structure of the flow downstream the Bluff-body burner, consisting in a highly turbulent internal recirculation zone surrounded by the centripetal annular air flow and pierced by the methane central jet. Comparison between high speed flame imaging and PIV measurements allow to determine correlations between the periodic burning pockets ejection and some features of the instantaneous velocity fields in this region, in order to fully describe this physical phenomenon controlling the stabilization of the turbulent flame.