INFLUENCE OF Co AND COUNTER SWIRL ON LEAN STABILITY LIMITS OF AN AIRBLAST NOZZLE

摘要

This paper presents the reacting flow field and the temperature distribution of two different airblast nozzles, namely co and counter swirl. To support the interpretation of the obtained results, previous measurements of the isothermal flow and mixture field of methane and combustion air are summarized. Velocities within the turbulent flow field were measured by using 3D-LDA, measurements of the field distribution of temperature were performed by means of thermocouple probes. The results show that the counter swirl arrangement provides formation of an additional vortex in the immediate vicinity of the nozzle, which has been observed as well within the isothermal as within the reacting flow. Furthermore, a dampening effect of the tangential velocity profiles towards turbulent exchange of momentum has been observed within the counter swirl configuration. Both effects cause preferential mixing of the fuel with the inner combustion air flow, thereby performing higher concentrations of methane in the near nozzle mixture field. As a consequence the counter swirl flow field exhibits larger areas of near-stoichiometric composition of fuel and air, resulting in an elevated temperature level within the stabilization zone at otherwise identical operation conditions. Therefore, application of a counter swirl nozzle allows a higher thermal load than the co swirl configuration, which offers a satisfying explanation for the wider operating range of the counter swirl burner.