Effect of co- and counter-swirl on the isothermal flow- and mixture-field of an airblast atomizer nozzle

Results of the performed experiments describe the influence of co- and counter-rotating airflows through the inner and outer ducts of an airblast atomizer on the turbulent flow and mixture field. Therefore, two different nozzles have been manufactured, which differ only in the orientation of rotation of the inner and outer airflow, and have been confined by a combustion chamber, which provides optical access for laser diagnostics. The isothermal flow field has been investigated using a 3D-LDV-system. The 30° off-axis forward scattering configuration allows a high temporal and spatial resolution of the three velocity components, thus determining not only mean velocities but the total Reynolds stress tensor. Determination of the mixture field has been performed by a suction probe and subsequent analysis of the local concentration by means of conventional gas analysis. Compared to the co-swirl configuration the flow field of the counter-swirl arrangement exhibits a marked increase of the mass flow recirculated in the internal recirculation zone and a reduction of its length in axial direction. The analysis of turbulence quantities shows a considerable attenuation of the turbulent exchange of momentum perpendicular to the main flow direction for counter-rotating airflows. According to Rayleigh’s criterion this effect is attributed to the weaker reduction of the radial profiles of the time mean tangential velocity within the domain of the near-nozzle outer jet boundary in case of the counter-swirl configuration. In analogy to the exchange of momentum the obtained mixture fields feature a reduction of turbulent mass transfer rate in radial direction with counter-rotating airflows.