The influence of swirler design parameters on the aerodynamics of downstream recirculation region

Abstract

The influence of swirler design parameters namely, vane shape (flat or curved), vane outlet angle, aspect ratio and space-to-chord ratio (number of vanes), on the pressure drop-mass flow characteristics and the size and turbulence characteristics of the downstream recirculation region has been determined experimentally. A five-hole spherical pressure probe and hot-wire anemometry have been used for the investigation. A technique for simple and reasonably accurate interpretation of the signals from the hot-wire anemometer has been formulated. Its performance has been tested at first in round jets for which experimental turbulence data are already available for comparison purposes. The method has been applied subsequently to swirling flows issuing from the swirlers; the mean and turbulence characteristics of the flow fields have been determined in this way. In addition to these, the turbulent viscosities Ĭrr, Ĭrz, Ĭrθ and Ĭθθ have been calculated. Also, detailed accounts of the previous experimental work on swirling flows and of some finite difference procedures for the computation of weakly and strongly swirling flows have been given. It has been shown that the hot-wire formulation developed provides a simple, economic and efficient procedure for the measurement of the turbulence quantities in swirling flows. The results have shown that curved vane type swirlers operate more efficiently than flat vane types such that a larger recirculation region and a stronger shear zone are induced at a lower pressure drop. The pressure drop through both the flat and curved vane type swirlers increases with increasing vane angle, decreasing aspect ratio and decreasing space-to-chord ratio. The size of the downstream recirculation region, the reverse mass flow and the decay rates of the axial and swirl velocities increase with increasing vane angle and decreasing aspect ratio in the case of curved vane type swirlers. The effect of the space-to-chord ratio has not been found very significant in the cases of I6 and I2 vanes for the vane angle of 60° and the aspect ratio 0.4. Higher turbulence and stronger shear zones can be obtained in swirling flows with increasing vane angle, decreasing aspect ratio and decreasing space-to-chord ratios. Turbulence is anisotropic in strongly swirling jets issuing from vane type swirlers. The most dominant stresses are in turn ww, uw, and uu; all the other stresses are smaller. In parallel to the anisotropy in the stresses, the turbulent viscosities are also anisotropic; the turbulent viscosity Ĭθθ has shown domination among the four viscosities which have been calculated. Ĭrr values have shown too much scatter. For these reasons, curved vane type swirlers should prove much more efficient than flat vane types in the application to combustion process.