Flow structure in lean premixed swirling combustion

Abstract

Swirling flows have been widely used in industrial burners, furnaces, and gas turbine combustors to improve blowoff characteristics, ignition stability, mixing enhancement, flame stabilization, and pollutant reduction. Mean flow structure of the swirling flows with and without combustion has been extensively studied over the past 40 years. Some important phenomena in swirling combustion, such as combustion instabilities, depend on the multipoint instantaneous flow structure. However, the multipoint instantaneous flow structure in swirling flows has not been revealed in the past work because of measurement difficulties in the complex flow environment. Motivated by this, a cross-correlation particle image velocimetry (PIV) system was used to measure the multipoint instantaneous velocity fields in strongly swirling flows with a theoretical swirl number of 2.4 and a Reynolds number of 72.000. The mean flow structure based on the present measurements is consistent with the literature. However, the multipoint instantaneous flow structures show significantly different characteristics compared with the mean flow structure. Many smaller scale vortices with both directions of vorticity appear in the instantaneous flow in contrast to a single large vortex observed in the mean flow. Heat release seems to help a breakdown of the multiple vortices to produce a more homogeneous field. The turbulence intensity has a spatial distribution similar to that of the mean velocity. The heat release significantly increases the magnitude of the mean and the instantaneous vorticity. The differences in the flow structure and length scales of the vortices between the mean and the instantaneous fields suggest that the latter should be used in evaluations of transient phenomena and simulations.