Abstract
Effective atomisation is a governing parameter in a liquid-based gas turbine. Incorporating the swirling air with the spray influences the spray structure, drop dispersion, and large-scale unsteadiness. Therefore, reliable measurement of drop sizes, spray structure, and large-scale instabilities are necessary for the detailed analysis of the complex interaction of the spray droplets in the swirl airfield. We aim to investigate the spray structure, drop sizing, and large-scale unsteadiness of a flow-blurring (FB) spray under swirl airfield conditions. In previous studies, the drop size distribution in FB sprays in a stagnant medium has been characterized based on the point measurement tool, namely Phase Doppler Particle Analyzer (PDPA). However, the PDPA measurement is time-consuming, suffers issues related to multiple scattering in the dense region of the spray, and provides only the time-averaged statistics. Herein, we characterize FB spray under the influence of an external swirling air stream using structured laser illumination planar imaging (SLIPI) technique. The SLIPI technique eliminates unwanted optical signals due to the multiple scattering and provides shot-to-shot variations of the drop size mapping and spray structure.
A lab-scale experimental setup consisting of a centrally mounted FB atomiser with co-annular swirl air passage is operated at a fixed air-liquid mass ratio but with different swirl velocities. Following 1p-SLIPI technique, simultaneous Mie and LIF signals are recorded using two cameras, which are eventually used to obtain two-dimensional droplet Sauter mean diameter (SMD) contours. The calibration constant for the LIF/Mie ratio is deduced using PDPA measurements. The ensemble of instantaneous images is utilized for the proper orthogonal decomposition (POD) analysis to investigate the spatio-temporal features of the large-scale structure. The role of the swirling air on the different types of large-scale instabilities is explained based on the POD modes and energy contributions by the individual modes.
