Saturation phenomenon of swirling spray flames at pressure antinodes of a transverse acoustic field

Abstract

This paper describes an experimental investigation of a linearly-arranged multiple-injection system of three swirling spray flames, simulating an unfolded sector of an annular combustor. Here we focus on the central flame placed in the basin of a pressure-antinode (PAN) of a standing transverse wave. Its response, quantified by means of the rms CH* amplitude filtered at the forcing frequency fr, is studied as a function of the acoustic pressure rms amplitude reduced by the bulk aerodynamic pressure, Π, and parameterized by the flame power P. As Π increases, three zones are identified: a linear growth, a transition zone and a saturation zone. Data in the linear growth and transition zone reduced by an adequate parameter merge into a unique self-similar curve whatever P. In the saturation zone, the time-frequency analysis applied to photomultiplier signals shows that the frequency signal at fr degrades in intensity level and uniformity, showing a loss of robustness in the flame response to the acoustic forcing over time. The gas phase displays strongly oscillating aerodynamics that can disrupt the central recirculation zone and destructure the mixing process. The liquid-phase shows a severe space-time modulation in droplet distribution and properties at high Π. Droplets are mainly found in an annular domain, characterized by a large time-dependence and inhomogeneity: a few quite big and slow fuel droplets are detected when the acoustic pressure P′(t) is positive, while a large population of small and rapid droplets is found when P′(t) is negative. In contrast, a little populated central core widens at positive P′(t) and fills up with big and slow droplets at negative P′(t). Such a periodic clustering might thus slow down the evaporation process.