Effect of Prefilmer Edge Thickness on Breakup Phenomena of Liquid Film in Prefilming Airblast Atomizer

Abstract

This paper describes the investigation of the effect of a prefilmer edge thickness on the breakup phenomena of a liquid film in a prefilmer airblast atomizer. The breakup phenomena of the liquid film at five prefilmer edge thicknesses (160, 500, 1250, 2000, and 3000 μm) under various conditions was observed using a high-speed camera. The breakup length of the liquid film was calculated by an image processing technique developed in this study. In order to quantitatively evaluate the effect of the prefilmer edge thickness on the breakup frequency, the Fast Fourier Transformation (FFT) analysis was conducted based on the time evolution of the breakup length. The results indicated that the breakup length increase and the breakup frequency decreases by increasing prefilmer edge thickness due to a larger volume of a liquid accumulation attaching to the prefilmer edge. The FFT analysis showed that the increase in prefilmer edge thickness causes the transition of the maximal power spectrum to a lower frequency (i.e. less than 100 Hz) due to the increase in the liquid accumulation at the edge as well. Finally, a dimensionless correlation has been proposed for the breakup length of a liquid film. Keywords Prefilming airblast atomizer, Breakup length, Breakup frequency, Prefilmer edge thickness, FFT analysis IntroductionIn the recent jet engine, the prefilming airblast atomizer has been widely used for the liquid fuel injection due to a good spray characteristics even at low fuel injection pressure. This type of atomizer uses the high-speed air stream flowing in the combustor for the atomization of a liquid fuel. Thus, the pressure loss of atomizing air in an atomizer is smaller than a conventional atomizer using the air stream, and leads to stable performances of the combustor over the entire range of operating conditions. The atomization mechanism of a prefilming type of airblast atomizer is well known. After injection, the liquid homogeneously wet the prefilmer surface, and flows down to the prefilmer edge in the form of a liquid film that is sheared by the high-speed air stream. The liquid film accumulates at the prefilmer edge. This accumulation is sheared by air and disintegrated into the bag-shaped liquid framed by a thicker rim. The bag-shaped part of the structure bursts and generates fine droplets. The rim is stretched and fragments into two elongated ligaments that disintegrate into larger droplets. Many researchers have studied the spray characteristics of a prefilming type of airblast atomizer, and revealed that a liquid film thickness, liquid flowrate, and air velocity are important parameters. Lefebvre reviewed the investigations on airblast atomizers, and pointed out the advantages of airblast atomizers such as lower fuel pressure, low soot formation, constant fuel distribution over the whole range of fuel flow rate, and fine mixing between fuel and air [1]. Inamura et al. experimentally investigated the spray characteristics of a prefilming type of airblast atomizer using the two-dimensional test a...