Flow Field Measurements of a Prefilming Airblast Atomizer

Abstract

In the primary zones of combustors featuring fuel injectors the pattern of burning is highly complex and depends on the atomization quality. The objective of the present study is to assess the cone angle by light scattering technique, planar droplet velocity field using 2DPIV, patternation using PLIF and droplet size distribution using PDPA of a prefilming airblast atomizer spray in a swirling flow field having a high swirl number (1.09). The results showed that the cone angle and the velocity of the spray field increases with increase in pressure drop across the atomizer. The dispersion of liquid is delayed when there is increase in liquid mass flow rate even though there is increase in air flow rate. The Sauter mean diameter (SMD) distribution of the spray becomes uniform at 40 and 50 mm down stream regions. I. Introduction uel injection process plays a major role in many key aspects of gas turbine performance. It is important to know the details of the fuel injector performance. Most of the gas turbine engines in service use prefilming type of air blast atomizer because of their potential for achieving significant reductions in soot formation and exhaust smoke. The prefilming concept for airblast atomization evolved from the studies of Lefebrve and Miller 1 . They showed that the minimum drop sizes were obtained by using atomizers designed to provide maximum physical contact between the air and the liquid, and increase in the thickness of the sheet produces larger size of droplets. Rizkalla and Lefebrve 2 investigated the effects of both air and liquid properties on atomization quality. Rizk and Lefebrve 3 studied the influence of the air velocity and liquid properties on the drop size distribution in an airblast atomizer. Han et al. 4 investigated the effects of the fuel nozzle displacement on the spray characteristics. The found that there was significant variation in cone angle, droplet number density and recirculation pattern of the spray. To obtain flame stabilization, a region of flow field must be found where the flame speed matches the forward flow velocity and also the heat supplied must be sufficient to initiate combustion process. High degree of swirl flow is characterized by the presence of central toroidal recirculation zone (CTRZ), high turbulence and combustion intensity. Brena de la Rosa et al. 5 conducted studies on the velocity and turbulence field of a liquid spray in the swirl air flow. Due to flow reversal, droplets of order of 5 to 10 microns recirculate in the core recirculation region at high swirl numbers. Swirl flows have large scale effect on flow fields. For example in the case of inert jets swirl affects the jet growth, entrainment and decay and for reacting flows flame size, shape, stability and combustion intensity. Reddy et al. 6 investigated the characteristics of swirl flows in a sudden expansion square chamber. They reported a decay of the tangential velocity along the flow path. The swirl motion decreased and the flow transitioned to an axial flow with vanishing radial and tangential velocities and a decay of turbulence intensity towards down stream of the flow from the swirler. The primary zone airflow pattern is of prime importance to the flame stability in gas turbine combustor. The most effective way of inducing flow recirculation in the primary zone is to fit a swirler in the dome around the fuel injector 7 . Vortex break down is a well-known phenomenon with high swirl number; it causes recirculation in the core region when the amount of rotation imparted to the flow is high. This type of recirculation provides better mixing than what is normally obtained by other means, such as bluff bodies because swirl component produce strong shear regions, high turbulence and rapid mixing rates.