Influence of Geometry on the Performance of Simplex Nozzles under Constant Pressure Drop

In this paper, the effect of atomizer geometry on the flow in simplex atomizers and atomizer performance is numerically investigated. A computational model based on the Arbitrary-Lagrangian-Eulerian method has been used. The effect of changes in four non-dimensional geometric parameters on the atomizer performance is studied. These geometric parameters are the atomizer constant (ratio of inlet slot area to the product of swirl chamber diameter and exit orifice diameter), the length to diameter ratio of the swirl chamber and that of the orifice, and the spin chamber to orifice diameter ratio. The variation in the atomizer performance is obtained keeping the pressure drop across the atomizer constant and is presented in terms of the dimensionless liquid film thickness at the exit of the orifice, spray cone half angle and discharge coefficient. * Corresponding author Introduction Simplex nozzles are widely used in air-breathing gas turbine engines for fuel injection because of their good atomization characteristics and relatively simple geometry. The fuel atomization process plays an important role in determining the eventual emissions from the combustor. It is highly desirable to design fuel atomizers that can produce sprays with a predetermined droplet size distribution at the desired combustor location (small mean droplet diameters and uniform local air/fuel ratios) to reduce emissions from the combustion process. Figure 1 shows a schematic of a simplex atomizer. High-pressure liquid enters the swirl chamber through virtually tangential inlet slots such that the liquid has a swirling motion. Due to high swirl velocity, an air core is formed around the centerline of the atomizer. The liquid exits through a small orifice with a high swirl velocity that results in liquid spreading out in the form of a hollow conical sheet. Further downstream the thin liquid sheet becomes unstable and disintegrates into droplets. The mean droplet diameter (which is roughly proportional to the square root of the film thickness) and the spray angle are the two important parameters governing atomizer performance. In this study, the effect of atomizer geometry on the flow in simplex atomizers and atomizer performance under constant pressure drop has been numerically investigated. A computational model based on the Arbitrary-Lagrangian-Eulerian method[1] has been used. The effect of changes in the four non-dimensional geometric parameters, viz., the length to diameter ratio of the swirl chamber (Ls/Ds) and that of the orifice (lo/do), the spin chamber to orifice diameter ratio (Ds/do), and atomizer constant (K), is studied. The variation in the atomizer performance is presented in terms of the dimensionless liquid film thickness at the exit of the orifice (t), spray cone half angle (θ), discharge coefficient (Cd), and ratio of average swirl velocity to average axial velocity at the exit (wav/uav). This study provides important insights into the physics of the flow in the atomizer and will be useful in developing guidelines to improve present atomizer designs. Computational Model The main difficulty in the numerical simulation of the flow in a simplex atomizer is the accurate tracking of the liquid/air interface. The location and shape of the interface are not known a priori and must be obtained as part of the solution. A fully Lagrangian approach is best suited to accurately track the interface. However A A