KEYWORDS: Atomization; Gas turbine combustors; pressure atomizers; simplex atomizer; modified atomizers; needle insert; rotating needle in-sert; turbo swirl atomizer; rotating spindle; primary atomization; secondary atomization; axisymmetric simulations; Volume of fluid model; Discrete phase model; liquid-sheet thickness measurement; internal flow of simplex atomizer; spray; fine droplets.
Atomizers are primarily used to convert bulk liquid into small droplets. Pressure swirl atomizer is one of the widely used atomizers for fuel atomization in gas turbine com-bustors. The design of fuel injector influences the combustion efficiency and pollutant emissions from the combustors. In the present work, numerical simulations are per-formed to model the flow inside the simplex atomizer and primary break-up of the liquid stream using a fixed-grid based technique of Volume Of Fluid (VOF) method. Axisymmetric simulations are considered, as they prove to be effective and economi-cal for such problems. The validated numerical method is used for evaluating different injector designs for better performance. Further, an experimental study is also carried out to highlight the change in spray characteristics for atomizers with different internal geometries. Experimental techniques like shadowgraphy using backlight or laser beam (for spray imaging) and Phase Doppler Interferometry (for measuring drop sizes) are used. Overall, the numerical model is used to understand the flow phenomena occuring inside the atomizer such as air core formation, whereas experimental study is primarily intended to highlight the effects of injector geometry on the resulting spray outside the orifice.
A parametric study for different simplex atomizer geometries is performed with a view of obtaining an optimized geometry. Modification of atomizer geometry including different convergence or divergence angles and its effects on liquid spray were investigated experimentally and numerically. Furthermore, modified atomizer geometries with needle inserts incorporated inside the atomizer body were also analyzed and the results were compared with those of conventional simplex atomizer. Here, the principle focus is to improve atomization by reducing resultant drop sizes and enable a wide dispersion of droplets in the combustion chamber.
As a sequel to the extensive studies carried out with a simplex atomizer, a novel Turbo Swirl Atomizer (TSA) is developed which exhibits better performance than pres-sure swirl atomizers in many aspects. It is based on the concept of imparting tangential momentum and kinetic energy to the fluid (fuel in the combustor) using a rotating spin-dle, rather than by high pressure drop, as in conventional pressure based atomizers. The spindle, which also has blades employed at the spindle end, is designed to break the monotonous flow of liquid sheet into droplets of moderate size (of the order of 100 mi-cron). The atomization behavior of the novel Turbo swirl atomizer has been thoroughly studied experimentally, especially at low injection pressures for which the performance of the pressure swirl atomizer is poor. The atomization characteristics of the turbo swirl atomizer and the conventional simplex atomizer have been compared under similar op-erating conditions. The mode primary liquid break-up changes from asymmetric jet break-up to film break-up mode, when the spindle speed is increased. With further increase in the rotational speed, the mode changes to prompt break-up, with direct pro-duction of droplets without forming a liquid film or ligaments. With the development of the new atomizer, an attempt has been made to achieve small drop sizes in an energy efficient mariner at low injection pressures. Also, an air-swirler is incorporated, which is co-annular to the rotating spindle, to induce swirling air in the combustion chamber. The main purpose of using swirler is to enhance the break-up of droplets which arc originally formed by rotating spindle utilized in the TSA. With this configuration, very small drop sizes (of the order of 20 micron) are achieved even at low injection pressures of the order of 1 bar.