Abstract

ABSTRACT Lean premixed prevaporized combustion (LPP) is a promising approach for the reduction of emissions in gas turbine combustion. Typically, modern gas turbines operate at high pressure and high temperature flow conditions, giving rise to very short self ignition times. Consequently, the residence time of the mixture in the premix duct has to be minimized. Complete fuel vaporization and mixing can only be achieved by an improvement of the atomization process towards a fine droplet size spectrum over the whole range of operating conditions. In this paper an air assisted pressure swirl atomizer is introduced and analyzed. The special design of the nozzle under investigation enhances the interaction between the liquid sheet of the atomizer and the co-flowing air around the nozzle. The visualization of the atomization process gives detailed insight into the fundamental atomization phenomena of the atomizer. In addition, extensive measurements of droplet size distributions describe the dependence of the atomization process on liquid flow rate and air velocity, respectively. All phenomena occurring are explained in detail by means of a theoretical analysis of the flow pattern. With the help of this kinematic model, it can be shown why co-rotating swirls of atomization air and liquid cone lead to better atomization than Counter rotating swirls. NOMENCLATURE ALR air liquid ratio Presented at the International Gas INTRODUCTION The design of combustors for lean premixed prevaporized combustion (LP?) is governed by different, partially opposing factors. First of all, liquid fuel has to be evaporated completely within the premixing zone and the lean fuel air mixture has to be as homogeneous as possible. To achieve complete evaporation over the whole range of operation conditions, the fuel droplets must be as small as possible, with typical Sauter mean diamters lower than 25 tan. An improvement in homogeneity usually requires additional elements giving rise to pressure losses or longer premixing channels. At high pressures and temperatures, on the other hand, the selfignition time becomes very small, (Spadaccini [ 12 ]). Consequently, the droplet residence times within the premixing zone must be reduced, demanding considerably higher air velocities. However, good atomization with droplet sizes in the range of 25 pint over a wide range of fuel flow is one key factor for the application of a fuel atomizer in a LPP combustor concept. From the numerous existing atomizer designs, it is possible to extract two major classes for the application in gas turbine combustors: Airblast atomizers and pressure swirl atomizers, both having their inherent advantages and disadvantages. Airblast atomization, for example is almost independent of the fuel flow rate, but decisively governed by the density and velocity of the atomization air, (Aigner [ 1 ], Sattelmayer [ 2 ]). Pressure atomizers, on the other hand, are characterized by superior atomization at high fuel pressures within a relatively small range of fuel flow. Similar to the atomizer concepts of Lefebvre [ 3 ] and Rizk [ 6 ], the pressure swirl atomizer in the present study is surrounded by a converging air flow, with the idea to combine both principles. A detailed analysis of the resulting atomization mechanisms, followed by a derivation of design rules for this special type of atomizer will be presented in this paper. In the first part of the text, the setup of an atmospheric Mg rig is described and both atomizer concepts are discussed. The test rig allows the optional use of a prefilming airblast atomizer or an air assisted pressure swirl atomizer. The visualization of the atomization process for the air assisted pressure swirl atomizer gives insight into the behavior of the cone depending on liquid mass flow and velocity level of the surrounding air flow. Detailed measurements of droplet sizes under varying operating conditions render the possibility for a comparison of the two different concepts. A theoretical analysis of the flow pattern is performed in order to interpret the governing physical effects.