Atomization in Gas - Centered Swirl - Coaxial Injectors ( POSTPRINT )

A preliminary study of atomization mechanisms in gas-centered swirl-coaxial injectors for use in rocket engines has been undertaken. Gas-centered swirl-coaxial injectors differ from other injectors in that atomization occurs from a wall-bounded liquid. Few studies of atomization mechanisms in wall-bounded flows exist; some probable atomization processes have been determined, however. These mechanisms include liquid turbulence, aerodynamic stripping and a process driven by gas-phase structures. The likely character of the gas and film undergoing these atomization processes is presented. Relevant nondimensional groupings based on simplified theoretical descriptions of select mechanisms are outlined. Preliminary experimental and numerical results taken at atmospheric pressure are qualitatively compared to the likely mechanisms and each other. Corresponding author Distribution A: Approved for public release, distribution unlimited Introduction Recently, gas-centered swirl-coaxial (GCSC) injectors have garnered attention for use in rocket engines [1, 2]. These injectors operate like pressure-swirl atomizers with the addition of gas flow. A schematic of the injector is shown in Fig. 1. Liquid moves from a plenum to the outer wall of the injectors where it is injected tangentially. As a result of the tangential injection the liquid rotates about the axis of the injector forming a swirling film along the wall. Gas is introduced axially through the center of this swirling flow. The large aerodynamic forces make this injector an effective atomizer [2]. GCSC injectors have several advantages over more traditional designs including the ability to decouple the fuel feed system from any variations in chamber pressure [3]. Design strategies for these injectors remain poorly understood, however, because their operation differs from the more common injectors which they superficially resemble, i.e. pressure-swirl and coaxial air-blast atomizers. At their standard operating conditions in rockets GCSC injectors do not produce a conical sheet, as the more common injectors do, an obvious departure from other injectors. Atomization occurs within the injector cup instead [2]. In other words, droplets are produced by a wall-bounded film, not from the more frequently encountered sheet or jet. A more thorough understanding of the film atomization process must be developed to enable the characterization and improvement of GCSC injectors. Part of developing this understanding involves determining the underlying causes, or mechanisms, of atomization from wall-bounded films. A recent literature review conducted by the authors identifies a variety of potential mechanisms, but concludes that there are three most-likely culprits: liquid turbulence, stripping of waves generated by hydrodynamic instabilities and behavior related to vortices in the gasphase [6]. The first two of these have been observed in flat films under differing inlet conditions [4, 7, 8]. The third was observed during recent studies and is reported on herein. Each has been observed (seemingly) on their own, so there appear to be at least three modes of film atomization. To properly predict atomizer performance one must know the mode in which the atomizer operates. Additionally, knowledge of the design parameters relevant to the operating mode is necessary in order to design efficient and effective injectors. In order to advance the understanding of GCSC injectors and film atomization in general a series of experiments and numerical simulations are being undertaken. This paper outlines these activities and reports some preliminary findings. The following section discusses the three modes of film atomization including the likely relevant design parameters. Subsequent to this introductory description the setup for the numerical simulations and experiments is given. The initial results of both the simulations and experiments are presented in the Results and Discussion section. Film Atomization Regimes Studies have suggested three causes of atomization from wall-bounded films: liquid turbulence, stripping of waves and stripping/tearing resulting from gas-phase vortices [6]. These three mechanisms produce different droplet sizes and proceed at different rates so that they constitute three different atomization behaviors. The three are not exclusive, that is multiple mechanisms may be operable at the same time, but there will be regimes in which one is dominant over the others. To properly design an atomizer the operating mode must be known. Prior to determining the bounding conditions for these modes, a better understanding of each mechanism is required. B A