Acoustically Forced Coaxial Hydrogen Liquid Oxygen Jet
Combustion instabilities can pose serious problems in the development of liquid rocket engines. In order to understand and predict them, it is necessary to understand how representative liquid rocket injector flames react to acoustic waves. In this study, a representative coaxial gaseous hydrogen liquid oxygen (LOX) jet flame is visualized for both reacting and nonreacting cases. The jet flame was studied unforced, without acoustics, and forced, with transverse acoustic waves in a pressure node and a pressure antinode configuration. For unforced flames, reactions are found to cause a significantly more expanded plume due to the vaporization and expansion of the LOX. Flame holding is established at the lip with a particularly dominant LOX recirculation zone. Nonreacting convective structures propagate downstream at relatively constant velocity, while reacting structures start at a slow speed and gradually accelerate with downstream distance. These structures never reach the velocity of the nonreacting structures. Reactions shift the spectral content to lower frequencies, consistent with trends observed in the linear stability literature. For forced flames, acoustics do not appear to affect the flame holding. Dynamic mode decomposition detects jet response not only at the fundamental frequency but at higher harmonics as well. Reactions produce inconsistent trends in the harmonics: reactions promote harmonics at a pressure antinode while they damp harmonics at a pressure node. *Corresponding author: Mario.roa.ctr@us.af.mil 1Now at University of St. Thomas, St. Paul, Minnesota ILASS Americas 28th Annual Conference on Liquid Atomization and Spray Systems, Dearborn, MI, May 2016 DISTRIBUTION A. Approved for public Release; distribution unlimited. PA 16168 Introduction Combustion instability associated with liquid rocket engines has been responsible for degradation of engine performance, increase in engine component stresses, and under extreme conditions, catastrophic engine failure. High frequency instabilities, which are generally the most harmful in liquid rocket engines, can be driven when the disturbances associated with transverse acoustic resonances couple with the combustion process to form a feedback loop. In order to understand and predict combustion instabilities, it is necessary to understand how representative liquid rocket injector flames react to acoustic waves. In general, the thermoacoustic instability phenomenon is understood to be an interaction between acoustic waves and unsteady combustion. The unsteady heat release can amplify the acoustic field if the conditions are favorable. Due to the broadband nature of hydrodynamic instabilities, avoiding a detrimental coupling between acoustics and fluid dynamic mixing is challenging. One way to try to mitigate the instabilities is to design an injector that does not couple with acoustic perturbation while it still provides the necessary mixing to sustain combustion. One such injector, used extensively for liquid rocket applications, is the shear coaxial injector [1 5] In this injector configuration liquid oxidizer is injected surrounded by a gaseous fuel, as shown in Figure 1. The aerodynamic interactions at both fluid interfaces bring about hydrodynamic instabilities and atomize the liquid oxidizer into small droplets. These droplets evaporate and mix with the surrounding fuel and react. There have been many models and efforts [6 9] to describe the atomization process with most being linear in nature and supported with empirical approximations to existing data. The models provide approximate liquid core lengths, time scales associated with growth rates of small disturbance, and droplet sizes and distributions. Due to the highly nonlinear nature of atomization, these models are limited in their range application and do not take reactions and the effects of high pressures into consideration. Accurate predictions of nonlinear, reacting high pressures atomization are left for intense computational studies. In an effort to further understand high pressure atomization and combustion of a shear coaxial injector, experimental efforts have been carried at academic and national laboratories [10 12] From these studies, there have been several proposed models of the breakup and dynamics of the liquid core of the shear coaxial injector under relevant rocket engine conditions. One such model is the classical phenomenological break up modFigure 1. The shear coaxial flow of a LOX center surrounded by gaseous hydrogen. The first row shows the unreacting condition under same acoustic amplitude (0.34 bar) in a PAN and PN configuration. The second row shows the same conditions but reacting. el by Reitz and Bracco [6] In this model, the intact liquid core has ligaments and droplets continually shed due to surface instabilities, with the rate of liquid atomization dependent on momentum flux and velocity ratios between the liquid and the gas. This model has been typically used for rocket applications. Experimental and computational results have since raised questions of Reitz and Bracco model on its applicability to coaxial jets at rocket relevant conditions [13 15] Another model is the core fragmentation model [16] where helical instabilities distort and breakup the liquid core. It is this helical mode that is responsible for some the large scale breakup of the LOX core observed for this investigation. Since the LOX core has been observed to be the most prominent feature of shear coaxial injectors and with studies suggesting that combustion instabilities couple with LOX post acoustics [17] an experimental effort was undertaken to study LOX core dynamics and breakup using a shear coaxial configuration subjected to high frequency acoustic forcing. A near field view at the injector exit was studied, limited to 6 LOX diameters, to capture the onset of the LOX core instabilities. The shear coaxial injector was studied both at reacting and nonreacting conditions. High frequency pressure anti node (PAN) and a pressure node (PN) acoustic forcing was applied at the injector exit. Dynamic mode decomposition (DMD) was used to extract frequencies and spatial modal shape of the LOX response to the acoustic forcing under reacting and nonreacting conditions. There were differences in LOX core convective speeds and LOX core length between the reacting and nonreacting conditions for the acoustically unforced conditions. The LOX core behaved similarly under reacting and nonreacting conditions for the PAN acoustic forcing. A coupling of hydrogen (GH2) and LOX core shear layers occurred and a LOX core helical model was observed for the PN acoustic forcing for both reacting and nonreacting cases. Experimental Setup and Conditions The experimental facility at the Air Force Research Laboratory (AFRL) at Edwards Air Force Base was used to investigate the coupling of transverse acoustic resonances with a single, shear coaxial, LOX/GH2 flow with varying acoustic perturbation amplitudes. Figure 2 shows the experimental chamber. The features of this facility were documented by Wegener et al. [12] The injector geometry was the same as studied by Wegener et al [12] Experiments were done at a mean chamber pressure of 34.45 bars (500 Psia) Nominal flow rates of liquid oxygen and hydrogen were 4.2 and 0.7 g/s, respectively, with a jet momentum ratio (GH2 to LOX) of 2.2. Both fluids had a Reynolds number on the order of 104. The LOX core had a Weber number on the order of 104. The LOX core was kept at constant temperature of 140 K and the GH2 at 250 K. The transverse acoustic resonance was induced through the use of carefullycontrolled piezo sirens, allowing excitation across a range of pressure amplitudes for discrete resonant chamber frequencies. The amplitude of the acoustic perturbations was varied incrementally from 0 bar (i.e. no acoustic forcing) to 0.34 bar (5 Psi) The pressure fluctuations were measured via Kulite high speed pressure transducers (model XCL 100) located at different transverse locations, including the middle of the chamber which is the location of the injector. The mode shape of the acoustic field was altered through the phase relationship between the two acoustic drivers. The control of the mode shape allows for the PAN and PN regions of the acoustic mode to be placed at the exit of the injector. Resonant chamber frequencies of 3060Hz for PAN forcing and 2890 Hz for PN were selected. Previous studies on nonreacting experiments have shown a dramatic difference in the coupling mechanism depending on the local acoustic environment [18] Shadowgraphy was selected as the basis of flow visualization due to the focus on the LOX core dynamic response to the acoustics, while providing an indirect view of the flame through the locations of density variations. A collimated light source and Vision Research v710 Phantom camera were used to acquire the highspeed shadowgraph images. The images were acquired at a 25 kHz frame rate with an exposure of 5 microseconds. The high speed shadowgraph images were then analyzed using dynamic mode decomposition (DMD) similar to that described by Schmid [19] to identify when and how the LOX core and its shear flow responded to acoustic perturbations with and without heat release. Results Figure 1 shows instantaneous shadowgraph images of unforced and acoustically forced cases under both unreacting and reacting conditions. The presence of the Figure 2. The experimental chamber and high speed camera. flame has a dramatic effect on the topology of the LOX surface. For nonreacting, unforced conditions, the interface is highly blurred and appears to be in a fibrous atomization regime, consistent with Farago and Chigier [20] diagram of fiber type of atomization for the given Reynolds and Weber numbers. Evidence of large scale breakup of the LOX core length for the nonreacting and unforced condition, shown on the upper left in Figure 1, is evident 6 di