Douglas G Talley

Injection of Fluids into Supercritical Environments

ABSTRACT This paper summarizes and compares the results of systematic research programs at two independent laboratories regarding the injection of cryogenic liquids at subcritical and supercritical pressures, with application to liquid rocket engines. Both single jets and coaxial jets have been studied. Cold flow studies provided valuable information without introducing the complexities of combustion. Initial studies utilized a single jet of cryogenic nitrogen injected into a quiescent room temperature nitrogen environment with pressures below and above the thermodynamic critical pressure of the nitrogen. Later, the work was extended to investigate the effects of a co-flowing gas. Parallel to this work, combustion studies with cryogenic propellants were introduced to understand high pressure coaxial injection phenomena with the influence of chemical reaction. Shadowgraphy and spontaneous Raman scattering were used to measure quantities such as growth rates, core lengths, turbulent length scales, fractal dimensions, and jet breakup regimes. It is found that jets injected at supercritical pressures do not atomize as they do at subcritical pressures. Rather, they behave in many respects like variable density turbulent gas jets.

Visual Characteristics and Initial Growth Rates of Round Cryogenic Jets at Subcritical and Supercritical Pressures

Cryogenic liquids initially at a subcritical temperature were injected through a round tube into an environment at a supercritical temperature and at various pressures ranging from subcritical to supercritical values. Pure N2 and O2 were injected into environments composed of N2, He, Ar, and various mixtures of CO+N2. The results were photographically observed and documented near the exit region using a CCD camera illuminated by a short duration backlit strobe light. At low subcritical chamber pressures, the jets showed surface irregularities that amplified downstream, exhibiting intact, shiny, but wavy (sinuous) surface features that eventually broke up into irregularly shaped small entities. A further increase of chamber pressure at constant jet initial and ambient temperatures caused the formation of many small droplets to be ejected from the surface of the jet similar to what is observed in the second wind-induced jet breakup regime. As the chamber pressure was further increased, the transition to a f...

Coupling between hydrodynamics, acoustics, and heat release in a self-excited unstable combustor

The unsteady gas dynamic field in a closed combustor is determined by the nonlinear interactions between chamber acoustics, hydrodynamics, and turbulent combustion that can energize these modes. These interactions are studied in detail using hybrid RANS/large eddy simulations (RANS = Reynolds Averaged Navier-Stokes) of a non-premixed, high-pressure laboratory combustor that produces self-excited longitudinal instabilities. The main variable in the study is the relative acoustic length between the combustion chamber and the tube that injects oxidizer into the combustor. Assuming a half-wave (closed-closed) combustion chamber, the tube lengths approximately correspond to quarter-, 3/8-, and half-wave resonators that serve to vary the phasing between the acoustic modes in the tube and the combustion chamber. The simulation correctly predicts the relatively stable behavior measured with the shortest tube and the very unstable behavior measured with the intermediate tube. Unstable behavior is also predicted for the longest tube, a case for which bifurcated stability behavior was measured in the experiment. In the first (stable) configuration, fuel flows into the combustor uninterrupted, and heat release is spatially continuous with a flame that remains attached to the back step. In the second (unstable) configuration, a cyclic process is apparent comprising a disruption in the fuel flow, subsequent detachment of the flame from the back step, and accumulation of fuel in the recirculation zone that ignites upon arrival of a compression wave reflected from the downstream boundary of the combustion chamber. The third case (mixed stable/unstable) shares features with both of the other cases. The major difference between the two cases predicted to be unstable is that, in the intermediate length tube, a pressure wave reflection inside the tube pushes unburnt fuel behind the back step radially outward, leading to a post-coupled reignition mechanism, while in the case of the longest tube, the reignition is promoted by vortex convection and combustor-wall interaction. Other flow details indicated by the simulation include the relative phase between flow resonances in the tube and the combustor, increased mixing due to baroclinic torque, and the presence of an unsteady triple flame.

Phase-Doppler interferometry with probe-to-droplet size ratios less than unity. II. Application of the technique

Practical limitations associated with the use of small probe volumes with respect to the droplet size that is being measured by the phase-Doppler interferometry technique are discussed. An intensity-validation scheme and corresponding probe volume correction factor have been developed that reject trajectory errors and account for the rejections in calculation of the probe cross-sectional area. The intensity-validation scheme also provides a tractable method of setting the photomultiplier tube gain and laser power. Volume flux measurements in dilute sprays have shown a significant improvement over those made by standard phase-Doppler interferometry techniques at small beam waist/droplet size ratios.

Influence of Boundary Condition Treatment on Longitudinal-Mode Combustion Instability Predictions

Abstract : Combustion instability in rocket chambers is strongly influenced by acoustic interactions at the boundaries of the configuration. Many CFD simulations employ approximate boundary conditions in order to simplify the geometry but the impact that they have on the solution is not well understood. The present study focuses on the use of detailed (exact) boundary representations and an approximate boundary condition in a given longitudinal mode test chamber. The actual inlet boundary of the injector is comprised of a series of choked slots while the approximate boundary condition is a uniform constant mass flow. Both two and three-dimensional simulations are carried out. Differences in the flowfield are evident in the combustion region away from the inlet, including the size of the recirculation region and location of the peak heat release. The amplitudes of the acoustic modes are well predicted for the first two modes especially in three-dimensional simulation, while higher modes are poorly predicted. These results suggest that such boundary condition approximations must be judiciously used and having access to more detailed treatments is important to verify accuracy.

Parametric Trends in the Combustion Stability Characteristics of a Single-Element Gas-Gas Rocket Engine

Abstract : Combustion instability continues to be a challenge in the design of rocket engines. The use of computational fluid dynamics (CFD) simulations screen potential designs offers the ability to reduce the number of costly tests and improve understanding of the underlying instability mechanism. In this study a series of axisymmetric CFD simulations are used to investigate the instability sensitivity to four design changes. The design changes are selected in an attempt to reduce the level of instability. The parameters considered are the combustor wall temperature, the effect of adding swirl to the fuel injector and two geometric changes, namely, fuel injector area reduction and the introduction of a chamfer in the injector face. The simulations show that both the wall temperature and swirl are able to significantly lower the amplitude by 70%. The results of the geometric changes are mixed with both decreases and increases in the instability amplitude. The parametric study has enhanced the understanding of the instability mechanisms by demonstrating that the when a continuous fuel supply to the combustor is maintained the instability amplitude is decreased.

Phase-doppler interferometry with probe-to-droplet size ratios less than unity. I. Trajectory errors.

Phase-Doppler interferometry in which a probe volume that is much smaller than the droplets being measured has been shown to work well when coupled with a phase-ratio and intensity-validation scheme that is capable of eliminating trajectory-dependent scattering errors. With ray-tracing and geometric-optics models, the type and magnitude of trajectory errors were demonstrated quantitatively through stochastic trajectory calculations. Measurements with monodispersed water droplet streams and glass beads were performed to validate the model calculations and to characterize the probe volume. Scattered-light intensity has also been shown to provide a robust means of determining the probe cross-sectional area, which is critical for making accurate mass flux measurements.

Comparison of Laminar and Linear Eddy Model Closures for Combustion Instability Simulations

Abstract : Unstable liquid rocket engines can produce highly complex dynamic flowfields with features such as rapid changes in temperature and pressure, increased heat transfer, local flame extinction and reignition, and local partially-premixed and non-premixed combustion. This type of flowfield represents a challenge for turbulent combustion models, which are typically associated with a number of assumptions that limit regime applicability. In the present study, the linear eddy model (LEM) is applied to an unstable single element liquid rocket engine to assess its performance and to contrast it with simple laminar combustion model (LCM). Two distinct operating conditions showing different dynamic behavior are used; the first is marginally stable and has peak-to-peak amplitudes of 12% of the mean, while the second is strongly unstable and has pressure amplitudes in excess of 40% of the mean. Results show that while the LEM is able to capture the general dynamics behavior, the trends are in the wrong direction when compared with the experimental results. In other words, the stable case becomes more unstable and the unstable case becomes more stable. The paper also examines the underlying assumptions of the LEM and suggests reasons for the observed behavior.

Nano-Ignition Torch Applied to Cryogenic H2/O2 Coaxial Jet

Abstract : A high-pressure photoignition torch has been developed which takes advantage of the photoignition properties of single wall carbon nanotubes (SWNTs). The goal was to initiate combustion in a cryogenic O2-H2 coaxial injector at about 35 atm (520 psi) at O2 temperature of about 130 K with SWNT-based solid fuel mixtures. Our investigation includes the effects of chamber pressure, the presence of different solid oxidizers such as BKNO3 and KMnO4, as well as solid fuels and solid propellants, on the functionality of the photoignition torch. We have shown that the ignition parameters such as burn temperature, burn duration and the ignition byproducts can be tailored to meet different ignition requirements. It is anticipated that photoignition provides a suitable method for ignition of systems that require the start of combustion at a high pressure up to about 135 atm (2000 psi) as well as ignition of certain thrusters and liquid rocket engines that require an extremely small and light weight torch igniter. This ignition method can be applied to large combustion chambers such as gas turbines, gas generators, liquid rocket engines and possibly multi grain solid rocket motors.

Application of Detailed Chemical Kinetics to Combustion Instability Modeling

Abstract : A comparison of a single step global reaction and the detailed GRI-Mech 1.2 for combustion instability modeling in a methane-fueled longitudinal-mode rocket combustor was performed. A single element shear-coaxial injector was operated under two different conditions corresponding to marginally stable and unstable operation in order to evaluate the performance of the chemical kinetics mechanisms on combustion stability. Results show improved prediction in the frequencies and amplitudes with the detailed kinetics but the underlying source of the instability phenomena remains the same. In contrast to previous two-dimensional results, these three-dimensional results demonstrate that the present non-premixed injector configuration is primarily mixing-controlled and that the global chemical kinetics are sufficient to capture the stability characteristics.