Bruce Chehroudi

Measurements in an Acoustically Driven Coaxial Jet under Sub-, Near-, and Supercritical Conditions

Abstract : An experimental investigation was conducted on a coaxial jet, similar to those used in cryogenic liquid rockets, under sub-, near-, and supercritical pressures, with the ultimate intent of gaining a better understanding of an aspect of combustion instability that pertains to interactions of an externally-imposed acoustic field with the jet. Past research works on this subject have shown both the relevance and importance of geometrical changes in an injectors exit-area and its nearby physical and fluid mechanical processes. On this basis, special attention is paid in collecting spatially-resolved time averaged temperatures and documenting the aforementioned interactions at the exit of this injector. Short-duration and high-speed framing digital images provided information on the behavior of this jet under a variety of conditions. Mean and rms values of the dark-core length fluctuations were measured from the acquired images via a computer-automated method and the ensuing results are discussed. There appears to be a good correlation between this length and the outer-to-inner jet momentum ratio, but the form of this dependence was found to be different at subcritical pressures than the rest of the conditions. The rms values of the dark-core length fluctuations suggested a possible explanation for the observed improvement in instability limit at increasingly higher outer-to-inner jet velocity ratios.

Recent Experimental Efforts on High-Pressure Supercritical Injection for Liquid Rockets and Their Implications

Pressure and temperature of the liquid rocket thrust chambers into which propellants are injected have been in an ascending trajectory to gain higher specific impulse. It is quite possible then that the thermodynamic condition into which liquid propellants are injected reaches or surpasses the critical point of one or more of the injected fluids. For example, in cryogenic hydrogen/oxygen liquid rocket engines, such as Space Shuttle Main Engine (SSME) or Vulcain (Ariane 5), the injected liquid oxygen finds itself in a supercritical condition. Very little detailed information was available on the behavior of liquid jets under such a harsh environment nearly two decades ago. The author had the opportunity to be intimately involved in the evolutionary understanding of injection processes at the Air Force Research Laboratory (AFRL), spanning sub- to supercritical conditions during this period. The information included here attempts to present a coherent summary of experimental achievements pertinent to liquid rockets, focusing only on the injection of nonreacting cryogenic liquids into a high-pressure environment surpassing the critical point of at least one of the propellants. Moreover, some implications of the results acquired under such an environment are offered in the context of the liquid rocket combustion instability problem.

Raman Scattering Measurement in the Initial Region of Sub- and Supercritical Jets

Abstract : A high-pressure chamber is used to investigate and further enhance our knowledge and physical understanding on effects of thermodynamical subcritical-to-supercritical transition of ambient condition on cryogenic liquid injection using two-dimensional scattering. Pure liquid N2 is injected into N2. The injector is a 508-micron diameter straight hole having a long length-to-diameter ratio of 100. The optical setup uses a pulsed Nd:Yag laser frequency-doubled to 532 nm. Difficulties arise with optical breakdown of the N2 molecules in drops and ligaments by local focusing of the laser beam dominating the Raman signal particularly at sub- and near-critical regions. The severity of this problem is reduced by stretching the laser pulse width using a double-loop design with mirrors and beam splitters. Careful and painstaking alignment is needed to take advantage of this pulse-stretcher design. Two-dimensional images are taken near the injector and results interpreted in terms of density plots. At subcritical ambient conditions a small number of images are needed for averaging and strong Raman signal is obtained.

Interaction of Acoustic Waves with a Cryogenic Nitrogen Jet at Sub- and Supercritical Pressures

Abstract : To better understand the nature of the interaction between acoustic waves and liquid fuel jets in rocket engines, cryogenic liquid nitrogen is injected into a room temperature high-pressure chamber having optical access on its sides. A piezo-siren capable of generating sound waves with an SPL of up to 180 dB is used under three chamber pressures of 1.46, 2.48, and 4.86 MPa. The reduced pressures for these pressures are 0.43 (subcritical), 0.73 (near-critical), and 1.43 (supercritical), respectively. The assembly consisting of the acoustic driver and the high- pressure chamber form a cavity that resonates at several frequencies, the strongest being at 2700 and 4800 Hz. Three different flow rates are considered and the nature of the aforementioned interaction has been documented via a high-speed imaging system using a CCD camera. It is found that the impact of the acoustic waves on the jet structure is strongest from low to near-critical chamber pressures and at low injectant flow rates. No significant effects of the acoustic waves are detected at the supercritical chamber pressure examined. It suggests that engine operation either near the critical point or in transition passing through the critical point could be troublesome and may lead to or feed combustion instabilities in liquid rocket engines. Further work is needed to directly relate these effects to the observed instabilities.

Shear-Coaxial Jets from a Rocket-Like Injector in a Transverse Acoustic Field at High Pressures (POSTPRINT)

Abstract : In order to gain a better understanding of some of the underlying physics associated with the interaction of high-amplitude acoustic waves and a coaxial-jet injector similar to those used in cryogenic liquid rockets, a non-reacting-flow experimental investigation was conducted under sub-, near-, and supercritical chamber pressures, with and without acoustical excitation. Past research works on this subject have shown both the relevance and importance of geometrical changes in an injectors exit-area and its nearby physical and fluid mechanical processes. On this basis, special attention is paid in collecting spatially-resolved mean temperatures and documenting the aforementioned interactions at the exit of this injector. Short-duration and high-speed digital cameras provided information on the dynamic behavior of this jet under a variety of conditions. Mean and root mean square (RMS) values of the coaxial-jet dark-core length fluctuations were measured from the acquired images via a computer-automated method. It is seen that as the outer-to-inner jet velocity ratio increases, the RMS of the dark-core length fluctuations decreases. It is hypothesized that a connection to rocket instability can be obtained from these data through examination of the RMS values of the dark-core length fluctuations. It is possible that decreases in the fluctuation levels, which were shown here to occur at higher velocity ratios, could weaken a key feedback mechanism for the self-excitation process that is believed to drive the combustion instability in rocket engines. This could offer a possible explanation of the combustion stability improvements experienced in engines under higher outer-to-inner jet velocity ratios. Additional analysis and data acquisition are planned to further investigate this finding.

Physical Hypothesis for the Combustion Instability in Cryogenic Liquid Rocket Engines

In this work, the author would like to portray a sketch of a fluid dynamical picture to describe the coupling nature/ strength between the chamber acoustics and the injectors. This new perspective is achieved through a physically intuitive argument combined with previously published test results for two popular injector designs, namely, coaxial and impinging jets. For the impinging jet injectors, it is shown that the dynamic behavior of the dark-core (or breakup) zone for each jet, their lengths and thicknesses, has a profound impact on injector sensitivity to disturbances in its surrounding. This information is used to offer a possible explanation for the trends seen on the Hewitt stability plot in impinging jet injectors.

Measurements in an Acoustically Driven Coaxial Jet under Supercritical Conditions

*† ‡ An experimental investigation was conducted on a coaxial jet similar to those used in cryogenic liquid rockets, under sub-, near-, and supercritical conditions, with the ultimate intent of gaining a better understanding of an aspect of combustion instability that pertains to interactions of an externally-imposed acoustic field with the jet. Past research works on this subject have shown both the relevance and importance of geometrical changes in an injector’s exit-area and its nearby physical and fluid mechanical processes. On this basis, special attention is paid in collecting spatially-resolved mean temperatures and documenting the aforementioned interactions at the exit of this injector. Short duration and high-speed framing digital cameras provided information on the behavior of this jet under variety of conditions. Mean and rms values of the “dark core” length fluctuations were measured from the acquired images via a computer-automated method and the ensuing results are discussed. There appears to be a good correlation between this length and the outer-to-inner jet momentum ratio, but the form of this dependence was found to be different at subcritical than the rest of the conditions. The rms values of the length fluctuations suggested a possible explanation for the observed improvement in instability limit at increasingly higher outer-toinner jet velocity ratios.

The Effects of Pressure and an Acoustic Field on a Cryogenic Coaxial Jet

Abstract : A coaxial injector was designed to inject liquid nitrogen (LN2) with a coflow of gaseous nitrogen (GN2) in its annular region as part of a program to better understand the nature of the interaction between acoustic waves and liquid the jets in cryogenic rocket engines. Backlit images were taken from the jets at various flow rates and at sub-, near-, and super-critical chamber pressures with and without the presence of a 2700 Hz standing wave acoustic field. injector exit plane temperature measurements were made in both the center jet and annular regions. Results indicate that when the jet core appeared short and thin, mostly under supercritical chamber pressures, the jet became insensitive to the external acoustic field. The strongest interaction was observed when the jet core looked long and thick. To explore their implications, the characteristic acoustic impedance of the central jet and fuel/oxidizer momentum ratios are considered to play a role in the observed interactions. It is feasible that they play a similar role in cryogenic rocket engine combustion instability of the coaxial jet.

Experiments on Spray Combustion in a Gas Turbine Model Combustor

Abstract A hollow-cone kerosene spray with a nominal cone angle of 30 degrees from a pressure-swirl fuel atomizer was used in a swirl-stabilized combustor. Swirling air flow with a calculated swirl number of 0.36 is generated with a swirl plate having an exit air velocity vector of 30 degrees with respect to the chamber axis. Effects of swirl and dilution air flow rates on the shape and stability of the flame are investigated. A Phase Doppler Particle Analyzer (PDPA) is used to measure drop size, mean and rms values of axial drop velocity, fuel volume flux, drop velocity and size distributions, and size-classified drop velocity profiles for two cases of with and without combustion and at six different axial locations from the nozzle. A thermocouple is used to measure the uncorrected average temperature at these axial positions. For the no-combustion case all air and fuel flow rates were kept at the same values as the combusting spray condition. A Laser Doppler Velocimeter (LDV) is employed to measure the ...

A rapidly scanning laser Doppler anemometer

A rapidly scanning directionally sensitive fringe-type laser Doppler anemometer (SLDA) which scans the measurement volume perpendicular to the optical axis of the transmitting optics is described. Scan frequencies up to 60 Hz over scan distances of 40 cm have been used, although scan frequencies up to 150 Hz are possible. The maximum scanning velocity of the measurement volume that can be used is directly proportional to the shift frequency of the Bragg cell since each signal-producing particle must cross a minimum number of fringes to produce a valid signal. Signal-averaging bias is lower with a scanning LDA than with pointwise measurements. Results obtained for a separating turbulent boundary layer show that very good mean and RMS velocity profiles can be obtained in less than 1 min of data acquisition.