A detailed understanding of liquid propellant combustion is necessary for the development of improved and more reliable propulsion systems. This article describes experimental investigations aimed at providing such a fundamental basis for design and engineering of combustion components. It reports recent applications of imaging techniques to cryogenic combustion at high pressure. The flame structure is investigated in the transcritical range where the pressure exceeds the critical pressure of oxygen (p > p c (O2 = 5.04MPa)) but the temperature of the injected liquid oxygen is below its critical value . Data obtained from imaging of OH* radicals emission, CH* radicals emission in the case of LOx/GCH4 flames and backlighting provide a detailed view of the flame structure for a set of injection conditions. The data may be used to guide numerical modelling of transcritical flames and the theoretical and numerical analysis of the stabilization process. Calculations of the flame edge are used to illust...

http://www2.eng.cam.ac.uk/~mpj1001/papers/CST_Candel_Juniper_2006.pdf

Structure and dynamics of cryogenic flames at supercritical pressure

Experimental investigations have been carried out to examine film cooling effectiveness of an accelerated hot gas in a subscale rocket combustion chamber. In support of future first-stage high-performance rocket combustion chambers, a Vulcain2-like test case has been examined with combustion pressure levels up to 12 MPa. The effectiveness of an almost tagentially injected film of hydrogen with an initial temperature of approximately 280 K has been determined. Axial distributions of temperature were measured inside the copper liner as well as on the chamber surface in the convergent and divergent parts of the nozzle segment. An existing film cooling model has been modified for application in a combined convective and filmcooled combustion chamber with an accelerated hot gas. The new model predicts film cooling effectiveness at different combustion-chamber pressures and film blowing rates at sub-, trans-, and supersonic conditions.

Film Cooling of Accelerated Flow in a Subscale Combustion Chamber

: In the past, liquid rocket engines (LRE) have experienced high-frequency combustion instability, which impose an acoustic field in the combustion chamber. The acoustic field interacts with the fluid jets issuing from the injectors, thus altering the behavior of the jet compared to that of stable operation of the LRE. It is possible that this interaction could be a substantial feed back mechanism driving the combustion instability. In order to understand the problem of combustion instability, it is necessary to understand the interaction of the jet with the acoustic waves. From past combustion instability studies of the liquid oxygen and hydrogen propellant combination in a shear-coaxial injector configuration, a design guideline of outer-to-inner jet velocity ratio greater than about ten was proposed in order to avoid high-frequency acoustic combustion instability problems. However, no satisfactory physical explanation was provided. To promote this understanding, a cold-flow experimental investigation of a shear-coaxial jet interacting with a high-amplitude non-linear acoustic field was undertaken under chamber pressures extending into the supercritical regime. Liquid nitrogen (LN2) flowed from the inner tube of a coaxial injector while gaseous nitrogen (GN2) issued from its annular region. The injector fluids were directed into a chamber pressurized with gaseous nitrogen. The acoustic excitation was provided by an external driver capable of delivering acoustic field amplitudes up to 165 dB. The resonant modes of the chamber governed the two frequencies studied here, with the first two modes being about 3 and 5.2 kHz. High-speed images of the jet were taken with a Phantom CMOS camera.

On the Behavior of a Shear-Coaxial Jet, Spanning Sub- to Supercritical Pressures, with and without an Externally Imposed Transverse Acoustic Field

: Sierra Engineering and the Air Force Research Laboratory Propulsion Directorate, have undertaken a program to develop gas-centered, swirl coaxial injectors. This injector design will be used in the multi-element Advanced Fuels Tester (AFT) engine to test a variety of hydrocarbon propellants. As part of this program, a design methodology is being developed which will be applicable to future injector design efforts. The methodology combines cold flow data, acquired in the AFRL High Pressure Injector Flow facility, uni-element hot fire data, collected in AFRL Test Cell EC-1, and a computational effort conducted at University of Alabama-Birmingham, to identify key design features and sensitivities. Only results from the experimental effort will be presented in this work. Three different gas-centered swirl coaxial element concepts are being studied: a converging design, a diverging design, and a pre-filming design. The cold flow experiments demonstrated that all three classes of elements produced an extremely dense, solid cone spray, with the highest mass density in the center. The atomization of all of these injectors was excellent, producing mean drop sizes 1/3 to 1/4 of that typically measured for shear coaxial elements operating under similar conditions. Uni-element hot fire testing has found that the converging designs produce C* efficiencies in excess of 90% over a wide-range of mixture ratios and pressure conditions. Near the design pressure, efficiencies exceeding 96% have been measured. In the diverging designs, a chamber oscillation of near 200 Hz has been noted. The cause of this oscillation is under investigation.

/pdf/swirl-coaxial-injector-development-55833q3sey.pdf

Swirl Coaxial Injector Development

On porous liquid propellant rocket engine injectors

An experimental investigation has been carried out to examine the effects of liquid-oxygen (LOX) post biasing of a shear coaxial injector on the behavior of the spray near a chamber wall. The experimental work was performed with inert propellant simulants in a high-pressure chamber. Injector flow rates and chamber pressure were designed to match the space-shuttle-main-engine (SSME) injector gas-to-liquid density and velocity ratio at the point of propellant injection. Measurements of liquid mass-flux, gas-phase velocity, and droplet size were made using mechanical patternation and phase Doppler interferometry techniques. The measurements revealed that the liquid mass-flux distribution shifts away from the wall with increasing LOX postbias away from the wall. The shift in the liquid flux distribution was much greater than that caused by the angling of the LOX post alone. Gas velocity near the wall simultaneously increased with increasing LOX post bias away from the wall. The increase in wall-side gas velocity was caused by the higher fraction of gas injected on the wall side of the injector as a result of the eccentricity at the injector exit. The net result was a decrease in mixture ratio near the wall. Estimates of heat transfer and engine performance relative to the unbiased case are presented.

Effects of liquid-oxygen post biasing on SSME injector wall compatibility

An experimental investigation has been carried out to examine the effects of LOX post biasing of a shear coaxial injector on the behavior of the spray near a chamber wall. The experimental work was performed with inert propellant simulants in a high-pressure chamber. Injector flow rates and chamber pressure were designed to match the Space Shuttle Main Engine (SSME) injector gas-to-liquid density and velocity ratio at the point of propellant injection. Measurements of liquid mass flux, gas phase velocity and droplet size were made using mechanical patternation and phase Doppler interferometry techniques. The measurements revealed that the liquid mass flux distribution shifts away from the wall with increasing LOX post bias away from the wall. The shift in the liquid flux distribution was much greater than that caused by the angling of the LOX post alone. Gas velocity near the wall simultaneously increased with increasing LOX post bias away from the wall. The increase in wall side gas velocity was due to the higher fraction of gas injected on the wall side of the injector as a result of the eccentricity at the injector exit. The net result is a decrease in mixture ratio near the wall. Estimates of heat transfer and engine performance relative to the unbiased case are presented.

K. I. Miner

Papers

Effects of liquid-oxygen post biasing on SSME injector wall compatibility

The Effects of LOX Post Biasing on SSME Injector Wall Compatibility