Doug Talley

Cryogenic shear layers: experiments and phenomenological modeling of the initial growth rate under subcritical and supercritical conditions

A jet of a cryogenic fluid, typically liquid N2, is injected into a chamber whose ambient pressure is varied to values exceeding the critical pressure of the injectant. The structure of the jet and the shear layer between the jet and the ambient have been examined. Results from visualization, jet initial growth rate, fractal analysis, and Raman scattering measurements indicate that the behavior of the injected fluid changes from liquid spray-like to gaseous jet-like behavior as pressure increased. This is attributed to the reduction of the surface tension and enthalpy of vaporization as the critical pressure of the injectant is approached. The initial divergence angle indicating the growth rate of the jet is measured at the jet exit. These values are then compared with those measured from a large number of other mixing layer flows, including atomized liquid sprays, turbulent incompressible gaseous jets, supersonic jets, and incompressible but variable density jets covering over four orders of magnitude in the gas-to-liquid density ratio, the first time such a plot has been reported over this large a range of density ratios. At and above the critical pressure of the injected fluid, the jet initial growth rate measurements agrees well with the theory and measurements of incompressible, variable density, gaseous mixing layers. This is the first time a quantitative parameter has been used to demonstrate that the similarity between the two flows extends beyond mere qualitative physical appearance. The initial growth rate using Raman scattering is also in reasonably good agreement with our measurements using shadowgraphy if twice the FWHM of the normalized intensity radial profiles are used. Finally, an equation based on a proposed physical mechanism combined with the characteristic gasification time (τg) and interfacial bulge formation/separation time (τb) is proposed, θ=0.27[τb/(τb+τg)+(ρg/ρl)0.5], that shows good agreement with the measured initial growth rate data. It is found that the transition point from sub- (liquid-jet like) to supercritical (gas-jet like) behavior occurs when the time scale ratio (τb/(τb+τg)) is approximately equal to 0.5.

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.

A Study of Acoustic Forcing on Gas-Centered Swirl-Coaxial Reacting Flows

Abstract : The reacting flow from a single gas-centered, swirl-coaxial injector was studied in an optically accessible, high-pressure chamber, with and without high-frequency acoustic perturbations. The gas-centered, swirl-coaxial injector employed liquid rocket engine relevant propellants of gaseous oxygen and RP-2. The reacting flow field behavior at an operating chamber pressure of 3.2 MPa and varying momentum flux ratios were investigated. High-speed shadowgraph images along with OH* and CH* chemiluminescence images were taken to capture the liquid fuel film, droplets, and flame response under acoustic excitation. For the acoustic forcing studies, low amplitude transverse standing waves typically below 5 of the chamber pressure were generated to simulate transverse combustion instabilities. Proper orthogonal decomposition and dynamic mode decomposition were performed on the high-speed shadowgraph and chemiluminescence images to detect the flame response to acoustic forcing, to which in-plane flapping motion was observed for acoustic forcing and rotating soot clouds were a large structures associated with the reacting flow field.

The Response of Cryogenic H2/O2 Coaxial Jet Flames to Acoustic Disturbances

Abstract : An experimental study has been conducted at the Air Force Research Laboratory(AFRL) at Edwards Air Force Base to explore the coupling between a coaxial jet flame and transverse acoustic perturbations. A new experimental facility at AFRL was used to expose a single H2/O2 shear coaxial diffusion flame to controlled acoustic resonances. A variety of chamber conditions including acoustic resonance properties were considered. The acoustic frequency and amplitude were selected relative to the characteristic frequency and dynamic pressure of the reacting injector flow. Placing the flame within the pressure node and antinode was also considered. Diagnostics employed high-speed imaging including backlit visualization and OH* chemiluminescence. The images were analyzed using proper orthogonal decomposition to identify the natural frequencies and organized structure of the unforced jet flame. These techniques were used to elucidate the effects of forcing, including the structure and relative importance of forced modes relative to the natural flame behavior.

Coaxial Injection under Supercritical Conditions

Abstract : This work reports on findings from the initial phase of a coaxial injection process under both subcritical and supercritical conditions. The results presented here are part of a systematic investigation of common rocket engine injectors, such as impinging and coaxial designs. Liquid nitrogen (LN2) is injected through a large length-to-diameter ratio circular hole and exposed at the exit to an annular jet of different gases including nitrogen, helium, and argon. The length-to-diameter ratio is sufficiently large to ensure fully-developed turbulent pipe flow at the exit plane. The behavior of the central LN2 jet has already been analyzed extensively and reported in our earlier published works, for example, Cheliroudi et al. 1, 2. Experiments were conducted by injecting LN2 into a room temperature, high-pressure chamber with full optical access from four directions. The stainless steel chamber can withstand pressures and temperatures of up to 13.6 MPa and 473 K, respectively. Liquid nitrogen is used to cool and/or liquefy the gaseous nitrogen passing through the cryogenic cooler prior to injection. The mass flow rate of the injectant is measured and regulated by way of a mass flowmeter, and a precision micrometer valve. A model K2 Infinity long distance microscope is used to form images of the injected jets on a high resolution CCD camera by the Cooke Corporation.