Stephen A Danczyk

Photothermal deoxygenation of graphene oxide for patterning and distributed ignition applications.

A xenon discharge tube, such as is used to produce a photographic flash, has been reported to cause the ignition of carbon nanotubes, silicon nanowires, and welding of nanofibers of the conducting polymer polyaniline. In these reactions, the high surface-to-volume ratio of the nanomaterials being irradiated, coupled with the inability of the small structures to efficiently dissipate the absorbed energy, leads to a rapid increase in temperature and subsequent ignition or welding of the materials. Although heating materials through the use of light energy is not a new phenomenon, achieving such a rapid and dramatic temperature change using only millisecond pulses of light demonstrates a tangible and technologically significant capability that is unique to nanoscale materials. Graphene oxide (GO) is a deeply colored, water dispersible, oxidized form of graphene obtained through the treatment of graphite powder with powerful oxidizing agents. Although GO has been known for over 150 years, only recently have scientists had access to the tools necessary to properly analyze its atomically thin sheet structure. This has rekindled interest in graphite oxide and has led to a number of recent discoveries, including: the stacking of GO platelets to form paper-like materials of high modulus and strength. Many studies have suggested that GO can be reduced to graphene-like carbon sheets by applying chemical reducing agents or by using thermal treatments. This has led to speculation that GO could find use as a precursor in a bulk route to dispersible graphene sheets. Already, several groups have succeeded in creating conducting polymer composites, transparent conducting films, and simple electronic devices based on reduced GO. In addition to the chemical reduction of GO, Aksay, et al. have reported the thermal deoxygenation of GO to create functionalized graphene sheets upon rapid heating to 1100 8C under an inert atmosphere. These organic solvent dispersible sheets have enabled the direct creation of polymer composites, without the need for surfactants. Thermal deoxygenation of GO to form graphitic carbon dates back to the 1960s when Boehm and Scholz first reported on the ignition and deflagration of graphite oxides prepared by different methods. Upon rapid heating to temperatures of 200 8C, GO decomposes to the most thermodynamically stable oxide of carbon, CO2. Along with the exothermic release of CO2, H2O, and CO also form as minor products. [31]

Quasi-continuous burst-mode laser for high-speed planar imaging

The pulse-burst duration of a compact burst-mode Nd:YAG laser is extended by one order of magnitude compared to previous flashlamp-pumped designs by incorporating a fiber oscillator and diode-pumped solid-state amplifiers. The laser has a linewidth of <2 GHz at 1064.3 nm with 150 mJ per individual pulse at 10 kHz. The performance of the system is evaluated by using the third-harmonic output at 354.8 nm for high-speed planar laser-induced fluorescence of formaldehyde in a lifted methane-air diffusion flame. A total of 100 and 200 sequential images of unsteady fluid-flame interactions are acquired at repetition rates of 10 kHz and 20 kHz, respectively.

Ultrafast-time-gated ballistic-photon imaging and shadowgraphy in optically dense rocket sprays

Time-gated ballistic-photon imaging is a form of shadowgraphy in which an ultrashort, optical-Kerr-effect (order 2 ps) time gate is used to enhance the relative intensity of ballistic versus multiply scattered photons. In the current work, this technique is adapted for what is believed to be the first time for use in the moderately dense environment (optical density approximately 1.5 to 2) of a high-speed 5 to 15 mm diameter rocket spray to improve image contrast and observe liquid-breakup phenomena. Unlike coherence gating, which is another form of ballistic imaging, the time-gating approach allows sufficient signal levels from ballistic and near-ballistic photons to enable time-resolved single-shot imaging. Direct comparisons with non-time-gated shadowgraphy indicate that the two techniques are sensitive to different features of the flowfield, with regions composed of a dense field of droplets being highly attenuated in conventional shadowgrams but appearing transparent to ballistic photons. This enables significant image contrast enhancement (approximately 6.6:1) of liquid-core structures and facilitates improved understanding of the primary and secondary breakup processes in sprays of moderate optical density.

Effect of Swirl on Gas-Centered Swirl-Coaxial Injectors

Abstract : Physics-based scaling laws and design methodologies for gas-centered swirl-coaxial injectors have been under consideration for several years. Prior work showed that scaling via a momentum flux ratio was promising. However, that work neglected the effects of liquid swirl on the atomization. This current work refines the definition of the momentum flux ratio by adding compressibility in the gas phase and using a total liquid velocity. The focus, however, is on the impact of swirl through the study of different liquid inlet configurations. The film length and character are studied for a range of swirl values with ratios of axial to total velocity ranging from 26 to 41 percent. The new definition of momentum flux ratio collapses film length data onto a single curve over the range of swirl levels and interior injector geometries. The character of the film is seen to change, particularly in regards to gas entrainment into the film, as the swirl is decreased. Changes in swirl have little impact on film length; this is anticipated because the centripetal forces are several orders of magnitude lower than the aerodynamic forces driving atomization. Some examination of an inlet with no swirl (axial to total velocity ratio of 100 percent) is also presented.

Effect of Cup Length on Film Profiles in Gas-Centered Swirl-Coaxial Injectors

Recent interest in LOX-hydrocarbon rocket engines has resulted in the need for design criteria and scaling laws for injectors which work well in such an environment. One injector type that has been shown to work well in LOX-hydrocarbon engines is a Gas-Centered Swirl-Coaxial (GCSC) Injector. While earlier work has focused on a number of the important parameters, one that has been left unexplored is the injector cup length. In this study the effect of the injector cup length on the atomizing film in GCSC injectors is explored. The length of the atomizing film is used as reference parameter for the overall spray quality. Using lasersheet illuminations along with water and nitrogen as the working fluid, film lengths were determined in six unique injector geometries and over a number of flow conditions. Each injector geometry and flow condition was tested with two injector cup lengths. Results will show that the injector cup length has little effect on the films for test conditions with momentum flux ratios greater than 400 as long as the film is completely broken up within the cup.

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.

Characterization of three-dimensional dense spray visualization techniques

Resolving the dense liquid core structure of sprays has required the development of novel imaging techniques to more thoroughly examine complex multiphase fluid phenomena. In the current work, we discuss complementary methods of investigating the dense core region of the spray, including measurements of the exterior three-dimensional surface topology using structured light and interior liquid mass distributions using X-ray radiography and Xray computed tomography. Three-dimensional shape measurement data were collected using a 3-D machine vision system. The information from this technique is then used to compute the exterior three-dimensional surface of the spray, which captures the timeresolved motion of impact waves generated by instabilities leading to primary breakup. Data from the Advanced Photon Source at Argonne National Laboratory and an X-ray tubesource are compared to evaluate the accuracy of internal liquid mass distribution measurements from time-averaged X-ray radiography and computed tomography. Measurements with a tube source were also extended using a flash X-ray tube-source and two simultaneous orthogonal detection systems for spray imaging of a gas-centered swirl coaxial injector. These complementary techniques are used to reveal the exterior and interior details of primary break-up dynamics in dense sprays of practical interest.

Interpretation of Core Length in Shear Coaxial Rocket Injectors from X-ray Radiography Measurements

Abstract : Shear coaxial injectors are so named because they rely on the shear between an outer lower-density high-velocity annulus and a higher-density low-velocity inner jet to atomize and mix a liquid and a gas. These injectors have an intact core, and the high amount of scatter its corrugated surface produces creates large optical densities. These high optical densities, in turn, make interrogation of the spray field in the region of the core difficult. In combustion applications, such as rockets, this region is also the area of flame holding, so is of primary importance in predicting combustion behavior. To overcome the problems of multiple scattering, the near-injector region was studied using X-ray radiography at Argonne National Laboratorys Advanced Photon Source. These results clearly show regions of differing behavior throughout the downstream distance examined. These regions correspond to changes in atomization behavior and can be used to quantify core length and understand more clearly what this term means. Three methods are explored to measure core length from X-ray radiography data and are compared to two-phase core length measurements from the literature. The core length nondimensionalized by the inner jet diameter was found to scale with the momentum flux to the -0.66 power.