Richard Branam

Performance Evaluation of an Iodine-Vapor Hall Thruster

The performance of a nominal 200 W Hall effect thruster fueled by iodine vapor was evaluated. The system included a laboratory propellant feed system, a flight-model Hall thruster, and breadboard power processing unit. Operation of the Hall thruster with iodine vapor was stable on both short and long time scales, enabling measurements of thruster performance across a broad range of operation conditions. Performance was found to be comparable with xenon. At 200 W, thrust is 13 mN, anode specific impulse is 1500 s, and anode efficiency is 48%. Plume-current measurements indicate a profile typical of xenon Hall thrusters, while E B probe measurements indicate the presence of ionic dimers.

Comparison of line-peak and line-scanning excitation in two-color laser-induced-fluorescence thermometry of OH

Two-line laser-induced-fluorescence (LIF) thermometry is commonly employed to generate instantaneous planar maps of temperature in unsteady flames. The use of line scanning to extract the ratio of integrated intensities is less common because it precludes instantaneous measurements. Recent advances in the energy output of high-speed, ultraviolet, optical parameter oscillators have made possible the rapid scanning of molecular rovibrational transitions and, hence, the potential to extract information on gas-phase temperatures. In the current study, two-line OH LIF thermometry is performed in a well-calibrated reacting flow for the purpose of comparing the relative accuracy of various line-pair selections from the literature and quantifying the differences between peak-intensity and spectrally integrated line ratios. Investigated are the effects of collisional quenching, laser absorption, and the integration width for partial scanning of closely spaced lines on the measured temperatures. Data from excitation scans are compared with theoretical line shapes, and experimentally derived temperatures are compared with numerical predictions that were previously validated using coherent anti-Stokes-Raman scattering. Ratios of four pairs of transitions in the A2Sigma+<--X2Pi (1,0) band of OH are collected in an atmospheric-pressure, near-adiabatic hydrogen-air flame over a wide range of equivalence ratios--from 0.4 to 1.4. It is observed that measured temperatures based on the ratio of Q1(14)/Q1(5) transition lines result in the best accuracy and that line scanning improves the measurement accuracy by as much as threefold at low-equivalence-ratio, low-temperature conditions. These results provide a comprehensive analysis of the procedures required to ensure accurate two-line LIF measurements in reacting flows over a wide range of conditions.

Ignition and Plume Characteristics of Low-Current Cerium and Lanthanum Hexaboride Hollow Cathodes

This research investigated cerium hexaboride (CeB 6 ) and lanthanum hexaboride (LaB 6 ) as emitters in 6.4-mm-diam hollow cathodes used for low-power electric propulsion applications. Each cathodes ignition performance and plume behavior is presented and discussed. Two identical cathodes, with the exception of the insert material, were assembled and integrated into a cylindrical anode configuration without an applied magnetic field. The experiments tested two orifice geometries and a wide range of xenon flow rates (1.5-4.5 sccm) and anode currents (2-6 A) for each cathode. Also investigated was cathode performance for these conditions with the keeper on and off. A langmuir probe collected plume data to include plasma densities, electron temperature, plasma potential, and floating potential. An oscilloscope monitored the voltage behavior of the keeper electrode and anode. Initially, both cathodes successfully started without excessive keeper voltages (<650 V), heating times (<15 min), or flow rates (<4.5 sccm). The LaB 6 cathode consistently started at these same conditions. However, the CeB 6 cathode demonstrated some degradation after time by requiring higher fiow rates and a longer heating time to start. Both cathodes demonstrated ideal (spot mode) operation for flow rates as low as 1.5 sccm at anode currents as high as 5 A with no heater or keeper power.

Ultrahigh-Speed Imaging of Hall-Thruster Discharge Oscillations With Krypton Propellant

The discharge oscillations of a 200-W Hall thruster, with krypton propellant, was captured optically with an ultrahigh-speed camera providing information up to 500 kHz. The sequential images provide a temporal 2-D description of the plasma field, which illustrate the direct emission (i.e., visible light emitted by the plasma discharge) of the plasma increasing and decreasing within the thruster channel. These periodic fluctuations of intensity have a frequency of 31 kHz, which is 10% less than the measured breathing-mode frequency utilizing xenon propellant under the same conditions.

Laser-Induced Fluorescence Measurements of Product Penetration Within an Ultra-Compact-Combustor

Stanislav Kostka∗ Spectral Energies, LLC, Dayton, Ohio 45431 Richard D. Branam Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433 Michael W. Renfro University of Connecticut, Storrs, Connecticut 06269 Patrick J. Lakusta Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433 James R. Gord U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433 and Sukesh Roy∗∗ Spectral Energies, LLC, Dayton, Ohio 45431

Characterizing the Effect of Radial Vane Height on Flame Migration in an Ultra Compact Combustor

The Ultra Compact Combustor (UCC) has shown viable merit for significantly improving gas turbine combustor performance. UCC models for small engines can provide centrifugal loading up to 4,000 gs. However, as the scale of the combustor increases, the g-load will necessarily decrease and the radial vane height will increase. Thus, the importance of understanding flame migration over increasing radial vane heights is pivotal to the applicability of this design to larger engine diameters. The Air Force Institute of Technology’s Combustion Optimization and Analysis Laser laboratory studied this effect with a sectional UCC model using three different vane heights. By varying the mass flow rates of the circumferential UCC section, the g-loading was varied from 500–2,000 gs. Two-line Planar Laser Induced Fluorescence at 10Hz was used for 2D temperature profiles. High-speed video at 2kHz was also used for qualitative flame migration characterization. Several cases were studied varying the radial vane height, the circumferential g-load, and the UCC/core mass flow ratio but specifically focusing on the interaction between matching the core mass flow and the core freestream velocity among the different vane heights. Finally, the decreased core flow velocity for the same mass flow weakened the shear layer between the main and cavity flows and this allowed deeper flame migration into the core flow from the UCC. Control of the overall flame migration is the key to produce desirable combustor exit temperature profiles. Increased spans lead to higher velocity gradients and increased flame injection angles at the same mass flow rates. However, at the same core flow velocities and UCC to core flow velocity ratios the flame injection angle was relatively independent of the radial vane height and almost entirely dependent on the core flow velocity alone.© 2011 ASME

Ultrahigh Speed Images of Hall Thruster Azimuthal Instabilities

Visible emission of the azimuthal plasma instabilities were observed within the plume of a low-power Hall thruster utilizing ultrahigh speed imaging. The captured images provide time-dependent and spatial information on the azimuthal instability in the axial-radial plane of the thruster, which have not been well characterized. These images are then compared with observations made based on their probe measurements of azimuthal instabilities termed spoke instabilities. Data from the images show that the spoke instabilities are more complex than originally described and have additional features not detectable with probe measurements in a single plane.

OH-PLIF Calibration and Investigation within the Ultra Compact Combustor

The AFIT Combustion Optimization and Analysis Laser (COAL) labs modular design and state-of-the-art diagnostic systems make it a flexible and important facility for the analysis of combustion processes. The objectives of the current research are to install several enhancements in the lab, validate the laser diagnostic system, characterize the igniter for AFITs Ultra-Compact Combustor (UCC) sections, and perform a non-intrusive laser diagnostic, performance, and high-speed video analysis of a flat-cavity UCC section. Validation of the laser system was accomplished using OH Planar Laser-Induced Fluorescence (PLIF) in a laminar hydrogen-air flame produced by a Hencken burner. Results are compared to previous research to show improvements. Both ratios of intensities and excitation scans in the OH (A-X) (1-0) electronic transition system are used to measure temperature and species concentrations. Igniter characterization was accomplished using open-air flammability and flame height observations to select an anticipated operating condition. That condition was validated by attaching the igniter to the UCC section and observing its performance. An operating procedure is recommended. A PLIF flame location study using optically-clear quartz windows on the combustor was performed in the cavity-vane area. Performance measurements and high- speed video footage were also acquired in order to analyze the system. Results are compared to previous experimental and Computational Fluid Dynamics (CFD) research. Future work will include instantaneous two-color PLIF and other laser diagnostic studies of several different locations inside AFITs flat- and curved-cavity UCC sections.

Effect of Radial Curvature in Rockets on Film Cooling Adiabatic Effectiveness and Jet Width

A computational study was performed to determine the effects of radial curvature on film cooling adiabatic effectiveness. This curvature is typically encountered in modern rocket combustion sections. Jet width and spanwise-averaged adiabatic effectiveness are observed to slightly decrease as radius of curvature decreases. This decrease impacts hole spacing requirements for adequate thermal management inside rocket combustors. Compound injection aggravates the effect so that the increased adiabatic efficiency usually associated is diminished due to a cradling effect of the radial curvature. The result is an increase in adia-batic efficiency immediately downstream of the hole and a decrease after 2 hole diameters, when compared to the flat plate. Furthermore, increasing the momentum ratio past 1 diminishes the effect of radial curvature on adiabatic effectiveness. Quantified understanding of radial curvature effects allow one to use flat plate effectiveness correlations for sizing of film cooling systems in internal flow situations, like rocket combustion chambers. This research is funded and supported by AFRL Propulsion Directorate.© 2008 ASME

Minimum drag and heating 0.3-caliber projectile nose geometry

Understanding the performance of penetrators and aerodynamic bodies of revolution (missiles, rockets, aircraft noses, etc.) requires a close look at the drag and the heat transfer characteristics at a wide range of supersonic flight conditions. This research utilizes computational study and compares the aerothermal loads of supersonic flows around a new penetrator geometry, derived based on the optimization of the nose factor, to those of other common projectile shapes: conical, tangent-ogive, and power series nose geometries. The abundance of research on 0.3-caliber projectile made the choice for this research simple in order to maximize our ability to compare to the existing data. The comparison of our 0.3 caliber cylindrical projectile with other geometries shows that within the range of 500–1500 m/s flight speed the new geometry has the lowest aerodynamic drag, lowest body temperature, and least amount of heating.