Grant A. Risha

Regression Rate Behavior of Hybrid Rocket Solid Fuels

An experimental investigation of the regression-rate characteristics of hydroxyl-terminated polybutadiene (HTPB) solid fuel burning with oxygen was conducted using a windowed, slab-geometry hybrid rocket motor. A real-time, x-ray radiography system was used to obtain instantaneous solid-fuel regression rate data at many axial locations. Fuel temperature measurements were made using an array of 25- πm e ne-wire embedded thermocouples. The regression rates displayed a strong dependence on axial location near the motor head-end. At lower mass e ux levels, thermal radiation was found to signie cantly ine uence the regression rates. The regression rates werealso affected by theadditionofactivated aluminum powder.A 20%by weightaddition of activated aluminum to HTPB increased the fuel mass e ux by 70% over that of pure HTPB. Correlations were developed to relate the regression rate to operating conditions and port geometry for both pure HTPB and for HTPB loaded with certain fractions of activated aluminum. Thermocouple measurements indicated that the fuel surface temperatures for pureHTPBwerebetween930 and1190 K.TheHTPBactivationenergywasestimatedat11.5 kcal/mole,suggesting that the overall regression process is governed by physical desorption of high-molecular weight fragments from the fuel surface.

Combustion of Nanoscale Al/MoO3 Thermite in Microchannels

Abstract : Microscale combustion is of interest in small-volume energy-demanding systems, such as power supplies, actuation, ignition, and propulsion. Energetic materials can have high burning rates that make these materials advantageous, especially for microscale applications in which the rate of energy release is important or in which air is not available as an oxidizer. In this study we examine the combustion of mixtures of nanoscale aluminum with molybdenum trioxide in microscale channels. Nanoscale composites can have very high burning rates that are much higher than typical materials. Quartz and acrylic tubes are used. Rectangular steel microchannels are also considered. We find that the optimum mixture ratio for the maximum propagation rate is aluminum rich. We use equilibrium calculations to argue that the propagation rate is dominated by a convective process where hot liquids and gases are propelled forward heating the reactants. This is the first study to report the dependence of the propagation rate with a tube diameter for this class of materials.Wefind that the propagation rate decreases linearly with 1=d. The propagation rate remains high in tubes or channels with dimensions down to the scale of 100 m, which makes these materials applicable to microcombustion applications.

Combustion and Conversion Efficiency of Nanoaluminum-Water Mixtures

An experimental investigation on the combustion behavior and conversion efficiency of nanoaluminum and liquid water mixtures was conducted. Burning rates and chemical efficiency of aluminum-water and aluminum-water-poly(acrylamide-co-acrylic acid) mixtures were quantified as a function of pressure (from 0.12 to 15 MPa), nominal aluminum particle size (for diameters of 38, 50, 80, and 130 nm), and overall equivalence ratios (0.67 < φ < 1.0) under well-controlled conditions. Chemical efficiencies were found to range from 27 to 99% depending upon particle size and sample preparation. Burning rates increased significantly with decreased particle size attaining rates as high as 8 cm/s for the 38 nm diameter particles above approximately 4 MPa. Burning rate pressure exponents of 0.47, 0.27, and 0.31 were determined for the 38, 80, and 130 nm diameter particle mixtures, respectively. Also, mixture packing density varied with particle size due to interstitial spacing, and was determined to affect the burning rates at high pressure due to inert gas dilution. The presence of approximately 3% (by mass) poly(acrylamide-co-acrylic acid) gelling agent to the nAl/H2O mixtures had a small, and for many conditions, negligible effect on the combustion behavior.

Characterization of Nano-Sized Energetic Particle Enhancement of Solid-Fuel Burning Rates in an X-Ray Transparent Hybrid Rocket Engine

Conventional polymeric fuels for hybrid rocket propulsion have relatively low regression rates. Two possible solutions were examined: utilization of energetic nano-sized particles, and adoption of non-polymeric paraffin fuel. Addition of 13 wt.% of nano-sized tungsten powder to HTPB-based fuel resulted in an increase of 38% in fuel regression rate compared to the pure HTPB fuel. The use of nano-sized tungsten powders in solid fuels for volume limited propulsion systems is greatly beneficial due to its high density, high heat of oxidation, and low oxidation temperature. SEM/EDS micrographs of the newly processed energetic paraffin-based solid fuels have shown that the nano-sized Silberline aluminum flakes are homogenously mixed in the fuel matrix. Paraffin-based solid fuels containing aluminum flakes showed a significant increase in regression rate over the non-aluminized paraffin fuel. A real-time X-ray radiography system enables the measurement of the instantaneous radius of the solid fuel grain. The radial increment of the regressing fuel surface can be correlated with time in a power-law form. An implicit relationship showing the dependency of instantaneous fuel regression rate on the total mass flux was obtained. The functional relationships for aluminized HTPB and paraffin fuels were obtained in graphical forms. Results show that the conventional power-law relationship between the average regression rate and average oxidizer mass flux cannot be applied to the instantaneous regression rates of solid fuel burning in hybrid rocket motor conditions.

Combustion of HTPB-Based Solid Fuels Containing Nano-sized Energetic Powder in a Hybrid Rocket Motor

An experimental investigation was conducted to determine the relative propulsive performance of various HTPB-based solid-fuel formulations containing nano-sized energetic metal particles. These particles include Alex® particles (diameter ~ 150 nm), WARP-1 aluminum particles (-70 nm), B4C (-120 nm), and a mixture of B4C and WARP-1. The nano-sized particles were cast in an HTPB solid-fuel grain and burned in the Long-Grain Center-Perforated (LGCP) hybrid rocket motor using pure oxygen as the oxidizer injected at the head-end of the motor. The LGCP hybrid rocket motor is capable of oxygen mass flow rates up to 0.36 kg/s (0.8 Ibm/s) and chamber pressures up to 12 MPa (1,750 psig). The oxidizer mass flux was varied from 140 to 850 kg/m-s at chamber pressures ranging from 2.3 to 4.6 MPa (320 to 650 psig). The addition of energetic powders showed an increase of up to 50% in mass burning rate compared to the pure HTPB fuel. Elemental compositions of quenched boron particles in oxygenated and fluorinated environments were determined by using the Energy Dispersion Spectroscopy (EDS) technique to compare the composition of quenched boron particles in oxygen-containing versus fluorine-containing environments. These results indicate no significant difference between particles recovered in oxygenated and fluorinated environments. It was found that there was no nitrogen on the particle surface, indicating no boron nitride formation.

Characterization of Nano-Sized Particles for Propulsion Applications

Energetic nano-sized particles have been shown to have a great potential for use in the aerospace propulsion applications. Some of the unique combustion properties of nano-particles such as very rapid ignition and short combustion times make them particularly valuable for propulsion systems; they can be included in solid fuels, solid propellants, or even as energetic gellant in liquid systems. However, due to the novelty of the application and rapid development of production techniques, there is no comprehensive understanding of what characteristics of a nano-sized particle are important in contributing to desirable performance and ease of processing into a final usable form. Previous studies have shown that HTPB-based solid fuels containing various types of nano-sized particles showed differing performance results when tested in the same hybrid rocket motor under identical conditions. Many of these particles have data available only on the basic composition (aluminum, boron, boron carbide, etc.), average diameter, and/or BET surface area. In order to better understand and correlate observed combustion behavior with intrinsic material properties, the particles of interest need to be better characterized. A variety of standard particle characterization techniques were applied to the fifteen types of particles examined in this study and the results tabulated. Some of the parameters measured were average particle diameter, specific surface area, amount of active content, and oxide layer thickness. Trends in propulsion performance measured using a parameter of great interest to the hybrid rocket community (fuel mass burning rate) in general matched trends in particle characteristics (i.e. active content, surface area), but there were some noticeable exceptions. This study indicates that there is still much more to learn about the correlation between physical and chemical properties and measured combustion performance. Other parameters that should be examined in the future include particle size distribution, degree of agglomeration, reactivity and thermal effects (oxidation rate, onset temperature for oxidation exotherm, heat release associated with any excess stored energy), etc.

Combustion of Aluminum Particles with Steam and Liquid Water

Experiments were conducted to study the effect of liquid and gas phases of water as the oxidizer combusting with both micron- and nano-sized aluminum particles. Combustion of aluminum with steam was achieved using a Bunsen-type dust cloud apparatus. During the experiment, the aluminum particles, ~5-8 microns in diameter, were entrained in a high- velocity steam flow while the aerosol velocity is regulated by an ejector system to maintain a stable flame at the end of the contoured nozzle. Al/steam/N2 flames were obtained for various equivalence ratios at atmospheric pressure. The flame luminosity was less than that of aluminum/air mixtures due to the reduction in flame temperature. Linear and mass- burning rates of mixtures of nanoaluminum (38 nm) and liquid water as a function of pressure and mixture composition at room temperature were measured using a constant volume optical pressure vessel. At the highest pressure studied (4.3 MPa), the linear burning rate was found to be 8.6±0.4 cm/s corresponding to a mass-burning rate of 6.1 g/cm 2 -s. The pressure exponent at room temperature was 0.47, which was independent of the overall mixture equivalence ratio for the cases considered.

Decomposition and Ignition of HAN-Based Monopropellants by Electrolysis

In this paper, electrolytically-induced HAN ignition has been studied. The purpose of the present study is to identify the various parameters and mechanisms dictating the electrolytic ignition of HAN-based monopropellants. Electrochemical mechanisms are established and incorporated into an existing chemical kinetics scheme developed by Lee and Litzinger. The ignition of HAN-water solution by electrolysis has been treated numerically using a constant pressure, homogeneous reactor model. A stiff ODE solver was used in the analysis to handle the highly stiff species conservation and the energy equations. The analysis focuses on the temporal evolution of temperature and condensed and gas phase species. Parametric studies were conducted to investigate the effect of electric current, voltage, volume, initial temperature, and HAN concentration on the ignition time delay. The ignition time delay is found to decrease with increase in current, temperature, and HAN concentration and increase with volume.

Hybrid Rocket Investigations at Penn State University's High Pressure Combustion Laboratory: Overview and Recent Results

The Pennsylvania State University has actively pursued hybrid rocket fuel regression rate and combustion research for 15 years. Initial work focused on developing and testing a high-pressure slab-burning hybrid motor with X-ray diagnostics for characterizing the local, real-time regression rates of hydroxyl terminated p olybutadiene (HTPB) with gaseous oxygen. Fuel decomposition and pyrolysis investiga tions were also carried out. Follow-on work is continuing to investigate high regression r ate hybrid fuels with various metal additives in center-perforated hybrid motors using both HTPB and paraffin binders. The addition of aluminum powders to paraffin-based solid-fuel formulations was shown to increase the regression rates by a factor of 4 comp ared to neat HTPB. This regression rate increase corresponds to a mass-burning rate increase ~7 times that of HTPB when the increase in fuel density is considered. This artic le reviews some of the more significant results from previous investigations and presents r ecent data from on-going test programs. Nomenclature

Analysis of Nano-Aluminum Particle Dust Cloud Combustion in Different Oxidizer Environments

The combustion of nano-sized aluminum particles with various oxidizers, including oxygen, air, and water, is studied in a well-characterized laminar particle laden flow by means of both numerical and theoretical approaches. In the numerical analysis, nano-sized aluminum particles are treated as large molecules, which can be regarded as a limiting case when the particle size approaches to zero. The particle laden-flow is then modeled as a onedimensional, laminar, steady flow of a premixed gas mixture. Emphasis is focused on the detailed chemical kinetics and its ensuing effect on the flame structure. A companion theoretical model is also established. The flame is assumed to consist of three zones: preheat, flame. and post flame regimes. By solving the energy equation in each regime and matching the temperature and heat flux at the interfacial boundaries, an algebraic equation for the flame speed is obtained. The analysis allows for the investigation of the effect of particle size, ranging from nano to micro meters in diameter, on the burning characteristics of aluminum oxidizer mixtures. In the molecular limit, the numerical results indicate that the flame speeds of an aluminum mixture with air are significantly higher than that with H2O, mainly due to the greater reaction rate of Al with O2 versus H2O. The kinetic bottleneck results from the Al2O3 formation, in which O atoms play an influential role, in the aluminum-steam system. For micro-sized particles, the theoretical analysis shows that the aluminum-steam flame speed is slightly larger than that of an aluminum-air mixture. The phenomenon may be attributed to the high diffusivities of oxidizer and the presence of H2 and H in reaction products for a H2O system.