COMBUSTION OF ALUMINUM AND ICE BASED SOLID PROPELLANTS FOR PROPULSION APPLICATIONS AND HIGH TEMPERATURE HYDROGEN GENERATION

Open Access
Author:
Connell Jr., Terrence Lee
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Dr Yetter, Thesis Advisor
  • Richard A Yetter, Thesis Advisor
  • Grant Alexander Risha, Thesis Advisor
Keywords:
  • hydrogen generation
  • nano particles
  • combustion
Abstract:
The aluminum/water reaction has been studied for over 50 years as a potential means of generating on demand hydrogen for use in propulsive applications. The use of cryogenic means to store and supply hydrogen is inefficient, as losses due to boil off are inevitable, making long term storage an issue. Furthermore, the low density of liquid hydrogen and the insulation requirements to maintain it results in large volumes and significantly increases the bulk system mass. The ability to manufacture bulk quantities of nano-sized particles have enabled their use in the development of propellants with increased performance properties, including faster burning rates. This performance increase is purely a result of the much higher surface area these particles possess, and thus enables their use in the manufacture of reactive compositions, which would otherwise remain inert. The motivation for the current work stems from the ability of nanometer sized aluminum particles, when combined with water to self-propagate, reacting to form high temperature molecular hydrogen and condensed phase aluminum oxide. Freezing these mixtures produces a solid propellant having applications of long term hydrogen storage for low earth orbit and for specialized missions. Furthermore, the simplicity of the chemistry and relative ease of manufacture promotes their use as in-situ propellants for lunar and Mars missions. Investigations include baseline frozen propellants consisting only of nanometer sized aluminum particles combined with water, and compositions which make use of fuel blends and energetic additives as a means of increasing the propulsive performance of the baseline composition. The particles used were characterized using thermogravimetric analysis and differential scanning calorimetry, scanning and transmission electron microscopy, Brunauer Emmett Teller analysis, and energy dispersive x-ray spectroscopy, to determine active content, nominal particle size and size distribution, oxide shell thickness, and surface composition. Generally, the particles used had a nominal diameter of 80 nm and an active aluminum content of 74.5 percent by mass. Propellant mixing, manufacture, and casting techniques were all developed specific to these compositions, using hand and acoustic mixing methods to blend reactants. Experimental analysis of the frozen compositions was conducted using a constant pressure strand burner, constant volume cell, and a series of scaled rocket motors, several of which were constructed for use during this investigation. Strand burning tests provide information on propellant linear and mass burning rates with respect to pressure and composition which were correlated using a Saint Roberts Law fit, while the cell provided an effective means to study combustion efficiency of compositions, through analysis of the gaseous products. Static-fired motors, instrumented with pressure and force transducers, were used to determine the propulsive performance of several propellant formulations. Additional compositions included substitution of micron sized aluminum and aluminum hydride (AlH3) for nano material, and ammonia borane (AB), and hydrogen peroxide (HP) were considered as additives. Two specific equivalence ratios (0.71 and 0.943) were chosen based on theoretical performance calculations and testing requirements. Baseline compositions were shown to self propagate and strand tests yielded pressure coefficients of 0.602 and 0.992 with exponential values of 0.785 and 0.405 for the two given equivalence ratios. Similar compositions containing several different batches of nanometer aluminum yielded measurable variations in burning rate, suggesting careful particle characterization should be conducted prior to use. A comparison between the baseline and classical hydroxyl-terminated polybutadiene (HTPB)/ammonium perchlorate (AP) and aluminized HTPB/AP/Al composite solid propellants yielded faster burning rates at higher pressures for the frozen compositions, and for the formulation having an equivalence ratio of 0.943, the resulting pressure exponent was very similar to the aluminized composite solid propellant. These compositions, successfully tested in a series of scaled rocket motors (in both end-burning and center-perforated grain configurations), were shown to produce repeatable results, with slightly lower pressures and longer burn durations exhibited with increasing equivalence ratio. Performance was also shown to increase with increasing motor size and equivalence ratio. Substitution of micron aluminum for nanometer material showed the increasing fraction of larger particles did not significantly impact the linear burning rates up to a certain loading fraction, following which the burning rates were observed to decrease. This limit was determined to be particle size dependent, decreasing with increasing nominal particle diameter. Compositions tested under varied pressure conditions yielded significantly reduced burning rates at low pressure, which was recovered as pressures were increased, yielding a higher pressure exponent. Combustion efficiency was shown to increase from the 72% measured for the baseline to approximately 80% for compositions containing 20 micron aluminum particles, both having an equivalence ratio of 0.943. Motor tests conducted for compositions containing 25 and 50% (by mass) 2 micron aluminum yielded lower chamber pressures, longer burn times, higher C* efficiencies and similar specific impulse (Isp) efficiencies compared to the baseline composition. Similarly, micron-sized AlH3 was introduced in place of micron aluminum to boost the hydrogen yield. Measured linear burning rates decreased with increasing AlH3 weight percent. Pressure tests yielded similar pressure exponents, with burning rates decreasing from the baseline as the fraction of AlH3 in the composition increased, following the same overall trend. Conversion efficiencies for compositions containing alane were similar to mixtures containing micron aluminum, however, with decreasing pressure (below approximately 7 MPa), combustion efficiencies were shown to decrease as low as 32%. Hydrogen peroxide was considered as an energetic additive to the baseline formulation. Burning rate measurements were obtained for several formulations at pressures up to approximately 7 MPa, however, at higher pressures the combustion process was observed to transition to an unsteady mode. Ammonia borane was also considered as an energetic additive, introduced to the baseline composition to increase the hydrogen yield, and ultimately the propulsive performance of the composition. Varied amounts of AB tested under constant pressure conditions, yielded a peak burning rate of approximately 3 cm/s corresponding to approximately 18.8 wt% addition of AB. Pressure tests conducted using a similar formulation yielded an increased burning velocity over the baseline, following the same linearly increasing trend.