Gregory Young

Combustion Behavior of Solid Fuels Based on PTFE/Boron Mixtures

An experimental study was conducted to understand the combustion behavior of polytetrafluoroethylene (PTFE)/boron–based solid fuels for future hybrid rocket motor applications. Fuels were loaded with 10–40% boron powder (w/w). Two different types of PTFE were examined in this study, while a single type of boron powder was considered. No significant differences in the decomposition mechanisms for PTFE and a candidate solid fuel mixture were observed by differential scanning calorimetry (DSC) and temperature-jump (T-jump)/Fourier transform infrared (FTIR) experiments. Diffusion flame studies between solid fuels and gaseous oxygen were carried out to measure regression rates and to develop a fundamental understanding of the combustion behavior. The fuels with the lowest boron content readily extinguished upon removal of the supplemental oxygen flow. The fuels with the highest loadings of boron self-propagated after ignition. X-ray diffraction on postcombustion residue of the self-propagating material revealed graphite and boron carbide as the remaining products, while particles captured leaving the surface of the fuel under normal burning conditions were found to be mostly boric acid. Boron oxidation and magnesium fluorination were observed in the flame zone of the diffusion flame by UV-Vis emission spectroscopy (magnesium is the major impurity in the elemental boron powder used). The results of this study suggest that solid fuels comprising PTFE and boron show promise for improving the energy density of hybrid rockets.

High Pressure Ignition and Combustion of Aluminum Hydride

An experimental study was conducted to determine the high pressure ignition characteristics of α-aluminum hydride. Aluminum hydride particles were heated on a platinum filament at heating rates of approximately 1 × 105 K/s in a pressure vessel for pressures ranging up to about 7 MPa, in order to quantify the ignition temperature and to observe the ignition process. Experiments were conducted in air, argon, and nitrogen as the pressurizing environment. This study revealed that the dehydrogenation of aluminum hydride is not a function of pressure under the conditions tested. In addition, ignition temperatures were found to be approximately linearly related to pressure until pressures exceeded about 0.4 MPa, at which point they remained constant through the highest pressures tested. High speed imaging of the ignition process showed a dramatic change in the ignition behavior for pressures above 0.4 MPa, corresponding to what we believe is a threshold for H2/air autoginition or perhaps even an explosion limit. We find that the combustion behavior of aluminum hydride particles shared many traits similar to what has been previously observed with aluminum particles including a diffusion flame surrounding the particle, spinning, jetting, and explosions/fragmentation. Quenched particles also showed clear evidence of gas phase combustion with parent particles containing nanofeatures, which were condensed from the gas phase. The results of this study provide additional understanding on the ignition and combustion process of aluminum hydride at extreme conditions, which may be useful in modeling efforts or in the development of solid propellants.

Combustion of Aluminum, Aluminum Hydride, and Ice Mixtures

The combustion of nano-aluminum, alane, and ice mixtures is theoretically studied. A multi-zone theoretical framework is established by solving the energy equation in each zone and matching the temperature distribution and the heat flux at the interfacial boundaries. The effect of replacing a portion of nano-aluminum particles with micron-sized aluminum and alane particles is examined in the pressure range of 1-10 MPa and for an additive mass fraction of 25%. The addition of micron-sized alane particles results in lower flame temperatures due to the dehydrogenation reaction of alane particles prior to their ignition. The lower flame temperatures and the longer burning times of micron-sized alane particles is responsible for lower flame speeds of these mixtures. For bimodal aluminum-ice mixtures, the lower mass fraction of alumina causes an increase in the flame temperatures. However, the mixtures exhibit mildly lower flame speeds, in view of the longer burning times of micron-sized aluminum particles. The flame thickness of a bimodal mixture of aluminum particle mixtures is higher than that of mono-dispersed mixtures due to the prevalence of two distinct reaction zones. The model results are well supported by experimentally measured burning rates.

Decomposition and Ignition Characteristics of Titanium Hydride at High Heating Rates

An experimental study was conducted to evaluate the decomposition, ignition, and combustion behavior of titanium hydride under high heating rate conditions. Samples were deposited on filaments, which were rapidly heated by joule heating under various conditions. Dehydrogenation experiments were conducted under vacuum conditions at heating rates of up to 4 × 105 K/s. The results of these experiments suggest that, at high heating rates, the onset of dehydrogenation is limited by intraparticle diffusion. The ignition and combustion behavior was studied in air and for ambient pressures ranging from atmospheric up to 7 MPa in a windowed pressure vessel. Broadband light emission was used to quantify the ignition temperature. The experiments revealed that the ignition temperature decreased linearly with increasing pressure from approximately 1700 K to 1475 K. Comparison of the dehydrogenation temperatures to the ignition temperatures over the entire pressure range suggests that the onset of the dehydrogenation process is not likely to be affected by ambient pressure. Finally, observation of the steady state combustion process by high speed imaging and post mortem analysis revealed many similar combustion characteristics to pure titanium. Particle explosions were observed and quenched particles were found to consist of titanium, nitrogen, and oxygen.