|M.Sc Student||Elkin Georgy|
|Subject||Combustion Model of a Metal Hydride Particle|
|Department||Department of Aerospace Engineering||Supervisor||Professor Emeritus Alon Gany|
|Full Thesis text - in Hebrew|
This work describes a theoretical combustion model of an aluminum hydride particle. The developed algorithm is based on Glassman's widely known model of metal particle combustion. In order to achieve better correlation with thermo-physical processes of aluminum hydride combustion, the basic model was retrofitted by adding two particular stages to it. It is shown that these stages are common for the combustion process of several hydrides (like magnesium hydride, for example).
Almost all hydrides are divided into three major groups: saline, interstitial and covalent hydrides. Aluminum hydride relates to the covalent group, which has very weak covalent bonds. Hydrides of this group are characterized by very low decomposition energy, and therefore they are not stable at the standard conditions, making their actual application difficult. Despite this fact, big hydrogen storage capacity makes them attractive candidates to be used as propellant additives.
Aluminum burns in the vapor phase. When aluminum particles are placed in a very hot environment, the temperature of the particle begins to rise until it reaches the melting point. The particle continues absorbing heat which leads to a phase change of the particle, forming a liquid droplet. Due to slow oxidation, aluminum oxide (solid) layer grows on the particle/droplet surface from the very beginning of aluminum particle heating. When the particle temperature reaches the aluminum oxide melting point temperature, the oxide layer melts and shrinks to form a molten oxide cap, exposing the fresh aluminum to oxidizing gases. This situation leads to fast oxidation reaction and ignition of the aluminum droplet. The droplet’s temperature jumps almost immediately to the aluminum boiling temperature, leading to a steady state spherical combustion envelop around the droplet between the aluminum vapors and the surrounding oxidizing gas.
In the model of aluminum hydride combustion which is developed in the present work, we assume two more stages in addition to those for aluminum particle combustion described before. At the beginning of the heating, the hydride particle temperature raises to its decomposition temperature (433K). Then decomposition of the hydride takes place. After the second stage the combustion process proceeds in accordance with Glassman's model as described above. The results of the new model in terms of burning time were compared to the extensive database on aluminum particle combustion. The algorithm's results of the time that takes combustion of aluminum hydride are located in the middle of wide-range distribution of aluminum particle combustion time.