Effects of Ammonia Borane on the Combustion of an Ethanol Droplet at Atmospheric Pressure

Abstract

Adding hydrogen containing energetics to liquid fuels has the potential to change the combustion behavior of the fuels, thereby enhancing performance. One potential energetic additive is ammonia borane (AB), which contains 19.6 wt.% hydrogen and can be dissolved in anhydrous ethanol (up to 6.5 wt.%). Single droplet combustion experiments are performed that vary the AB concentration in ethanol from 0 to 6 wt.%. These experiments indicate that the addition of 6 wt.% of AB increases the burn rate of the fuel by 16% during normal regression. As the ethanol droplet containing AB burns, gas formation within the droplet causes it to expand slightly. Gas bubbles generated during burn reach the surface of the droplet and rupture, jettisoning liquid and increasing the burn rate. High speed (5 kHz) planar laser-induced uorescence (PLIF) measurements show an increase in OH production during combustion, indicating that the reaction rate of the fuel increases with the addition of AB. PLIF is also used to capture the jetting dynamics. The eects of hydrogen gas and other decomposition products of AB in the fuel vapor surrounding the droplet are evident in these dynamic measurements that aect the burn rate. AB addition to the fuel causes the droplet to shatter towards the end of the droplet lifetime causing atomization and rapid combustion of the remaining fuel. This droplet shattering is similar to some other systems, but has not previously been reported for ammonia borane systems.