Dependence of Nano-Aluminum and Water Propellant Combustion on pH and Rheology

Abstract

Over the past few years, the combustion of nano-aluminum/water (nAl/H2O) propellants has been widely reported, but further progress has been slowed for the following reason: the loosely correlated trends in combustion data are insufficient in guiding further research efforts, and they cannot be used to significantly improve the Isp observed in static rocket motor tests. It was previously found that different mixing techniques (hand, planetary and resonant mixers, duration and temperature), or equivalence ratio gave rise to different burning rates, but the influence of pH and rheology on nAl/H2O propellants was not considered. We find that the effects of pH on nAl/H2O propellants are profound, and correlate well with viscosity, low pressure deflagration limits, burning rate exponents, and rocket motor performance. Our findings suggest that coagulation can influence the pressure exponent over a wide range of values (0.34–0.68). For particle diameters <1 µm, dispersion during mixing is affected more by electrostatic repulsion from charged ions than from mechanical agitation, and this is reflected through zeta potential and viscosity measurements at different pH levels. Additionally, we observe that pH has an influence on nAl/H2O reaction kinetics during ignition, as the propellant transitions from low temperature oxidation to high temperature combustion.