Burn rate characterization of iso-propyl nitrate – A neglected monopropellant

An experimental investigation was conducted to characterize the combustion of a monopropellant isopropyl nitrate (IPN). Quartz tubes of 3, 5, 7.5, and 9 mm inner diameter and 1 mm thickness were utilized to create the strands of varying dimensions. The experiments were conducted in a quiescent atmosphere of air and the ambient pressure was varied from atmospheric pressure to 50 bar. Ignition of the IPN strands was achieved by using a heated 60 μm Nichrome wire dipped in the liquid and was retracted after ignition. The effect of ambient pressure and strand diameter on the linear burning rates of pure IPN was elucidated. The ambient pressure variation was investigated for a diameter of 5 mm, while the diameter-variation was studied at 1, 10, 20, and 40 bar. The linear burning rate of IPN strands of 5 mm diameter was found to exhibit two distinct regimes, where a steep rise in burning rate with pressure was observed after an initial decreasing trend. The values of the burn rates increased with a pressure exponent of 1.54, from approximately 0.2 mm/s at atmospheric pressure to 1.6 mm/s at 50 bar. In the first stage, the burn rates were found to be controlled by diffusion-dominated combustion, characterized by a single bipropellant flame up to a pressure of 10 bar, beyond which monopropellant-dominated combustion was found to be dominant in the second stage, with a distinct monopropellant flame and a bipropellant flame. The bipropellant flame was found to exhibit an oscillatory behavior in the second stage of combustion. A semi-empirical model was utilized successfully to predict the experimental burn rates of IPN in both regimes. The burn rates were found to depend on the strand diameters under the conditions applied, with a generalized decreasing trend at high pressures and atmospheric pressures, and an initial decrease followed by an increase at intermediate pressures.