Heat and mass transfer induced by the ignition of single gel propellant droplets

This experimental research studies the gas-phase ignition of single droplets of several gel propellant compositions based on ethyl alcohol with a gellant, liquid and fine solid combustible components. Droplets 2 mm in diameter were located on a holder and heated in a muffle furnace at a temperature ranging from 873 to 1073 K. A software and hardware system of high-speed video recording (4200 frames per second at full resolution) allowed the analysis of consistent patterns in the physical and chemical processes occurring during the induction period. For the compositions under study, we determined the threshold conditions (minimum ambient temperature of 873-943 K) required for the gel propellant ignition as well as the dependences of the ignition delay times versus air temperature. The ignition delay times range from 0.1 to 3.3 s. If the ignition does not start within this period, it will not occur after a longer heating time, since the propellant droplets evaporate completely. For the first time, using the shadow methods, we analyze the characteristics of vapor jetting during the induction period as a result of microexplosions caused by the differences in the boiling points of fuel components. The average vapor jetting speed is about 3 m/s. The size of the zones, in which the vapors slow down to zero, ranges from 6 to 8 mm. We determine the consistent patterns of changes in the diameter of the sphere-shaped gas-vapor envelope around the propellant droplet at the moment of ignition at different ambient temperatures. The higher the temperature, the higher the intensity of physical and chemical processes. This shortens the ignition delay times. At relatively high air temperatures (over 1050 K), the diameter of the flammable gas-vapor envelope around the propellant droplet at the moment of ignition is three times smaller than this value at the near-threshold ignition conditions, when the diameter of the fuel vapor envelope is about 9 mm (more than four typical initial droplet diameters). The results obtained helped us formulate a physical model of the process, which may serve as the basis for the development of a mathematical model simulating the ignition of gel propellant droplets under rapid heating. Such a mathematical model will make it possible to reliably forecast the characteristics of the process in a wide variation range of propellant properties, droplet configurations and parameters of the heating source. (C) 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.