The characterization of disruptive combustion of organic gellant-laden ethanol fuel droplets

Abstract

The future of high-performance rocket propulsion systems depends on superior burn rates, shorter ignition delay times, and eco-friendly nature. Gelled fuel propellants are such candidates which are pivotal to achieving these performance parameters. Gel fuels are multi-component fuels, and their combustion behavior is different from conventional solid and liquid propellants. To develop a fundamental understanding of the vaporization and combustion of gel fuels, single droplet experiments in a pendant mode setup are performed for ethanol-based organic gel fuels by capturing high-speed video data. Primarily, two gel fuel combinations are taken for experiments. The gel fuels are non-metalized ethanol-based fuels containing organic gellants Hydroxypropyl methylcellulose (HPMC) and methylcellulose (MC). Both fuels have shown three distinct combustion stages namely transient heat up (stage I), disruptive burning (stage II), and carbonization (stage III). The demarcation of the stages is done by visual evidence backed by high-fidelity high-speed video data. In stage I, phase separation occurs which leads to the formation of gellant shells. Organic gellants tend to form viscoelastic shells which significantly influence the jetting behavior during the disruptive burning (stage II). The combustion of gel droplets in stage II exhibits characteristic differences in bursting mechanism, for instance, oscillatory and transient bursting in HPMC and transient bursting in MC. Moreover, droplet-bursting behaviors influence jetting attributes and burn rates. Besides, the bursting of gel droplets during combustion is also dependent on the thickness of the gellant shells, the thin and weak shells are prone to bulk droplet motion due to active jetting, whereas the thick and rigid shells act as kinetic energy dampeners and thereby inhibit the bulk motion of droplets. Apart from disruptive burning, carbonization is a significant part of the combustion during which the suppressed jetting behavior is seen due to the traces of fuel trapped in the gellant shells. A significant rise in burn rate is observed due to jetting during the disruptive burning stage in contrast to the other two stages of combustion for both the gel fuels under study. Furthermore, combustion residue analysis is carried out to assess the variation in shell morphology across the stages. Overall, the current study provides detailed insights into the combustion characterization of organic gel fuels, which can help in fabricating gel fuel compositions.