Abstract
The increasing demand for high energy density fuels and the concern for their safety have propelled research in the field of gelled propellants, where understanding the combustion of single gelled fuel droplets is the first stage to predict the spray combustion characteristics. The experiments utilized single-isolated freely falling gelled unsymmetrical dimethylhydrazine (UDMH) droplets instead of the conventional suspended droplet approach, in order to eliminate the perturbation associated with the suspension mechanism. Morphological transformations of the gelled droplet involved in the combustion processes were monitored by employing a high-speed digital camera, while the effects of ambient pressure and oxygen concentration on burning rate constants were also studied. The experimental results show that four main phenomena (droplet deformation, bubble formation and growth, vapor jetting and luminous jetting flame with “horn” shape) and three distinct phases were identified in the droplet combustion process; the high yield stress and polymer chain structure of polymer gellant are responsible for the appearance of bubbles with almost the same order of magnitude as the droplets. Increasing the ambient pressure can increase the burning rate, postpone the appearance of microexplosions, and weaken microexplosion intensity; while increasing the ambient oxygen concentration can promote the appearance of microexplosions, strengthen microexplosion intensity and increase the burning rate.
Highlights
Gelled propellants offer good storability without sacrificing high energy performance and operability for rocket applications, and are being considered for certain future rocket and atmospheric propulsion applications such as tactical missiles, ramjets, etc. [1]
Yanan and Qiao conducted a series of experimental studies on the combustion characteristics of liquid fuel droplets with additional nano- and micro-sized aluminum particles [17,18,19]; the results indicated five distinctive stages for the n-decane/nano-Al droplet, they carried out theoretical analysis to understand the effects of particle size on particle collision mechanisms and aggregation rate
Morphological transformations of the gelled unsymmetrical dimethylhydrazine (UDMH) droplet involved in the combustion processes are obtained by employing a fast-speed camera, whilst the effects of ambient pressure and oxygen fraction on burning rate are analyzed
Summary
Gelled propellants offer good storability without sacrificing high energy performance and operability for rocket applications, and are being considered for certain future rocket and atmospheric propulsion applications such as tactical missiles, ramjets, etc. [1]. The study of the combustion mechanisms of single droplets is necessary for characterizing and understanding the phenomena of spray vaporization and combustion. Extensive theoretical and experimental work has been carried out in the field of multi-component droplet evaporation/burning to understand the combustion mechanisms. Ghassemi et al studied the single droplet evaporation at elevated pressure and temperature [5]. The results showed a three-staged evaporation: (1) a period of more volatile component evaporation; (2) the period of increasing droplet temperature and almost no evaporation; (3) a quasi linear evaporation period. They observed microexplosions at low pressure, but no microexplosions at high pressure
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