Experimental study of hypergolic ignition by droplet collisions

Abstract

Hypergolic ignition induced by binary droplet collisions of propellants of WFNA/TMEDA and H₂O₂/Ethanolamine solution of NaBH₄, and the dynamics of droplet collision of the shearing thinning non-Newtonian liquids have been experimentally studied in the present thesis. A new set of experimental system has been designed and established, in which two special types of droplet generators were in-house made and integrated together with the precisely controlling system to ensure the stable collision of droplets of hypergolic propellants with high chemical activity. A method based on the grayscale levels on the shadowgraphs was developed and employed to calculate the collision conditions and also to help describe the ignition process. The ignition delay time is determined by applying this method in high accuracy. The hypergolic ignition by head-on collision of TMEDA/WFNA droplets is firstly studied and phenomenologically described based on five stages. A monotonic dependence of IDT on We has been identified and explained by the non-monotonic occurrence of jet-like internal mixing patterns. The droplet size ratio was discovered to affect the ignition by influencing both the mixing and the equivalence ratio. Non-monotonic dependence of IDT on B also is identified and explained as the competence between stretching and reflexive impact inertia. Droplet size effects on the hypergolic ignition was experimentally studied and theoretically analyzed through multiple time-scaling analysis. Two contradictory effects are identified that when the droplet size is diminishing, on one hand, the heating period will be shortened tending to lead to a shorted IDT; on the other hand, the vaporization will be weakened due to the enhancement of heat lost. As been found, It may even lead to non-ignitability if the droplet size continues decreasing. Similar phenomena exhibit in the collision of droplets of hydrogen peroxide and ethanolamine solution of sodium borohydride. The internal reaction of H₂O₂/Ethanolamine solution of NaBH₄ within the merged droplet could be clearly observed during the pre-ignition period. The reaction products would first generate and expand inside the merged droplet and then break through droplet surface before ignition. Non-Newtonian effect of the droplet collision of with shear thinning Carbomer aqueous solution is not obvious at moderate range of We but will exhibits at larger value of We when plashing happens. In the off-center collisions, a distinct strap-shape bouncing regime and exclusive stretching separation modes are exhibiting, caused by the non-Newtonian effect.