Hypergolic ignition of paraffin-based hybrid rocket fuels by sprays of liquid oxidizer

Abstract

An experimental study is conducted to measure the ignition delay of ammonia borane doped paraffin wax with sprays of white fuming nitric acid as oxidizer. The injection pressure and injector diameter are varied to investigate the effects of spray characteristics on the ignition dynamics. A range of Weber (We) number for the oxidizer jet covering 100 < We <2700 and Reynolds (Re) number 1300 < Re <8900 is investigated, whereas the We number at the droplet scale varies from 0.8 < Wedrop <24.1. The sprays are characterized using phase-doppler anemometry to obtain the size and speed distribution of the oxidizer droplets across multiple positions. In addition to conventional high-speed imaging, the hypergolic ignition is also observed using Schlieren imaging and a high-speed mid-infrared camera. The results show that the ignition delay of a hypergolic hybrid fuel under spray conditions is unaffected by the mean droplet size of the oxidizer. The ignition delay decreases with increasing We number at the droplet scale and increasing droplet velocity. The variability in the ignition delay also decreases with Weber number and droplet velocity. The values obtained for the ignition delay when using a spray are generally higher than those from simple oxidizer drop experiments, highlighting the need to investigate the hypergolic ignition in configurations representative of engine conditions.