Ignition processes and characteristics of charring conductive polymers with a cavity geometry in precombustion chamber for applications in micro/nano satellite hybrid rocket motors

The arc ignition system based on charring polymers has advantages of simple structure, low ignition power consumption and multiple ignitions, which bringing it broadly application prospect in hybrid propulsion system of micro/nano satellite. However, charring polymers alone need a relatively high input voltage to achieve pyrolysis and ignition, which increases the burden and cost of the power system of micro/nano satellite in practical application. Adding conductive substance into charring polymers can effectively decrease the conducting voltage which can realize low voltage and low power consumption repeated ignition of arc ignition system. In this paper, a charring conductive polymer ignition grain with a cavity geometry in precombustion chamber, which is composed of PLA and multiwall carbon nanotubes (MWCNT) was proposed. The detailed ignition processes were analyzed and two different ignition mechanisms in the cavity of charring conductive polymers were revealed. The ignition characteristics of charring conductive polymers were also investigated at different input voltages, ignition grain structures, ignition locations and injection schemes in a visual ignition combustor. The results demonstrated that the ignition delay and external energy required for ignition were inversely correlated with the voltages applied to ignition grain. Moreover, the incremental depth of cavity shortened the ignition delay and external energy required for ignition while accelerated the propagation of flame. As the depth of cavity increased from 2 to 6 mm (at 50 V), the time of flame propagating out of ignition grain changed from 235.6 to 108 ms, and values of mean ignition delay time and mean external energy required for ignition decreased from 462.8 to 320 ms and 16.2 to 10.75 J, respectively. The rear side of the cavity was the ideal ignition position which had a shorter ignition delay and a faster flame propagation speed in comparison to other ignition positions. Compared to direct injection scheme, swirling injection provided a more favorable flow field environment in the cavity, which was beneficial to ignition and initial flame propagation, but the ignition position needed to be away from the outlet of swirling injector. At last, the repeated ignition characteristic of charring conductive polymers was also investigated. The ignition delay time and external energy required for ignition decreased with repeated ignition times but the variation was decreasing gradually.