High Repetition Rate OH Planar Laser Induced Fluorescence of Gelled Propellant Droplet

Abstract

A novel measurement technique based on 5 kHz Hydroxide Planar Laser Induced Fluorescence (OH PLIF) was used to investigate the combustion of gelled propellant droplets. The diagnostic provides time resolved OH PLIF images; OH is a key species for chemical kinetics modeling and flame front positioning. Jets are a unique phenomenon of gels, where vapor ruptures an unburned gellant shell, forming a flame with a high local velocity. Droplets of methanol and monomethyl hydrazine gelled with hydroxypropyl cellulose were studied by varying ambient pressure and gellant concentration. The high speed OH PLIF can capture three distinct types of jets that could not be imaged with a visible high speed camera or with a low speed OH PLIF system. It was observed that the radial jet velocity is proportional to the gellant concentration and inversely proportional to the ambient pressure. Also, jets occurred at the same location repeatedly. A possible explanation is the initial jet weakens the shell, causing more jets to occur at the weakened location. The frequency of jets increased when the ambient pressure or the gellant concentration was increased. It is hypothesized that this is from a faster accumulation of the unburned gellant layer, which makes the shell recover faster after jetting.