Imaging dynamic multiphase combusting events is challenging. Conventional techniques can image only a single plane of an event, capturing limited details. Here, we report on a three-dimensional, time-resolved, OH planar laser-induced fluorescence (3D OH PLIF) technique that was developed to measure the relative OH concentration in multiphase combustion flow fields. To the best of our knowledge, this is the first time a 3D OH PLIF technique has been reported in the open literature. The technique involves rapidly scanning a laser sheet across a flow field of interest. The overall experimental system consists of a 5 kHz OH PLIF system, a high-speed detection system (image intensifier and CMOS camera), and a galvanometric scanning mirror. The scanning mirror was synchronized with a 500 Hz triangular sweep pattern generated using Labview. Images were acquired at 5 kHz corresponding to six images per mirror scan, and 1000 scans per second. The six images obtained in a scan were reconstructed into a volumetric representation. The resulting spatial resolution was 500×500×6 voxels mapped to a field of interest covering 30  mm×30  mm×8  mm. The novel 3D OH PLIF system was applied toward imaging droplet combustion of methanol gelled with hydroxypropyl cellulose (HPC) (3 wt. %, 6 wt. %), as well as solid propellant combustion, and impinging jet spray combustion. The resulting 3D dataset shows a comprehensive view of jetting events in gelled droplet combustion that was not observed with high-speed imaging or 2D OH PLIF. Although the scan is noninstantaneous, the temporal and spatial resolution was sufficient to view the dynamic events in the multiphase combustion flow fields of interest. The system is limited by the repetition rate of the pulsed laser and the step response time of the galvanometric mirror; however, the repetition rates are sufficient to resolve events in the order of 100 Hz. Future upgrade includes 40 kHz pulsed UV laser system, which can reduce the scan time to 125 μs, while keeping the high repetition rate of 1000 Hz.

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Combustion of a nonmetallized organic gelled droplet was studied using high-speed (5 kHz) hydroxide planar laser induced fluorescence. Monomethylhydrazine was gelled with 3 and 6 wt % hydroxypropyl cellulose and burned in a pressure vessel with 35-414 kPa of ambient air. Fuel jetting is observed during the combustion of the gelled droplets. Three distinct types of jetting events were observed for the first time: 1) jets that distort the flame front, 2) jets that break the flame front, and 3) jets that form a fire ball outside of the flame front. The average jet speed was the highest for type 3 and the lowest for type 1. Speeds and locations of the fuel jets were quantified. It was observed that the radial jet velocity increases with increasing gellant concentration and decreases with increasing ambient pressure. Also, jets were observed to occur at the same location repeatedly. A possible explanation is that the initial jetting event weakens the shell, causing more jets to occur at the weakened location. Understanding the fuel jets of gelled droplet combustion is important because they are likely to influence the oxidizer-fuel ratio, chemical equilibrium, gas-phase mixing, and droplet trajectory in gel spray combustion.

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