Kevin Y. Cho

High-repetition-rate three-dimensional OH imaging using scanned planar laser-induced fluorescence system for multiphase combustion

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.

Ignition of Advanced Hypergolic Propellants

The hypergolic ignition of droplets of monomethyl hydrazine, 2-Azido-N,Ndimethylethanamine, N,N,N’N’-Tetramethylethylene-1,2-diamine, a blend of 2-Azido-N,Ndimethylethanamine and N,N,N’N’-Tetramethylethylene-1,2-diamine, 1-Butyl-3-methylimidazolium dicyanamide, and Fuel A with red fuming nitric acid were investigated using a controlled descent under a nitrogen environment and via standard drop tests under a fume hood. For the controlled-descent experiments, ignition was only observed for monomethyl hydrazine and Fuel A. Ignition was achieved with monomethyl hydrazine, Fuel A, N,N,N’N’Tetramethylethylene-1,2-diamine, and a blend of 2-Azido-N,N-dimethylethanamine and N,N,N’N’-Tetramethylethylene-1,2-diamine for the drop tests. The controlled-descent, inert atmosphere tests provided some insight into the importance of liquid-liquid reactions on hypergolic ignition.

High speed OH PLIF applied to multiphase combustion (Review)

Multiphase reactive systems can exhibit highly dynamic combustion phenomena that could be better understood by using recently developed high-repetition-rate optical diagnostic and imaging approaches. Here, we present an overview of recent activities using high-speed (5 kHz) OH planar laser-induced fluorescence to visualize and make measurements in several multiphase reactive systems. This technique is used to visualize the dynamically changing OH concentration in the gas phase near the surface of solids, liquids, and gels. In addition to gas-phase OH imaging, condensed phases of various solid propellants, gels, and liquids are found to fluoresce when exposed to the laser radiation centered at 283.2 nm. Simultaneous imaging of condensed phases and gasphase OH radical fluorescence has proven to be particularly useful for various measurements, and several examples are presented.

Combustion Performance of Porous Silicon-Based Energetic Composites

The combustion performance of oxidizer filled porous silicon(PSi) was studied. PSi samples with diameters of 2.54 cm were fabricated by electrochemical etching. The porosity of the samples ranged from 55 to 82%. The samples were cut into 3-5 mm strips and filled with an oxidizer under a vacuum for 5 min to 15 min. The PSi was filled with the oxidizers NaClO4 x 1H2O, Ca(ClO4)2 x 4H2O, sulfur and perfluoropolyether (PFPE). The filled PSi was then burned by igniting the sample with a hot Nichrome TM wire. The experiments were recorded using high speed photography from which burning rates were calculated. The porosity did not appear to have a direct on affect the burning rates for those studied however burning rates did show a strong dependency on quality of the oxidizer loading. The oxidizer loading appeared to be best for the samples with a porosity of 55% that were placed under a vacuum for 15 min. PSi loaded with NaClO4 x 1H2O produced burning rates that ranged from 216-349 cm/s. PSi loaded with Ca(ClO4)2 x 4H2O had burning rates of 154-285 cm/s. A sulfur filled PSi sample burned a rate of 16 to 290 cm/s, and perfluoropolyether loaded PSi burned at a rate of 1.4 cm/s. We believe this is the first time burning rates in PSi have been reported.

Flame Structure Study of a Bimodal Ammonium Perchlorate Composite Propellant Using 5 kHz PLIF

The self-deflagration of a bimodal ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB) propellant was studied using high speed planar laser induced fluorescence (PLIF) for the first time. The qualitative OH concentration was characterized near the surface. In addition to OH, it was found that the larger AP particles can be imaged as they fluoresce when exposed to laser radiation centered at 283.2 nm. Single AP particle ignition delay, lifetime, and flame height are determined as a function of particle diameter over a range from 100 to 500 μm at 1 atm within the burning sample. High speed visible imaging was also completed to confirm the trends seen during PLIF experiments, although the fluoresced particles have much improved contrast. Ignition delay times and single particle burn times were compared with a model proposed by Shannon and Peterson. The measured final diffusion flame height above a regressing AP crystal was compared with an expression used by the Beckstead, Derr, and Price (BDP) model. It was found that the AP/HTPB propellant flame structure varies significantly with particle size, even at 1 atm. The models are found to adequately predict the observed trends, but do not capture the interaction of adjacent particles. It is shown that high speed OH PLIF can be a valuable tool to characterize AP composite propellant combustion.

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

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.