Influence of the laser beam characteristics on the laser-induced breakdown of air was studied. Energy needed for the breakdown was precisely evaluated from the transmitted laser energy from the breakdown area. As the laser beam characteristics was varied, the breakdown threshold was unexpectedly found to be increased when the narrowest pulse width (8 ns) was used, and the threshold value was approximately 2.5 mJ under a focusing lens of f = 100 mm. In addition, the aberration of the lens is found to be an important factor for the breakdown threshold when the laser-induced breakdown is performed under the lens having a short focal length, such as f = 50 mm.
We observed chemiluminescence spectra from the plasma formed by laser-induced breakdown of air at atmospheric pressure using Cassegrain optics and a streak camera. The streak images showed three characteristic regions in the plasma when the energy of the laser pulse was 23mJ and was focused by an f=100mm lens. The central region of the plasma was characterized by its weakest emission, but the onset of the laser breakdown was found to initiate in this area. In addition, two emission peaks of the chemiluminescence were observed in this area. The phenomenon was seem to be originated from the propagation of the shock wave in plasma, and the speed was calculated to be approximately 12.7km/s.
We discussed the flame structure of premixed flames in high intensity turbulent fields, where Damköhler numbers were around or lower than unity. In the former study, low Damköhler number flames were successfully formed in relatively weak turbulent reactant flows by using very lean premixture of sufficiently long chemical characteristic time, where the transition of flame structure from 'Distributed reaction zone' to 'Wrinkled laminar flame' was observed. We, in the present study, discussed microstructures of turbulent premixed flames in high intensity turbulent fields of low Damköhler number. As a result, we have suggested two types of microstructures of intense turbulent premixed flames. One is a propagating flame with local extinct protrusions resulting in cylindrical or warped flamelets surrounded by a pseudo-distributed reaction zone, and the other is a propagating flame with local extinction spots resulting in a pseudo-distributed reaction zone. The proposed models are consistent with experimental observations, and the classifications of the two types of flame structures are understood to correspond the boundary separating 'Distributed reaction zone' and 'Well-stirred reactor' on Borghi's diagram.
We carried out numerical studies on methane/oxygen diffusion flames of counter-flow configuration to elucidate the influence of pressure on flame structure, heat release rate and reaction mechanisms. The chemistry in gas-phase was based on GRI-Mech 3.0 database. The thickness of diffusion flame became thinner with increasing strain rate a , with its characteristic flame thickness varying inversely with √a, especially its relation became significant with increasing pressure. Flame temperature increased with increasing pressure. Enhanced H2O production reactions, especially chain terminal reactions for H2O production, were found to be important in determining the flame temperature at high pressures. The small reduction in the flame temperature with increasing strain rate at high pressures, compared to the atmospheric pressure, is caused by the capacitor effect of product dissociation. From QRPDs, the third body dependent reactions were enhanced in high pressure conditions, hence C2 pathway was enhanced.
We carried out the flow field measurement of methane-oxygen turbulent nonpremixed flame in non-combusting and combusting situations at high pressures using LDV. The main objectives are to study the influences of combustion on the turbulence structure at high pressures and to provide detailed data on which numerical predictions on such flows can rely. Direct observation and CH* chemiluminescence detection are conducted at high pressures up to 1.0MPa. It was found that the flame length at elevated pressures became constant. From flow field measurements, the following features of flames at elevated pressure were found: (1) the existence of flame suppressed turbulence in the upstream region of the jet and enhanced it in the downstream region with increasing pressure; (2) Turbulence in the flame was more anisotropic than in the corresponding cold jet in all regions of the flow with increasing pressure; (3) Reynolds shear stresses did not change at elevated pressure; (4) Combustion processes had a marked influence on the turbulence macroscale under high pressures, however, the turbulence macroscale was not changed even with the increase in pressure.
This study proposes a realizable technology for an emulsion combustion method that can reduce environmental loading. This paper discusses the effect on spray combustion for W/O emulsion fuel properties with an added agent, and the ratio between water and emulsifier added to a liquid fuel. The addition of water or emulsifier to a liquid fuel affected the spray combustion by causing micro-explosions in the flame due to geometric changes in the sprayed flame and changes to the temperature distribution. Experimental results revealed that the flame length shortened by almost 40% upon the addition of the water. Furthermore, it was found that water was effective in enhancing combustion due to its promoting micro-explosions. Results also showed that when the emulsifier was added to the spray flame, the additive burned in the flame's wake, producing a bright red flame. The flame length was observed to be long as a result. The micro-explosion phenomenon, caused by emulsifier dosage differences, was observed using time-dependent images at a generated frequency and an explosion scale with a high-speed photography method. Results indicated that the micro-explosion phenomenon in the W/O emulsion combustion method effectively promoted the combustion reaction and suppressed soot formation.
Spray characteristics of hypergolic propellants injected by unlike impingement nozzles, which is used for the attitude control or the orbit transfer of a spacecraft, have been investigated under combusting conditions. The atomization process of combusting hydrazine and nitrogen tetroxide (NTO) was visualized by means of a laser sheet and a high-speed camera, and the droplet size distributions were calculated by image processing based on the Mie-scattering theory. Compared with the non-reactive flow of simulants, the atomization process of the reactive flow was accelerated by the immediate ignition of the hypergolic propellants. The atomization mechanisms in the reactive and the non-reactive cases was not similar, for the empirical constants to approximate the experimental results was significantly different in both cases.
A new CFD-code "3d-MSW" to predict the waste bed combustion in the stoker-type Municipal Solid Waste Incinerators (MSWI) was developed. 3d-MSW was validated by comparison with the combustion tests in a pilot incinerator. The calculation results could predict the waste bed combustion under not only normal air but also oxygen enrichment air. Grate feeding velocity in an actual incinerator was also evaluated by applying the new code. 3d-MSW could present the influence of the grate feeding velocity on gas temperature and carbon monoxide concentration. 3d-MSW is very useful as a prediction tool for understanding the phenomenon of the waste bed combustion in the stoker-type MSWI.
In magneto-plasma-dynamic (MPD) arcjet generators, plasma is accelerated by electromagnetic body forces. Silicon nitride reactive deposition was carried out using an MPD arcjet generator with crystal silicon rods and nitrogen gas. Because the MPD arcjet generator produces higher-velocity, higher-temperature, higher-density and larger-area plasmas than those with conventional thermal plasma torches, nitriding of silicon can be enhanced. A dense and uniform β-Si3N4 film of 30μm in thicknes was formed after 200 shots at a repetitive frequency of 0.05 Hz with a discharge current of 9 kA and a substrate temperature of 700 °C. The Vickers hardness reached about 1300. It was found that film thickness was highly sensitive to discharge current and that microstructure of the film was sensitive to the substrate temperature. These results show that the MPD arcjet generator has high potentials for silicon nitride deposition.