Takashi Tsuruda

Growth of Flame Front Turbulence During Flame Propagation Across an Obstacle

Abstract The growth of flame front turbulence, when a flame was propagating through a methane-air mixture acceleratedly flowing across a block, was examined by high-speed schlieren photography. Three types of flame front turbulence induced by different mechanisms were observed to appear. The turbulence induced by the Taylor-Markstein mechanism appeared at the leading flame front normal to the direction of mixture acceleration and became of a needle-like structure over the block. In the present particular situation, the turbulence that appeared at the flame front parallel to the direction of acceleration and that near the walls grew later and their rales of growth were much smaller than that which appeared at the leading flame front

Local flame front disturbance development under acceleration

Abstract The development of a local flame front disturbance in an accelerating gas flow was explored using high-speed schlieren photography. The movement of the local flame front was examined in detail, and, based on these results, the flow field around the disturbed flame front was inferred. From this, it was confirmed that in an accelerating flow field, the unburned gas penetrates into the burned gas region at a velocity about 30 times higher than the laminar burning velocity. Flame front acceleration generates intense shear flow layers across disturbed flame fronts.

The potential of visualization for studies on flames and furnaces

Abstract The results of research into the potential of visualization for studies on flames and furnaces are presented. The visualization techniques used in previous studies are briefly described, with examples of visualization and facts derived from them. A number of findings indispensable for an understanding of the characteristics of flames and furnaces are pointed out. The possibility of further development and improvement of visualization techniques is discussed.

Flame front disturbance induced by a weak pressure wave

An experimental study has been conducted on the effect of unburned mixture properties on flame front disturbance induced by acceleration. Experiments were performed using a rectangular combustion chamber of 80×80×440 mm. The flame front disturbance was observed in two different directions by high-speed schlieren photography. Mixtures used are of three different concentrations (Φ=0.8, 1.0, and 1.25) of methane/air and two different concentrations (Φ=1.0 and 1.5) of propane/air. For the methane/air mixture of Φ=1.0, experiments were performed at three different initial pressures ( P i =50, 70, and 101 kPa). The observed disturbance was of a very fine structure of circular spikes, which penetrated into the burned gas. The scales of disturbance were measured and indicated to be in the range of 1.7–4.0 mm. The circular spike shape is a typical structure induced by accelerating the flame front where the density changes steeply. However, the shape of the disturbance observed for a rich propane/air mixture was not of circular spikes but of a net of ridges. For the rich propane/air mixture, the effect by preferential diffusion was remarkable and the disturbance of a different structure was generated. For the methane/air mixtures, the scale was the smallest at Φ=1.0 and larger at Φ=0.8 and 1.25. The scale for the propane/air mixture of Φ=1.0 was slightly larger than that in the methane/air mixture of Φ=1.0. The scale became larger with the decrease of the intial pressure. The measured scales were compared with the preheat zone thicknesses of corresponding flames. It was shown that the scale is closely related with the flame thickness. The scale of disturbance is found to be about 15 times as large as the preheat zone thickness.

Translated from Nensho no Kagaku to Gijutsu, 1, 59 )1992( Detailed Structure of Flame Front Disturbance

Abstract The detailed three-dimensional structure of the flame front disturbance induced by a weak pressure wave has been studied experimentally using a small combustion chamber. The flame front disturbance was recorded by a newly developed high-speed Schiieren system, which gives a view from the direction normal to the flame front. The results were compared with those induced by preferential diffusion mechanisms. The image of a flame front disturbance induced by acceleration caused by weak pressure waves was found to be circular when it was observed from the direction normal to the flame front. This image was much different from that induced by the preferential diffusion mechanisms, which was observed as a net composed of thick lines. These images of the flame front disturbances were compared with those observed in our previous studies, in which the disturbances were observed from the direction normal to the flame propagating direction. Consequently, the flame front disturbances induced by acceleration w...

Numerical simulation of flame disturbance growth induced by a pressure gradient

Abstract As a premixed methane-air flame is accelerated by a pressure wave, spikes penetrating into the burnt gas appear on the flame front, eventually leading to the formation of a turbulent flame. A model of a two-dimensional, inviscid flow in which the flame is considered to be a source of volume and vorticity is shown to develop spikelike features. The source of volume models the thermal expansion occurring inside the flame. The vorticity inside the flame is generated by the pressure gradient across the flame front. In the course of the computations, Fourier transform techniques were used to eliminate small-scale disturbances and hence simulate the stabilizing effects of thermodiffusive factors. A simulation was carried out with, as the initial flame configuration, a flame shape taken from experiment. The simulation resulted in the growth of a spike into the burnt gas that is in good agreement with the experimental observation, at least in the first stages of the simulation.

A Study On Extinction Of RDF (Refuse Derived Fuel) Pile

Tests were conducted with the objectives of observing and recording the characteristics of fire growth and extinction of RDF fire, and to establish a tentative basis for the development of safety and fire control requirements. An RDF sample used was a pellet type made from municipal solid waste. A pile of RDF pellets was ignited and burned for predetermined burn time. By applying water mist, flame over the pile was extinguished. When burn time was longer than 30 min, hot area remained on the surface after extinguishing flame. The hot area expanded with time. Smoke was emitted from the hot area. After the pile was cut open for inspection, hot area was found inside the pile. RDF pellets are aggregated by heating since shredded plastics contained in them melt and work as adhesive. By applying water, aggregated pellets are cooled to form a layer which prevents water from soaking inside. RDF pellets remain hot under the layer. If air is supplied to hot RDF pellets under the layer, oxidation continues and heat and smoke are emitted.

Effect Of Combustible Vapors In Air On Extinction Of Cup Burner Flames By Hfc And Fc Fire Suppressants

HFC and FC agents were developed as replacements of halon 1301 (CF3Br), however, some fluorohydrocarbons decrease the lower flammability limits of combustible gases and vapors. To use trifluoromethane (CF3H), 1,1,1,2,3,3,3 heptafluoropropane (CF3CFHCF3), and perfluorobutane (CqF10) as the replacements safely in plants containing combustible vapors, cup-burner flame extinguishing concentrations of the agents, halon 1301, and nitrogen for nheptane flames were investigated as functions of concentration of methane or methanol vapor in air. The following conclusions have been obtained: (1) When the air contains a combustible less than the lower flammability limit, blue flame appeared around n-heptane diffusion flame by addition of the HFC and FC agents in air. Such flame did not appear through the addition of CF3Br and nitrogen. (2) Methanol vapor in air increased the flame extinguishing concentrations of all agents when increasing the vapor concentration, while methane did not affect the flame extinguishing concentrations of all the agents until the blue flame was formed. (3) If a combustible vapor is mixed in the air of a space protected by a total flooding fire extinguishing system employing the HFC or FC agenf a design concentration determined by the flame extinguishing concentration is not always sufficient to extinguish fires in the space. An inerting concentration may be desirable for the design concentration in such case.