Shigeharu Ohyagi

Diffusion flame stabilized on a porous plate in a parallel airstream

Effects of an obstacle on the structure and stability of a laminar diffusion e ame established on a porous plate in a parallel airstream have been investigated experimentally. The obstacle, a backward-facing step or a rectangular cylinder, is located upstream of the porous plate through which gaseous methane is injected uniformly. Structures of the e ame are elucidated by the direct and schlieren photography. Flame shapes are described and stability diagrams are plotted for the freestream velocity and the fuel injection velocity, which are discussed with e ow structures.

A high-speed photographic study of the transition from deflagration to detonation wave

Experiments were conducted to investigate the DDT process of the oxyhydrogen gas in the rectangular detonation tube of 3 m long. The repeated obstacle was installed near the ignition plug and the effects of the obstacle on the DDT process were investigated. The behaviour of the combustion and detonation wave were visualized utilizing Imacon high-speed camera with the aid of Schlieren optics.As a result, DDT process was visualized, i.e. (i) multiple shock waves were induced by the expanding combustion wave, because the combustion flame played a role as a piston and compressed the unburned gases. (ii) The acceleration of the combustion wave was occurred and the distance between the shock wave and the combustion flame became shorter. (iii) Eventually, the local explosion was occurred and cause overdriven detonation wave to propagate at the velocity of about 3 kms−1.

A study on DDT processes in a narrow channel

One of the fundamental problems to be studied on a Pulse Detonation Engine (PDE) is the deflagration to detonation transition (DDT). For the development of the PDE, it is essential to shorten a distance of detonation transition that is called a detonation induction distance (DID). We carried out an experimental study of DDT in a narrow channel with height of 1-5mm by using pressure and soot track records in oxyhydrogen mixtures. Detonation limits was discussed according to height of tube and equivalence ratio. According to pressure history and soot track record, detonation velocity, DDT process and DID was discussed. Over driven detonation and attenuated detonation was observed in the narrow channel. DID that measured by soot track record applied to experimental formula for oxygen and hydrogen system.

Re-initiation of detonation wave behind slit-plate

The study to investigate a quenching mechanism of detonation wave utilizing a slit is of particular importance by considering safety devices to suppress the detonation wave in industries where flammable gases are handled [1, 2]. The detonation wave propagated through the slit is disintegrated into a shock wave and a reaction front, since expansion waves generated at a corner of the slit have effects to decrease a temperature and reaction rate behind the shock wave. However, it is understood that the shock wave diffracted from the slit causes re-initiation and transited to detonation wave at downstream region, even though a diameter of open-area is smaller than critical tube diameter [1,2]. It is also well known that reaction front can accelerate rapidly to supersonic velocity when propagating over obstacles. Mitrovanov and Soloukhin [3] reported and was also confirmed by Edward et al. [4] that the critical value to distinguish the propagation of detonation wave is about 13λ for circular tube and about 10λ for rectangular channel, where λ is a cell size of stable detonation wave. A fundamental observation carried out by Moen et al. [5] clarified if the turbulence intensity is maintained by placing obstacles, the reaction rate and degree of turbulence become highly coupled. Furthermore, experiment and numerical simulation of decoupling and re-coupling processes behind sudden expansion of a tube were conducted by Pantow et al. [6] and Ohyagi et al. [7] to show re-initiation processes of detonation wave after decoupled by diffraction process. These results showed that reflected shock wave and Mach reflection could be a source to re-initiate a detonation wave. However, fundamental mechanisms of re-initiation processes of detonation wave by the interaction of shock wave with another shock wave or tube wall are still open questions. In this study, experiments are carried out in order to elucidate the re-initiation mechanisms of detonation wave by installing the slit-plate into a detonation tube filled with premixed gas of hydrogen and oxygen. A width of slit w, a distance between two slits x and initial pressure of test gas p0 are varied and re-initiation processes are visualized using high-speed image converter camera with schlieren optical system.

Propagation Behavior of Combustion Wave Induced by a Shock Wave Propagated into a Premixed Gas of Oxygen and Hydrogen

In this paper, experimental results were reported to investigate a behavior of combustion wave when a shock wave was transmitted into a combustible premixed gas of oxygen and hydrogen. In general, phenomena occurring in the premixed gas would be classified into four types, i.e. (a) the shock wave was just transmitted without causing ignition for the shock wave propagated with low-Mach number, (b) the gas was ignited behind the shock wave and a deflagration wave was propagated following the shock wave, (c) the deflagration wave was transited to a detonation wave behind the shock wave, (d) a detonation wave was directly initiated just behind incident shock wave having high-propagation Mach number. In this study, a shock wave produced by a detonation-driven shock tube was transmitted into a premixed gas of oxygen and hydrogen varied with an equivalence ratio, initial pressure of premixed gas and Mach number of the shock wave. As a result, the phenomena of combustion wave were classified using a cell-size of steady-propagating detonation wave. For sensitive gases having small cell-size, the detonation wave was directly initiated behind the shock wave even though the Mach number of the shock wave was relatively low. Empirical equations to evaluate a Mach number and temperature behind shock wave were obtained, which are threshold parameters to cause detonation wave behind transmitted shock wave.

A simulation of initiation process of detonations

A numerical simulation of gaseous detonations is performed by using the Random Choice Method (RCM) to predict an initiation process and, as a result, a detonation induction distance (DID) for a given mixture strength and initiation energy. The flow field is assumed to be unsteady, one‐dimensional, and non‐dissipative. A chemical reaction is assumed to be one‐step irreversible with a reaction order being unity. The experiments were performed by using a straight detonation tube with a driver section filled with a driver gas (initiator) to initiate detonation in a driven section of the tube. Gaseous mixtures to be tested were hydrogen‐oxygen mixtures with various initial compositions. A strength of the initiator was represented by an initial pressure of the initiator gas, i.e., a stoichiometric hydrogen‐oxygen mixture. The DID was estimated from sooted track records on which traces of explosions during the initiation processes was clearly observed. The agreement between the simulation and the experiment is q...

Behavior of Combustion Wave Induced by Propagation of Shock Wave into Premixed Gas of Hydrogen and Oxygen

Experiments were conducted in order to investigate a behavior of comcustion wave when a shock wave was propagated into a combustible premixed gas of hydrogen and oxygen. A phenomenon occurring in the premixed gas can be classified into four types, i.e. (a) the shock wave just transmitted into the gas without causing ignition for the shock wave of low-Mach number, (b) the gas was ignited behind the shock wave and a deflagration wave was propagated following the shock wave, (c) the deflagration wave transited to a detonation wave behind the shock wave, (d) a detonation wave was directly initiated just behind incident shock wave of high-Mach number. In this study, a shock wave produced by a detonation-driven shock tube was transmitted into a hydrogen-oxygen premixed gas varied with an equivalence ratio φ, initial pressure p1 and Mach number of the shock wave Msi. As a result, the phenomena observed in the gas was classified using a cell-size λ for steady detonation wave, since the cell-size was inversely proportional to a chemical reaction rate of the gas. For the case of sensitive gases having small cell-size, the detonation wave was directly initiated behind the shock wave even though the Mach number of the shock wave was relatively low. An empirical equation to evaluate a pressure was obtained, which is a threshold pressure to ignite the gas behind incident shock wave.

Re-initiation Processes of Detonation Wave Behind Slit-Plate (Visualization of Re-initiation and Quenching Processes of Detonation Wave)

A propagation of detonation wave shows particularly interesting phenomena, since the detonation wave is constituted from a three-dimensional shock wave system accompanied with a reaction front. Thus, the passage of a detonation wave draws cellular patterns on a soot-covered plate. Pressure and temperature behind the detonation wave are extremely high and have a potential to cause serious damages around it. Therefore, it is necessary from safety engineering point of view to quench the detonation wave with short distance from the origin. In this study, experiments applied high-speed schlieren photography are conducted to investigate behaviors of the detonation wave diffracting from two pieces of slits. The detonation wave produced in a stoichiometric mixture of hydrogen and oxygen is propagated through the slits and behaviors behind slit-plate are experimentally investigated. As a result, when the detonation wave diffracted from the slits, a shock wave is decoupled with a reaction front. Since two shock waves progagated from the slits interact each other at centre behind the plate, the detonation wave is re-initiated by generating a hot-spot enough to cause local explosion. It is also clarified that the shock wave reflected from a tube-wall induces detonation re-initiation.

A Study of the Initiation Process of Dust Layer Detonation

Detonations of dust-oxidizer gas systems have recently attracted considerable attentions because of their possible role in explosion hazards of combustible dusts such as corn starch, wheat, aluminum, coal and so on. In these detonations in a heterogeneous system, the structure of the reaction zone behind the shock front may be very complex because physical processes such as momentum, heat and mass exchanges between particles and the gas phase are coupled with chemical processes. The present study aims to elucidate the initiation process of dust layer detonation using corn starch in a horizontal tube. Experimentally, the behaviour of shock waves and reaction waves are elucidated in the initiation process of dust layer detonation.

Instabilities of Flame Front Propagating in a Constant-Volume Chamber. Effects of Dilution by Inert Gases.

The present work is a continuation of our previous study to obtain a fundamental understanding of instabilities of a flame front propagating in a constant-volume combustion chamber. In this report, attention was focused on effects of dilute gases on this phenomenon. Experiments were conducted in a disk-shaped constant-volume chamber of 100 mm diameter and 30 mm height with a window. The flames were visualized by instantaneous schlieren photography. It was found that the instabilities arise for lean hydrogen/oxygen mixtures diluted with argon as well as helium, wich can be explained as thermodiffusive effects. For methane or propane/oxygen mixtures, the diluent plays an important role. Using argon as a diluent renders the flame unstable in accord with the thermodiffusive theory, while using helium renders it stable which cannot be explained by this theory. For these mixtures diluted by helium, initial disturbances might be suppressed by viscosity.