Katsuo Asato

Characteristics of flame propagation in a vortex core: Validity of a model for flame propagation

Abstract Characteristics of flame propagation for methane-, propane-, and hydrogen-air mixtures and the validity of a model for flame propagation in a vortex core were investigated experimentally using a vortex ring generated by pulsing a quantity of the mixture through a circular nozzle. The experimental results show that the ratio of the flame speed to the maximum tangential velocity of the vortex core decreases as the maximum tangential velocity is increased. This experimental observation agrees qualitatively with predictions of a model that considers the shape of the flame tip in the vortex core. The flame speeds calculated by the proposed model are proportional to the square root of the ratio of the density of the unburned gas to that of the burned gas ( ρu ρb ) 1 2 , which agree with the experimental results. The present predictions better match the experimental results than those calculated using Chomiaks model. The constant of proportionality between flame speed and ( ρu ρb ) 1 2 depends on the type of fuel used. This experimental observation is not predicted by Chomiaks model. The flame speed in the vortex core can be predicted by a model of flame propagation taking into account the shape of the flame tip. The calculated flame speeds, however, are still higher than those obtained in experiments. In order to predict the flame speed exactly, not only the shape of the flame tip in the vortex core, but also the effects of baroclinic torque, curvature of the vortex core, unsteadiness of the propagation velocity, and structure of the flame in the vortex core should be considered.

Explanation of the blowoff of inverted flames by the area-increase concept

Abstract An experimental study of a laminar, two-dimensional inverted flame established in the wake of a thin plate inserted into the interior of the premixed gas stream was performed, and the effects of the radius of curvature of the very base of the flame on the blowoff limit were examined in detail. Experimental results show that as the boundary velocity gradient at the trailing edge of the plate is increased, the distance from the trailing edge to the flame base increases, whereas the radius of curvature of the flame base decreases. The critical radii of curvature at the blowoff limits tend to approach a constant value independent of the plate thickness when it is small. A single curve for the critical area-increase factors-namely, for the fractional area increases from the ignition plane to the front edge of the preheat zone for the flame base at the blowoff limits-can be drawn through the data points except for the thicker plate, and the blowoff is more directly correlated to the factor based on the area-increase concept than to the socalled Karlovitz number that has been used so far.

Reexamination of the blowoff mechanism of premixed flames: inverted flames

Abstract As a start of a reexamination for the blowoff mechanism of premixed flames, an experimental study of a laminar, two-dimensional inverted flame established in the wake of a thin plate was performed. Streamlines and velocities in the wake were measured in detail, and they were analyzed to obtain the stretching along the flame front at the base of the flame, namely, at a “stabilization point,” and the flow velocity gradient along a streamline passing through that point. At the stabilization point, the flow velocity must be balanced by the burning velocity, and the flow velocity gradient must be smaller than the burning velocity gradient along the streamline. When these conditions are not satisfied, the blowoff will occur. The experimental results show that the stretching along the flame front at the stabilization point of inverted flames does not occur, and it is confirmed that the flame-stretch concept can not be applied to the blowoff of inverted flames. The dimensionless flow velocity gradient at the stabilization point is equivalent to the area-increase factor, which is proportional to the curvature of the flame base. The critical flow velocity gradient for the blowoff will be caused by an excessive increase in the curvature of the flame base resulting in a lowering of the burning velocity there.

Structure Analysis of the Stabilizing Region of Plane, Laminar Fuel-Jet. Flames.

Abstract An experimental study was made of the structure of the stabilizing region of plane, laminar fuel-jet flames of methane ejected parallel to the air stream. Velocity and temperature profiles were measured in detail and were analyzed to yield the distributions of the heat-flux vector and the heat-release rate throughout the stabilizing region. Results show that, a region of the higher heat-release rate appears near the flame base. This is due to combustion of a small volume of premixed gases produced as a result of fuel and air coming into direct contact. Another region of the negative heat-release rate appears on the fuel side of the flame zone. This is attributed to pyrolysis-type reactions. Towards the fuel-nozzle wall the heat-release rate decreases remarkably, still the heat-release rate in the dark space is substantial.

Effects of curvature on extinction of premixed flames stabilized in stagnation flow

Characteristics of laminar premixed flames convex to uniform stream of unburned mixture, which were established in the stagnation region of a jet and the uniform stream opposing each other, were studied experimentally. The effects of flame curvature and the combined effects of the curvature and the Lewis number (Le) on the extinction of the convex flame were discussed, and the results were contrasted with those obtained from premixed flat flames established in the stagnation flow. The main conclusions obtained are summarized as follows: (1) In the lean methane flames of Le=1.0, the convex flames are extinguished close to the stagnation point in the same manner as the flat flames. In the lean propane flames of Le

Evaluations of grid currents of an ion engine by particle analysis

To evaluate the effects of inflowing-high-energy electrons on the grid erosion, three-dimensional full-PIC analyses of ion engine were conducted. A comparison of the results of full-PIC results show that the inflowing high energy electrons expand the sheath region downstream and blurs the sheath edge. However, only a slight influence of the electrons on grid currents is observed.

Characteristics of Combustion of Rich-Lean Burner Controlled Boundary Region between Rich and Lean Flames (2nd Report, Effects of Mixture and Air Supplied into Boundary Region on Practical Combustion Region)

Effects of the velocity and the air ratio of the mixture and the air supplied into the boundary region between the rich and lean flames on the characteristics of the rich-lean combustion were investigated in order to develop a new type of a domestic boiling water heater and a warm heater with low emission of NOx and high TDR. The characteristics of flame stabilization of the rich-lean combustion are improved even under the condition of the practical combustion when the stoichiometric, lean mixture and the air are supplied into the boundary region between the rich and lean flames. The stable combustion is achieved under the condition of 1.34 times of the normal load combustion by ontrolling the boundary region. The emission of NOx is decreased when the air is suplied into the boundary region between the rich and lean flames. Especially, the effects of NOx reduction are noticeable under the conditions of low load combustion. Therefore, it is possible to prevent combustion oscillation and combustion noise with low frequencies from occurring, and to develop the new type of burners with high performance of high TDR and further low emission of NOx by supplying the mixture and the air into the boundary region between the rich and lean flames in the rich-lean combustion.

An experimental study on combustion of ultralean-fuel and air mixture by heat recirculation.

An experimental study was carried out on the combustion of ultralean-fuel/air mixture by preheating the mixture by means of a heat recirculating burner. The combustion chamber of the burner was made of transparent glass to allow the observation of flame appearences. A laminar, continuous flame was stabilized in the combustion chamber. The conclusions obtained are as follows : (1) Almost completely burned gas is produced with a negligibly small amount of nitrogen oxide. (2) If the combustion of the mixture is incomplete in the combustion chamber and the residual of the mixture burns secondarily in the inner cylinder, the temperatures of the preheated mixture and the flame drop leading to inferiority of the burner performance. (3) The flame blows off prior to the approach to the proper limit of flammability for the preheated mixture, so that the flame stabilization in the combustion chamber is important since the mixture at the blow-off limit is still flammable.