Yoshiro Otsuka

Unsteady Behavior of Diffusion Flames and Premixed Flames for Counter Flow Geometry

Abstract Unsteady analysis was carried out for both diffusion flames and premixed flames stabilized in the counter flow stagnation region when the velocity normal to the stagnation plane pulsates in a sinusoidal motion around its steady state value with small amplitude. Fluctuating and time mean components are investigated by adopting Arrhenius second order reaction rate model, assuming that the flow is inviscid incompressible and Lewis number unity. Furthermore, the theoretical results are compared with the experiment performed under the conditions involved in theoretical analysis.

The one-dimensional diffusion flame in a two-dimensional counter-flow burner

A counter-flow burner has been used to stabilize one-dimensional diffusion flames of a large size, and temperature treasurements on these flames have been made by the “deflection method.” The distance between the stagnation point and the flame shows a discontinuity when the velocity gradient along the flame reaches a critical value e tr [1] and there is a corresponding discontinuity in the temperature. Analysis of experimental results indicates that, though the ratio of oxygen to fuel mass flux into the flame front is held constant below this value it shifts towards more fuel rich ratios when e tr is exceeded. This can be ascertained from the measured concentrations of combustion products around the flame front. Furthermore, a kinetic analysis of the flames indicates that such discontinuities are to be expected.

An Analysis of a Stretched Cylindrical Premixed Flame

Abstract For the basic investigation of a premixed flame subjected to the effects of both flame stretch and flame curvature, the characteristics of the outward propagating cylindrical flame formed in an axially stretching flow were theoretically analyzed and compared with those of counter-flow twin flames, which have no curvature, and other types of non-stretched flames. The flame sheet model was first adopted for the parametrical investigation of the effects of Lewis number and preferential diffusion. Then, the finite reaction rate model was used and the extinction characteristics were analyzed over a wide range of equivalence ratios, resulting in the agreement of the extinction limit curves with the experimental data reported before.

A study of cylindrical premixed flames with heat loss

Abstract An axially stretched cylindrical flame with heat loss to the upstream porous wall, through which a fresh mixture was assumed to percolate, was theoretically analyzed and the characteristics of the flame, which was simultaneously subjected to every effect of stretch, curvature, and heat loss, were investigated. The results were discussed in comparison with those of flames subjected to only one or two conditions. The extinction limit curve was also predicted for an outward propagating flame and was compared with the curve measured in a previous experiment, showing a qualitative agreement.

On the deviation of the flame from the stagnation point in opposed-jet diffusion flames

In this study measuring the deviation of the opposed-jet diffusion flame or the distance between the flame and the impingement plane, it is found that the deviation varies discontinuously at a certain jet velocity. The analytical results on the relation between the deviation and the jet velocity, which are obtained from thin flame kinetics following the usual concept that the ratio of O 2 mass flux to fuel mass flux into the flame front is stoichiomatric, are qualitatively consistent with the measured values at lower jet velocities than the velocity of discontinuity. From these results it is possible to say that the stability of diffusion flames stretched violently at higher jet velocities follows a different concept from usual.

On Flammability Limits and Flame Shapes of Counterflow Premixed Flames

Abstract Flammability limits for methane-air and propane-air mixtures were measured using the counterflow type burner which consisted of a couple of converging-diverging nozzles made of quartz glass. As several types of flames other than the usual twin flames were obtained in this burner, a simple theoretical analysis on the shape and nature of the flame developing in the counterflow field was first made, and flame stability and correspondence between the theoretically obtained and experimentally observed flame shapes were discussed. Flammability limits were then discussed with respect to the flame stretch effect and the preferential diffusion effect.

Suppression Effects of Stretching Flow on Soot Emission from Laminar Diffusion Flames

Abstract The comparison of soot limits of axisymmetrical laminar diffusion flames stabilized in two different flow fields, i.e., stretching and non-stretching (parallel) flow, revealed that soot emission is suppressed more effectively in the former flow. From the experimental results that flame heights were hardly affected by the flow field difference, it was inferred that such an effect of the stretching flow is attributed to the flow acceleration which leads to the shortening of fuel residence time in the soot-forming region and, therefore, to the effective suppression of soot particle growth Burner scale effects on the soot suppression were also investigated using the stretching flow burners of several sizes and some empirical relations at the soot limit were derived among burner size, air flow rate (stretch rate), and fuel flow rate.