Satoru Ishizuka

An experimental study of effect of inert gases on extinction of laminar diffusion flames

The limiting fuel concentration and the limiting oxygen concentration required to maintain the diffusion flame and the limit flame temperatures were measured using a counterflow diffusion flame established in the forward stagnation region of a porous cylinder. The fuels used were methane and hydrogen, and three kinds of inert gas (nitrogen, argon, and helium) were used as the diluent. The flame temperature at the limiting fuel concentration coincides with that at the limiting oxygen concentration, and therefore, the controlling factor with diffusion flames under limiting conditions is the limit flame temperature. The limit flame temperatures for methane and for hydrogen, diluted with nitrogen, are 1,200°C and 740°C, respectively. The limit flame temperature is considerably higher with helium than with nitrogen or argon. The limit flame temperature of a diffusion flame diluted with an inert gas is closely related to the flame temperature at the lean flammability limit of the premixed combustible gas of the fuel and “air”, including the same inert gas. The lean flammability limit of a premixed flame and the limiting concentrations of the reactants for a diffusion flame are primarily controlled by the same factor. In the special case of the hydrogen diffusion flame where the flame lies on the fuel side of the stagnation point, the nonuniform extinction of the flame caused by the preferential diffusion of hydrogen occurs before the flame temperature is reduced to the limit temperature.

Characteristics of tubular flames

Abstract A tubular flame is established in a stretched vortex flow. This flame is long and its cross-sectional shape is cylindrical; hence, the shape is exactly tubular. From a fundamental viewpoint, tubular flames have two different aspects from the usual flames. First, the tubular flame belongs to a kind of stretched flame, but its cross-sectional shape is cylindrical. Therefore, its characteristics are similar to, in the sense of ‘stretched’, but different from, in the sense of ‘cylindrical’, those of the stretched flat flames such as the plane flame in a stagnation-point flow and the twin flames established between two opposing gas streams. Second, the tubular flame can be established in a rotating, axisymmetric flow field as well as in a non-rotating axisymmetric flow field. These two aspects are quite interesting when we consider combustion in turbulent flows, which involve both straining and rotational motions. In this paper, the experimentally found characteristics of tubular flames are presented. Especially, the effects of the Lewis number on the stability and diameter of tubular flames are described. Based on knowledge of the tubular flame characteristics, turbulent combustion is discussed.

On the flame propagation in a rotating flow field

Abstract Using a glass tube into which a combustible mixture is tangentially injected from one end, the phenomenon of flame propagation in a rotating flow field has been experimentally investigated. The regions for the flame propagation are mapped as functions of the fuel concentration and the flow velocity, and the flame speeds are also determined. The results show that, when the tangential injection velocity is small, the propagation range in the fuel concentration becomes narrower with an increase of the flow velocity, whereas when the injection velocity is large, and thereby the rotation is strong, the range becomes wider as the flow velocity is increased; an axisymmetric flame, convex towards the oncoming mixture, is formed at the open end, and it projects into the glass tube eventually to the closed end. An interesting point is that the concentration limit is extended by the flow rotation to that for a quiescent mixture in lean methane or rich propane mixtures, but not in rich methane or lean propane mixtures. In the former mixtures, the head of the flame is intensified in burning, whereas in the latter mixtures, the head is weakened. In addition, it is found that the flame speed almost linearly increases with an increase of the tangential velocity. These results are discussed on the basis of the vortex bursting mechanism proposed by Chomiak and Lewis number considerations.

Determination of flammability limits using a tubular flame geometry

Abstract Using a porous cylinder through which a combustible mixture is injected, the flammability limits in a tubular geometry have been investigated experimentally. Results show that flame behaviour in a tubular flame geometry is strongly dependent on the Lewis number (Le) of the deficient species in the mixture. For mixtures with Le 1, the flame does not become small at extinction. Therefore, in the flame of Le > 1 mixtures the heat loss to the burner surface is increased, resulting in narrowing of the flammable range. Results also show that the flame is significantly deformed due to buoyancy when the burner is horizontally mounted. For mixtures with Le

Effects of pressure on structure and extinction of tubular flame

Abstract A numerical study on effects of pressure on structure and extinction of a tubular flame was made. The adopted approach was the detailed kinetics theory combined with the exact similarity solution for the flow field. The calculation was made for stoichiometric methane-air mixture, and the structure and the extinction limits were determined for pressures ranging from 1 to 8 atm by using the so-called C1 chemistry. It was found that at elevated pressures the flame radius is decreased because of the decrease in the burning velocity, while the reaction zone width is decreased due to the decelerated transport properties. The flame temperature is increased, approaching the equilibrium flame temperature due to the accelerated reaction rates. The extinction flame temperature increases, whereas the extinction flame radius decreases with pressure, with a minimum radius of around 0.2 mm at the highest pressure of 8 atm. The critical strain rate at extinction limit increases with pressure initially, reaching a maximum at around 2.7 atm and then decreasing for higher pressures. It appears that this behavior is closely related to the effects of curvature on extinction, which can be explained in terms of the simplified asymptotic analysis.

Effects of rotation of the stability and structure of tubular flame

The effect of rotation on the stability and structure of a tubular flame has been experimentally investigated. The concentration field of OH radical has been measured by laser-induced fluorescence, and the temperature field was examined using a fine thermocouple. Results show that, with increasing intensity of rotation, the lean limit of methane flame is decreased, and the stable flame region is expanded. The flame diameter is increased, and the inner flame structure is also changed. Namely, when the rotation is weak, the radial OH distribution takes an M-shaped profile; the concentration is low in the outer region of the unburned gas, increases and takes a local maximum value in the reaction zone, then falls around the center. With decreasing the fuel concentration Ω, the peak positions approach and the profile becomes Π-shaped. With further decreasing Ω, the profile becomes Λ-shaped; eventually, the flame is extinguished. Correspondingly, the peak value is decreased, increased, and decreased with decreasing Ω. However, when the rotation is intensified, the variation of the peak value becomes moderate, and the M-shaped profile is maintained close to extinction. The temperature measurement shows that, with increasing intensity of rotation, a temperature dip appears around the center and its magnitude becomes larger. Although this dip is usually attributed to radiative heat loss, the temperature at the center remains unchanged while the reaction zone temperature is increased; i.e., a protrusion in the reaction zone temperature occurs. This aspect resembles the excess enthalpy flame. Although the details of the extension mechanisms are unclear, these results suggest that the stability of the flame is enhanced by rotation through a mechanism like the excess enthalpy flame.

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.

Extinction characteristics of tubular flames diluted with nitrogen in a rotating stretched flow field

Extinction characteristics of tubular flames diluted with nitrogen in a rotating flow field have been experimentally investigated. The extinction limit was mapped as functions of the equivalence ratio φ and the added nitrogen ratio χ , and the flame diameter and temperature at extinction limits, the flame structure and the stable species concentration in the tubular flame near the extinction limits have been measured. Results show that, as compared with other results for the usual propagating flames in a tube or the flat, twin flames, the flammable range shifts towards the fuel-lean side for methane and towards the fuel-rich side for propane; among others, the lean limit of methane becomes minimum and the rich limit of propane becomes maximum; the maximum allowable dilution by nitrogen occurs at the lowest equivalence ratio for methane flames ( φ =0.87) and at the highest equivalence ratio for propane flames ( φ =1.75). This shift is more remarkable than that obtained by the tubular flame in nonrotating flow field. Measurements on the concentrations of the product and intermediate species in the inner core have also shown that these values are shifted towards the fuel-lean side in methane flames and the fuel-rich side in propane flames. From these results, it seems that although the characteristics of the tubular flame in the rotating flow field are influenced by the stratification through preferential diffusion and the Lewis number through stretch as those of the other stretched flames, this effect is promoted by its curvature and the rotating motion of the mixture.

Elementary processes governing behavior of turbulent premixed flames

Present knowledge on elementary processes governing the behavior of turbulent premixed flames is summarized. The movements and configurations of local reaction zones or flame fronts in the overall reaction zones of stationary turbulent premixed flames and typical aspects of the turbulence behavior of propagating flames under various conditions and situations are presented. To give basic knowledge on the processes governing behavior of turbulent premixed flames, recent results of experimental studies on flame behavior under several simple sets of conditions, including rotating and accelerating flow fields, are introduced. Finally, effective processes governing the behavior of turbulent premixed flames are discussed. It is pointed out that presently available data are not sufficient to understand the structure of turbulent premixed flames. The transport phenomena within local reaction zones and aerodynamical effects on the local reaction zones seem to be the most important subjects to discuss.