Masahiko Mizomoto

Transition from laminar to turbulent free jet diffusion flames

Abstract The transition phenomena in free jet diffusion flames of pure hydrogen and hydrogen-nitrogen mixtures were studied in unconfined still air using the schlieren technique. The break-point lenghts of both the inner fuel jet and outer flame zone were measured as the jet exit Reynolds number was varied by using different diameter burner tubes of 1.42, 1.97, and 2.39 mm and by varying the nitrogen concentration. Two distinct types of the diffusion flame jets were found. The break-point length-jet exit Reynolds number dependence of the flames with large kinematic viscosity ( ν j > 0.6 cm 2 /s) exhibits four distinct regimes, controlled by (1) minimum Reynolds number for instability, (2) free jet instability, (3) flow transition in the feed tube, and (4) turbulence in the burner wake, associated with three possible “triggers.” The dependence of the flames with small kinematic viscosity ( ν j 2 /s) lacks regimes associated with the laminar jet instabilities and is composed of only two regimes. It is shown that the most important parameter is the Reynolds number based upon the local gas dynamic parameters at the break-point and that the critical condition at which a transition to turbulence in a laminar jet occurs can be related to a threshold value of this effective Reynolds number.

Lifting mechanism of free jet diffusion flames

The lifting phenomena of free jet diffusion flames of pure hydrogen and hydrogen-inert gas mixtures stabilized on a thin-walled burner tube were studied in unconfined, quiescent, oxygen-inert gas atmospheres. Nitrogen, argon, and helium were used as the inert gases. A laminar flame base, which exists even in a fully-developed turbulent jet diffusion flame, controls the flame stability. A simplified flame stability model is presented based on the concept that the flame base is the very end of the diffusion flame and it provides a continuous ignition source to propagate towards the combustible zone between the flame base and the burner tip. The local burning velocity parallel to the diffusion flame zone is considered at the flame base to be equilibrated with the entrained stream velocity component opposite to the direction of the burning velocity. Velocity measurements were made in the flame stabilizing region under near-limit conditions by means of the laser-Doppler technique. The measured velocity component parallel to the flame zone at the flame base was found to be directly proportional to the maximum laminar burning velocity obtainable by mixing the jet and external fluids. The result is independent of whether the mixture strength was obtained by varying the hydrogen content of the jet fluid or the oxygen content of the external fluid.

Velocity and Temperature Fluctuations in a Flat Plate Boundary Layer Diffusion Flame

Abstract Measurements of mean value and turbulent intensity of fluctuating velocity as well as fluctuating temperature have been made on a turbulent boundary layer diffusion flame over a flat plate. Results show that the turbulence production occurs along the flame zone. This behavior is reflected in the high gradient of mean velocity. That is, the variation of velocity fluctuation due to the flame is primarily attributed to the variation of the mean velocity profile. Therefore it can be concluded that an ordinary shear-generated turbulence plays an important role on turbulence in the whole region of the boundary layer. Results also show that the temperature fluctuation is attributed to the gradient of mean temperature in a turbulent flow. At the same time, the temperature fluctuation in the air stream side is intensified due to the penetration of room temperature gases into the burnt gas region.

A comprehensive examination of the structure and extinction of turbulent nonpremixed flames formed in a counterflow

Abstract An experimental investigation of turbulent counterflow nonpremixed flames has been undertaken in order to clarify the interaction between the properties of the nonpremixed flames and the characteristics of the turbulent counterflow field. In particular, to distinguish between the effects of turbulence caused by the air and fuel streams, the turbulent characteristics of each flow in an opposed jet flow were controlled individually. From the visualization by laser tomographic technique, it was found that the width of the diffusion region along the centerline regarded as a macroscopic parameter of the local structure of nonpremixed flames was not changed by the flow turbulence, and was determined by the mean flow condition characterized by the bulk velocity gradient, while whole diffusion regions spatially showed the typical wrinkled motion within the turbulent counterflowing stream. On the other hand, the mixture fraction fluctuations which were estimated by measurements of the behavior of the flame and the diffusion region, depended mainly on turbulence and were not affected by the bulk velocity gradient. The mean scalar dissipation rate χturb due to the turbulence, estimated by combining the turbulent strain rate of the air side stream and the rms of mixture fraction fluctuation, increased with an increase in the turbulent strain rate of the air side stream, that is, with a decrease in the turbulent Damkohler number, Da. However, it is known that in a counterflow field the strain caused by the mean flow is also effective for properties such as the transport phenomena. Then, the total scalar dissipation rate χtotal, which is derived from the turbulence and the mean flow velocity gradient, was suggested as the characteristic quantity of nonpremixed flames formed in counterflow geometry. The total scalar dissipation rate of flames at extinction showed almost constant value regardless of the initial turbulent conditions. The present results agree with the laminar flamelet concept.

Evaporation and ignition of a fuel droplet on a hot surface (Part 4, model of evaporation and ignition)

Abstract In order to clarify the relation between the evaporation process and the ignition mechanism of a fuel droplet (approximately 2 mm in diameter) on a hot surface, the mass of fuel (n-cetane) evaporated in an arbitrary short duration soon after the droplet comes into contact with the hot plate surface whose temperature is higher than the Leidenfrost temperature has been measured. Then the profiles of fuel concentration around the droplet are obtained by the numerical integration of the unsteady diffusion equation in spherical coordinates using the experimental results mentioned above. It is shown that the location calculated as the place where the stoichiometric mixture is formed at the ignition delay time is in good agreement with the observation of the initial flame position at the ignition.

Extinction mechanism of lean methanes/air turbulent premixed flame in a stagnation point flow

Effects of flame stretch, flame curvature and heat loss to the solid wall on the extinction of turbulent premixed flames formed in a stagnation point flow have been studied experimentally. Lean methane/air mixture, whose Lewis number is less than unity, was used. Bulk stretch rate was varied from 15 s −1 to 60 s −1 , while the turbulent intensitys of velocity fluctuation in the approach flow was varied from laminar condition to 0.6 m/s. The flame stretch due to flow divergence was estimated by measuring the mean centerline velocity profile with LDV and the flame curvature and flame location were measured by using laser tomographic technique. The total stretch rate is defined as a sum of flame stretch due to flow divergence and that due to flame curvature. The lean methane/air flame is intensified by a Lewis number effect and the flame can locate close to the wall. As a result, the heat loss to the stagnation plate affects the flame and finally the flame is extinguished under the influences of both heat loss and the total flame stretch. On the other hand, the flame of premixed gas with Le>1 (e.g. lean propane/air flame) is extinguished due to total flame stretch. From these results, it can be concluded that the extinction mechanism of wrinkled laminar flame is supposed to be fundamentally the same as that of laminar flame when the flame stretch is reasonably estimated by taking into account the effect of flame curvature.

Flame-Spreading over the Surface of a Solid Propellant Part 1: Experimental Results

Flame-spreading over the surface of a flat solid propellant in a turbulent boundary layer has been investigated experimentally. Nitrogen gas was used as a main flow gas, and noncatalyzed and catalyzed double-base propellants were used as specimens. Experimental measurements of the flame-spreading were conducted as a function of the main stream velocity in a small scale wind tunnel under atmospheric pressure. After the upstream edge of the specimen was ignited, the flame-spreading phenomena were investigated photographically. The temperature of the unburned propellant surface and the heat flux through the gas-phase to the unburned surface were measured with thermocouples and a calorimeter. It is found that the flame does not always spread downstream continuously but a secondary (and/or tertiary) ignition occurs on the unburned surface far downstream of the main burned region at the lower main stream velocity or for a propellant with higher normal burning rate, and that in general the history of the unburne...

Response of flame displacement speeds to oscillatory stretch in wall-stagnating flow

Abstract The response of the flame displacement speeds of stagnating flat premixed flames to the periodical fluctuation of stretch is investigated experimentally, regarding two mixtures with different Lewis numbers: lean C 3 H 8 /air and lean CH 4 /air in the range of the Strouhal number 2.3 to 4.8 (24 to 51 Hz). The stagnation wall is oscillated sinusoidally along the stagnation streamline. The oscillation of the stagnation wall induces the periodic fluctuations of flow velocity and flame stretch, and hence the fluctuations of flame displacement speed. The amplitude of the flame stretch fluctuation increases with increasing frequency of the wall in the Strouhal number greater than unity owing to the increase in the amplitude of the oscillatory wall velocity, when the stagnation wall is oscillated along the stagnation streamline at constant amplitude. The displacement speed is measured as the propagating velocity of the flame relative to the unburned gas velocity at the cold edge of the flame by using the history of flow velocity fluctuation. The response of the flame displacement speed is discussed from two viewpoints: its amplitude response and phase response. The significant dependence of the flame displacement speed on the flame stretch has been shown by the amplitude response. It is shown that the sinusoidal fluctuations of flame displacement speeds for both mixtures follow the sinusoidal fluctuation of flame stretch with a phase delay from about 20° to 90° in the present frequency range. Hence, the displacement speeds for both mixtures, although these mixtures have the contrary ratio of thermal to mass diffusivity against unity, show the same increasing dependence on unsteady stretch because the flow divergence affects the displacement speed with unsteady stretch as well as steady stretch.

Interaction of adjacent flame surfaces on the formation of wrinkling laminar premixed flame

To gain further understanding of the flamelet regime of turbulent combustion, the interaction betweenadjacent flame surfaces of a wrinkling laminar premixed flame has been investigated experimentally. The wrinkling flame was formed with three adjacent laminar V-shaped flames stabilized on a multiple-slitburner of a propane/air mixture. In addition, a single V-shaped flame was also formed on the burner. We compared flame shapes and flow velocity distributions between those two types of flames. The results show that the unburned flow field was expanded outwardly by the convex flame surface toward unburned side. In the case of the wrinkling flame, however, the outward deviation of the flow field caused by the existence of the inner convex was prevented by the existence of the surrounding flames. Consequently, the inner convex flame surface of the wrinkling flame is more oblique, and the curvature is sharper than that of a single convex flame (single V-shaped flame) due to the interaction of adjacent flame surfaces. Moreover, the formation of the inner convex flame of the wrinkling flame is more unstable than that of a single convex flame. This is due to the curvature of the inner convex of the wrinkling flame being sharper than that of the single convex flame so that the burning velocity is decreased as a result of the curvature effect.

Effect of slot gas injection to the flow field and coherent structure characteristics of a backstep flow

Abstract The flow field and coherent structure of the flow over a backstep with slot-gas-injection were investigated. The mean flow, turbulence properties and coherent structure characteristics are significantly influenced by the increase of injection momentum near the step ( l f /H=2). The mean flow field is characterized by recirculation zone splitting, with mass transfer of injected gas toward each part depending on the specific momentum ratio of injection. More mass entrainment, turbulence structures alteration in the shear layer and the hydrodynamic disturbances from the injection flow suppress the coherent structure velocity, size and detachment. These results provide basis information for achieving the balance of competing factors between geometrical and fluid dynamical parameters of injection to establish an efficient design of a combustor from the perspective of enhancing the gas–air mixing process.