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The effect of the non-uniform profile of scalar variables, such a fuel at the upstream and temperature at the downstream of the flame zone was discussed theoretically to elucidate; (1) the deviation of motion from the steady state case and (2) the hysteresis of premixed flames response to the equivalence ratio oscillations seen in an experimental and numerical works. One-dimensional integral model for the non-uniform scalar variable profile with low frequency equivalence ratio oscillation has been developed. Here, the wavelength of the oscillation is assumed to be larger than the nominal flame thickness. Through the integral analysis, we obtained the relation of the flame propagation speed for steady and unsteady cases depending on the non-uniform scalar profile at the upstream and downstream of the flame zone. Hysteresis of the flame propagation speed is found due to the transport of fuel and heat by the non-uniform scalar profile at the upstream and downstream of the flame zone. This result qualitatively agreed with the numerical results of a response of the stagnation laminar CH4/air premixed flames for a low equivalence ratio oscillation frequency.

This study investigates the effect of fuel concentration oscillation on laminar stagnating premixed flames by both experiment and numerical simulation. The numerical analysis is conducted using ANSYS Fluent 14.5. The equivalence ratio oscillation in the experiments is formed by a novel oscillator with two cylinder piston units that can produce alternating ejections of leaner and richer pre-mixtures. Velocity fluctuation is well suppressed by installing screens on the burner exit. The fuel concentration oscillation between the stagnation plate and the burner exit is visualized and analyzed by acetone ultraviolet light-induced fluorescence in the isothermal condition. The oscillator frequency is varied in the range 2–20 Hz, and the oscillation wavelength is much longer than the flame thickness. The flame oscillates with the fuel concentration, and in the experiment, the amplitude of the flame oscillation attenuates as the frequency of fuel concentration oscillation increases above 5 Hz, which corresponds to a Strouhal number of unity. This indicates that the Strouhal number distinguishes quasi-steadiness for St St > 1. The flame oscillation pattern is a closed loop, which might be attributable to variation of the back support effect on the flame. The numerical results show a similar trend for the flame response to oscillations in fuel concentration. This study finds the flame motion is significantly affected by fuel concentration oscillations, even at low frequencies; in other words, the oscillation wavelength is much longer than the flame thickness, as a result of the back support effect.

Flame spreading over pure methane hydrate in a laminar boundary layer is investigated experimentally. The free stream velocity ( U ∞ ) was set constant at 0.4 m/s and the surface temperature of the hydrate at the ignition ( T s ) was varied between −10 and −80 °C. Hydrate particle sizes were smaller than 0.5 mm. Two types of flame spreading were observed; “low speed flame spreading” and “high speed flame spreading”. The low speed flame spreading was observed at low temperature conditions ( T s = −80 to −60 °C) and temperatures in which anomalous self-preservation took place ( T s = −30 to −10 °C). In this case, the heat transfer from the leading flame edge to the hydrate surface plays a key role for flame spreading. The high speed flame spreading was observed when T s = −50 and −40 °C. At these temperatures, the dissociation of hydrate took place and the methane gas was released from the hydrate to form a thin mixed layer of methane and air with a high concentration gradient over the hydrate. The leading flame edge spread in this premixed gas at a spread speed much higher than laminar burning velocity, mainly due to the effect of burnt gas expansion.

Responses of the premixed methane/air mixture flames under equivalence ratio oscillations were numerically investigated assuming axi-symmetric stagnation flow fields. The flame motion was numerically investigated at three different oscillation frequencies (10, 20 and 50 Hz) and with three oscillation cases namely: lean case, rich case and lean rich crossover case. Methane/air mixture with equivalence ratio oscillation was issued from the burner exit with 1.0 m/s uniform velocity profile. The effects of frequency and amplitude of the equivalence ratio oscillation were discussed. The amplitude of the equivalence ratio oscillation is attenuated between the burner exit and the upstream edge of the preheat zone. The attenuation is much significant for higher frequency. The amplitude of the flame temperature oscillation attenuates following the attenuation of the equivalence ratio oscillation. The flame location makes the closed cycle around the flame location of correspond equivalence ratio in the steady state condition. The formation of the cycle can be explained by the back support effect. It was further demonstrated that, the back support effect influences the dynamic response of the flame location, in that, the direction of the cycles of the dynamic response in the lean case and the rich case are different. Furthermore, the time variation of the flame location plays a significant effect to the flame displacement speed.

The effect of the equivalence ratio oscillation on a premixed laminar CH4/air flame motion was studied experimentally with equivalence ratio oscillation frequencies of 2 to 15 Hz at lean equivalence ratio using stagnation flow field burner. Novel oscillator does the oscillation conditions and turbulence reduction method is used to suppress the velocity perturbation. The flame position variations at 2, 5, 10 and 15 Hz oscillation frequencies were significantly small when the amplitude of the equivalence ratio oscillation was zero. On the other hand, increase in amplitudes of the equivalence ratio oscillation increased the flame position variation significantly. The flame moved in sinusoidal shape and it can be clearly seen that the flame movement’s amplitude was proportional to the amplitudes of the equivalence ratio variations. This result showed that the velocity perturbation is significantly suppressed by turbulence reduction method in the examination range.

Effects of burner rotation on the shapes and dynamics of premixed Bunsen flames have been investigated experimentally in normal gravity and in microgravity. Mixtures of CH{sub 4}-air and C{sub 3}H{sub 8}-air are issued from the burner tube with mean flow velocity U = 0.6 m/s. The burner tube is rotated up to 1400 rpm (swirl number S = 1.58). An oscillating flame with large amplitude is formed between a conical-shape flame and a plateau flame under the condition of Lewis number Le > 1 mixtures (rich CH{sub 4}-air and lean C{sub 3}H{sub 8}-air mixtures). In contrast, for Le = 1 mixtures (lean CH{sub 4}-air and rich C{sub 3}H{sub 8}-air), asymmetric, eccentric flame or tilted flame is formed under the same swirl number range. Under microgravity condition, the oscillating flames are not formed, indicating that the oscillation is driven by buoyancy-induced instability associated with the unstable interface between the hot products and the ambient air. The flame tip flickering frequency {nu} is insensitive to burner rotation for S 0.11, {nu} decreases linearly with increasing S. As S exceeds 0.11, a minimum value of axial mean velocity along the center line uj,m due to flow divergence ismore » found and it has a linear relationship with {nu}. This result shows that uj,m has direct control of the oscillation frequency. When S approaches unity, the flame oscillation amplitude increases by a factor of 5, compared to the flickering amplitude of a conical-shape flame. This is accompanied by a hysteresis variation in the flame curvature from positive to negative and the thermo-diffusive zone thickness varying from small to large. With S > 1.3, the plateau flame has the same small flickering amplitudes as with S = 0. These results show that the competing centrifugal and buoyancy forces, and the non-unity Lewis number effect, play important roles in amplifying the flame-tip oscillation.« less

Unsteady motions of a Bunsen-type premixed flame tip with burner rotation are experimentally investigatedfrom the viewpoint of nonlinear dynamics. The mean velocity from burner tube U is varied from 0.6 to 1.2 m/s, and the rotational speed of the burner tube N is varied from 0 to 2800 rpm. A rich methane/air mixture with the equivalence ratio of =1.43 is used. With the Lewis number Le larger than unity, an axisymmetric oscillating flame is formed between aconical flame and a plateau flame at U=0.6m/s and swirl number S=1.14, As U and N increase, but with S constant, the oscillating flame tip motion becomes unstable. This variation in the flame tip motion is shown qualitatively by drawing an attractor and evaluated quantitatively by estimating the correlation dimension. For U≤0.8 m/s, the attractor is a limit cycle and the correlation dimension Dc is estimated at about unity, indicating periodic motion. When U reaches 1.0 m/s, the trajectories of the attractor become rolled up slightly and Dc approaches about 2, indicating quasi-periodic flame tip motion. With a further increase in U, the attractor becomes much more complicated and Dc, is estimated as a non-integer value, indicating a deterministic chaos. These results indicate that the flame tip motion with the burner rotation under the condition of Le>1 varies from periodic to non-periodic (i.e., to chaotic). The present results also show that an analysis based on deterministic chaos theory, such as the correlation dimension, is valid for quantifying the motion of unsteady flames.

Stabilizing mechanism of a methane-air edge flame that is a partially premixed flame, is investigated experimentally using the axisymmetric wall jet burner. Measurements of stability limits, flame locations and velocity fields with PIV, and visualization of themal boundary layer around of the edge flame have been performed. As a result, it is found that Peclet number Pe, which is expressed as Pe= Ut× pe/D, where Ut is tangential velocity toward the edge flame, Pe is curvature radius of thermal boundary layer of the edge flame and D is mass diffusivity, shows constant value at stability limits. The Pe physically means the ratio between partially premixing time to form flammable mixing layer and residential time of a fluid element in partially premixing region. Therefore, the result obviously indicates that the edge flame's stability is determined due to the balance between the partially premixing time and the residential time in partially premixed region. Thus, it is clear that a partially premixing, which is absent in a pure premixed flame, is important for stabilizing mechanism of the edge flame in the wall jet.

The relation between blowoff characteristics of the edge flame in a methane-air diffusion flame and its flame structure has been investigated by using our original burner. The burner can form an edge flame without premixed flame, as a hole, in the stagnation region of an axisymmetric impinging jet. Varying the hole diameter, blowoff limits and maximum flame temperature were measured for an edge flame's blowoff character, and also, for an edge flame's structure, temperature profile and flame location were measured and a thermal boundary layer around the edge flame was visualized with laser tomographic technique. It is found that all the edge flames in the stagnation flow have a critical stagnation velocity gradient, beyond which the flame can never be existed. The critical stagnation velocity gradient that represents the overall reaction rate in the edge flame zone decreases as the hole diameter is increased. The increase in a hole diameter leads to change of the edge flame's structure and to addition of two heat loss factors to the edge flame. One heat loss factor is the edge flame-wall interaction, which occurs due to decrease in the edge flame's location. Another is penetration of cold flow into a hole in the flame, which occurs due to decrease in the overlapping range of thermal boundary layer of the edge flame in the hole. These additional heat losses occur at lower stagnation velocity gradient and have stronger influence on the edge flame as a hole diameter becomes larger. Consequently the overall reaction rate in the edge flame zone is reduced by the increase in a hole diameter. Finally it is clarified that the edge flame shows qualitatively same extinction as a pure diffusion flame, when the flame zone of the edge flame lies spatially as the boundary that divides between oxidizer side and fuel side in spite of existence of a partially premixture of fuel and oxidizer ahead of the reaction zone in the edge flame region.

Responses of stagnating laminar methane/air premixed flames under fuel concentration oscillation, i.c., equivalence ratio oscillation, were numerically investigated using a one-step overall reaction mechanism and a four-step reaction mechanism that included CO and H2 formation. The flame motion was numerically investigated for three different oscillation cases namely: lean, rich and lean-rich crossover case. Methane/air mixtures with sinusoidal equivalence ratio oscillations were issued from the burner exit with uniform 1.0 m/s velocity profiles. In the steady state condition, the one-step overall reaction mechanism and the four-step reaction mechanism had nearly the same characteristics in the lean region, while in the rich region variations in characteristics such as flame location and flame displacement speed for the four-step model were much more significant than those for one-step model. When the equivalence ratio was oscillated, the flame location oscillated. The amplitude of the flame location oscillation did not change with the equivalence ratio oscillation when the frequency of equivalence ratio oscillation was less than 8Hz, while it decreased monotonically when it exceeded 8 Hz. Here 8 Hz corresponds to a Strouhal number (St) of unity. Thus, this result indicates that the flame was in a quasi-steady state when St1.0. The variation in flame location and the flame displacement speed did not follow those for the steady state condition and made a limit cycles. This was due to the back support effect. The cycles were significantly inclined at higher frequencies. In the lean condition, the limit cycle was inclined similarly for both the one-step and four-step reaction mechanisms. In the rich condition, however, the limit cycle for the four-step reaction was more inclined than that for the one-step reaction. These results show that the formation of CO and H2 played an important role in the rich condition.

Transition processes from the edge flame without premixed flame to the edge flame with premixed flame, the triple flame, of methane-air edge flame are experimentally investigated by using the novel burner system. The burner system forms a diffusion flame in an axisymmetric wall jet. The measurements of the stability limits and the flame locations, and the observation of the flame shapes have been performed. The burner can make three types of the stable edge flame. First one is the edge flame without premixed flame, whose location is determined by the wall interaction with the edge flame, that is, the thermal factor. Second one is the edge flame with premixed flame, whose location is still determined by the thermal factor. Third one is the edge flame with premixed flame, and the location is determined by a balance between the propagation rate of the edge flame and the local velocity of unburnt mixture, that is, the fluid dynamical factor. The change of the dominant factor for flame location is due to the increase in the partially premixing region ahead of the edge flame. The increase in the partially premixing region makes it possible to form premixed flame and the first edge flame changes into the second one. Further increase in the partially premixing region lead to an increase in a total amount of the heat release around the edge flame region and the flow field ahead of the edge flame diverges. Finally, the edge flame is lifted up by the flow divergence and the dominant factor to determine the edge flame location changes from the thermal factor to the fluid dynamical factor.

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.

The effect of unsteady stretch on the wall-stagnating lean methane, premixed flame was investigated experimentally. The influence of the oscillatory stretch on the flame speed was examined when the stagnation wall was oscillated sinusoidally in the direction of the main flow at a constant amplitude. Using LDV, the velocities of unburned gas and of flame in laboratory coordinate were measured to obtain flame speed (propagating velocity relative to unburned gas). The oscillatory characteristic of the flow field is indicated by the Strouhal number, representing the ratio of the angular frequency of the wall times the mean wall location to the flow velocity at the nozzle exit. There exist two factors dominating the oscillatory stretch, that is, the change of the distance between the wall and the nozzle, which is a quasi-steady factor, and the change of the velocity of the wall, which is an unsteady factor. When the flow is oscillated at low Strouhal number (≤1), the quasi-steady factor, which is independent of the Strouhal number, is dominant. And the oscillation of the flame speed is not observed. In this case, quasi-steady factor is not large enough to oscillate the flame speed since the ratio of the amplitude of the wall oscillation to the mean distance between the wall and the nozzle is small. On the other hand, when the flow is oscillated at high Strouhal number (≤1), the unsteady factor, which is proportional to the Strouhal number, becomes dominant. In this case, the oscillation of the flame speed is observed and the amplitude of the oscillation of flame speed increases with increasing Strouhal number.

A stability limit of the diffusion flame with fuel injection from a porous wall in a laminar flat plate boundary layer is measured as functions of fuel (CH4) concentration of CH4/N2 injectant mixture (x) and its injection velocity (v). The free stream velocity (U∞) is set as 0.6 m/s. The thermal condition at the wall is controlled by setting temperature at the upstream end of the porous wall as a reference temperature. When v>20 mm/s, the flame becomes unstable with the separation of leading flame edge with decreasing x. The value of x at the stability limit is constant without regard to v as long as the wall temperatrure is kept constant. As the wall temperature is decreased the value of x increases. The separation is supposed to take place as a result of the limit of the reaction rate. When v

Extinction conditions and flame structures for methane-air turbulent non-premixed flames are investigated experimentally for a counterflow nozzle-type burner system. Extinction limits are measured by varying fuel concentrations diluted by nitrogen, mean, flow velocities of both burners, and turbulent characteristics generated by perforated plates. In particular, the flow turbulence of each burner is controlled individually. The extinction limits for the same fuel concentration can be expressed by approximately the same bulk velocity gradient. At the condition of lean fuel concentration or high intensity of flow turbulence, the flame strength is weak. It is shown that the flame strength is influenced by the turbulence of the air stream rather than that of the fuel stream. The flame shape was observed by a laser tomographic technique under various conditions. Because the methane-air counterflow non-premixed flames in the present study are formed in the air stream, the diffusion region can be visualized as the vanishing strip of seeding particles between near flames and the stagnation plane. In the case of laminar flames, there is no difference of mean flame locations, or the width of the diffusion region, for a certain range of fuel concentration. In the case of turbulent non-premixed flames, it is also observed that relative flame locations in the diffusion region are almost the same, and the width of the diffusion region is not changed for various turbulent flow conditions. It is shown that the diffusion region containing turbulent flames keeps the structure, as laminar flamelets.

Effects of heat loss and flame stretch on the lean-limit extinction of laminar premixed flames in a stagnation point flow have been studied experimentally. The bulk stretch rate was varied from 40s-1 to 420s-1, while the Lewis number (Le) was varied from 0.85 to 1.8. The extinction of the lean propane/air flame (Le>1) is mainly caused by the flame stretch. On the other hand, the extinction of lean methane/air and lean methane/CO2-O2 flames (Le

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.

The burning intensity of inverted flame base has been studied by using Infrared Rays Emission Computed Tomography. The results substantiate the general concept of the influence of flame stretch and preferential diffusion on the burning intensity. Specifically it was found that the burning intensity increases (decreases) towards the inverted flame base for mixtures whose effective Lewis number is smaller (greater) than unity: this behavior is qualitatively opposite to that observed for regular Bunsen flame tip because of the reversed flame curvature. It is further demonstrated that a heat loss to the stabilizing rod and a flow acceleration toward the flame base appear to have very small influences on the flame behavior, while flame curvature exerts strong influence on these phenomena.

The temperature has been measured in a field in which laminar hydrogen (6.0 m/s at the center of a burner exit) was injected into a co-flowing air stream (6.0 m/s) with some kinds of grid turbulence. It is shown that the ratio of a laminar flame length to the turbulent one is proportional to the turbulent Reynolds number, Reι, (integral scale is considered a characteristic length) in the measured extent (Reι≤1000), and therefore that the integral scale of a reactant flow is an important parameter for the flames in this paper. The width of a flame displacement is estimated from the temperature profiles when the flame is assumed to be a wrinkled laminar flamelet. The width is also measured directly by a flow visualization method. Both the widths obtained by the two different methods agree well with each other. This fact indicates that the turbulent flame in this paper can be treated as a wrinkled laminar flamelet.

Axial, radial and tangential velocities were measured using an LDV in a field in which a laminar hydrogen jet (6.0 mls at the center of a burner exit) was injected into a co-flowing air stream (6.0 m/s) with or without grid turbulence. The conditional sampling based on the origin of the jet and the co-flowing stream was applied. The interaction between grid turbulence and the flame is discussed in detail. It is found from the radial profile of the mean velocity that with the grid the jet and the coflowing stream are mixed intermittently. Inside the flame the jet flows inward and the co-flowing stream flows outward. The profiles of axial and radial turbulent intensities of the co-flowing stream have a peak near the flame and at this location the histograms of velocity are bimodal. This suggests wrinkled laminar f1amelets; that is, the fuel, the air and the f1amelets are mixed intermittently. Moreover, the mean axial/radial velocities of the air and the f1amelets are different from each other but the mean t...

Numerical analysis on a aerodynamic structure of a laminar boundary layer diffusion flame over a horizontal flat plate has been done, taking into account a local pressure gradient in a boundary layer. The calculation were carried out, using a 'Simpler' algorithm. a flame sheet approximation was applied to describe the combustion process. When the flame is formed, the pressure field in the boundary layer is varied locally and the stream wise pressure gradient is formed even if the effects of gravity of of acceleration of the free stream velocity are not taken into account. This streamwise pressure gradient induces the volocity overshoot. This shows that the flame formation plays an important role on the pressure variation in the boundary layer and the velocity overshoot. Furthermore, the numerical calculation, taking into account the effects of gravity and the free stream accerelation in the present equations, shows good agreement with the corresponding experimental results.

An aerodynamic structure of a laminar boundary layer over a flat plate with uniform fuel injection from the flat plate and with diffusion flame is investigated numerically. Elliptic type conservation equations are used to take into account the pressure variation within the boundary layer. Velocities and the pressure are solved numerically by ‘SIMPLER’ algorithm. One step irreversible chemical reaction of methane is assumed. An Arrhenius type chemical reaction rate model is assumed and the pre-exponential factor is varied from 1.0 × 1012 to 1.0 × 1030 m3/(kg ▪ s) as a parameter of the reactivity in order to elucidate the effect of the reactivity on the structure of the boundary layer. When the chemical reaction is very fast, the leading edge of the reaction zone reaches the flat plate. As the chemical reaction rate is decreased with a decrease in the pre- exponential factor, the leading edge of the reaction zone parts from the flat plate and it shifts downstream. The fuel is injected in front of the leading edge of the reaction zone, where the air is dominant, and the oxygen penetrates into the fuel dominant zone through the region between the leading edge and the flat plate. As a consequence, a premixed gas is formed around the leading edge of the reaction zone. The premixed gas seems to react in the region apart from the main visible flame.