Flame Stabilization Mechanism Research Articles

On burner-stabilized cylindrical premixed flames in microgravity

The structure and response of the curved but unstretched cylindrically symmetric one-dimensional premixed flame generated by a cylindrical porous burner has been studied using (1) activation energy asymptotics with one-step reaction and constant properties, (2) numerical computation with detailed chemistry and transport, and (3) drop-tower microgravity experimentation. The study emphasizes the relative importance of heat loss (to the burner surface) vs flow divergence as the dominant mechanism for flame stabilization, the possibility of establishing a one-dimensional, adiabatic, unstretched, premixed flame in microgravity, the influence of curvature on the upstream and downstream burning rates of the flame, and the relation of these burning rates to those of the inherently nonadiabatic flat-burner flame as well as the freely propagating adiabatic planar flame. Results show that, with increasing flow discharge rate, the dominant flame stabilization mechanism changes from heat loss to flow divergence, hence demonstrating the feasibility of establishing a freely standing, adiabatic, one-dimensional, unstretched flame. It is further shown that, in this adiabatic, divergence-stabilized regime in which the burner discharge flux exceeds that of the adiabatic planar flame, the downstream burning flux is equal to the (constant) burning flux of the adiabatic planar flame while the upstream burning flux exceeds it, and the upstream burning velocity exhibits a maximum with increasing discharge rate. Based on the property of the downstream burning flux, it is also proposed that the laminar burning velocity of a combustible can be readily determined from the experimental values of the burner discharge rate and flame radius. Microgravity results on the flame radius compare favorably with the computed values, while the corresponding laminar burning velocity also agrees well with that obtained from independent numerical computation.

The mechanisms of flame stabilization and low NOx emission in an eccentric jet pulverized coal combustor

The mechanisms of flame stabilization and low NOx emission features of an eccentric jet pulverized coal combustor were studied through numerical modelling and experimental investigation. The results show that the formation of the unique flowfield structure is closely related to the interaction among combustor configuration, the primary jet and the control jet; and that certain rules should be followed in order to obtain the optimum condition for flame stabilization. The distributions of temperature and concentrations of NO, O2, CO and CO2 inside the combustor were experimentally measured. The effects of structural and operational parameters on combustion and NO formation were studied. It was found that reduction of primary air, suitable use of control jet and reasonable uptilt angle of the primary jet all contributed to the reduction of NOx at the combustor exit. A new hypothesis, that reasonable separation of oxygen and fuel within the fuel-rich zone is beneficial to further reduction of NOx emission, is given. The study showed that good compatibility existed between the capability of flame stabilization and low NOx emission for this type of combustor.

Behavior of the lifted jet flame under acoustic excitation

The importance of large coherent vortical structures on lifted jet flame stability under acoustic excitation was extensively demonstrated in this investigation. By employing visualization and digital image processing methods, the most probable stabilization locations of a lifted flame base in the hysteresis region were found to be located at the roll-up and pairing positions of the coherent vortices. The flame stabilization mechanism in the hysteresis region was identified by phase-averaged concentration and velocity measurements. Phase-averaged results show that the strong entrainment induced by the large vortices during roll-up and pairing processes plays a key role in preventing the lifted flame from propagating upstream and causes the hysteresis phenomenon. Modification of lifted flame stability by acoustic excitation was demonstrated to be feasible. Acoustic excitation at frequencies in the turbulence amplification region of the cold jet is helpful in enhancing the flame stability in the high exit velocity region where the flame base zone is stabilized downstream of the end of the potential core; such excitation could extend the blowout limit by more than 25%. Excitation at frequencies in the turbulence suppression region is helpful when the flame is stabilized upstream of the end of the potential core.

An analysis of flame stabilization mechanism in radiation burners

An analysis was executed to reveal the flame stabilization mechanism in the surface combustion burners (porous radiant burners) which can emit a high intensity of thermal radiation depending on the flow velocity as well as the equivalence ratio of combustible mixture. Numerical calculations have shown some detailed behaviors of flame with respect to time for understanding the stabilization mechanism and have also illustrated the criteria of the three critical limits of blowoff, flashback and extinction correlated to the flow velocity, flame structure and thermal radiation propagation. These results have all been successful more or less for predicting the empirical behaviors of flames stabilized around the porous surface for a wide range of working parameters.

Flame Stabilization by a Transverse Jet

Abstract The present paper reports the results of an experimental study of flame stabilization by a jet of air, normal to the flow of homogeneous propane-air mixture. The objective of the present work was to study in detail the mechanism of flame stabilization by a transverse jet. The experiments were carried out in a 40 mm × 40 mm square combustion tube equipped with schlieren quality glass windows. Critical jet flow from a nozzle was used to stabilize the flame. The basic characteristics of the system were determined: —the flame stabilization process was recorded on shadowgraph pictures taken by means of a high speed camera, —the area of intense mixing process behind the stabilizing jet was denned by the method of gas analysis, —the turbulence intensity field was determined in gas flowing without combustion, —the flow picture behind the jet interacting with the flowing mixture was visualized by means of suspended threads. The experimental results obtained were used to describe the mechanism of flame sta...

A mixedness-reactedness flamelet model for turbulent diffusion flames

A novel, non-equilibrium, combustion model is proposed, to simulate turbulent diffusion flames, based on a laminar flamelet approach that was originally developed for premixed combustion. The model embodies a mixedness factor to describe the mixture strength and a reactedness parameter to describe the degree of completeness of combustion. The effect of straining is accounted for in prior laminar flame computations and the imposition of a known distribution of flame straining on the flamelets. Profiles of laminar heat release rate against temperature and the assumed independence of fluctuations in mixedness, reactedness and flame straining enable a simple evaluation of turbulent mean heat release rate to be made. This approach is used to predict numerically the field solutions for lifted, turbulent jet, methane-air, diffusion flames, in conjunction with the k −e turbulence model. The computations predict an approximately linear relationship between lift-off height and fuel jet velocity, in good agreement with available experimental data. Results suggest that the mechanism of flame stabilization is associated with the complex interactions between convection, turbulent mixing, heat release rate under strain and thermal expansion, rather than with a single phenomenological parameter such as, for example, the flame strain extinction limit.

The investigation on the mechanism of the flame stabilization in the combustor with flat inlet jet

This paper resents a newly developed aerodynamic technique for combustion improvement, Flat Inlet Jet Combustion, which is suitable for gaseous fuel, liquid spray fuel, and powder fuel (especially pulverized coal). It is based on the Coanda Effect, which causes the flow to veer and attach to a wall, and forms a recirculation zone on both sides of the main forward flow. These recirculation zones are of different strengths, one is small and the other is large with strong turbulence and mixing. The inertia of the fuel particles carries these particles into the large high-temperature recirculation zone where they are rapidly ignited, creating stable flames and intense combustion. The resulting flow in the combustor also produces no ash or slag accumulation in the combustor. LDV studies were made in models of the flat jet combustor; numerical modeling corroborated the experimental results. Two-phase flow tests, measurement of the distribution of fuel particle density, and the laboratory combustion tests also revealed the fine properties of the method. Because its usefulness, reliability, and simplicity have been demonstrated in industrial installations, some utility boiler operators in China are using the flat jet combustor to startup and partload flame stabilization and low grade coal combustion. This enables them to burn coal instead of the large quantity of oil presently used for these purposes.

Particle Dispersion and Pulverized Coal Combustion in a Single-Sided Backward-Facing Step Flow.

Experiments were performed in a backward-facing step flow to study the flame stabilization mechanism. Two types of nozzles, square-shaped and baffle plate added on the nozzle exit, were used in these experiments. The baffle plate changed the step flow in the following three ways. (1) Near the nozzle exit, the width of recirculating flow was decreased, and the corner eddy was expanded to the main stream. (2) The reversing velocity of gas and solid spheres was increased, and recirculation gas was well mixed with the main stream. (3) Particles slipped against the air flow due to the impingement of particles on the baffle plate. The baffle was effective in accelerating the coal ignition and stabilizing the coal flame under high-speed conditions. Furthermore, streamwise velocity of coal particles in a flame was higher than solid particles in a cold flow stream. Streamwise and transverse components of coal particle turbulent velocities were decreased in the main stream side of the mixing layer.

Flame-Turbulence Interactions in a Freely-Propagating, Premixed Flame

Abstract Previous experimental investigations of turbulence measurements in conventional laboratory premixed flames are extensively reviewed. Because of numerous disadvantages of these configurations, primarily associated with flame stabilization mechanisms, a freely-propagating flame configuration is proposed for the study of flame-turbulence interactions. The unperturbed, unrestrained flow field in this configuration is ideal for measuring the effect of the flame on turbulence properties. Temporally resolved, ensemble averaged measurements of turbulence intensity, integral length scale, integral time scale, and energy spectrum have been measured both normal and parallel to the mean flame surface with laser Doppler velocimetry (LDV) in a propane-air flame at an equivalence ratio of 1.0, in a flow with incident turbulence intensity of 25 cm/s and length scale of 8 mm. The following changes in turbulence parameters were observed across the flame: a factor of five to six increase in the normal turbulence in...

Flame structure and stabilization in a divergent flow. Eddy structure behind a circular cylinder in a cold divergent flow.

In order to elucidate the effects of positive pressure gradient on flame structure and stabilization mechanism, it is first necessary to clarify those on turbulence characteristics and eddy structure not only in a cold shear layer, but also in a cold wake flow behind a bluff body. In this study, the influences of pressure gradient in the flow direction on the surface pressure around a circular cylinder and the eddy-shedding process are investigated in a two-dimensional cold air flow without combustion. Flow visualization and an FFT analysis of the oscillating surface pressure are made to examine time-averaged and fluctuating properties concerning the eddy-shedding process. It is found that increasing the free stream pressure gradient causes a transition of the wake flow pattern from an alternative eddy-shedding type to a symmetric one without Karman vortex. An increase in the pressure gradient extends the wake region, and simultaneously lowers the pressure drag force, which may be regarded as one of the most important characteristics of the divergent flow.

Coflowing turbulent jet diffusion flame blowout

We present results from an experimental and theoretical investigation into the blowout mechanism in turbulent diffusion flames. The blowout stability limits of coflowing turbulent jet diffusion flames are formulated in terms of a recently proposed flame stabilization mechanism based on the large scale organization of entrainment and mixing observed in turbulent shear flows. In contrast to the linear similarity scaling of the more commonly studied simple turbulent jet flames, the nonlinear scaling of coflowing turbulent jets allows an essential element of this stabilization mechanism to be investigated. Results show that when the flame stability criterion is evaluated for the last large structure in the flame as is consistent with the underlying physical picture for this stabilization mechanism, a large reduction in the blowout limit is expected for even a small coflow velocity. This phenomenon is experimentally verified and good quantitative agreement is demonstrated with a set of measurements for the blowout limits of such coflowing turbulent jet flames.

Flame stabilization and turbulent exchange in strongly swirling natural gas flames

An experimental study has been made in the near field of a variable swirl burner under isothermal and reacting conditions in order to quantify the effect of combustion on the isothermal flow field, and to explain the mechanism of flame stabilization due to internal recirculation zones. A two colour Laser-Doppler velocimeter was used to determine turbulence properties in-/and outside of the recirculation zone. Temperature and species concentrations were measured by conventional measuring techniques. Turbulent exchange coefficients for momentum, matter and heat have been obtained by measurements and by calculations from time mean value distributions, respectively. The results show that, with strong swirl, the intensity of recirculation is reduced by the presence of combustion due to a marked decrease of the effective swirl number. Divergent burner nozzles at the burner exit lead to a radial extension of the reverse flow zone, but do not affect the reverse flow density. Reynolds number similarity of the diffusion flame is confirmed as long as reaction kinetic effects are negligible. On the other hand, blow-off occurs as a result of a reduction of local residence time in the ignition zone when increasing burner load. LDA-measurements exhibit high turbulence intensities and strong turbulent fluxes in the region of the stabilization zone. All turbulent fluxes agree with the gradient law, so that no countergradient diffusion exists under the conditions studied. The presence of combusion leads to a damping of turbulent exchange as compared with the isothermal flow of same swirl intensity.

Experimental investigation on the stabilization mechanism of jet diffusion flames

Two contrary concepts have been suggested in order to explain the mechanism of flame stabilization based on premixed and diffusion flamelet combustion respectively. To contribute to the understanding of which model represents the true stabilization mechanism, two different natural gas jet diffusion flames, at exit velocities between flame detachment and blow-off, were investigated. The measured profiles of gas composition and velocity around the stabilization zone were used to derive the rate of mixing and fuel burnout. The results show that, for the flames investigated, about forty to fifty percent of the total fuel flow is already mixed at a molecular level upstream of the flame stabilization zone. This mixture then reacts over a very short distance, supporting the concept of premixed combustion in lifted jet diffusion flames.

Experimental Study on Flame Stabilization Mechanism behind Cylindrical Rods : Relations between Pressure Distribution around Rods and behavior of Flame

Experimental Study on Flame Stabilization Mechanism behind Cylindrical Rods : Relations between Pressure Distribution around Rods and behavior of Flame

Mechanism of Flame Stabilization for Gas Burners of Radial Injection Type with a Bluff Body

Mechanism of Flame Stabilization for Gas Burners of Radial Injection Type with a Bluff Body

The mechanism of flame stabilization in an inhomogeneous mixture by a blunt body

This article examines the flame stabilization mechanism with a nonuniform distribution of the fuel in the flow before the stabilizer and the extent of fuel-concentration equalization in the circulation zone. Discusses what values of the excess-air coefficient in the circulation zone in terms of the vaporized fuel are responsible for stability of the combustion when there are nonuniformities in the circulation zone (local ones or averages over the zone). Finds that with any distribution of the mixture composition along the stabilizer, the decisive role for flame stabilization is played by regions in the circulation zone with the highest combustion temperature.

Stabilization of spray flames in a high-tempeature stream

The structure of the flame front and the mechanism of flame stabilization were studied for spray flames stabilized in a high-temperature stream. The flux of the fuel droplets and the composition of the interdroplet gas were measured in flames using a specially designed sampling probe and analyzing system. The rate of entrainment of the surrounding stream into a spray was determined by the flow visualization technique. The hydrocarbon gas produced by evaporation of the droplets around the spray boundary plays a major role in the process of flame stabilization. Therefore the convective heat flux entrained through the spray boundary is a principal parameter governing the location of the flame front. Spray flames are stabilized in a high-temperature stream principally by the flame propagation mechanism. The flame front, therefore, approaches the injection nozzle far more closely than would be expected from the ignition delay data.

Flame propagation in laminar boundary layers

This paper presents the results of an experimental program aimed at providing a critical test of the equilibrium theory of flame stabilization in boundary layers previously proposed by the senior author. Using schlieren photography and a particle-track technique, the characteristics of a two-dimensional flame stabilized on a cylindrical rod and propagating into a laminar boundary layer adjacent to a flat plate are investigated. The results presented include the velocity distribution of the unburned mixture in the free stream, the velocity profile in the boundary layer, and the experimental burning-velocity profiles for three different plate temperatures. The latter profiles are compared with the adiabatic profiles computed on the basis of a previous theoretical analysis, and the effect of quenching due to the presence of the flat plate is discussed. The steps which lead to the experimental burning-velocity profiles are carefully examined and compared with available theoretical results. In each case the comparison adds weight to the experimental findings. Finally, it is shown that the burning-velocity profiles are indeed affected by the growth of the thermal boundary layer when the flat plate is heated, and that these profiles assume shapes postulated in the theoretical study of the mechanism of flame stabilization. Quantitatively, the flame-holding characteristics of a smooth surface predicted by the use of these measured profiles agree with reported experimental results.

On the mechanism of bluff body flame stabilization at low velocities

Previous workers on bluff body laminar flame stabilization have observed, under a wide variety of circumstances, flame sheets that were wrinkled into ‘nodal’ waves with well-defined wavelengths and velocities of propagation. In this paper, experiments are described that explain the observed wave forms. It is believed that they are forced, unstable, propagating, interfacial waves—forcing being caused by the oscillation of the point of attachment of the flame in the boundary layer and wake surrounding the flameholder. Spark shadowgraphs are presented to support this conclusion. Data are presented to show the dependence of the frequency of oscillation of flow velocity, air/fuel ratio, and sphere diameter. Flameholding phenomena and the formation of ‘envelope’ flames are also discussed.

Some experimental techniques for the investigation of the mechanism of flame stabilization in the wakes of bluff bodies

This report describes experimental techniques developed at the Applied Physics Laboratory, The Johns Hopkins University, for investigating the mechanism of flame stabilization in the wakes of bluff bodies. The powder techniques previously used by other investigators for visualization of flow in Bunsen burners have been extended to much higher velocities by the use of new illumination sources. Flow patterns at high velocities about baffles in ducts with and without combustion have been recorded using this technique. High speed motion pictures by schlieren and shadow