Flame Top Research Articles

Experimental investigations on the jet flow field propagation characteristics and ignition mechanisms of stoichiometric CH4/O2/N2 premixed turbulent jet flames

The jet flow field (JFF) and flame propagation of stoichiometric CH4/O2/N2 mixtures under variable thermodynamic conditions were investigated experimentally using Schlieren and natural flame luminosity imaging techniques. A two-color method by wavelength integration was used to characterize the temperature field. The turbulent flame velocity (ST), Damköhler number (Da), Reynolds number (Re), and jet flame regime were explored by a combination of experimental data and theoretical calculations. The flame morphology shows that the propagation process can be subdivided into three stages: first, the JFF enters the main combustion chamber, then the flame propagates in the JFF, and finally, the JFF disappears after the flame front and the JFF front coincide. The vortex motion changes the thin reaction zone and the local high temperature region distribution. The normal jet shifts to the Mach disk jet at higher oxygen content. The ignition location changes from the jet tail to the shear layer. The top secondary vortex is more likely to generate at higher ambient temperatures, which enhances the ignition probability on the flame top. In addition, the degree of orifice blockage determines the jet flame type. Since the initial jet flame is more affected by orifice blockage, the variation in ST is strongly influenced by the change in the length-scale ratio. As the jet flame gradually moves away from the orifice, ST is mainly governed by turbulence intensity. Finally, a strong correlation between the Re number and Da number of the jet flame was found.

Blow-off mechanism of a holder-stabilized laminar premixed flame in a preheated mesoscale combustor

In a previous study, we fabricated a well-designed mesoscale combustor with a flame holder and preheating channels to improve the flame stability under extreme combustion conditions. This combustor can also act as a model for studying the synergistic effect of heat and flow recirculation on the flame dynamics. This paper systematically illustrates and discusses the blow-off mechanisms of a holder-stabilized laminar premixed flame in a preheated mesoscale combustor via three-dimensional numerical simulation. The flame blow-off processes that have been observed experimentally at various Reynolds number (Re) are well reproduced. Pinch-off appears at large Re but is absent at small Re. First, the moving characteristics of the flame root and flame top in the blow-off process at small Re are discussed in detail. Then, the flame pinch-off mechanisms are carefully analyzed in terms of the flow field, conjugate heat exchange, and stretch effect. The results indicate that the influence of conjugate heat exchange, which exists between the flame holder and flame root, is not significant in the flame pinch-off phenomenon, but a larger recirculation zone (which helps to stabilize the flame root) and flame stretch (which splits the flame) both induce flame pinch-off. This work can aid in gaining insight into the flame blow-off dynamics when the effects of heat and flow recirculation work synergistically in many practical combustors.

PLIF measurements of instantaneous flame structures and curvature of an acoustically excited turbulent premixed flame

In this work, we experimentally measured the instantaneous flame structure and curvature of a methane fueled premixed turbulent acoustically-excited flame in a longitudinal combustor. For this, we implemented simultaneous formaldehyde (CH2O) and hydroxyl (OH) planar laser-induced fluorescence (PLIF) and high-speed schlieren imaging techniques to quantify the dynamic response of this acoustically-excited flame, which was fixed at 260 Hz. The Reynold number of the jet flow was varied from 1216 to 8108. Both imaging techniques revealed that the vortex appeared at the flame top in the presence of acoustic excitations. As the jet velocity increased, the jet flame was squeezed dramatically along the axial direction. The CH2O-PLIF is found to successfully capture the evolution of vortex shedding process. The vortex appears as a sharp angle, when the acoustically-excited flame is squeezed. The vortex size is then increased gradually over an acoustic fluctuation period. To measure the heat release rate from the flame, we used the pixel-by-pixel product of OH×CH2O as a quantitative index. Based on the experimentally measured CH2O, OH and their product characterizing the heat release, it is shown that the more the flame is squeezed by the acoustic waves, the more CH2O is produced and the more unsteady heat is released. Finally, the characteristics of the jet flame curvature were investigated. The variation of the probability density function of the flame curvature revealed that the acoustic excitation with a proper phase could smooth the flame front.

Effect of thermal condition of solid wall on the stabilization of a preheated and holder-stabilized laminar premixed flame

A novel combustor with a flame holder and preheating channels was developed to investigate the synergistic effect of heat and flow recirculation on combustion characteristics. This paper mainly focuses on the thermal condition of flame holder and combustion chamber wall (CCW) and their interaction on flame stabilization. Based on the distribution characteristics of temperature and flow fields, it can be deduced that the interaction between the flame holder and CCW can further increase the incoming fresh mixture temperature and decrease the height of recirculation zone, which subsequently affect the anchoring locations and temperatures of the flame root and top. To be more specific, the flame holder and CCW synergistically improve the preheating temperature of fresh mixture, making it higher than simple linear superposition. Moreover, the anchoring location of the flame root is mainly determined by the heat exchange of flame holder, but CCW matters more significantly for the flame top. Additionally, the anchoring temperature of the flame root is higher than that of the flame top. This study can help us to gain insights into the effect of conjugate heat exchange on flame stability when the effect of heat recirculation and flow recirculation works synergistically in practical combustors (like aero-engine).

Study of the influence of water vapour and carbon dioxide dilution on flame structure of swirled methane/oxygen-enriched air flames

In this work, the effect of dilution with water vapour H2O and carbon dioxide CO2 on the structure and stability of a methane/enriched air premix flame, confined and swirled at atmospheric pressure was studied. Measurements were carried out at constant adiabatic temperature (1773, 1873, 1973 and 2073 K), from air to oxycombustion (enrichment OI ranging from 21 to 100%), for two inlet temperatures (T0 = 373 K and 473 K), at constant equivalence ratio maintained at 0.91 and inlet aerodynamic conditions maintained constant: average inlet velocity at 30 m·s−1 and geometric swirl number at Sn = 0.90. The experimental setup is a burner used consisting of a swirled stainless steel single injector, mounted in a combustion chamber operating at atmospheric pressure. The flames were visualized by CH* chemiluminescence. From instantaneous images, the flame contour, detected with a Matlab® processing, was determined. From this flame contour, flame height (Hf) and lift-off height (Hlo) were extracted by measuring respectively the maximum height of the flame top and the minimum height of the bottom flame. The evolution of the mean macrostructure of the flame was then studied. The results of the steam dilution were detailed and compared with those obtained with carbon dioxide. Links between the laminar flame speed and the flame structure and stability were clearly established.

Anchoring mechanisms of methane/air premixed flame in a mesoscale diverging combustor with cylindrical flame holder

A mesoscale diverging combustor with a cylindrical flame holder is developed to improve the flame stabilization in present work for the first time. The experimental results show that the flame in the combustor with holder can remain symmetrically stable within a quite wide range of equivalence ratio. Then, the flame-anchoring mechanisms are analyzed systematically in terms of flow structure, conjugate heat exchange and preferential transport effect via numerical simulation. As the flame root and top attach to the flame holder and combustor, it can be deduced that the flow boundary layer very near the wall has the anchoring effect on the flame, so does the conjugate heat exchange. On the one hand, the upstream flame holder and combustor wall can preheat the incoming unburned mixture via conducting heat from the downstream solid walls, which provides a beneficial ignition condition. On the other hand, there are flame-wall coupling effects between the flame root/top and flame holder/combustor wall providing the suitable locations for flame anchoring. The anchoring locations of flame root and top show the almost same sensibility responding to the thermal conductivity of solid material. However, the anchoring temperature of flame top is more sensitive than that of flame root. In addition, the local equivalence ratios at the locations of the flame root and top are lower than the corresponding incoming equivalence ratio, which means that a better heat recirculation effect is not good for the flame-anchoring if only considering the preferential transport effect. In summary, the addition of flame holder can remarkably improve the flame stabilization and suppress the unstable flame in the combustor. This is beneficial for the application of the combustor with the diverging structure.

G601 渦糸ソリトンによる高速火炎伝播メカニズムの検証(GS6 反応を伴う流れ,一般セッション)

In order to verity the high-speed flame propagation mechanism by a vortex filament soliton, 3-D numerical simulation was carried out. In the 3-D visualization of vortex line and heat release rate distributions, the helical vortex line was seen near the flame top. Furthermore, in the 1-D analysis along the vortex line, the peak of curvature (the solitary wave like the vortex filament soliton) and the peak of heat release rate (the flame zone) overlapped and propagated together along the vortex line. This result also proved the validity of the original idea of high-speed flame propagation mechanism proposed by Shinoda et al. (1993).

G603 渦糸ソリトンによるマイクロ・スワール燃焼器の安定性向上(GS6 反応を伴う流れ,一般セッション)

The axisymmetrical 2-D numerical simulation of a cylindrical-shape micro swirl combustor was carried out. By the 3-D visualization inside the combustor and the 1-D analysis along a certain vortex line, the helical vortex line with the peak of curvature (the solitary wave like the vortex filament soliton) was observed near the flame top. This result showed the effectiveness of the vortex bursting mechanism by the vortex filament soliton in the micro swirl combustor. Therefore, by changing the axial and azimuthal gas flow velocities at the combustor inlet, the size of solitary wave and the flame propagation speed could be controlled, and the design of a high-load and high-stability micro swirl combustor might become achievable.

Experimental study of limit lean methane/air flame in a standard flammability tube using particle image velocimetry method

Lean limit methane/air flame propagating upward in a standard 50 mm diameter and 1.8 m length tube was studied experimentally using particle image velocimetry method. Local stretch rate along the flame front was determined by measured gas velocity distributions. It was found that local stretch rate is maximum at the flame leading point, which is in agreement with earlier theoretical results. Similar to earlier observations, extinction of upward propagating limit flame was observed to start from the flame top. It is stated that the observed behavior of the extinction of the lean limit methane/air flame can not be explained in terms of the coupled effect of flame stretch and preferential diffusion. To qualitatively explain the observed extinction behavior, it is suggested that the positive strain-induced flame stretch increases local radiation heat losses from the flame front. An experimental methodology for PIV measurements in a round tube is described.

Aerosol behavior in chromium waste incineration

Cr 2O 3 is considered as the dominant incineration product during the combustion disposal of chromium waste. A hydrogen/air diffusion flame was employed to simulate the industrial process of incineration. Cr 2O 3 aerosols were generated inside the flame by the gas phase reaction of chromium and oxygen. Chromium came from the rapid decomposition of chromium hexacarbonyl (Cr(CO) 6) at room temperature and was carried into the combustion chamber by hydrogen. Aerosol and clusters can then be easily formed in the flame by nucleation and coagulation. A two dimensional Discrete-Sectional Model (DSM) was adopted to calculate the Cr 2O 3 aerosol behavior. The experimental measurement method was Dynamic Light Scattering. The numerically predicted results agreed well with those of the experimental measurement. Both results show that the Cr 2O 3 aerosol size reached about 70 nanometers at the flame top.

Experiments on the instability modes of buoyant diffusion flames and effects of ambient atmosphere on the instabilities

Large scale dynamic behavior of buoyant diffusion flames were studied experimentally. It was found that buoyant diffusion flames originating from circular nozzles exhibit two different modes of flame instabilities. The first mode results in a sinuous meandering of the diffusion flame, characteristic of flames originating from small diameter nozzles. This instability originates at some distance downstream of the nozzle exit in the contraction region of the buoyant flame envelope and develops into a sinuous motion of the flame. The second mode is the varicose mode which develops very close to the nozzle exit as axisymmetric perturbations of a contracting flame surface. In this mode, flame oscillations result in the formation of toroidal vortical structures that convect through the flame and cause periodic burn out at the flame top resulting in the observed flame height fluctuations. The average flame heights are found to be typically shorter for these flames. The oscillation frequencies and their scaling for the two modes are also different with the sinuous mode having higher frequencies than the varicose mode. It was also observed that the instability can switch from one mode to the other and the probability of observing the varicose mode appears to increase with increasing Richardson number. Additionally, the feasibility of altering the behavior of buoyant diffusion flames was explored through variation of the oxidizer medium density. It was found that the flame oscillations can be completely suppressed for flames burning in helium rich helium–oxygen mixtures. At lower helium concentrations, the oscillation frequency can be significantly reduced. In order to enhance the buoyancy effect, CO2–O2 mixtures were also studied. However, the density increase and its effects on flame oscillation frequency were found to be small compared to those flames burning in air. These experiments point towards the feasibility of altering buoyant flame behavior under earth gravity and studying the large scale dynamical aspects of buoyant flames without the need of variable gravity environment.

Radical Emissions and Anomalous Reverse Flames Appearing in Upward-Increasing Magnetic Fields

Upward-increasing magnetic fields of lower intensities increased flame dimensions and liberations of carbons, and decreased radical emissions, temperatures and a bluing tendency of the flame. These phenomena were opposite to those occurring in the upward-decreasing magnetic fields. The upward-increasing magnetic fields of stronger intensities caused an anomalous inverse burning, where the positions of the blue and the yellow-orange regions are reversed, and the highest temperature of the flame appeared at flame surfaces distant from the flame top. The above phenomena are ascribable to the fact that the flame shapes and the flow directions of combustion products, air and the flame were changed due to the magnetic forces originating from magnetic gradient fields, thereby resulting in changes in the mixing and the diffusion conditions.

Visible structure of buoyant diffusion flames

Natural gas diffusion flames stabilized on 0.10, 0.19 and 0.50 m. diameter porous bed burners have been studied for heat release rates ranging from 10 to 200 kW. Flame heights were measured from video tape recordings and by eye averaged techniques. The dependence of flame height on a dimensionless heat addition parameter shows a transition for values of the parameter around unity. For flames taller than three burner diameters, the initial diameter of the fire does not affect the length of the flame whereas for short flames, initial geometry becomes important. Another prominent feature of these flames is the presence of large scale axisymmetric structures which are formed close to the burner surface with more or less regular frequency and which rise through the flame region. These structures are responsible for the fluctuations of the flame top and strongly influence the geometry of the flame.

Entrainment in the Near and Far Field of Fire Plumes

This paper describes entrainment measurements made in fire plumes with a new technique. Measurements were in plumes rising from natural gas diffusion flames stabilized on 0.10, 0.19 and 0.50 m diameter burners and the heat release rates ranged from 10 to 200 kW. The heights examined ranged from elevations starting very close to the burner surface to distances about five times the average flame heights. Experiments indicate the presence of three regions: a region close to the burner surface where plume entrainment rates are independent of the fuel flow (or heat release) rates; a far field region above the flame top, where a simple point source model correlates the data reasonably well; and an intermediate region where entrainment appears to be similar to that of a turbulent plume.

Structure of Bunsen flames

The structure of Bunsen cone flame tops is studied using the thermal diffusion flame model. Assuming a strong temperature dependence of the reaction rate, the shape of the flame front as well as the temperature distribution at the front are found. It is shown that for the case in which the Lewis number of the limiting component is greater than unity, the front is a smooth surface with a radius of curvature at the flame top substantially exceeding the width of the preheating zone. Towards the flame top, the temperature in the reaction zone increases along with the velocity of the flame relative to the gas. When the Lewis number of the limiting component is less than unity, the temperature at the front decreases towards the flame top, the velocity of the flame relative to the gas drops, and the flame is extinguished at distances close to the top.