Blow-off Limit Research Articles

The stability of turbulent hydrogen jet flames with carbon dioxide and propane addition

Abstract In the study the lift-off, blow-out and blow-off stability limits of hydrogen/propane flames and hydrogen/carbon dioxide flames were tested in three different mixing arrangements. The first was to premix hydrogen with carbon dioxide or propane to form a jet flame. The second was to add the gas as an annular jet around the hydrogen flame. The third was to inject into the centre of the hydrogen flame. Propane and carbon dioxide have the same density but create very different chemical kinetic changes when added to hydrogen flames. The results showed that when premixed with hydrogen, propane is more effective in flame lift-off and blow-out. The analysis of kinetic mechanisms revealed that the propane is the dominating fuel in determining the burning rate of the hydrogen/propane while carbon dioxide mainly acted to dilute the hydrogen/CO 2 mixture. Comparing the three mixing arrangements, the experiments showed that hydrogen flame can be effectively lifted or blown out when gases were in annular flow around the hydrogen flame. The isothermal mixing process of the co-flow configuration was discussed.

Experimental studies of flame stability limits of CNG–air premixed flame

Abstract The stability aspects of laminar premixed CNG–air flames are investigated experimentally. Bunsen burners with two different port diameters, such as 12 and 15 mm, are employed to characterize the blowoff and flashback limits of the CNG–air premixed flames. For the two burners, the peak flashback limits occurs at a fuel–air ratio slightly richer than stoichiometric. However, the flashback limits are enhanced with increase in diameter of the burner, but the blowoff limit increases with increase in fuel concentration. Similar trends in increasing blowoff limits are obtained with further increase in burner diameter. The stability plots for burners with two different port diameters have been established, which can be used for designing and developing CNG/air combustion systems.

Determining factor for the blowoff limit of a flame spreading in an opposed turbulent flow, in a narrow solid-fuel duct

Abstract This study clarified the blowoff mechanism for a flame spreading in an opposed turbulent flow in narrow solid fuel ducts. To clarify this mechanism, two experiments were conducted. The first experiment was to investigate the influence of ambient pressure and fuel duct size on the blowoff limit. The results indicated that the flow velocity at the point when blowoff occurred, V g , t , increased with ambient pressure. This tendency could not be confirmed by a well-known expression for the Damkohler number, which is defined as the ratio of the characteristic flow time to the characteristic chemical time. Subsequently, to clarify the determining factor for the blowoff, the second experiment, which observed the flow field near the flame leading edge, was conducted. The results show that the flow separation in front of the flame leading edge, which provided sufficient residence time of oxidizer and gaseous fuel, is necessary for the flame to spread in an opposed oxidizer flow. From the results, it is found that the oxidizer friction velocity, u ∗ , which is an indicator of the turbulent momentum transfer, is the determining factor for the flame blowoff limit. When the friction velocity is larger than a critical value, flame blowoff occurs in the fuel duct, due to the absence of flow separation.

Active control of lifted diffusion flames with arrayed micro actuators

Active control of a lifted flame issued from a coaxial nozzle is investigated. Arrayed micro flap actuators are employed to introduce disturbances locally into the initial shear layer. Shedding of large-scale vortex rings is modified with the flap motion, and the flame characteristics such as liftoff height, blowoff limit, and emission trend, are successfully manipulated. Spatio-temporal evolution of large-scale vortical structures and fuel concentration is examined with the aid of PIV and PLIF in order to elucidate the control mechanisms. It is found that, depending on the driving signal of the flaps, the near-field vortical structures are significantly modified and two types of lifted flames having different stabilization mechanisms are realized.

The influence of pressure on the control of premixed turbulent flames using an electric field

Previous investigations of the effects of electric fields on flames have shown the potential for stabilizing flames and reducing emissions with comparatively little effort, but were restricted to atmospheric pressure. In the present work the influence of the electric field on premixed turbulent jet flames at increased pressure is investigated. Besides the question of whether field effects persist at elevated pressure, it is of interest to find physically based scaling laws. The current work describes experiments with premixed turbulent seven-hole Bunsen-jet flames for pressures between 1 and 10 bar, where the exit velocity was held constant, and where electric fields of varied strength and direction were applied to the flame. Concentrations of CO, NO, and NO 2 were measured in the exhaust gas section. Experiments show that the electric field influence is clearly visible for increased pressures, without any indication that 10 bar should be an upper limit. CO emissions could be reduced by about 95%, irrespective of pressure. The decrease of CO was accompanied by an increase of NO x by about 25%. Both of these effects can be understood qualitatively within the framework of a one-dimensional model. For reduced voltages up to 3.5 kV/bar the model correctly describes the current–voltage characteristics and leads to the conclusion that high pressure should favor rather than hamper electric field effects on flames. The electric power required for a CO reduction of 95% amounted to 0.1% of the thermal power. The improvement of the lean blowoff limit upon application of an electric field observed so far ranges from 1 to 3% and increases with pressure.

Experimental Study of Surface and Interior Combustion Using Composite Porous Inert Media

Combustion using silicon carbide coated, carbon–carbon composite porous inert media (PIM) was investigated. Two combustion modes, surface and interior, depending upon the location of flame stabilization, were considered. Combustion performance was evaluated by measurements of pressure drop across the PIM, emissions of NOx and CO, and the lean blow-off limit. Data were obtained for the two combustion modes at identical conditions for a range of reactant flowrates, equivalence ratios, and pore sizes of the PIM. Results affirm PIM combustion as an effective method to extend the blow-off limit in lean premixed combustion.

801 低カロリーガス燃焼火炎の安定限界(G06-1 燃焼,G06 熱工学)

This paper describes flame stability limits of low calorific gas fuel combustion for two types of fuel flow to the combustion air, one is a parallel fuel injection and another is a cross fuel supply. The experimental results showed that the blow-off stoichiometric ratios for the former are higher than those for the latter, and that the logarithmic Damkohler numbers at blow-off limit exist on one line for each fuel gas injection pattern. It was also shown that the gradient of the line for the logarithmic Damkohler numbers at blow-off limit increased with an increase in the flame stability.

A comparison of computational and experimental lift-off heights of coflow laminar diffusion flames

As a sensitive marker of changes in flame structure, the number densities of excited-state CH (denoted CH ∗), and excited-state OH (denoted OH ∗) are imaged in coflow laminar diffusion flames. Measurements are made both in normal gravity and on the NASA KC-135 reduced-gravity aircraft. The spatial distribution of these radicals provides information about flame structure and lift-off heights that can be directly compared with computational predictions. Measurements and computations are compared over a range of buoyancy and fuel dilution levels. Results indicate that the lift-off heights and flame shapes predicted by the computations are in excellent agreement with measurement for both normal gravity (1 g) and reduced gravity flames at low dilution levels. As the fuel mixture is increasingly diluted, however, the 1 g lift-off heights become underpredicted. This trend continues until the computations predict stable flames at highly dilute fuel mixtures beyond the 1 g experimental blow-off limit. To better understand this behavior, an analysis was performed, which indicates that the lift-off height is sensitive to the laminar flame speed of the corresponding premixed mixture at the flame edge. By varying the rates of two key “flame speed” controlling reactions, we were able to modify the predicted lift-off heights so as to be in closer agreement with the experiments. The results indicate that reaction sets that work well in low dilution systems may need to be modified to accommodate high dilution flames.

MODELING GRAVITY EFFECT ON DIFFUSION FLAMES STABILIZED AROUND A CYLINDRICAL WICK SATURATED WITH LIQUID FUEL

The familiar flames from candles and oil lamps are representative examples of wick-stabilized diffusion flames. The shape and burning characteristics of these flames depend strongly on gravity-induced buoyant flows. To obtain a better understanding of the gravity effect, a diffusion flame model has been formulated and numerically solved. In the computation, gravity is treated as a parameter, spanning from zero to the high blowoff limit. The model includes realistic wick and candle geometry but assumes that the wick surface is coated with liquid fuel. The gas-phase formulation consists of the full Navier–Stokes equations with a one-step finite-rate overall chemical reaction. Gas-phase flame radiation, important in low gravity near the quenching limit, is included by solving the axisymmetric radiation transfer equation using the S N discrete ordinates method. The computed results include flame and flow structures, heat flux distributions, the energy budget, and the overall burning characteristics as a function of gravity and wick diameter.

Direct numerical simulation of hydrogen-enriched lean premixed methane–air flames

The effect of hydrogen blending on lean premixed methane–air flames is studied with the direct numerical simulation (DNS) approach coupled with a reduced chemical mechanism. Two flames are compared with respect to stability and pollutant formation characteristics—one a pure methane flame close to the lean limit, and one enriched with hydrogen. The stability of the flame is quantified in terms of the turbulent flame speed. A higher speed is observed for the hydrogen-enriched flame consistent with extended blow-off stability limits found in measurements. The greater flame speed is the result of a combination of higher laminar flame speed, enhanced area generation, and greater burning rate per unit area. Preferential diffusion of hydrogen coupled with shorter flame time scales accounts for the enhanced flame surface area. In particular, the enriched flame is less diffusive-thermally stable and more resistant to quenching than the pure methane flame, resulting in a greater flame area generation. The burning rate per unit area correlates strongly with curvature as a result of preferential diffusion effects focusing fuel at positive cusps. Lower CO emissions per unit fuel consumption are observed for the enriched flame, consistent with experimental data. CO production is greatest in regions which undergo significant downstream interaction. In these regions, the enriched flame exhibits faster oxidation rates as a result of higher levels of OH concentration. NO emissions are increased for the enriched flame as a result of locally higher temperature and radical concentrations found in cusp regions.

SOFCとスターリングエンジンコンバインドシステムの研究 : 起動用燃焼器の性能試験(スターリングエンジン及び関連要素と応用システム(1))

Blow-off limit and unburned gas concentrations were measured for a mult-hole combustor designed for start-up of 4kW class SOFC-Stirling engine combined system. The blow-off limit strongly depended on the preheat temperature and the equivalence ratio and the released heat at the blow-off limit was insufficient for the 4kW system if the operation at higher combustion temperature is avoided to supresss the NOx emission level. The concentrations of CO2 and CH4 at the combustion temperature of 1700K showed the tendency to increase toward the radial direction and with the increase of the flow velocity. Also, the higher preheat temperature showed the lower concentrations of unburned gas. The unburned gas concentrations were still high at 1700K and the higher combustion temperature is needed to improve the combustion efficiency.

Flame Stability Limit and Exhaust Emissions of Low Calorific Fuel Combustion in Turbulent Diffusion Combustor for a Small-Scale Fuel Cell

This paper describes an investigation conducted on flame stability and exhaust emissions from a turbulent diffusion combustor, fueled with low-calorific gas, for a small-scale fuel cell. It is important to maintain flame stability in the combustor, even under lean fuel conditions, and to suppress CO emission in the exhaust gas. An imitation off-gas, in which hydrogen and methane were diluted by adding nitrogen, with Wobbe indices ranging from ca. 4 400-8 700, corresponding to the fuel utility ratio of 90%-60% in the fuel cell, was supplied to the combustor, and the blow-off limits, CO, and NO x emissions were experimentally investigated. The results show that the blow-off excess air ratios increases with an increasing Wobbe index and with decreasing fuel input to the combustor, and that they are proportional to the hydrogen concentration in the fuel to the power of 0.5-1.0. In addition, it was found that the Damkohler numbers at blow-off limits decreased with decreasing fuel input and with increasing Wobbe indices, and that the product of (S S /V M )A[H 2 ][O 2 ] 0.5 was constant at blow-off limits. Furthermore, NO x emissions from the combustor were low, less than 20 ppmV (O 2 = 0%), it was also found that the apparent activation energy of NO x emission derived from Arrhenius plots was almost equal to that of prompt NO in the combustion of imitation off-gas.

FUNDAMENTAL SCALING EFFECTS OF ACETYLENE AND NATURAL GAS FREE-JET FLAMES

The aerodynamic and turbulent structures of combusting, partially premixed free-jet flames, with varying size and for different fuels at constant equivalence ratio, are compared. The first fuel, natural gas, produced predominantly blue flames and was investigated using laser Doppler anemometry measuring techniques for three round-jet exit nozzle diameters to a distance X = 40 diameters downstream. Each jet had the same exit velocity, which was limited only by the blowoff limit. A similar set of experiments was repeated with acetylene, a fuel of higher calorific value producing highly radiative yellow flames. Constant exit velocities were maintained for all cases; in this case the higher flame speed of acetylene allowed higher values of exit velocity to be used. The natural gas flames were then revisited and the flow was seeded with fine coal particles of constant mean diameter and their effect on the flow dynamics was explored using particle image velocimetry for acetylene jet flames it was found that mean velocities scale to X/D = 10, whereas fluctuating velocities are seen to increase across the jet from just beyond the nozzle exit. Normalized flame length and potential core decrease with increasing scale. Self-similarity was seen for mean velocity and different nozzle diameters, less so for fluctuating velocities. Relaminarization of the flames occurred between X/D = 5 and 10. Compared to acetylene, natural gas flames produce self-similarity to greater distances downstream due to longer flame lengths. They also have lower fluctuating velocity levels and exhibit only a slight increase in large-scale turbulence. Comparison of results with other studies shows that induced turbulence into the jet at small scale may be needed to simulate larger scale systems. Finally, the addition of fine coal particles to natural gas flames is shown to delay vorticity formation and mixing, the latter increasing with nozzle diameter.

Experimental investigations and laser based validation measurements in a gas turbine model combustor

A swirled CH4/air diffusion flame has been operated in a gas turbine model combustor at the lean blow-off limit. Laser Doppler velocimetry, planar laser-induced fluorescence of OH and CH, and laser Raman scattering were applied in order to measure velocities, flame structures, and the joint probability density functions of major species concentrations, mixture fractions and temperatures. The main goals of the investigations were a better understanding of the complex physical and chemical processes in flames with swirl, e.g., mixing, stabilisation, heat release, and finite-rate chemistry effects, and the generation of an extensive data base for the validation of numerical flame codes.

Stability characteristics and flowfields of turbulent non-premixed swirling flames

A simple, yet representative, burner geometry is used for the investigation of highly swirling turbulent unconfined, non-premixed, flames of natural gas. The burner configuration comprises a ceramic faced bluff-body with a central fuel jet. The bluff-body is surrounded by an annulus that delivers a swirling primary flow of air. The entire burner assembly is housed in a wind tunnel providing a secondary co-flowing stream of air. This hybrid bluff-body/swirl burner configuration stabilizes complex turbulent flames not unlike those found in practical combustors, yet is amenable to modelling because of its well-defined boundary conditions. Full stability characteristics including blow-off limits and comprehensive maps of flame shapes are presented for swirling flames of three different fuel mixtures: compressed natural gas (CNG), CNG–air (1:2 by volume) and CNG–H2 (1:1 by volume). It is found that with increased fuel flow, flame blow-off mode may change with swirl number, Sg. At low swirl, the flame remains stable at the base but blows off in the neck region further downstream. At higher swirl numbers, the flames peel off completely from the burner's base. Swirling CNG–air flames are distinct in that they only undergo base blow-off. In the low range of swirl number, increasing Sg causes limited improvement in the blow-off limits of the flames investigated and (for a few cases) can even lead to some deterioration over a small intermediate range of Sg. It is only above a certain threshold of swirl that significant improvements in blow-off limits appear. Six flames are selected for further detailed flowfield and composition measurements and these differ in the combination of swirl number, primary axial velocity through the annulus, Us, and bulk fuel jet velocity, Uj. Only velocity field measurements are presented in this paper. A number of flow features are resolved in these flames, which resemble those already associated with non-reacting swirling flows of equivalent swirl obtained with the present burner configuration. Additionally, asymmetric flowfields inherent to some flames are revealed where the fluidic centreline of the flow (defined in the two-dimensional (U–W velocity pair) velocity field by the ⟨ω⟩ = 0 tangential velocity contour), meanders strongly on either side of the geometric centreline downstream by about one bluff-body diameter. Flow structures revealed by the velocity data are correlated to flame shapes to yield a better understanding of how the velocity field influences the flames physical characteristics.

동축 확산 부상화염의 Blow-off와 연소 특성

An experiment was performed to investigate lift-off, blow-off and combustion characteristics of a lifted coaxial diffusion flame according to fuel jet and air velocity. A jet diffusion flame which is attached on the nozzle rim begins to be lifted with increase of air velocity, and finally becomes blow-off at higher air velocity. In experiment, blow-off limit increased with increase of fuel jet velocity, however lift-off occurred at lower air velocity. Flame structure and combustion characteristics were examined by schlieren photos, temperature distributions and emisions concentration distributions. Flame temperature became higher at midstream and its RMS became larger at up and downstream with increase of air velocity. Local NO concentration decreased but CO₂ concentration increased with increase of air velocity, which shows combustion reaction becomes close to be stoichiometric at higher air velocity in spite of lift-off.

A numerical analysis of ignition to steady downward flame spread over a thin solid fuel

A numerical analysis using an unsteady combustion model is presented to study the ignition and subsequent downward flame spread over a thermally thin solid fuel in a gravitational field. The solid-fuel temperature rises gradually in the heat-up stage and the pyrolysis becomes more intense. Ignition, including the induction period and thermal runaway, occurs as soon as a flammable mixture is formed and the gas-phase temperature, heated by the solid fuel, becomes high enough. During the induction period, the reactivity and temperature in the gas phase are mutually supportive. The thermal runaway consists of a burning premixed flame as the flow moves with the flame front. This is followed by a transition from a premixed flame into a diffusion flame. The flame front extends along and toward the upstream virgin fuel as the diffusion flame is formed. Finally, steady flame spread takes place as burnout appears. The ignition delay time is found to be controlled mainly by the time required to form the flammable mixture and is almost independent of the gravity level and the ambient oxygen index. The ignition delay time increases nearly linearly with an increase in solid-fuel thickness within the range of 0.005\\,{\\rm cm}\\le {\\bar \ au}\\le 0.02\\,{\\rm cm} and is proportional to ({\\bar Q}_{\\max})^{-1.11} within 2\\,{\\rm W/cm}^2\\le\\bar Q_{\\max}\\le 8\\,{\\rm W/cm}^2 . The steady downward flame-spread rate decreases with increases in the gravity level or fuel thickness and with decreases in the ambient oxygen index but is independent of the incident peak heat flux. The blowoff limit is around 6.7\\,{\\bar g}_{\\rm e} and the extinction limit is found to be Y O X = 0.131.

선회도가 평면화염버너의 유동과 연소 특성에 미치는 영향

In this study, the flow and combustion characteristics of flat flame burner with twirler were investigated. There are several factors that define the characteristics of burner. Among them, the experiments was focused on swirl effect by four types of twirler in terms of flow structure, distribution of temperature and emission characteristics. In PIV(Particle Image Velocimetry) experiment, the less of swirl number, axial flow is dominant at the center. As swirl number increases, the flow develops along the burner tile and backward flow becomes stronger at center. From the combustion characteristics, as long as combustion load increases, blow-off limit was improved. But at the higher swirl number, the limit is decreased. At swirl number 0, the temperature is shown typical distribution of long flame burner. but swirl number increases, the temperature distribution is uniform in front of round tile. Therefore, the temperature distribution is coincided with flow structure. As excess air ratio increases, NO concentrations are high. But high swirl number gives rise to become low NO concentrations. The flame characteristics are comprised in wrinkled laminar-flame regime according to turbulence Reynolds number(Rel) and Damkohler number(Da).

On solid trioxane combustion in stagnation point flows

Response, flame structure, and extinction limits of solid trioxane (C 3 H 6 O 3 ) combustion in stagnationpointflows are computationally studied with detailed chemistry, transport properties, and radiation representation. While a solid surface radiation model addresses emission and absorption by the surface, a narrowband radiation model, with carbon dioxide, carbon monoxide, and water vapor as the gas-phase participating media, is employed to describe the gas-phase and surface radiation is presented over the flammable regime, with emphasis on the low-stretch regine of the radiatively participating flames. When only surface radiation is included, two extinction limits exist, namely the blow-off limit and the low-stretch radiative limit, and the burning rates and maximum flame temperatures are lower than those of the adiabatic counterpart, as expected. With the inclusion of surface and gas-phase radiation, results show that, while flame temperatures are even lower, the burning rate of the trioxane diffusion flame may actually exceed the adiabatic limit and increase at low stretch rate due to radiative feedback from the flame to the surface. Reaction pathways leading to trioxane oxidation are also analyzed. High-temperature and lowtemperature reaction pathways identified previously in homogeneous kinetics studies are sustained in heterogeneous trioxane combustion. Furthermore, potential effects of radical recombination at the solid surface are assessed and discussed.

Blowoff Characteristics and Flame Structure of Edge Flame in the Stagnation Flow.

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.