Blowout Limit Research Articles

Prediction and Validation of Blowout Limits of Co-Flowing Jet Diffusion Flames—Effect of Dilution

The blowout limits of a co-flowing turbulent methane jet diffusion flame with addition of diluent in either jet fuel or surrounding air stream is studied both analytically and experimentally. Helium, nitrogen, and carbon dioxide were employed as the diluents. Experiments indicated that an addition of diluents to the jet fuel or surrounding air stream decreased the stability limit of the jet diffusion flames. The strongest effect was observed with carbon dioxide as the diluent followed by nitrogen and then by helium. A model of extinction based on recognized criterion of the mixing time scale to characteristic combustion time scale ratio using experimentally derived correlations is proposed. It is capable of predicting the large reduction of the jet blowout velocity due to a relatively small increase in the co-flow stream velocity along with an increase in the concentration of diluent in either the jet fuel or surrounding air stream. Experiments were carried out to validate the model. The predicted blowout velocities of turbulent jet diffusion flames obtained using this model are in good agreement with the corresponding experimental data.

The effects of hydrogen addition on the stability limits of methane jet diffusion flames

The flame stability limits of confined jet diffusion flames (JDFs) flowing into a co-axial oxidizing stream was studied both experimentally and analytically. Methane and hydrogen and their mixtures were used as the fuel. The experiments were conducted with two different jet diameters and within a wide range of co-flowing stream velocities and hydrogen concentrations in methane or air stream. The hydrogen diffusion flame was found to have a much larger region of stable operation than the methane JDF. Higher stability of the methane JDFs was achieved by the addition of hydrogen to either the jet fuel or the surrounding air stream. A hysteresis phenomena were observed in the reattachment process of lifted flames. It was found that the conditions prior to ignition of the flame, such as the value of co-flowing stream and jet velocities and position of the ignitor, have significant effect on the type of flame stabilization mechanism and flame blowout limits. The optimum ignition conditions for achieving higher blowout limits were investigated. The blowout limits of lifted JDFs were significantly affected by the velocity of co-flowing stream. The present study also reports the results of the calculation of the blowout limits of lifted JDFs using as a criterion the ratio of mixing time scale to characteristic combustion time scale. The agreement of the experimental and calculated data was satisfactory.

A parametric study of NO 2 emission from turbulent H 2 and CH 4 jet diffusion flames

Nitrogen dioxide (NO 2) emission levels of fuel jets were experimentally studied for H 2, H 2/He mixtures, a H 2/He/CH 4 mixture, and CH 4. The study was undertaken to understand the dependence of NO 2 emission in turbulent diffusion flames on parameters other than the widely known effects of rapid mixing. These parameters are fuel types (CH 4 vs H 2), the initial NO level, and the flame temperature. The fuel mixtures were chosen such that these factors could be investigated independently. In all the flames studied, NO 2/NO x increases with decreasing NO concentration in the flame and with decreasing adiabatic flame temperature. The CH 4 fuel demonstrates a qualitatively different influence on the NO 2/NO x ratio than H 2. Its effects are most pronounced when the flame blowout limit is approached. Adding a small amount of CH 4 to the H 2 flames also qualitatively affected the NO 2/NO x ratio.

Characteristics of a Trapped-Vortex Combustor

The characteristics of a Trapped-Vortex (TV) combustor are presented. A vortex is trapped in the cavity established between two disks mounted in tandem. Fuel and air are injected directly into the cavity in such a way as to increase the vortex strength. Some air from the annular flow is also entrained into the recirculation zone of the vortex. Lean blow-out limits of the combustor are determined for a wide range of annular air flow rates. These data indicate that the lean blow-out limits are considerably lower for the TV combustor than for flames stabilized using swirl or bluff-bodies. The pressure loss through the annular duct is also low, being less than 2% for the flow conditions in this study. The instantaneous shape of the recirculation zone of the trapped vortex is measured using a two-color PIV technique. Temperature profiles obtained with CARS indicate a well mixed recirculation zone and demonstrate the impact of primary air injection on the local equivalence ratio.

The Effects of Burner Geometry and Fuel Composition on the Stability of a Jet Diffusion Flame

The effect of the burner configuration and fuel composition on the stability limits of jet diffusion flames issuing into a co-flowing air stream is presented. Circular and elliptic nozzles of various lip thicknesses and aspect ratios were employed with methane as the primary fuel and hydrogen, carbon dioxide, and nitrogen as additives. It was found that the effects of nozzle geometry, fuel composition, and co-flowing stream velocity on the blowout limits were highly dependent on the type of flame stabilization mechanism, i.e., whether lifted or rim-attached, just prior to blowout. The blowout behavior of lifted flames did not appear to be significantly affected by a change in the nozzle shape as long as the discharge area remained constant, but it was greatly affected by the fuel composition. In contrast, attached flame stability was influenced by both the fuel composition and the nozzle geometry which had the potential to extend the maximum co-flowing stream velocity without causing the flame to blow out. The parameters affecting the limiting stream velocity were studied.

97/03065 Pulverizer air flow measurement aids combustion optimization

The blowout limits of a number of swirl-stabilized, nonpremixed flames were measured, and the observed trends are successfully explained by applying certain concepts that previously have been applied only to nonswirling flames. It is shown that swirl flame blowout limits can be compared to well-known limits for nonswirling simple diffusion flames by using the proper nondimensional parameter, i.e., the inverse Damkohler number (UFdF)(SL2αF). The fuel velocity at blowout (UF) was measured while four parameters were systematically varied: the fuel tube diameter (dF), the fuel type and thus reaction rate, which is related to the maximum laminar burning velocity (SL), the coaxial air velocity (UA), and the swirl number. Results show that for zero swirl, the blowout curves agree with curves predicted by previous analysis. However, as swirl is added, the flame becomes five times more stable (based on maximum fuel velocity). To explain the effect of swirl, a simple analysis is presented that is an extension of previous nonswirling flame blowout theory. It shows that the conventional swirl number is not the appropriate governing parameter. Instead, a Damkohler number based on swirl velocity is suggested by the analysis and is found to help collapse the data at the rich blowout limit to a single, general curve. Swirl causes a jet-vortex interaction; the recirculation vortex reduces the fuel jet velocity on centerline, which strongly stabilizes the lifted flame. As one increases the fuel tube diameter or the reaction rate (by adding hydrogen), the swirl flame becomes more stable, in a manner similar to a nonswirling flame. Another advantage of swirl is that it makes overall fuel-lean operation possible; the present flame is unstable without swirl for fuel-lean conditions.

Combustor Stability and Emissions Research Using a Well-Stirred Reactor

The design and development of low-emissions, lean premixed aero or industrial gas turbine combustors is very challenging because it entails many compromises. To satisfy the projected CO and NOx emissions regulations without relaxing the conflicting requirements of combustion stability, efficiency, pattern factor, relight (for aero combustor), or off-peak loading (for industrial combustor) capability demands great design ingenuity. The well-stirred reactor (WSR) provides a laboratory idealization of an efficient and highly compact advanced combustion system of the future that is capable of yielding global kinetics of value to the combustor designers. In this paper, we have studied the combustion performance and emissions using a toroidal WSR. It was found that the toroidal WSR was capable of peak loading almost twice as high as that for a spherical WSR and also yielded a better fuel-lean performance. A simple analysis based upon WSR theory provided good predictions of the WSR lean blowout limits. The WSR combustion efficiency was 99 percent over a wide range of mixture ratios and reactor loading. CO emissions reached a minimum at a flame temperature of 1600 K and NOx increased rapidly with an increase in flame temperature, moderately with increasing residence time, and peaked at or slightly on the fuel-lean side of the stoichiometric equivalence ratio. Finally, emissions maps of different combustors were plotted and showed that the WSR has the characteristics of an idealized high-efficiency, low-emissions combustor of the future.

Blowout Limits of a Jet Diffusion Flame in the Presence of Small Surrounding Jet Pilot Flames

The blowout limit of a methane jet diffusion flame is examined in the presence of a number of much smaller pilot jet flames of different fuels arranged within an experimental burner assembly in a co-flowing stream of air. It is shown that the blowout limit of the central jet flame can be extended very appreciably by increasing the flow rate through the smaller pilot jets. The basis for this extension to the blowout limit and the role of some changes in the operating parameters are discussed. It is suggested that the extension to the blowout limit observed is due mainly to the thermal contribution of the pilot jet flames.

Effect of Burner Geometry on the Blowout Limits of Jet Diffusion Flames in a Co-Flowing Oxidizing Stream

The effects of changes in the jet nozzle geometry, i.e., nozzle shape and lip thickness, on the blowout limits of jet diffusion flames in a co-flowing air stream were experimentally investigated for a range of co-flow air stream velocities. Circular and elongated nozzles of different axes rations were employed. Preliminary results showed that nozzles with low major-to-minor axes ratios improved, while high ratios reduced, the blowout limit of attached flames compared with that for an equivalent circular nozzle. The nozzle shape had no apparent influence on the blowout limits lifted flames and the limiting stream velocity. The experimental blowout limits of lifted flames were found to be a function of the co-flowing stream velocity and jet discharge area. On the other hand, the stability of attached flames was a function of the co-flowing stream velocity, jet discharge area as well as the nozzle shape. The effect of premixing a fuel with the surrounding air was also studied. Generally, the introduction of auxiliary fuel into the surrounding stream either increased or decreased the blowout limit depending on the type of flame stabilization mechanism prior to blowout. The stability mechanism of the flame was found to be a function of the co-flow stream velocity and the auxiliary fuel employed.

Shock-wave-enhancement of the mixing and the stability limits of supersonic hydrogen-air jet flames

A supersonic, non-premixed, jetlike flame was stabilized along the axis of a Mach-2.5 wind tunnel, and wedges were mounted on the sidewall in order to interact oblique shock waves with the flame. Schlieren photographs show how the interaction occurs, and measurements quantify how the flame length and the flame blowout limits are affected by the shocks. An optimum shock-interaction location was investigated by adjusting the wedge position. It was found that shock waves enhance the fuel-air mixing such that flame lengths decreased by 20% when an optimum shock location and shock strength were chosen. Enhanced mixing resulted, in part, because the shocks turn the flow and induce radial inflows of air into the fuel jet. A Mach disk sometimes occurs, which appears to split the reaction zone into two parts and severely distorts the flame shape. Substantial improvements in the flame stability (i.e., changes in the blowout limits) were achieved by properly interacting the shock waves with the flame-holding recirculation zone. The reason for the significant improvement in flame stability is believed to be the adverse pressure gradient caused by the shock, which can elongate the recirculation zone. Excessive shock strength (or poor shock placement) caused thermal choking to occur, and the flame base moved upstream of the fuel tube exit, leading to dangerously high wall heat transfer rates. Optimization of the mixing and stability limits requires a careful matching of the shock-flame interaction location and the shock strength. The experimental results show that the best mixing and stability correspond to 10° wedges placed at an upstream position (4 d F ) such that the primary shocks create radial inflow near the flame base and interact with the recirculation zone. This upstream wedge position also allowed the second (recompression) set of shocks to provide radial inflow near the flame tip.

An Analytical Examination of the Combustion of a Turbulent Jet in an Environment of Air Containing a Premixed Fuel or a Diluent

A simple analytical model for the mixing and combustion of an axisymmetric turbulent gaseous fuel jet discharging into a co-flowing streaming gaseous environment of an auxiliary fuel and/or a diluent homogeneously mixed with air is presented. A number of gaseous fuels and diluents are considered. It is shown that the combustion characteristics of a fuel jet can be modified significantly by the presence of a relatively small amount of a fuel in the surrounding air at concentrations well below the corresponding local flammability limits. Correlative procedures are presented for estimating changes in the flame length, the size of the combustion zone, and the blowout limits with changes in the type and concentration of the fuel in the surroundings. Predicted values showed generally good agreement with the corresponding experimental values.

Blowout Stability Limits Of a Hydrogen Jet Flame In a Supersonic, Heated, Coflowinq Air Stream

Abstract An extensive set of flame blowout limit curves has been measured for the case of a hydrogen jet flame surrounded by a heated, supersonic, coflowing air stream and some ideas are proposed to explain the observed trends. The stagnation temperature of the Mach 2.2 air stream was varied from 294 K up to the autoignition temperature of 900 K; hydrogen injection velocities were varied up to 1191 m/s. It was found that the flame blowout curves display two distinct stable regions which are bounded by the: (a) far-field blowout limit and (b) near-field blowout limit. Far-field blowout occurs after a flame first lifts off and is associated with a sufficiently large fuel velocity; the shape of the far-field blowout curve can be explained by previous subsonic lifted flame analyses. Near-field blowout is a sudden blowout for which no liftoff occurs; it results from a sufficiently large air velocity. In all cases the flame attachment point is in the shear layer at a distance of about 1 em from the fuel tube li...

An Experimental Investigation of the Blowout Limits of a Jet Diffusion Flame in Co-Flowing Streams of Different Velocity and Composition

The blowout limits of a methane diffusion flame in a co-flowing air-fuel or air-diluent stream were determined for a range of surrounding co-flow stream velocities, both laminar and turbulent, up to ~ 1.50 m/s. Methane, ethylene, propane and hydrogen were used as the fuels in the surrounding co-flow stream while nitrogen and carbon dioxide were used as diluents. The experimental results show that the velocity of the surrounding stream affects the blowout phenomena significantly. An increase in the stream velocity has a detrimental effect on the blowout limits at very low velocities up to 0.30 m/s (essentially laminar flow) and at velocities higher than 1.50 m/s (turbulent flow). The addition of a fuel to the air stream in most cases enhances the blowout limit of a methane diffusion flame. However, different trends in the variation of the blowout limits with the surrounding fuel concentration were observed, depending on the type of fuel used and on whether the surrounding coflow stream was laminar or turbulent. The addition of nitrogen or carbon dioxide to the air stream results in decreasing the blowout limits. The effect is more severe at the higher velocities.

Weak Extinction Limits of Large-Scale Flameholders

Weak extinction data obtained from an experimental apparatus designed to simulate the characteristics of practical afterburner combustion systems are presented. The apparatus supplies mixtures of varied composition (equivalence ratio and degree of vitiation), temperature and velocity to Vee-gutter flame holders of various widths and shapes similar to those found in jet engine systems. The fuel employed is a liquid hydrocarbon whose chemical composition and physical properties correspond to those of aviation kerosine, JP5. An equation for predicting weak extinction limits which accounts for upstream vitiation and the chemical characteristics of the fuel is derived from stirred reactor theory. The correlation between the predictions and experimental results indicates that the stirred reactor approach can provide a framework for predicting the lean blowout limits of practical flameholders over wide ranges of engine operating conditions.

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.

Simultaneous OH and Schlieren visualization of premixed flames at the lean blow-out limit

The structure of an air-propane premixed flame was studied experimentally at the lean flammability limit, using Schlieren photography synchronized with OH-imaging done with the Planar Laser Induced Fluorescence (PLIF) technique. The flame was studied in a wide range of fuel equivalence ratios. Various steps in the process of the flame destabilization were investigated, including partial lift-off, stable lift-off, and final blow-out conditions. The flame structure was visualized for each stage showing the transition from a flame held at the nozzle to a flame held by the flow structures. In order to study the latter conditions in more detail the flame was acoustically excited at the preferred mode frequency generating large, stable, coherent structures in the core region. The modified flame structure was visualized to understand the interaction between the flame and vortical flow dynamics. It is shown that for the flow conditions when the flame cannot be stabilized at the nozzle, a new anchoring point is reached at the location of the initial vortex roll-up in the jet shear layer. At this point the flow reversal and transition to turbulence produce stagnation points with relatively low local velocities and velocity gradients where the flame can be stabilized. When the flame jet is being forced at the jet most unstable frequency, large coherent structures are formed and the flame is stabilized intermittently on these vortices.

Blowout of nonpremixed flames: Maximum coaxial air velocities achievable, with and without swirl

The present study demonstrates how to optimize parameters in order to maximize the amount of coaxial air that can be provided to a nonpremixed jet flame without causing the flame to blow out. Maximizing the coaxial air velocity is important in the effort to reduce the flame length and the oxides of nitrogen emitted from gas turbines and industrial burners, a majority of which use coaxial air. Previous measurements by the latter two authors have shown that a sixfold reduction in the NO x emission index of a jet flame is possible if sufficient coaxial air can be provided without blowing the flame out. The coaxial air shortens the flame and forces the reaction zone to overlap regions of higher gas velocity, which reduces the residence time for NO x formation. The present work concentrates on demonstrating ways to prevent flame blowout when the following two constraints are imposed: (1) the coaxial air velocities must be sufficient to shorten the flame to a specified length (in order to reduce NO x emissions) and (2) the coaxial air flow rate must be sufficient to complete combustion without the need for ambient air, which is a common practical constraint. The zero swirl case is considered first, and the effects of adding swirl are measured and directly compared. The following were systematically varied: fuel velocity, air velocity, fuel tube diameter, air tube diameter, fuel type, and swirl number. Measurements demonstrate that coaxial air alone (with zero swirl) can cause up to a twofold reduction in flame length. However, the flame is stable only if the velocity-to-diameter ratio of the fuel jet does not exceed a critical value. It is found that the addition of swirl improves the maximum-air blowout limits by as much as a factor of 6. The results identify a strain parameter, based on the ratio of air velocity to air tube diameter ( U A d A ), which collapses the blowout curves for ten different conditions (burner size, swirl number) approximately to a single curve. A physical mechanism that explains the swirl flame data is presented. Swirl is believed to be beneficial because it reduces the local velocities, and thus the local strain rates, near the forward stagnation point of the recirculation vortex, where the flame is stabilized.

Blowout limits of turbulent jet diffusion flames for arbitrary source conditions

We present a formulation for the blowout limits of turbulent jet diffusion flames issuing from sources with arbitrary geometries and exit conditions into otherwise quiescent environments. It is argued that, while the liftoff characteristics of turbulent diffusion flames appear likely to be controlled by the straining out of flame sheets, the molecular mixing rate at the flame tip controls their blowout characteristics. The concept of a equivalent is introduced, and the local molecular mixing rate in the flow is expressed in terms of the associated far-field scaling laws. Blowout is expected when a resulting algebraic expression reaches a critical value. Results of a flip experiment verify the far-field equivalent source formulation. Measurements of the blowout limits over a range of geometries, fuels, and diluents show good agreement with the predictions from this formulation.

Enhancement of flame blowout limits by the use of swirl

The blowout limits of a number of swirl-stabilized, nonpremixed flames were measured, and the observed trends are successfully explained by applying certain concepts that previously have been applied only to nonswirling flames. It is shown that swirl flame blowout limits can be compared to well-known limits for nonswirling simple diffusion flames by using the proper nondimensional parameter, i.e., the inverse Damkohler number ( U F d F ) ( S L 2 α F ) . The fuel velocity at blowout ( U F ) was measured while four parameters were systematically varied: the fuel tube diameter ( d F ), the fuel type and thus reaction rate, which is related to the maximum laminar burning velocity ( S L ), the coaxial air velocity ( U A ), and the swirl number. Results show that for zero swirl, the blowout curves agree with curves predicted by previous analysis. However, as swirl is added, the flame becomes five times more stable (based on maximum fuel velocity). To explain the effect of swirl, a simple analysis is presented that is an extension of previous nonswirling flame blowout theory. It shows that the conventional swirl number is not the appropriate governing parameter. Instead, a Damkohler number based on swirl velocity is suggested by the analysis and is found to help collapse the data at the rich blowout limit to a single, general curve. Swirl causes a jet-vortex interaction; the recirculation vortex reduces the fuel jet velocity on centerline, which strongly stabilizes the lifted flame. As one increases the fuel tube diameter or the reaction rate (by adding hydrogen), the swirl flame becomes more stable, in a manner similar to a nonswirling flame. Another advantage of swirl is that it makes overall fuel-lean operation possible; the present flame is unstable without swirl for fuel-lean conditions.

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.