Blow-off Limit Research Articles

Combustion dynamics and control: Progress and challenges

Combustion dynamics constitutes one of the most challenging areas in combustion research. Many facets of this subject have been investigated over the past few decades for their fundamental and practical implications. Substantial progress has been accomplished in understanding analysis, modeling, and simulation. Detailed laboratory experiments and numerical computations have provided a wealth of information on elementary dynamical processes such as the response of flames to variable strain, vortex rollup, coupling between flames and acoustic modulations, and perturbed flame collisions with boundaries. Much recent work has concerned the mechanisms driving instabilities in premixed combustion and the coupling between pressure waves and combustion with application to the problem of instability in modern low NO x heavyduty gas turbine combustors. Progress in numerical modeling has allowed simulations of dynamical flames interacting with pressure waves. On this basis, it has been possible to devise predictive methods for instabilities. Important efforts have also been directed at the development of the related subject of combustion control. Research has focused on methods, sensors, actuators, control algorithms, and systems integration. In recent years, scaling from laboratory experiments to practical devices has been achieved with some successebut limitations have also been revealed. Active control of combustion has also evolved in various directions. A number of experiments on laboratory-scale combustors have shown that the amplitude of combustion instabilities could be reduced by applying control principles. Full-scale terrestrial application to gas turbine systems have allowed an increase of the stability margin of these machines. Feedback principles are also being explored to control the point of operation of combustors and engines. Operating point control has special importance in the gas turbine field since it can be used to avoid operation in unstable regions near the lean blowoff limits. More generally, closed loop feedback concepts are useful if one wishes to improve the combustion process as demonstrated by applications to automotive engines. Many future developments of combustion will use such concepts for tuning, optimization, and emissions reduction. This article proposes a broad survey of these fast-moving areas of research.

TIME-DEPENDENT MODEL OF A BUOYANT DOWNWARD FLAME SPREAD OVER A THERMALLY THIN SOLID FUEL

A time-dependent model was developed and solved numerically to study a purely buoyant downward flame spread over a thermally thin solid fuel in various gravity environments. According to the specified burn-out solid density and no retained ash, the flame propagation behavior over a thin solid fuel surface could be simulated. At ignition, the flame is premixed. After several transition burnouts the flame transitions into a self-sustained steady spread diffusion flame. When the gravity level was varied, the Damkohler number effects were verified. An unstable flame spread was noted near the extinction limit at which the flame spread rate decelerated. Blow-off extinction was predicted after the flame spread a short distance. The ignition delay time increased with increasing gravity level. Compared with the experimental measurements, the predicted blow-off extinction limit was closer than that predicted by the steady combustion model.

Effects of partial premixing on pollutant emissions in swirling methane jet flames

Experimental measurements of visible flame heights, temperatures, and pollutant emission indices in partially premixed swirling flames with a broad range of central fuel tube equivalence ratios (Φ F ) are reported. Two cases of partially premixed swirling flames are studied; one with a constant fuel tube exit velocity and the other with an increased exit velocity. With increasing partial premixing, the visible flame height decreases and the overall flame color changes from yellow to blue. Temperature measurements indicate that the flame structures become thinner and temperatures increase continuously with increasing partial premixing. Emission index, EINO x and EICO, values decrease with increasing levels of partial premixing and reach a minimum value at Φ F ≈ 3, followed by an increase as Φ F approaches the blowoff limits. The reduction in EINO x and EICO at optimum partial premixing (Φ F ≈ 3), as compared with that for the non-premixed swirling flame is at least 23% and 77%, respectively. The emission index for NO x scales very well with the fuel mass fraction and the fuel-air momentum flux ratio. Good agreement is achieved between the predictions and measured results.

Extension of Turbulent Jet Diffusion Flame Blowoff Limits by Doublet Flows.

A series of experiments was performed to investigate the effect of a new combustion technique, named doublet flows, on the extension of jet diffusion flame blowoff limits. Doublet flows were formed in a concentric annulus, in which fuel jet was ejected from a nozzle set at the center of the annulus and air was sucked into an outer cylinder encompassing the nozzle. Visualization of various changes of flame morphologies triggered by doublet flows and measurement of flame lift off heights were achieved by means of normal video recording as well as image processing and treatment devices. Results showed that the blowoff limits could be extended dramatically by an increase in air-suction flow rates. The limits could be further extended by an increase in rim thickness of the nozzle. Frequent flame lift-off and reattachment occurred only in the interim of doublet flow application.

The modelling of direct chemical kinetic effects in turbulent flames

Combustion chemistry-related effects have traditionally been of secondary importance in the design of gas turbine combustors. However, the need to deal with issues such as flame stability, relight and pollutant emissions has served to bring chemical kinetics and the coupling of finite rate chemistry with turbulent flow fields to the centre of combustor design. Indeed, improved cycle efficiency and more stringent environmental legislation, as defined by the ICAO, are current key motivators in combustor design. Furthermore, lean premixed prevaporized (LPP) combustion systems, increasingly used for power generation, often operate close to the lean blow-off limit and are prone to extinction/reignition type phenomena. Thus, current key design issues require that direct chemical kinetic effects be accounted for accurately in any simulation procedure. The transported probability density function (PDF) approach uniquely offers the potential of facilitating the accurate modelling of such effects. The present paper thus assesses the ability of this technique to model kinetically controlled phenomena, such as carbon monoxide emissions and flame blow-off, through the application of a transported PDF method closed at the joint scalar level. The closure for the velocity field is at the second moment level, and a key feature of the present work is the use of comprehensive chemical kinetic mechanisms. The latter are derived from recent work by Lindstedt and co-workers that has resulted in a compact 141 reactions and 28 species mechanism for LNG combustion. The systematically reduced form used here features 14 independent C/H/O scalars, with the remaining species incorporated via steady state approximations. Computations have been performed for hydrogen/carbon dioxide and methane flames. The former (high Reynolds number) flames permit an assessment of the modelling of flame blow-off, and the methane flame has been selected to obtain an indication of the influence of differential diffusion effects among gaseous species. The agreement with experimental data is excellent. The predicted blow-off, velocity is within 10 per cent of the experimental value and it is further shown that experimental levels of major and minor species are well reproduced. Interestingly, comparisons of experimental data with prediction indicate only a modest influence of differential diffusion effects on gaseous species. A comparison with previous modelling efforts, featuring smaller scalar spaces, permits the conclusion that accurate chemistry is a prerequisite for quantitative predications of finite rate chemical kinetic effects.

Lean blowout model for a spray-fired swirl-stabilized combustor

In aero-engine applications, the lean blowoff (LBO) limit plays a critical role in the operational envelope of the engine. The geometry of the combustion chamber primary zone plays a critical role in establishing LBO limits. This is especially true for advanced lean burn concepts which introduce the majority of the combustion air in a manner designed to enhance the rate of mixing with the fuel. In the present study, specific mixer hardware has been designed to develop a systematic, statistically sound test matrix to study the effect of mixer components (primary swirl vane, secondary swirl vane, Venturi, and co-and counterswirl) on LBO. A strategy is employed to develop, based on an existing model, a new predictive model for LBO which accounts for a heterogeneous swirl-stabilized reaction and explicitly relates the geometry of the hardware to the LBO limit. The model predicts the LBO fuel/air ratio at three operating temperatures to within 14% of the measured value. The multivariate experiments used to relate LBO to geometry were also further analyzed to establish the main hardware parameters affecting LBO. Specifically, the Venturi and swirl sense (co- versus counter-swirl) were found to impact LBO at lower air inlet temperatures (294 and 366 K). The Venturi and counter-swirl enhance the atomization and mixing processes which are more rate limiting at lower temperatures, and, as a result, improve stability. At a higher inlet air temperature (477 K), the secondary swirl vane angle also plays a role in determining LBO, with larger angles (75°) generating better stability, which is associated with a stronger recirculation zone. The hardware configuration with the best LBO performance over the different conditions was identified (45° primary swirler, 55° secondary swirler, counter-swirl with Venturi).

916 音響励振によるメタン噴流拡散火炎の吹飛限界への影響(GS.11 熱工学I)

メタンなどを燃料とする噴流拡散火炎において,吹飛限界は噴射管口径に依存することが知られている.また,これまでの研究により音波による外部撹乱に対して特定のレイノルズ数の範囲で火炎形状の変化や火炎先端の分岐などが見られた.しかし,音響励振による火炎内部の噴流への影響及び火炎の吹飛限界への影響については詳しく調べられていない.本研究では外部側面から音響励振をおこなった際の噴流拡散火炎の流れ場を可視化することにより,火炎及び内部噴流の音響励振による変化を調べた.また音響励振による火炎の吹飛限界への影響を実験的に調べた.ストローハル数0.15〜0.6の周波数-噴流流速領域において火炎内部の分岐噴流が観測された.また音響励振時の火炎の吹飛限界の変化は全ての噴射管において見られなかった.

Low-temperature natural-gas-fired combustors and NOx formation

This paper discusses the characteristics of low-temperature cooled combustors as currently being developed and utilized in situations where very low NOx emissions are required with natural gas firing. Cyclone or swirl-type combustors are commonly used owing to their very wide blow-off which when translated to a cooled situation results in a system with acceptable blow-off limits for flame front temperatures as low as 1000 °C with only 20 per cent excess air. This depends on the careful integration of the heat transfer surface with the flame front region such that the products of combustion are directed away, inwards from the cooled surface, thus permitting complete fuel burn-out to occur. The combustion aerodynamics of a multi-inlet pre-mixed natural gas-air cyclone combustor has been characterized using laser Doppler anemometry together with detailed mapping of NOx, temperature and gas species concentration. This has been undertaken in 32 different operational cases involving five different multi-inlet cyclone combustors operated over the equivalence ratio range 0.45 ≤ φ ≤ 1.25. The NOx emission data has been examined in light of the prompt NO model of De Soete and modified in light of the known combustion temperatures, NO and species concentration levels. A correlation was derived of similar form to that of De Soete and found to give a reasonable data fit over the temperature range from 900 to 1500 °C, especially for lean equivalence ratios. The constants in this correlation are quite different from those of De Soete; this is felt to reflect the much lower temperature levels over which the data were gathered. Conclusions are then drawn as to the configuration of very low NOx emission combustors.

98/04027 An experimental study of the fate of char nitrogen in pulverized coal flames

A twin-fluid air-assist atomizer coupled with a Hartmann type stem and cavity air-jet acoustic generator has been used to atomize water, kerosene, diesel oil and furnace oil. The effects of atomizing air pressure, air to liquid mass ratio and liquid viscosity on the mean drop size of the spray have been investigated. From the experimental results a suitable correlation has been obtained to estimate the Sauter mean diameter (SMD) of such sprays. The influence of resonating cavity on the spray pattern and lean blow-off limits for the combustion of kerosene has also been studied. Burners incorporating this type of twin-fluid acoustic atomizer are found to be particularly suitable for highly viscous oils.

98/04036 Ignition and burnout time of coal particle in fluidized bed combustor

A twin-fluid air-assist atomizer coupled with a Hartmann type stem and cavity air-jet acoustic generator has been used to atomize water, kerosene, diesel oil and furnace oil. The effects of atomizing air pressure, air to liquid mass ratio and liquid viscosity on the mean drop size of the spray have been investigated. From the experimental results a suitable correlation has been obtained to estimate the Sauter mean diameter (SMD) of such sprays. The influence of resonating cavity on the spray pattern and lean blow-off limits for the combustion of kerosene has also been studied. Burners incorporating this type of twin-fluid acoustic atomizer are found to be particularly suitable for highly viscous oils.

Results of Experiments and Models for Predicting Stability Limits of Turbulent Swirling Flames

Swirling flames are used in many industrial applications, such as process furnaces, boilers, and gas turbines due to their excellent mixing, stability, emission, and burnout characteristics. The wide-spread use of swirl burners in the process and energy industries and, in particular, new concepts for the reduction of NOx emissions, raises the need for simple-to-use models for predicting lean stability limits of highly turbulent flames stabilized by internal recirculation. Based on recently published experimental data of the first author concerning the reaction structures of swirling flames operating near the extinction limit, different methods for predicting lean blow-off limits have been developed and tested. The aim of the investigations was to find stabilization criteria that allow predictions of blow-off limits of highly turbulent recirculating flames without the requirement for measurements in those flames. Several similarity criteria based on volumetric flow rates, burner size, and material parameters of the cold gases were found to be capable of predicting stability limits of premixed and (in some cases) nonpremixed flames at varying swirl intensities, burner scales and fuel compositions. A previously developed numerical field model, combining, a k–ε model with a combined “assumed-shape joint-PDF”/eddy-dissipation reaction model was also tested for its potential for stability prediction.

A Model for Calculating Heat Release in Premixed Turbulent Flames

A unified reaction model, being valid in all turbulent combustion regimes, has been developed and tested. Based on a thoroughly validated model for the turbulent burning velocity the Kolmogorov, Petruvski and Piskunuv (KPP) theorem was applied, thus leading to the formulation of the mean reaction rate as a function of local turbulence and kinetic parameters in the flow. Numerical calculations, comprising all flame structures (0.2 < Dat < 2.8) of premixed flames were performed and compared with experimental data. At first, an unsteady spherical flame, propagating in a defined turbulence field was simulated, thus emphasizing the correct representation of the turbulent burning velocity. Thereafter, the mean flame lengths of two stationary, highly turbulent Bunsen-type flames were compared with experiments, varying the affecting turbulence parameters. The last premixed example was a strongly swirling flame in a combustor, related to kinetically controlled flame structures. Although those very different flame configurations cover a broad range in the Borghi diagram, the agreement with measured temperature profiles was found satisfactory. Finally, the model was extended to partially premixed systems by accounting for the influence of the mean mixture fraction on local kinetic parameters. The predictions of stability limits of unconfined, strongly swirling flames show satisfying agreement with experiments, supporting the earlier formulated opinion [25] that the blow-off limits of strongly swirling flames are determined by chemical kinetic limitation of the mean reaction rate. The results demonstrate the high performance of the reaction model and recommend its application also in complex 3-D flows, because of its simplicity and numerical robustness even in conjunction with higher order turbulence models.

Flame Stability and NOx Formation in a High-Intensity Swirl Burner.

Flame modes and blow-off limits at a high-intensity swirl burner are systematically studied by changing various parameters such as swirl intensity, fuel-flow rate, nozzle structure, combustion tube length and combustor pressure. Results show that higher-intensity swirl well stabilizes the flame above a critical swirl number S=0.6. In the case of fuel injection perpendicular to the burner axis under atmospheric pressure, the flame of premixed pattern appears and an extremely low NOx concentration is realised at a lean combustion with long combustion tube. On the other hand, in the coaxial fuel injection along the burner axis, the flame of diffusion pattern appears and NOx concentration becomes almost equal to that of premixed-pattern flame near the stoichiometric fuel-air ratio. As combustor pressure is elevated under a constant air-mass flow rate, NOx concentration is increased even at a lean combustion due to less local misfiring.

NOx and CO Formation Characteristics in Rich-Lean Combustion.

NOx and CO formation characteristics in rich-lean flames at a multi-slit burner were studied experimentally and numerically. In the rich-lean combustion, rich and lean slit burners are set side by side. The overall NOx and CO concentrations were measured by using a water-cooled quartz microprobe to investigate the effect of various pairs of air ratios for the rich and lean flames. To understand the NOx formation mechanism, two-dimensional reactive flow calculations were also performed. The results suggest that the thermal and prompt NO in the rich flames are the main source of NOx production, and NOx increases when the rich flames air ratio becomes lower or the fuel flow rate ratio for the rich burners increases. It is also found that the burner plate thickness affects the blow-off limits of the burner. These results are useful for improving the low pollutant and stable combustion method.

5632143 Gas turbine system and method using temperature control of the exhaust gas entering the heat recovery cycle by mixing with ambient air

Combustion dynamics constitutes one of the most challenging areas in combustion research. Many facets of this subject have been investigated over the past few decades for their fundamental and practical implications. Substantial progress has been accomplished in understanding analysis, modeling, and simulation. Detailed laboratory experiments and numerical computations have provided a wealth of information on elementary dynamical processes such as the response of flames to variable strain, vortex rollup, coupling between flames and acoustic modulations, and perturbed flame collisions with boundaries. Much recent work has concerned the mechanisms driving instabilities in premixed combustion and the coupling between pressure waves and combustion with application to the problem of instability in modern low NOx heavyduty gas turbine combustors. Progress in numerical modeling has allowed simulations of dynamical flames interacting with pressure waves. On this basis, it has been possible to devise predictive methods for instabilities. Important efforts have also been directed at the development of the related subject of combustion control. Research has focused on methods, sensors, actuators, control algorithms, and systems integration. In recent years, scaling from laboratory experiments to practical devices has been achieved with some successebut limitations have also been revealed. Active control of combustion has also evolved in various directions. A number of experiments on laboratory-scale combustors have shown that the amplitude of combustion instabilities could be reduced by applying control principles. Full-scale terrestrial application to gas turbine systems have allowed an increase of the stability margin of these machines. Feedback principles are also being explored to control the point of operation of combustors and engines. Operating point control has special importance in the gas turbine field since it can be used to avoid operation in unstable regions near the lean blowoff limits. More generally, closed loop feedback concepts are useful if one wishes to improve the combustion process as demonstrated by applications to automotive engines. Many future developments of combustion will use such concepts for tuning, optimization, and emissions reduction. This article proposes a broad survey of these fast-moving areas of research.

Heat Transfer Pathways in Flame Spreading Over Thick Fuels as a Function of the Flame Spread Regime: Microgravity, Thermal, and Kinetic

The mechanism of flame spread over thermally thick solid fuels in an opposing flow is explored. The pathways of forward heat transfer are computed with mathematical models of increasing complexity in all regimes. New formulas are developed that reproduce numerical results for the dominant forward heat transfer pathways in the thermal and chemical kinetic regimes and compare well with experimental data. The heat transfer structure at the leading edge of the flame computed by the comprehensive model agrees quite well with available experimental and numerical results. In the thermal regime, forward heat transfer through the gas is dominant, while in the chemical kinetic and microgravity regimes. forward heat transfer through the solid may assume a significant role. In the latter regime the contribution of forward radiation heat transfer is found to be negligible, although gas-phase radiation has a profound effect on the flame structure. The sensitivity of the dimensionless contribution of forward heat transfer through the solid fuel to that through the gas. Rsfc, on the location of the flame leading edge reconciles seemingly contradictory results found in the literature. A semi-empirical predictive formula for Rsfc, is developed supported by numerical and experimental results. The result, expressed as: Rsfc,=Csfc, (Qsfc/Qgfc)=csfc(λs/λp) (1/F(Ls/Lg))0.5, requires the flame spread rate, Vf, to be available directly from expenment or correlation. Two limiting forms of this quantity are obtained for the thermal regime and the chemical kinetic blowoff limit; Rsfc, thermal=Csfc(λs/λg and Rsfc.blowoff=Csfc(λs/λg(1/F). Values of both limiting forms can be determined from known parameters of the problem.

97/02441 New development of F-T synthesis: MFT industrial test status

Previously, we developed a model to predict lean flammability limit L and flame propagation velocity Sb for pulverized coal. In the present paper, we have extended the model to apply it in development of oxy-fuel combustion systems. The basic model consists of two particles. One of the two particles burns first, then, the other particle is ignited by the heat of combustion of the one burning particle. We analyzed at what distance the first burning particle could ignite the next particle, and how fast the first burning particle could ignite the next particle. The model was verified both for air and oxy-fuel combustion conditions. Next, a method to support burner development was examined by using the model. Local Sb and L near the ignition points of the burner could be analyzed from the concentration and temperature profiles of CFD results. Flame stability was judged by the calculated Sb and L profiles, and past results of blow-off limits obtained with actual- and pilot-scale experiments. A DS®T-burner was developed by Hitachi Power Europe, and installed at the Schwarze Pumpe pilot plant. Combination of the technique and large eddy simulation was applied to confirmation of the system.

Study of Flame Stability in a Step Swirl Combustor

A prime requirement in the design of a modern gas turbine combustor is good combustion stability, especially near lean blowout (LBO), to ensure an adequate stability margin. For an aeroengine, combustor blow-off limits are encountered during low engine speeds at high altitudes over a range of flight Mach numbers. For an industrial combustor, requirements of ultralow NOx emissions coupled with high combustion efficiency demand operation at or close to LBO. In this investigation, a step swirl combustor (SSC) was designed to reproduce the swirling flow pattern present in the vicinity of the fuel injector located in the primary zone of a gas turbine combustor. Different flame shapes, structure, and location were observed and detailed experimental measurements and numerical computations were performed. It was found that certain combinations of outer and inner swirling air flows produce multiple attached flames, aflame with a single attached structure just above the fuel injection tube, and finally for higher inner swirl velocity, the flame lifts from the fuel tube and is stabilized by the inner recirculation zone. The observed difference in LBO between co- and counterswirl configurations is primarily a function of how the flame stabilizes, i.e., attached versus lifted. A turbulent combustion model correctly predicts the attached flame location(s), development of inner recirculation zone, a dimple-shaped flame structure, the flame lift-off height, and radial profiles of mean temperature, axial velocity, and tangential velocity at different axial locations. Finally, the significance and applications of anchored and lifted flames to combustor stability and LBO in practical gas turbine combustors are discussed.

Development and assessment of correlations for predicting stability limits of swirling flames

Swirling flow systems are being used in many industrial processes like separation of particles in gas flows (cyclones), atomisation and spreading of liquids (water, oil) and fast mixing and high-intensity reaction in reactor systems as, e.g. stable and intense flames of swirl burners. The widespread use of swirl burners in the process and energy industries and, in particular, new concepts for the reduction of NO x-emissions (ultralean premixed combustion) raise the need for simple-to-use models for predicting lean stability limits of highly turbulent flames stabilized by internal recirculation. Based on recently published experimental data of the first author concerning the reaction structures of swirling flames operated near to the extinction limit, different methods for predicting lean blow-off limits have been developed and tested for different burner sizes and fuel gas compositions. The aim of the investigations was to find stabilization criteria that allow predictions of blow-off limits of highly turbulent recirculating flames without the requirement for expensive and time-consuming measurements in those flames. Several similarity criteria based on volumetric flowrates, burner size and material parameters of the cold gases, were found to be capable to predict stability limits of premixed and (in some cases) nonpremixed flames at varying swirl intensities, burner scales and fuel compositions. A previously developed numerical field model, combining a k,ε-model with a combined “assumed-shape Joint-PDF”/Eddy-Dissipation reaction model for the determination of the time mean reaction rates in turbulent flows was also tested for its potential for stability prediction. All the methods presented have specific advantages and limitations: the similarity criteria are restricted to geometrically similar systems, but they are easy to use, fairly precise and can take the detailled chemistry into account in an integral manner. The numerical field model necessitates large computational effort and is limited to very much simplified, global reaction mechanisms, but it offers the opportunity to make predictions for different burner geometries and swirl conditions.

A Theory for Downward Flame Spread Over a Thermally-Thin Fuel

Abstract A theoretical analysis is developed to study the downward flame spread over the thermally-thin solid fuel vertically in the gravitational field. The combustion model is basically similar to that of Chen (1990). The effect of Damkohler number (Da), as a function of gravity level (g), is under investigation. Beyond the blowoff limit, the flame spread rate is found to be proportional to (g)−1/3. A combination of the isotherm and velocity vector distributions in gas phase and the solid fuel pyrolysis are presented together to illustrate flame structures at normal gravity. Subject to the same Da, the flame is weaker than that in forced convection. Within the elevated gravity domain of experiment, the computed flame spread rate agree very well with the measurements obtained by Altenkirch el at. (1980).