Flame Stability Limits Research Articles

Characteristics of hollow bluff body-stabilized natural gas-air premixed flames with heat recirculation

The combined effects of flow recirculation and preheating on the premixed flame stability limits were employed via two flow configurations. In the first configuration, the natural gas-air mixture was set to flow around a bluff body whose orientation, number of vertices and vertex angle were varied. In the second configuration, an innovative design was developed where the fuel–air mixture was set to flow inside the bluff body as two opposing jets whose resultant flow is redirected to pass around the bluff body. The peak turbulent kinetic energy (T. K. E.) decreased from 7.24 to 4.94 m2/s2 with the star shape of 4 vertices as its orientation changed from 45 to 0° and such decrease was limited to 5.27 m2/s2 upon having a smaller vertex angle. A smaller drop from 7.24 to 6.0 m2/s2 was experienced when the 4 vertices were replaced by 5 vertices at 0° angle of orientation, while the drop continued to 4.3 m2/s2 upon having 6 vertices due to the reduction in the space available for eddies. A smaller peak T. K. E. of 2.33 m2/s2 was found with the star shape of 3 vertices because the sites for turbulence development were limited. With the star cross-section of 4 vertices at 45° angle of orientation, a maximum blow-off velocity as high as 51.5 m/s and a minimum lean limit as low as 0.40 were obtained. A further extension in the flame stability limits was provided via the second flow configuration due to the favorable effects of impingement and prolonged path of preheating. As the distance between the bluff body and the combustor separating wall decreased to 1.7 times the jet diameter, the blow-off velocity increased to 79.1 m/s; while the lean limit decreased to 0.31 with the star shape of 4 vertices at 45° angle of orientation.

Critical conditions at liftoff limit of a laminar n-butane-air jet diffusion flame

The stability mechanism of laminar coflow jet diffusion flames in normal gravity has been studied computationally and experimentally. N-butane, the heaviest alkane in a gaseous state at ambient temperature and pressure, is used as the fuel since the reaction mechanism is similar to that of higher (liquid) hydrocarbons. The critical mean n-butane jet and coflowing air velocities at flame stability limits are measured using a small fuel tube burner (0.8 mm inner diameter). The time-dependent, axisymmetric numerical code with a detailed reaction mechanism (58 species and 540 reactions), molecular diffusive transport, and a radiation model, reveals a flame structure. A fuel-lean peak reactivity spot (i.e., reaction kernel), possessing the hybrid nature of diffusion-premixed flame structure at a constant temperature of ≈1560 K, is formed at the flame base and controls the flame stability. In a near-quiescent environment, the flame base resides below the fuel tube exit plane and thereby premixing is limited. As the coflowing air velocity is increased incrementally under a fixed fuel jet velocity, the flame base moves slightly above (≈1 mm) the burner exit and vigorous premixed combustion becomes prevailing. The local heat-release rate at the reaction kernel nearly doubles due to the increased convective oxygen flux (i.e., a blowing effect). The local Damköhler number, newly defined as a ratio of the square root of the local heat-release rate and the local velocity, decreases gradually first and drops abruptly at a critical threshold value and the flame base lifts off from the burner rim. The calculated coflow air velocity at liftoff is ≈0.38 m/s at the fuel jet velocity of 2 m/s, which is consistent with an extrapolated measured value of 0.41 m/s. This work has determined the critical Damköhler number at the stability limit quantitatively, for the first time, for laminar jet diffusion flames.

Effects of ethanol and methyl formate additions on characteristics of pre-vaporised palm methyl ester laminar diffusion flame

Motivated by the growing interest in the application of renewable fuels and combustion efficiency, gaining a detailed understanding of the effect of oxygenated additives on the structure and shape of the biodiesel laminar coflow diffusion flame is of fundamental and practical importance. In the experiment, the amount of ethanol and methyl formate in the nitrogen-diluted biodiesel mixtures was 10% vol. while maintaining a constant fuel mole fraction. The results showed that adding ethanol and methyl formate decreased the flame height regardless of the variation of the fuel jet velocities, with methyl formate demonstrating a greater reduction effect on the flame height. Besides that, the addition of these oxygenated additives has a significant influence on the flame stability, which was characterised by the parameters such as liftoff height, liftoff velocity, and blowout velocity. The oxygenated additives addition reduced the flame stability limit, with methyl formate showing a greater decrease in flame stability limit based on the observation of the blowout velocity. The dilution effect of the oxygenated additives was identified as the dominant effect on the flame structure and stabilisation through the decoupling analysis when compared to the thermal and chemical effects. Further analysis revealed that the difference in the influence of the oxygenated additives on the combustion characteristics was attributed to the physicochemical properties of the fuel mixtures, such as the molecular diffusivity, energy density, and fuel decomposition. These parameters can be fundamentally related to the difference in the molecular structures between ethanol and methyl formate. On the other hand, the addition of these oxygenated additives decreased both carbon monoxide and nitrogen oxide concentrations; however, the differences in the pollutant concentrations between ethanol and methyl formate were insignificant.

Effect of hydrogen addition on flame stability and structure for low heating value coaxial nonpremixed flames

ABSTRACT An investigation of effect of hydrogen addition on flame stability and structure i.e. combustion characteristics of low heating value gases (LHVGs) that consist of methane, propane, nitrogen is carried out experimentally and numerically. For the experiment, a coaxial non-premixed jet-type combustor and Planar Laser Induced Fluorescence (PLIF) system are used to evaluate the flame stability limit and flame shapes and visualize OH radical distribution, respectively. For the numerical analysis of hydrogen added LHVGs, the calculation using the FLUENT software with decreased GRI 3.0 mechanism is performed to study the 2D flame temperature distribution and radial flame structure. As a result, the flame length of LHVGs reduces as hydrogen is added. CH* and OH* radicals visualized by chemiluminescence system are distributed closer to the nozzle as hydrogen is added to LHVGs. OH-PLIF images show that OH intensity of LHVG 6000 is higher with hydrogen addition. The flame stability of LHVGs is improved with hydrogen addition. As for the numerical result, the reduction of flame length and width of LHVG 6000 are predicted with adding the hydrogen.

RECENT ADVANCEMENTS IN MICROCOMBUSTION-BASED POWER GENERATORS

The limitations posed by electrochemical batteries due to their lower power densities, bulky volumes, and environmental hazards from their disposal have promoted the development of small-scale combustion-based power generators as an alternative power source for various microelectromechanical systems applications. Microcombustors are considered to be a viable heat source for these power generators due to the high power density of hydrocarbon and hydrogen fuels and the recent innovations in micromachining and microfabrication techniques. Because of their high heat loss to heat generation ratio, flow and heat recirculation techniques are used to improve the flame stability limits of microcombustors. In the present study, two types of direct micropower generators are reviewed in detail. There is a consensus among researchers that favors thermoelectric generators (TEGs) and thermophotovoltaic systems over micro-gas turbines due to the presence of moving parts, lower power densities, and complex manufacturing techniques involved in the development of the latter. Progress in the field of TEGs has been analyzed by highlighting the challenges faced in terms of thermoelectric materials and efficient thermal management. A maximum power output of 30 W and maximum conversion efficiency of 5% have been reported by various researchers using TEG-based power generators.

Experimental investigation of the flame stability limits for H2 + C3H8, H2 + C2H6 and H2 + CH4 jet flames

Experimental investigation of the flame stability limits for H2 + C3H8, H2 + C2H6 and H2 + CH4 jet flames

Flame stability optimization of cavity primary air-jet form in an augmentor

A trapped vortex cavity with a radial V-gutter flameholder is adopted to expand the flame stability of an augmentor. The improvement of flame stabilization limits is achieved experimentally by replacing the traditional slotted air-jet with the discrete-hole air-jet. The atomization characteristics of the air-assisted multi-point injector and the numerical fluid-structure are conducted to explain the results. Results indicate that a remarkable enhancement of the flame stability with discrete-hole air-jet is obtained in all conditions, while the slotted air-jet for the cavity leads to a failed ignition at 343 K except for the Mach number of 0.3. As the same passing area of the air-jet, the larger the aperture of the discrete hole, the better the flame stabilization performance. An increasing trend of lean blowout equivalence ratio is contributed by the increased Mach number. Whereas, the Mach number growth will lead to a reduction of lean ignition equivalence ratio at 343 K and 473 K, and a downward parabola is observed at 573 K. Eventually, the optimal flame stability performance is achieved in by the larger discrete space distance, which promotes the fuel/air mixture with the help of the recirculation zones between discrete holes.

Fundamental Numerical Analysis of a Porous Micro-Combustor Filled with Alumina Spheres: Pore-Scale vs. Volume-Averaged Models

Inserting porous media into the micro-scale combustor space could enhance heat recirculation from the flame zone, and could thus extend the flammability limits and improve flame stability. In the context of porous micro-combustors, the pore size is comparable to the combustor characteristic length. It is insufficient to treat the porous medium as a continuum with the volume-averaged model (VAM). Therefore, a pore-scale model (PSM) is developed to consider the detailed structure of the porous media to better understand the coupling among the gas mixture, the porous media and the combustor wall. The results are systematically compared to investigate the difference in combustion characteristics and flame stability limits. A quantified study is undertaken to examine heat recirculation, including preheating and heat loss, in the porous micro-combustor using the VAM and PSM, which are beneficial for understanding the modeled differences in temperature distribution. The numerical results indicate that PSM predicts a scattered flame zone in the pore areas and gives a larger flame stability range, a lower flame temperature and peak solid matrix temperature, a higher peak wall temperature and a larger Rp-hl than a VAM counterpart. A parametric study is subsequently carried out to examine the effects of solid matrix thermal conductivity (ks) on the PSM and VAM, and then the results are analyzed briefly. It is found that for the specific configurations of porous micro-combustor considered in the present study, the PSM porous micro-combustor is more suitable for simplifying to a VAM with a larger Φ and a smaller ks, and the methods can be applied to other configurations of porous micro-combustors.

Combustion performance in a cavity-based combustor under subatmospheric pressure

Combustion performance and flame stabilization mechanism of the flameholder under the subatmospheric pressure are the foundation for the design of high-altitude operating aircraft. An experimental cavity-based combustor was developed to explore the combustion performance in low pressure. Four low pressures of 0.03, 0.04, 0.05 and 0.06 MPa were validated to investigate the effect of inlet pressure on the flame stability limits, lean ignition and blowout process, combustion efficiency, and outlet temperature profiles. A unique lean spark ignition process was discovered at 0.03 MPa that the flame downstream of the V-type flameholder promotes the formation of the piloted flame in the cavity. Results indicate that the pressure reduction decreases the spray flame propagation, subsequently weakening flame stability and combustion efficiency. With the inlet pressure decreases, the pilot fuel injected into the cavity still maintains effective combustion, while the declining combustion is achieved in the mainstream fuel. This phenomenon suggests that increasing the pilot fuel proportion in the combustion chamber will enhance the combustion in ultra-low pressure.

Flame stabilization and soot emission of methane jet flames for CO2 diluted oxy-combustion at elevated pressure

Recently, oxy-combustion cycles with CO2 dilution have been of interest to eliminate NOx emission and to facilitate CO2 capture. Theoretically, they are operated in an overall stoichiometric condition. In addition, high pressure is preferred not only for better thermal efficiency but also for easier CO2 capture and storage. In this study, a triple-coaxial-tube was selected as the simplest conceptual design for an oxy-combustion burner at elevated pressure. Flame stabilization, soot emissions, and combustion efficiencies were investigated in relation to variation of the tube size, CO2 dilution ratio, and pressure. As a result, a jet Reynolds number and CO2 diffusion could explain the flame stabilization limits. Soot emission could be reduced at atmospheric pressure when the fuel-tube tip was moved into the middle oxidant tube, which also improved the combustion efficiency. Nevertheless, serious soot emissions recurred at excessive fuel flow rates and at elevated pressures. In such conditions, a lifted flame detached from the fuel-tube tip could be employed by increasing the flow rate or enhancing the CO2 dilution, and the soot emission was reduced again without losing combustion efficiency. Finally, the flame-stabilization mechanisms of the attached flame and of the lifted flame were discussed and optimized designs and operating conditions were proposed for less soot emission and better combustion efficiency at higher pressures.

Hybrid Large Eddy Simulation/Reynolds-Averaged Navier–Stokes Analysis of a Premixed Ethylene-Fueled Dual-Mode Scramjet Combustor

Hydrocarbon fuels offer optimal high energy per volume for scramjet applications for sustained hypersonic flight but require additional residence time due to slower ignition delays (compared with hydrogen fuel). The injection of ethylene at the start of the isolator of a dual-mode scramjet combustor, operating in ramjet mode, allows sufficient mixing to achieve efficient premixed turbulent combustion. A cavity flameholder anchors the flame, supplying sufficient radicals to sustain a stable flame in the high-speed environment. This work investigates flame structure and stabilization limits of a new configuration with a scaled-down cavity embedded in the flow path of the combustor with a strut and insert. The cavity is reduced in size by one-third to enable eventual direct numerical simulations of the flame stabilization process. This work, however, focuses on modeling the full isolator/combustor geometry using a hybrid large eddy simulation/Reynolds-averaged Navier–Stokes simulation strategy. Particle image velocimetry, planar laser-induced fluorescence, coherent anti-Stokes Raman spectroscopy, and pressure measurements are compared with numerical predictions to analyze and characterize the conditions within the combustor, including flame structure, flow velocities, species composition, and wall pressure. An adjustable air throttle in the extender is able to control the placement of the shock train in the isolator and maintain a stable flame at various equivalence ratios. The simulations show reasonably good agreement with the experimental scalar and velocity data and predict a flame angle consistent with premixed turbulent flame-speed correlations.

Spark ignition and stability limits of spray kerosene flames under subatmospheric pressure conditions

Flame stability limits and ignition delay time of the flameholder are experimentally studied in this work at the pressure of 0.03 – 0.06 MPa, Mach number of 0.1 – 0.3, and temperature of 320 K – 800 K. The results indicate that the reduction in equivalence ratios of lean ignition and blowout can be achieved by the increase in pressure or Mach number. Temperature rise will first increase and then decrease the lean ignition equivalence ratio. While increasing lean blowout equivalence ratio was observed with increasing temperature. The envelope of the stable combustion between the pressure and Mach number is measured, and the lowest temperature of successful ignition is 300 K, acquired at Mach 0.1 and 0.06 MPa. Besides, the correlation between the equivalence ratios of flame stability and inlet parameters, such as pressure, Mach number, temperature, is developed. The ignition process of the flameholder is imaged, and the ignition delay time decreases with increased pressure or temperature. The ignition delay time is shown first to decrease and then increase with the increase in Mach number.

Elevated pressure increases the effect of electric fields on ionic wind in methane premixed jet flames

Electric fields are useful for enhancing stability limits of flames, increasing the overall burning rate and reducing soot emissions. The electric body force has been known as a key element behind the aforementioned augmentation, and recent studies have provided clear picture for the resulted flow modification. In this study, we investigate the effects of pressure on an ionic wind by applying transverse electric fields to methane premixed jet flames in a pressurized chamber up to 2 atm. We investigated the voltage-current response by varying the pressure, the equivalence ratio of the mixture, and the flow rate. We found that the saturated current for lean and stoichiometric mixtures was not affected by the pressure, while the saturated current of rich premixed flames changed significantly. We developed a model to predict the voltage-current behavior and the ion-production rate, which we validated using experimental results. Based on our flow field measurements, we found that elevated pressure conditions enhanced ionic wind-driven mass transport. These results support the use of electric fields in a high-pressure environment.

Effect of non-uniform inlet profile on the combustion performance of an afterburner with bluff body

The quenching problem caused by the inlet distortion of the combustor during emergency attitude adjustment of an aircraft has not been adequately solved by a lack of experimental evidence in non-uniform inflow. Six non-uniform inlet velocity profiles with different velocity peak positions and values were built by perforated plates to explore the combustion performance of a bluff body. The results indicate that the velocity profile with a peak and off-center can produce the asymmetry of the dual-vortex structure downstream of the bluff body. The significant phenomenon is that the fluid flows from one vortex to another. More asymmetrical fluid-structure is facilitated by larger peak values or further off-center positions. Then, the asymmetrical flow field leads to non-uniform fuel distribution, which results in an asymmetrical flame and outlet temperature profile. Interestingly, the non-uniformity of outlet temperature increases first and then decreases with the rise of velocity peak position, while it just increases with the growth of the velocity peak value. Compare to uniform inflow, the non-uniform inlet velocity profile will destroy the ignition performance and combustion efficiency in most cases. However, the partial fuel accumulation may occur in the asymmetric fluid structure, which can widen the lean blowout limit. Overall, the flame stability limits and combustion efficiency under the non-uniform inflow in this study are beneficial to the design and application of combustor.

The blow-off and transient characteristics of co-firing ammonia/methane fuels in a swirl combustor

Recent studies have demonstrated that ammonia could be one of the most promising hydrogen carrier candidates which can be used in large-scale power plants. However, it is challenging to burn ammonia in gas turbines due to its narrow flame stabilization limits. This study investigates the blow-off characteristics and flame macrostructure transition behavior of ammonia/air flame (i.e. NH3 flame) and ammonia/methane/air flame (i.e. 50%NH3 flame) in a swirl combustor. Methane/air flame (i.e. CH4 flame) is also demonstrated for comparative purposes. The flow field and instantaneous OH profile are measured with PIV and OH-PLIF technique, respectively. Large eddy simulation (LES) is conducted to extend understandings of the experimental findings. The results show that the NH3 flame possesses a poor lean flame stability limit which can be largely extended by adding CH4 in the fuel. Moreover, changing swirl number (S) shows no apparent effect on the lean blow-off limit (ϕb) for the NH3 flame. On the contrary, a clear extension on ϕb is found for the 50%NH3 flame when increasing S. Four flame macrostructure modes can be identified when decreasing equivalence ratio (ϕ). The transition from flame II to flame III (ϕt describes the transition equivalence ratio) can be considered as the early warning of blow-off for a swirl stabilized flame. It is found that for the NH3 flame, there is no clear flame macrostructure transition at small inlet velocities (U < 3.8 m/s), i.e., ϕb ≈ ϕt, while the difference between ϕb and ϕt will be observed as the inlet velocity increases. However, for the 50%NH3 and CH4 flames, a clear flame macrostructure transition from flame II to flame III is observed even for a lower inlet velocity. The LES results show that the NH3 flame has a faster blow-off process compared to the CH4 flame, which is mainly attributed to the excessive stretch causing local extinction during the blow-off process.

An Experimental Investigation on Flame Stability and Lean Extinction Limit in Tubular Flame Burners Operating on Jet Fuel

ABSTRACT To solve problems of the low burn-off rate and the flame instability in a conventional rapidly mixed tubular flame (RMTF) burner and problems of the flashback and the narrow extinction limits in a premixed tubular flame burner, a new fuel/air mixing mode with multi-staged inlets was proposed and applied to a tubular flame burner, called the multi-staged tubular flame (MSTF) burner. To understand the mechanism of the fuel/air mixing mode of MSTF burner and their effects on the combustion, a series of experiments and simulations were carried out in comparison with the RMTF burner, operating on RP-3 jet fuel in this work. High-speed photography was applied to examine the flame characteristics of the two burners based on the flame irradiance intensity and flame morphology analysis combined with statistic methods. The results show that the tubular flames can be established stably without flash-back for both two burners in a wide range of equivalence ratios. The lean extinction limits can be extended to a global equivalence ratio as low as Фg = 0.26 for MSTF burner, while Фg = 0.40 for RMTF burner. Planar laser induced fluorescence (PLIF) measurements and 3D CFD simulations were also conducted to see the difference in fuel/air distributions for the two mixing modes of MSTF and RMTF burners at cold conditions. One of the findings is that the MSTF burner has better flame stability and wider extinction limits than the RMTF burner. The physics is that there stably forms a fuel-rich zone in the upstream of the inlet section which increase the resident time of rich fuel/air mixtures. The reveal of this mechanism for the multi-staged mixing mode is important as it has swept the main hurdles for the burner to find more applications to the industries.

Enhancement of the lean stability and blow-off limits of methane-air swirl flames at elevated pressures by nanosecond repetitively pulsed discharges

This paper examines the ability of nanosecond repetitively pulsed (NRP) plasma discharges to improve stabilization and extend the blow-off limit of lean premixed methane-air swirl flames at pressures up to 5 bar. The effect of two discharge regimes, NRP glows and NRP sparks, was investigated. The electrical characterization of the discharges was performed and direct images at 60 Hz of the flames, with and without NRP discharges, were collected to assess the effect of the discharges on flame stabilization. Results showed that NRP discharges efficiently extended the lean blow-off and stability limits of premixed methane-air swirl flames, at pressures up to 5 bar. These results were obtained for a ratio of NRP discharge power to flame thermal power of 0.7% or less. Moreover, the peak voltage necessary to maintain constant this power ratio did not increase linearly with increased pressure, even though the reduced electric field should linearly decrease with the pressure. It was also observed that the relative effectiveness of the NRP glows and NRP sparks changed by increasing the pressure. Based on discharge physics and current knowledge of the effect of pressure on the electrical properties of flames, explanations for these results are proposed.

Visualisation and performance evaluation of biodiesel/methane co-combustion in a swirl-stabilised gas turbine combustor

While dual fuel firing of power generation combustion systems can improve the fuel flexibility of such systems, several studies on compression ignition engines have also shown a positive impact on NOX and PM emissions. Previous multiphase fuel combustion studies for combustion turbines are limited, thus the present study addresses that gap by fuelling a model swirl stabilised gas turbine combustor with a blend of waste cooking oil-derived biodiesel and methane. Methane was increasingly injected into swirling combustion air flow while simultaneously reducing the biodiesel spray flowrate across a pressure atomiser, thus maintaining an overall equivalence ratio of 0.7 while delivering a thermal power output of 15 kW in all cases, except for flame stability range trials. Direct flame imaging, CH* and C2* chemiluminescence imaging, post combustion emissions as well as stability performance of the flames were evaluated. NOX emissions were found to decrease by 29% and unburnt hydrocarbons increased by 10% as the fraction of methane in fuel mix increased to 30%. Further, flame images suggest increased wrinkling and perturbing of the flame front as gas fraction of the biodiesel/methane flame increases. However, the temporal variation of integral intensity of CH* and C2* species chemiluminescence point to at least an 8% improvement in flame stability when 30% of flame heat output is supplied by methane compared to neat biodiesel burn. Also, it was found that flame stability limits reduce as methane partly replaces biodiesel in the flame.

The effects of hydrogen addition on the chemical kinetics of hydrogen-hydrocarbon flames: A computational study

In this paper, the effects of hydrogen addition on the chemical kinetics of hydrogen–hydrocarbon flames were investigated numerically. Profiles of maximum OH, O and H radical mole fractions, flame temperature and emission levels of hydrogen-methane, hydrogen-ethane and hydrogen-propane flames were computational obtained using Kintecus solver code. The simulations were performed by incorporating Konnov's hydrocarbon combustion mechanism at the stoichiometric condition and the constant pressure of 1 atm. It was found that a small hydrogen increase in the flame mixture can modestly affects the temperature and mole fraction profiles, however, the significant increase can be observed upon 40%, 60% and 80% of hydrogen addition for hydrogen-methane, hydrogen-ethane and hydrogen-propane flames respectively can lead to decrement of CO and CO2 emissions but increment in the combustion kinetics and the adiabatic flame temperature plus an extension of the flame stability limits. A comparison of the flames free radical profiles and the laminar burning velocity of the flames showed a strong correlation.

Experimental analysis of the effects of porous wall on flame stability and temperature distribution in a premixed natural gas/air combustion

AbstractIn the present analysis, the flame stabilization and temperature distribution within a premixed burner contain porous wall are studied experimentally. The effects of inner diameter, length, and pore density of the porous wall, thermal load, equivalence ratio, and the inlet velocity of the fuel‐air mixture on these are studied. The fuel used in this study is natural gas and the porous wall is SiC (silicon carbide) ceramic foam. The experimental results clearly indicate that the axial temperature along the porous wall increases when the inner diameter of the porous wall decreases and its length increases. The porous wall temperature with an inner diameter of 40 mm, length of 66 mm, and pore density of 30 PPI (pores per inch) has the highest temperature among the examined states. The results of studying the effect of the porous wall on flame stability show that the flame stability limit has a direct relationship with the length and pore density of porous wall and an inverse relationship with the inner diameter of the porous wall. Also, it is found that the porous wall has the highest temperature causes the maximum flame stability limit.