Shorter Flame Length Research Articles

Experimental and numerical study on a cavity-swirler-based combustion strategy for advanced gas turbine engine

A cavity-swirler-based combustor has been found to possess the capability of achieving robust stability, high combustion efficiency, low emissions, and a short flame length. However, the effect of cavity air-injection mode on the combustion and emission performances of the cavity-swirler-based combustor was not well understood to date. In this work, numerical and experimental investigations were conducted to analyze the flowfield, combustion efficiency, combustion stability, pollutant emissions, temperature distribution, and flame characteristics of the combustor under two different cavity air-injection modes, namely case-1 and case-2. The air-injection hole of case 1 was designed on the cavity inner-wall, while the air-injection hole of case 2 was designed on the cavity outer-wall. The results indicate that the combustion performance of the combustor is significantly influenced by the air-injection mode, leading to different flow fields within the combustor. Case 1 demonstrates notable advantages in terms of lean blowout limit compared to case 2. However, case 2 achieves a much more uniform distribution of outlet temperature. Higher combustion efficiency was achieved by case-1 under low fuel equivalence conditions, whereas case-2 exhibited greater combustion efficiency under high fuel equivalence conditions. Both case-1 and case-2 displayed distinct combustion zones that were separate from each other.

Investigation of evaporation and combustion characteristics of diesel and fatty acid methyl esters emulsified fuel droplets

To address the problems of energy shortages and environmental pollution, biodiesel and its emulsified fuel have gained wide attention. In this work, the fatty acid methyl esters (FAME) emulsified water-in-oil fuel (BW) is thoroughly investigated using the droplet suspension experiment with a focus on the main characteristics of puffing/micro-explosion, droplet ignition delay and flame regime, etc. The role of water content at different ambient temperatures is examined in comparison to a counterpart blend of diesel/water (DW) fuel. Results show that the increase in water content and temperature has a significant promotion effect on the strength and number of puffing/micro-explosion process, however, the higher viscosity of BW leads to the lower values. And owing to the heat absorption by water evaporation, only the micro-explosion with high intensity induced by water nucleation has a favorable effect on the evaporation rate. There exists a critical concentration of water addition, after which the enhancement of evaporation is expected. Moreover, it is indicated that the diesel with water additives has a better atomization effect, and the size of child droplets generated by DW droplets breakup is smaller, which is in the range of 0.08–0.1 mm compared to the 0.09–0.13 mm for BW droplets. It has been shown that the increase in water content hinders the rise of droplet flame temperature, extends the ignition delay time, and shortens the maximum premixed-flame length. Compared to diesel droplets, the larger combustion rate of BW droplets is correlated with their shorter flame length and the higher boiling point of FAME.

Combustion and emission performance of a trapped-vortex gas turbine combustor fueled by RP-3 kerosene

In this study, a trapped-vortex gas-turbine combustor was designed and investigated to simultaneously achieve robust stability, low pollutant emissions, and the desired temperature distribution within a short flame length. In the combustor, a trapped-vortex combustion is established using two separated cavities. A swirler was adopted in the mainstream to improve mixing between the hot products of the cavity and mainstream air. Experimental tests conducted under atmospheric pressure indicate that the combustor can achieve encouraging performance in terms of flame stability, pollutant emissions, outlet temperature profiles, and flame length. Improvements of 19.2% and 23.3% were achieved by the lean ignition and blowout performances, respectively, as the combustor inlet temperature increased from 373 K to 473 K. As the fuel equivalence ratio increased from 0.25 to 0.55, the NOx emissions increased slightly, whereas the CO emissions first decreased and then increased slightly. Furthermore, two separated short flames with uniform outlet temperature profiles were obtained using the combustor. Interestingly, an “M” pattern temperature profile is obtained corresponding to the swirler center plane as compared to the parabola form outlet temperature profile corresponding to the 20-mm left side of the swirler center plane.

Grazing Effects on Fuels Vary by Community State in Wyoming Big Sagebrush Steppe

Limiting fire in Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis [Beetle & A. Young] S.L. Welsh) steppe is often a management priority as fires threatens its ecological integrity and rural economies that depend on it. However, the Wyoming big sagebrush steppe is vast and occurs in different community states from intact (sagebrush-bunchgrass dominated) to exotic annual grass dominated. Grazing has been suggested as the only tool that is likely feasible to apply across such large landscapes to manage fine fuels, but there is concern that over time grazing may induce undesirable shifts in plant community composition (e.g., increases in exotic annuals) that increase fire risk. Therefore, we evaluated the longer-term (+10 yr) effects of contemporary, moderate grazing by cattle compared with grazing exclusion on fuel characteristics in three community states: intact, degraded, and exotic annual grass states. We accomplished this by measuring fuel characteristics in five grazed and ungrazed areas in each community state in 2020 and 2021. Grazing generally decreased fine fuel continuity and biomass and increased the live-to-dead ratio. These fuel alterations are consistent with decreasing the probability of fire ignition and, if ignited, producing a slower spreading fire with shorter flame lengths. The response of several fine fuel characteristics to grazing varied by community state. Fine fuel characteristics also commonly varied among community states and between years. These results suggest that fuel management plans need to recognize that grazing effects will vary by community state and be flexible because fuel characteristics vary spatially and temporally. Overall, our results suggest that contemporary grazing in the Wyoming big sagebrush steppe reduces the probability of wildfire and likely increases the effectiveness and safety of fire suppression. Consequently, grazing exclusion in these communities increases the probability of frequent, large wildfires that are difficult and dangerous to suppress.

Acoustic-Convective Interference in Transfer Functions of Methane/Hydrogen and Pure Hydrogen Flames

Abstract We investigate the occurrence and source of modulations in the gain and phase of flame transfer functions (FTF) measured in perfectly premixed, bluff body stabilized CH4/H2 and pure H2 flames. The modulations are shown to be caused by flow disturbances originating from the upstream geometry, in particular the grub screws used to center the bluff body, indicative of a more generalized phenomenon of convective wave propagation. Velocity measurements are performed at various locations around the injector dump plane, inside the injector pipe, and in the wake of the bluff body to provide detailed insight into the flow. Peaks corresponding to natural shedding frequencies of the grub screws appear in the unforced velocity spectra and it is found that the magnitude of these convective modes depends on their location. Flame imaging and PIV measurements show that these disturbances do not show up in the mean velocity and flame shape which appear approximately axisymmetric. However, the urms and vrms fields capture a strong asymmetry due to convective disturbances. To further quantify the role of these convective disturbances, hydrodynamic transfer functions are constructed from the forced cold flow, and similar modulations observed in the FTFs are found. A strong correlation is obtained between the two transfer functions, subsequently, the modulations are shown to be centered on the vortex shedding frequency corresponding to the first convective mode. The reason behind the excitation of the first mode is due to a condition that states that for acoustic-convective interaction to be possible, the shedding (convective) frequency needs to be lower than the cutoff frequency of the flame response. This condition is shown to be more relevant for hydrogen flames compared to methane flames due to their shorter flame lengths and thus increased cutoff frequency.

Numerical simulation on velocity and temperature of a slotted swirl combustor

Cyclone combustion chamber has the advantages of compact structure, short flame length, good combustion stability, high combustion efficiency and low emissions of pollutants, etc. As one of the key components of the cyclone combustion chamber, the cyclone has a direct impact on the working performance of the combustion chamber. In this paper, we propose a slotted swirler and describe its structure in detail. We simulate the cold state flow field and the hot state temperature field in the combustion chamber of a cyclone with open seam using the CFD method, taking into account the effect of open seam on the air flow field and temperature distribution after the cyclone at different angles. The simulation shows that the temperature distribution in the slotted swirl combustor exhibited a center-focused flame of higher temperature, compared with the non-slotted swirler, the slotted swirler exhibits certain advantages in terms of drag reduction, combustion stability and combustion efficiency, which has a certain guiding effect on the improvement of the working performance of the combustion chamber.

Effect of H2 enrichment ratio and N2/CO2 dilution on swirl-stabilized partially premixed H2/CH4/C3H8 SNG combustion

This paper discusses the N2 and CO2 dilution effects and H2 enrichment ratio of H2/CH4/C3H8 synthetic natural gas (SNG). As H2 enrichment ratio increases, the dump plane temperature and the combustion zone temperature increase because of shortened length of H2 flame. Even with a shorter flame length and a narrower chemical reaction regime, NOx emissions increase at same heat load because energy density is inversely proportional to flame length. As the CO2 diluent has a higher specific heat than the N2 diluent, the flame temperature is more greatly reduced by the effect of diluent heat capacity. Even a small amount of CO2 dilution can effectively reduce NOx emissions. The H2 enrichment ratio increases the OH radicals in the flame due to the increase in flame temperature. In addition, the burning velocity increases because the reaction rate in the flame increases, resulting in a decrease in flame length. NOx emissions increase exponentially with respect to global OH∗ chemiluminescence intensity per heat load, and these fit well into a single line. The results show that N2 and CO2 diluents can be used in SNG turbine combustors to control NOx emissions and that the H2 enrichment ratio in SNG.

Effect of solid inertants and sample inclination angle on fire hazard of metallic powder layers

Compared to non-metallic powders, metallic powders usually have a greater layer fire hazard because of high flame temperatures. In this paper, the effects of solid inertants and sample inclination angle on the fire hazard of three metallic powder (iron, titanium, and magnesium) layers were investigated. Flame spread velocity (FSV) was mainly dependent on the particle size and enthalpy of combustion of metallic powders. For example, FSV of iron powders jumped from 0.4 to 4.7mm/s when the particle size decreased from micro scale to nano scale. The FSV of micro Ti powder layers reached 12.8mm/s, which was much higher than the value (1.7mm/s) for micro Mg powder with similar particle size. Inclination angle significantly influenced the FSV of nano Ti powder because of a long flame length resulting from a predominantly homogeneous reaction. For other tested powders, sample inclination angle had little effect on FSV because of a short flame length resulting from a predominantly heterogeneous reaction. Addition of small amounts of solid inertant decreased the layer fire hazard of iron and titanium powder with high melting points, while increasing the fire hazard of magnesium powder because of its low melting point. The melting characteristics of metallic powders should be considered when preventing or mitigating fire hazard using solid inertant technology.

Radiant Heat Flux Profile of Horizontally Oriented Rectangular Source Fuel Jet Fires

A series of numerical simulations were performed to investigate the flame radiant heat flux of horizontally oriented jet fires by rectangular source fuel with the same orifice area (400 mm2) but different aspect ratios (1, 2, 4). The natural gas was carried out as fuel, of which the jet velocity varied from 27.5 to 205.8 m/s. The nondimensional radiant heat flux was defined to reveal the effect of fuel jet velocity and orifice aspect ratio on radiant heat flux. Results show that the flame size increases with the increasing fuel jet velocity and the larger orifice aspect ratio results in shorter flame length. The radiant heat flux is affected by the orifice aspect ratio as the different soot particle distribution. The predicted model to characterize the radiant heat flux is proposed by taking the orifice aspect ratio into account. The prediction results of proposed model agree well with the simulated data and previous experimental results.

KESAN SUDUT PUSARAN TERHADAP PEMBAKARAN MENGGUNAKAN PEMUSAR JEJARIAN DWI ALIRAN

Development of combustion systems which involves retrofitting or design of new burners, is made to reduce the formation of pollutant emissions. The reduction of this pollutant emission results from the complete mixing of fuel and air during the combustion process. Meanwhile, non-complete mixing of fuel and air during combustion process can cause ignition problem and create problems in terms of flame stability and combustion efficiency. This article describes a study on the effects of swirling flow generated by a double radial swirler on flame characteristics that is related to the emission of NO. The double radial swirlers used in this study have the angles of 30°/40°, 30°/50° and 30°/60°. Diesel is used as a fuel in this study. The results show that all double radial swirlers used have different effects on the flame characteristics and temperature profile. From all these double radial swirlers, the one with an angle of 30°/60° produces flame with high temperature, short flame length with blue colour and wide spread.

Effect of Flame Angle Using Various Swirler Angle in Combustion Performance

The abbreviation N is commonly used as a reference to all oxides of nitrogen and the examples of oxides of nitrogen are NO, N and O. These emission gases mostly come from the combustions of fossil fuels and biofuels in industrial activities and also in road traffics globally. N as a pollutant deserves special attention because of its various issues on human’s health and environment. Some of these issues are global warming, acid rain, ozone layer depletion and photochemical smog. To address these issues, some studies are concentrating on the reduction of NO emission from the combustion process. This paper describes a study in the effects of swirling flow generated by using a radial swirler on flame characteristics that is related to the emission of NO. The radial swirlers used in this study have the angles of 40°, 50° and 60°. Diesel is used as a fuel in this study. The results show that all radial swirlers used have different effects on the flame characteristics. From all these radial swirlers, the one with an angle of 60° produces flame with high temperature, short flame length with blue colour and wide spread. The results also show a short time residence during combustion process that could reduce the formation of CO and N .

Fundamental Studies of NOx Emission Characteristics in Dimethyl Ether (DME)/Air Non-premixed Flames

The NOx emission characteristics of dimethyl ether (DME) in laminar co-axial jet and counterflow non-premixed flames were investigated using experimental and numerical approaches, respectively. The flame structure and NOx emissions of DME were compared to those of C2H6, which has equivalent methyl structures but lacks an oxygen atom. Experimental results showed that, in the co-axial jet case, the combustion of DME had the characteristics of a partial premixed flame. Additionally, it had a shorter flame and lower NOx emissions compared to the C2H6 flame. It is thus concluded that the major cause of low NOx emissions from DME co-axial jet flames may be the short flame length because of the lower stoichiometric air/fuel ratio. The activation of reburning NO chemistry because of the characteristics of the partially premixed flame may also play a role. The computational results of the DME counterflow non-premixed flame revealed that the EINO decreased by approximately 50% relative to that of the C2H6 flame. Although the overall NOx reaction path of the DME flame is similar to that of the C2H6 flame, it is concluded that the DME non-premixed flame has a distinct NO reduction mechanism. This is associated with reburning NO chemistry in the fuel-rich region because of fast pyrolysis and oxidation reactions in comparison to that of the C2H6 flame.

OS1-5 超小型ガスタービン用アニュラー型燃焼器内の高負荷プロパン火炎の燃焼特性(OS1 マイクロエネルギー変換,循環型社会における動力エネルギー技術)

Combustion characteristics of propane flame with high space heating rate in an annular-type combustor for a several hundreds W-class micro gas turbine were investigated. The flame shape, flame stability limit, flame length and temperature distribution in the combustor were measured varying the orifice ratios, which were 0, 0.19 and 0.36, as the experimental parameter. It was considered that the flame shape was influenced by the combustor axial velocity at the orifice, orifice ratio and fuel/air concentration in the orifice upward region. In the case of orifice ratio of 0.19, a circular propane flame with short flame length was observed in the orifice upward region. Temperature distribution in the combustor showed that this flame could have high space heating rate.

The influence of geometric nozzle profile on the global properties of a turbulent diffusion flame

A detailed comparison of the global performance characteristics of flames issuing from a long pipe and smooth contraction nozzle is presented for the first time. The measurements reveal consistently that the well-established differences in the turbulent source flow propagate far downstream to influence every performance parameter of the flame. This gives further proof to recent findings in non-reacting flows that a widely believed classical theory, arguing that differences in emerging flow do not propagate into the far field, is flawed. Measurements show that the pipe jet has a shorter flame length and smaller projected area than the contraction nozzle, implying a smaller volume and a higher rate of molecular mixing. This is explained by a new analysis of existing cold-flow data, which shows that a segregation parameter, characterising molecular mixing, is closer to zero for the pipe jet, even though the smooth contraction nozzle has a higher rate of mean decay and spread of the scalar field. The radiant fraction and emission indices are also consistent with the pipe jet having higher mixing at the molecular level. New evidence is also presented of differences in the base of the flame, with significant differences in their lift-off behaviour identified.

CO/UNBURNED HYDROCARBON EMISSIONS OF STRONGLY-PULSED TURBULENT DIFFUSION FLAMES

The CO and unburned hydrocarbon (UHC) emissions of pulsed turbulent diffusion flames were examined by injecting unheated ethylene into a combustor with an air coflow at atmospheric pressure. In all cases the flames were fully modulated (fuel flow fully shut off between injection intervals). Video imaging was performed and time-averaged emissions were measured on the combustor centerline. For short injection times ( ≤ 46 ms), compact, puff-like structures were generated. Longer injection times produced elongated flame structures with flame lengths closer to that of steady flames. The highest emission indices of CO and UHC were found for compact, isolated puffs. The emissions for all flames approached the steady-flame levels for a duty cycle (jet-on fraction) of approximately 0.4. This suggests that there are combinations of injection time and duty cycle for fully modulated, turbulent diffusion flames that produce emissions comparable to the steady flame but with a significantly shorter flame length.

Combustion Air Jet Influence on Primary Zone Characteristics for Gas-Turbine Combustors

Results are presented of emissions and lean blowout measurements in a generic combustor primary zone of an aircraft gas-turbine engine for the effects of opposed, circular combustion air jets of a range of sizes, positioned half a dome height downstream from the dome. The primary zone was operated at simulated engine high-power conditions and representative dome and liner pressure drops. Most of the NO x is generated in the jets. Although an optimum jet size is identified, the lowest overall emissions resulted from a no-combustion air-jet configuration; air jets are necessary, however, to confer lean blowout stability and short flame length.

D126 噴霧燃焼器におけるエア供給方法の改善によるエミッション低減

To reduce its size and improve exhaust emission property, it is effective for spray type combustor to increase burning velocity by promoting such as fuel evaporation, diffusion, mixture of fuel and air. Excessive swirl inhibit the mixture in centrifugal direction of fuel and air to deteriorate combustion condition. This paper reports the coexistence of high load combustion and emission reduction experimentally confirmed having shorter flame length, compared to swirl type combustor, by making many air injection holes along combustion cylinder axis which causes creating small turbulence.

Structure and Flame Length of Fully-Modulated, Turbulent Diffusion Flames

The turbulent flame structure and flame length of fully-modulated diffusion flames was examined over a range of pulsing frequencies, injection flow rates, and duty-cycles. An injection system employing an electronically-controlled solenoid value was used to discharge puffs of unhealed natural gas and ethylene fuel into still air at one atmosphere pressure. Video imaging of the luminosity from the sooting regions of the flame revealed two distinct types of flame structure. For small injected volumes and short injection times, compact, puff-like structures with a short flame length were generated. More elongated “cigar-shaped” structures, with a longer flame length closer to that of steady-state flames, resulted from longer injection times and larger injected volumes. An injection parameter which characterizes the transition from puff-like to cigar-shaped flame behavior is presented. For puffs, an increase in duty-cycle generally led to an increase in flame length. This increase was less for cigar-shaped flames. The downstream location of the puff-like flame structures increased roughly with time to the 1/2 power, in agreement with buoyant thermals and starting jets.

Effects of Flame Lifting and Acoustic Excitation on the Reduction of NOx Emissions

Abstract The effect of the combustion fluid dynamics of a lifted jet flame with and without acoustic excitation on the control of NOx formation in the flame is investigated. A partially premixed jet was used, operated under lift-off flame bifurcation condition in the hysteresis region. Results show that flame lifting and acoustic excitation are effective in reducing the emission index of NOx (EINOx). A lean premixed condition, achieved by the strong upstream mixing of the lifted flame base, can further reduce NOx formation without increasing CO emissions. The prompt NO mechanisms of super-equilibrium OH concentrations and three-body recombination through N20 intermediates dominate in the initial region close to the flame base of the lean premixed, lifted flame. A lower initial prompt NOx and shorter flame length with reduced flame temperature, caused by the enhanced upstream mixing due to flame lifting and acoustic excitation, result in the low NOx and CO emissions in the present lean premixed jet flame.

Studies of Lean Blowout in a Step Swirl Combustor

The design requirements of a modern gas turbine combustor are increasingly dictated by wide stability limits, short flame length, and uniform mixing. To achieve the best trade-off between these three factors, flame characteristics (length, shape, mixedness), lean blowout (LBO), and optimum combustor configuration should be investigated over a wide range of inner and outer air velocities, inner and outer vane angles, and co- versus counterswirl arrangements. Such an investigation was performed in a step swirl combustor (SSC) designed to simulate the fuel–air mixing pattern in a gas turbine combustor dome fitted with an airblast atomizer. It was found that an increase in the outer vane angle and a decrease in inner air velocity decreased the flame length. LBO was improved when outer flow swirl intensity was increased. An optimum hardware and velocity configuration for the SSC was found for inner swirl = 45 deg, outer swirl = 60 deg, coswirl direction, and inner air velocity = outer air velocity = 16 m/s. This optimum SSC configuration yielded: (i) low values of LBO, (ii) short flame length, (iii) uniformly mixed stable flame, and (iv) little or no variation in these characteristics over the range of operation of SSC. Finally, the co- versus counterswirl arrangements and the operation of the optimized combustor configuration are discussed.