Effects of recessed air jet on turbulent compressed natural gas inverse diffusion flame shape and luminosity

Effects of the recession of the central air jet on the visible flame height, necking zone, and luminosity of a turbulent compressed natural gas-air inverse diffusion flame in a coaxial burner are investigated in this experimental study. The inner circular tube of the coaxial burner is recessed by 0.25d _a , 0.5d _a , and 1.0d _a , where d _a is the central tube inner diameter. From the visual observation, the flame height and the necking zone height are observed to decrease exponentially with the air jet Reynolds number with no recession of the central air jet. However, only a marginal reduction in the visible flame height is observed with an increase in the recession height of the air jet as compared to the necking zone height. Interestingly, the necking zone at the flame base disappears beyond the critical recession height of the central jet. Moreover, the recession is found to be effective in eradicating the fuel rich zone and soot ring at the flame base of turbulent compressed natural gas inverse diffusion flame at lower air jet Reynolds numbers.     

Spatial Structure of a Reacting Turbulent Swirling Jet Flow with Combustion of a Propane–Air Mixture

Results of an experimental study of the spatial structure of a reacting flow during combustion of a propane–air mixture in a turbulent swirling jet escaping into atmospheric air are presented. The fuel-to-air equivalence ratio is φ = 0.7, and the Reynolds number of the jet is Re = 5 · 10³. The time-averaged spatial distributions of velocity, local density, and concentrations of the main species of the gas mixture are measured in low-swirl and high-swirl flows. In both cases, the flame front is stabilized in the internal mixing layer formed by the axial region of jet retardation, where hot combustion products are concentrated. In a high-swirl flow, the temperature distributions in the cross section y/d = 0.5 show that the region with the maximum temperature of the gas is located at the periphery of the central recirculation zone. Moreover, in the case of a high-swirl flow, there exists a recirculation zone at the axis, and the CO₂ concentration is twice higher than in a low-swirl jet. The opposite situation is observed for O₂.     

Characteristics of the boundary layer with hydrogen combustion with variations of thermal conditions on a permeable wall

The paper describes a numerical study of the influence of thermal and boundary conditions on the structure of laminar and turbulent diffusion flames in the cases with hydrogen injection through a porous surface and with hydrogen combustion in an air flow. Two types of boundary conditions are compared: with a given constant temperature T _w = const over the length of the porous surface for arbitrary intensities of fuel injection and with a constant temperature T′ = const of the fuel injected through the porous wall. The first case occurs during combustion of a liquid fuel whose burning surface temperature remains unchanged. Injection of gaseous fuel usually leads to the second case with T′ = const. Despite significant differences in velocity and temperature profiles, the skin friction coefficients in the laminar flow are close to each other in these two regimes. In the turbulent regime, the effect of the thermal boundary conditions on friction and heat transfer is more pronounced. Moreover, the heat flux to the wall as a function of fuel-injection intensity is characterized by a clearly expressed maximum. A principal difference of the effect of combustion on friction and heat transfer in the laminar and turbulent flow regimes is demonstrated. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 3–11, May–June, 2009.     

Comparison of the structures of methane-air and propane-air partially premixed flames

A comparison of flame structures between methane-air and propane-air laminar partially premixed flames has been made through the centerline concentration distributions of selected species measured using gas chromatography. The concentrations of fuel, major species like O₂, CO and CO₂ and those of the intermediate hydrocarbons like C₂H₆, C₂H₄, C₂H₂ and CH₄ (for the propane flame only) have been compared. Distinct double flame structures are observed for the experimental conditions under study. With approximately the same equivalence ratio and jet velocity for the primary mixture, the height of the inner flame for propane is less than that of methane. The peak concentration of C₂H₆ in the propane flame is found to be only a little higher than that in the methane flame, while the peak concentrations of C₂H₄ and C₂H₂ are much greater in the propane flame than in the methane flame. In a methane partially premixed flame, the hydrocarbon concentrations drop from their peak values very rapidly at the inner flame tip, but in the propane flames it is more gradual. In a methane partially premixed flame, CO is formed at the inner flame and burns at the outer flame to CO₂. Similar distributions of CO and CO₂ are found in the propane flame also. However, the peak CO concentration in the propane flame is found to be higher than in methane flame. A radial measurement of species distribution for a particular case in the propane partially premixed flame is also done to ascertain the species distributions across the flame.     

Combustion of residual steel gases: laminar flame analysis and turbulent flamelet modeling

In recovery combustion systems operating in the steel industry, energy is provided by boilers burning residual gases of blast furnace and coke oven. To help understand combustion of this particular type of fuels, a numerical study is conducted where the major chemical properties of steel gas flames are collected. The chemical composition of representative fuel and oxidizer steel gas is varied over a large range in calculations using detailed chemistry and complex transport properties. The chemical equilibrium compositions, premixed flame speeds and diffusion flame extinction strain rates are determined. The advantages and shortcomings of the use of vitiated air emerge, and its introduction into the boiler appears as an interesting alternative to reduce NO_x emission. The detailed information obtained with laminar flame calculations is also introduced in flamelet turbulent combustion modeling. Reynolds Averaged Navier Stokes (RANS) simulations of a test case burner are performed and some comparisons between numerical predictions and experimental results are presented.     

旋流燃烧室内冷态和热态流场的三维数值模拟分析

采用RNGk-ε湍流模型以及有限化学反应速率涡破碎模型(Eddy break up)对Allied Signal公司75 kW微型燃气轮机旋流燃烧室内冷态和热态时气流三维流动过程进行了模拟计算。结果表明,燃烧室火焰筒内热态时与冷态时的流场相似,烧室火焰筒内都有3个回流区,3个回流区的存在将有助于燃料的连续点火和火焰的稳定;燃烧室内热态三维流场的中心回流区长度小于冷态时的中心回流区长度,气流的轴向速度也大于相应的冷态轴向速度,而且掺冷孔空气射流对环向速度分布的影响小于冷态时的影响。     

Characteristics of diesel fuel combustion in a burner with injection of a superheated steam jet

Basic characteristics of combustion of the diesel fuel in a novel autonomous burner with injection of superheated steam into the combustion region are studied. The temperature distribution in the flame is obtained. Calorimetric measurements of heat release and gas analysis of combustion products are performed. The environmental effects of fuel combustion are compared for regimes with injection of a steam jet and an air jet. It is demonstrated that the combustion regime with steam gasification ensures high combustion intensity and combustion efficiency; moreover, the combustion process becomes more environmentally friendly.     

Relationship between the dust flame propagation velocity and the combustion mode of fuel particles

A possibility of determining the regime of combustion of individual fuel particles on the basis of the dependence of the flame velocity on the fuel and oxidizer concentrations is considered by an example of a dust flame of microsized metal particles with diameters d ₁₀ < 15 μm and particle concentrations from ≈10¹⁰ to 10¹¹ m^?3 in oxygen-containing media at atmospheric pressure. The combustion mode (kinetic or diffusion) is responsible for the qualitative difference in the character of the normal velocity of the flame as a function of the basic parameters of the gas suspension. The analysis of such experimental dependences for fuel-rich mixtures shows that combustion of zirconium particles (d ₁₀ = 4 μm) in a laminar dust flame is controlled by oxidizer diffusion toward the particle surface, whereas combustion of iron particles of a similar size is controlled by kinetics of heterogeneous reactions. For aluminum particles with d ₁₀ = 5–15 μm, there are no clearly expressed features of either kinetic or diffusion mode of combustion. To obtain more information about the processes responsible for combustion of fine aluminum particles, the flame velocity is studied as a function of the particle size and initial temperature of the gas suspension. It is demonstrated that aluminum particles under the experimental conditions considered in this study burn in the transitional mode.     

Gas concentration and temperature in acoustically excited Delft turbulent jet flames

This paper shows the experimental results for changes in the flame structure when acoustic fields are applied in natural gas Delft turbulent diffusion flames. The acoustic field (pulsating combustion) generates zones of intense mixture of reactants in the flame region, promoting a more complete combustion and, consequently, lower pollutant emissions, increase in convective heat transfer rates, and lower fuel consumption. The results show that the presence of the acoustic field changes drastically the flame structure, mainly in the burner natural frequencies. However, for higher acoustic amplitudes, or acoustic pressures, a hydrogen pilot flame is necessary in order to keep the main flame anchored. In the flame regions where the acoustic field is more intense, premixed flame characteristics were observed. Besides, the pulsating regime modifies the axial and radial combustion structure, which could be verified by the radial distribution of concentrations of O₂, CO, CO₂, and NO_x, and by the temperature profile. The experiments also presented the reduction of flame length with the increase of acoustic amplitude.     

Flame length scaling in a non-premixed turbulent diluted hydrogen jet with coaxial air

The effect of fuel composition on flame length was studied in a non-premixed turbulent diluted hydrogen jet with coaxial air. Because coaxial air entrained in a fuel stream enhances the mixing rate of fuel and air, it substantially reduces flame length. The observed flame length was expressed as a function of the ratio of coaxial air to fuel jet velocity and compared with a theoretical prediction based on the velocity ratio. Four cases of fuel mixed by volume were determined: 100% H₂, 80% H₂/20% N₂, 80% H₂/20% CO₂, and 80% H₂/20% CH₄. In addition, fuel jet air velocity and coaxial air velocity were varied in an attached flame region as u_F = 86-309 m/s and u_A = 7-14 m/s. In this study, we derived a scaling correlation for predicting the flame length in a simple jet with coaxial air using the effective jet diameter in a near-field concept. The experimental results showed that the visible flame length was in good relation to the theoretical prediction. The scaling analysis is also valid for diluted hydrogen jet flames with varied fuel composition, which affects flame length by varying the density of the fuel.     

The effect of CO2 dissolved in a diesel fuel on the jet flame characteristics

This paper is concerned with an experimental study of the jet diffusion flame characteristics of fuel containing CO₂. Using diesel fuel containing dissolved CO₂ gas, experiments were performed under atmospheric conditions with a diesel hole-type nozzle of 0.19 mm orifice diameter at constant injection pressure. In this study, four different CO₂ mass fraction in diesel fuel such as 3.13%, 7.18%, 12.33% and 17.82% were used to study the effect of CO₂ concentration on the jet flame characteristics. Jet flame characteristics were measured by direct photography, meanwhile the image colorimetry is used to assess the qualitative features of jet flame temperature. Experimental results show that the CO₂ gas dilution effect and the atomization effect have a great influence on the flame structure and average temperature. When the injection pressure of diesel fuel increased from 4 MPa to 6 MPa, the low temperature flame length increased from 18.4 cm to 21.7 cm and the full temperature flame length decreased from 147.6 cm to 134.7 cm. With the increase of CO₂ gas dissolved in the diesel fuel, the jet flame full length decreased for the jet atomization being improved greatly meanwhile the low temperature flame length increased for the CO₂ gas dilution effect; with the increase of CO₂ gas dissolved in the diesel fuel, the average temperature of flame increases firstly and then falls. Experimental results validate that higher injection pressure will improve jet atomization and then increased the flame average temperature.     

Experimental studies of bluff-body stabilized LPG diffusion flames

Bluff-body stabilized turbulent jet diffusion flame has received renewed attention in recent years due to its practical applications. An experimental study is carried out to investigate the effect of coaxial air velocity, U_a, and lip-thickness, δ of the bluff-body on the flame stability limits and emission levels. The stability limits of a typical diffusion flame can be characterized in terms of two parameters namely flame lift-off height and blow-off velocity. It is experimentally observed that lift-off height is not linearly dependent on the fuel exit velocity, U_f, as compared to the simple jet. The flame stability is found to be improved for larger lip-thickness bluff-body because of the presence of lower pressure in the wake region behind the bluff-body. Flame length is observed to be dominated by buoyancy and momentum regimes. The transition from buoyancy to momentum regime is found to be extended with increase in lip-thickness. It is also observed that the blow-off limit is also extended further by 10% as compared to simple jet diffusion flames under similar conditions. The emissions data are reported in terms of mass based emission index, EINO_x (g [NO_x]/kg [fuel]) for a wide range of flow conditions. It is concluded that the addition of coaxial air in the larger lip-thickness bluff-body flames causes a marginal reduction in emission levels relative to smaller lip-thickness bluff-body.     

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

In the study the lift-off, blow-out and blow-off stability limits of hydrogen/propane flames and hydrogen/carbon dioxide flames were tested in three different mixing arrangements. The first was to premix hydrogen with carbon dioxide or propane to form a jet flame. The second was to add the gas as an annular jet around the hydrogen flame. The third was to inject into the centre of the hydrogen flame. Propane and carbon dioxide have the same density but create very different chemical kinetic changes when added to hydrogen flames. The results showed that when premixed with hydrogen, propane is more effective in flame lift-off and blow-out. The analysis of kinetic mechanisms revealed that the propane is the dominating fuel in determining the burning rate of the hydrogen/propane while carbon dioxide mainly acted to dilute the hydrogen/CO₂ mixture. Comparing the three mixing arrangements, the experiments showed that hydrogen flame can be effectively lifted or blown out when gases were in annular flow around the hydrogen flame. The isothermal mixing process of the co-flow configuration was discussed.     

Effect of addition of CO2 to the fuel on combustion of gas jets in air

Results of an experimental study of the effect of addition of CO₂ to methane and to a propane-butane mixture on conditions of stabilization of the diffusion flame in air are reported. It is shown that the flame lift-off length substantially increases with increasing fraction of CO₂ in the fuel; the main reason is the changes in the velocity and concentration in the region of the “ignition” points; a secondary factor is an increase in the characteristic combustion time. Data on the influence of the CO₂ admixture in the fuel in amounts greater than the fuel flow rate up to ≈1.5 times on the characteristic combustion time are obtained.     

OH concentration fluctuations in turbulent natural gas jet flames

OH radical concentrations in a turbulent non-premixed natural gas flame were measured using laser-induced fluorescence. Instantaneous concentration profiles along a line were obtained using a diode array camera. Investigation of the molecular transitions during laser excitation shows that concentrations, where calibrated with a one-dimensional laminar premixed flame, are biased with a factor of about two. This bias is similar for different flames, so that results of different flames can be compared with an accuracy of about 20%. Three different flames were studied, with fuel jet Reynolds numbers of 9.7 × 10³, 6.8 × 10³ and 4.9 × 10³. Average concentrations and probability density functions show that concentrations close to the nozzle in the flame with highest turbulence are low, which may indicate local extinction. Integral length scales and Taylor micro scales, derived from spatial correlation, exhibit minima at radical locations where OH fluctuations exhibit maxima.     

Investigation of feedpipe backmixing in an agitated vessel

Feedpipe backmixing in an agitated vessel was investigated using a newly developed conductivity technique. By this technique, the onset of feedpipe backmixing could be detected and the penetration depth of the vessel fluid into a feedpipe was determined. For a given feedpipe flowrate. critical agitator speeds to eliminate feedpipe backmixing were determined using Rushton six-bladcd disk turbine impeller (6BD) and high efficiency, axial-flow type 3-bladed impeller (HE-3) of 8.89 and 12.70 cm diameters in 11.2 liter reactor. The ratio of the feedpipe velocity to the critical agitator speed (v _f /v _i ) was determined as a function of feedpipe Reynolds number (N _R,T ). The conductivity technique was successful either in the laminar regime, the transitional regime, or in the turbulent regime in the feedpipe.     

Hydrodynamics Characterization of an Ellipse Gate Impeller by Experimental and Numerical Studies

Hydrodynamics characteristics like flow pattern, shear rate distribution, power consumption, axial pumping capacity, mixing time, and mixing efficiency of an ellipse gate (EG) impeller were investigated by experimental and numerical methods. The numerical simulation results were validated by experimental data of power consumption and mixing time. Results indicate that the axial pumping number of the EG impeller is larger than that of any other reported large‐scale impeller under laminar regime, and that the shear rate formed by this impeller is less sensitive to Reynolds numbers. In‐depth analysis reveals the different function of each part of the EG impeller under different flow regimes. This impeller provides an almost similar mixing efficiency like the double‐helical ribbon impeller under laminar regime, but much higher mixing efficiency both under transitional and turbulent flow regimes.     

Laminar flame speeds and extinction limits of conventional and alternative jet fuels

Experimental results of laminar flame speeds and extinction stretch rates for the conventional (Jet-A) and alternative (S-8) jet fuels are acquired and compared to the results from our earlier studies for neat hydrocarbon surrogate components, including n-decane and n-dodecane. Specifically, atmospheric pressure laminar flame speeds are measured using a counterflow twin-flame configuration for Jet-A/O₂/N₂ and S-8/O₂/N₂ mixtures at preheat temperatures of 400, 450, and 470 K and equivalence ratios ranging from 0.7 to 1.4. The flow field is recorded using digital particle image velocimetry. Linear extrapolation is then applied to determine the unstretched laminar flame speed. Experimental data for the extinction stretch rates of the nitrogen diluted jet fuel/oxidizer mixtures as a function of equivalence ratio are also obtained. In addition, the experimental data of Jet-A are compared to the computed values using a chemical kinetic mechanism for a kerosene surrogate reported in literature. A sensitivity analysis is further performed to identify the key reactions affecting the laminar flame speed and extinction stretch rate for this kerosene surrogate.     

Emission studies of impinging premixed flames

This paper reports the results of an experimental investigation of emissions from an impinging flame to a flat cold surface. This study pertains to the fuel rich premixed flame issuing from a 8-mm diameter nozzle burner. The effects of burner to plate spacing, equivalence ratio and Reynolds numbers on the flame structure and emission are investigated. The flame structure is characterized by temperature measurement. The axial temperature profiles show that it is strongly dependent on the distance between the burner and cold surface. The temperature in the radial direction declines sharply after certain distance from the center of the plate which is also seen to depend on the separation distance. It has been found out that the CO level increases for separation distance greater than 12 nozzle diameter for all Reynolds number which may be attributed to the excess entrainment of air leading to the dilution of mixture. However, the CO level also increases with increase in equivalence ratio for same separation distance. The NO level decreases with increase in equivalence ratio and Reynolds number. The separation distance of 12 nozzle diameter case gives higher NO level for all Reynolds number. The CO₂ level increases with equivalence ratio for all Reynolds number. It is believed that these experimental data will be useful for designing and developing rapid heating devices.