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.     

Liftoff height behavior in a non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen lifted flames, the experimental approximation of liftoff height in non-premixed turbulent conditions was studied. The objectives were to analyze liftoff height behavior and to derive the normalized expression for lifted jet with the effective diameter (d_F,eff). Hydrogen flow velocity varied from 100 m/s to 300 m/s. Coaxial air velocity was regulated from 12 m/s to 20 m/s. For the simultaneous measurement of velocity field and reaction zone, particle image velocimetry using hydroxyl radicals (PIV/OH) planar laser-induced fluorescence (PLIF) techniques with neodymium-droped yttrium–aluminum-gamet (Nd:YAG) lasers and charge-coupled device/intensified charge-coupled device (CCD/ICCD) cameras were used. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The stabilization point was defined as the point where local flow velocity is balanced with turbulent flame propagation velocity. On the basis of the far field concept, we could derive the experimental approximation of the liftoff height divided by the effective diameter.     

The effect of diluent on flame structure and laminar burning speeds of JP-8/oxidizer/diluent premixed flames

Experimental studies have been performed to investigate the flame structure and laminar burning speed of JP-8/oxidizer/diluent premixed flames at high temperatures and pressures. Three different diluents including argon, helium, and a mixture of 14% CO₂ and 86% N₂ (extra diluent gases), were used. The experiments were carried out in two constant volume spherical and cylindrical vessels. Laminar burning speeds were measured using a thermodynamics model based on the pressure rise method. Temperatures from 493 to 700 K and pressures from 1 to 11.5 atm were investigated. Extra diluent gases (EDG) decrease the laminar burning speeds but do not greatly impact the stability of the flame compared to JP-8/air. Replacing nitrogen in the air with argon and helium increases the range of temperature and pressure in the experiments. Helium as a diluent also increases the temperature and pressure range of stable flame as well as the laminar burning speed. Power law correlations have been developed for laminar burning speeds of JP-8/air/EDG and JP-8/oxygen/helium mixtures at a temperature range of 493-700 K and a pressure range of 1-10 atm for lean mixtures.     

Measurement of laminar burning velocity of dimethyl ether-air premixed mixtures

Measurement of laminar burning velocity of dimethyl ether-air mixtures was taken under different initial pressures and equivalence ratios using a constant volume bomb and high-speed schlieren photography. The stretched laminar burning velocity increases with the increase of stretch rate. At equivalence ratio of 1.0, low initial pressure gives high stretched flame speed. At initial pressure less than 0.1 MPa, the stoichiometric mixture gives the higher value of stretched flame speed than those at ? = 1.2 and ? = 0.8. The Markstein numbers decrease with the increase of equivalence ratio, and this reveals that lean mixture will maintain higher stability of flame front surface than that of rich mixture in dimethyl ether-air premixed flames.     

Determination of schmidt number of mixed fuels by the characteristics of laminar lifted jet flames

A method to determine the Schmidt number of fuel is proposed from the behavior of laminar lifted jet flames. Based on the observation of a laminar lifted flame edge, the flame stabilization point is located along the stoichiometric contour in the mixing layer of fuel and air in laminar jets, since a tribrachial (triple) flame structure exists which is composed of a diffusion flame, a rich premixed flame and a lean premixed flame, all extending from a single location. For the flame edge to be stationary, the axial velocity at the edge should balance with the propagation speed of the tribrachial flame. Since the region between the flame stabilization point and the nozzle exit can be treated as a cold jet, the jet theory of momentum and species can be applied to obtain the correlation of liftoff height with jet velocity and nozzle diameter of . Using this relation, the mixture of fuels having Sc<1 and Sc>1 are tested. The dependence of liftoff height on jet velocity is curve-fitted to extract the effective Schmidt number of mixed fuels. Experimentally determined Schmidt numbers agree satisfactorily with the theoretical predictions from the kinetic theory.     

Burning velocity of methane/diluent mixture with reformer gas addition

The motivation of this study is to explore the feasibility of extending the EGR (exhaust gas recirculation) diluent tolerance for methane/air mixtures with reformer gas (CO and H₂). A preheated cylindrical combustion chamber was used to measure the laminar burning velocity of methane/air mixture with variations of EGR diluent, reformer gas, temperature and pressure. The experiments were carried out at the range of initial temperature from 298 K to 498 K and initial pressure from 1 atm to 5 atm. The maximum EGR fraction is 40%. Reformer gas was introduced to raise the burning velocity of methane/EGR mixture to the undiluted level. Results indicate that the reformer gas has potential to improve the burning velocity while reducing the nitric oxide emission.     

The confined impeller stirred tank (CIST): A bench scale testing device for specification of local mixing conditions required in large scale vessels

A new stirred tank geometry, the confined impeller stirred tank (CIST), was designed to provide repeatable testing of the effect of mixing on the performance of chemical additives at the bench scale. The CIST (T = 0.076 m, H = 3T) is filled with five or six impellers. Three impeller geometries were tested: A310, Rushton and Intermig. This paper presents the following hydrodynamic characteristics of the CIST: power number, flow number, momentum number, velocity profiles at different locations in the tank and the transition point from fully turbulent to transitional flow. Based on the scaled velocity profiles, the CIST was able to keep the flow turbulent at Re < 2000 for Rushton turbines and 3200 for Intermigs. The ratio ?_max/?_average was lower for the CIST than for a conventional stirred tank, indicating that the energy dissipation is more uniformly distributed in the CIST. The CIST consistently maintains turbulent flow down to a Reynolds number 10× smaller than that needed in a conventional stirred tank.     

Estimation of lower flammability limits in high-pressure systems. Application to the direct synthesis of hydrogen peroxide using supercritical and near-critical CO2 and air as diluents

A method for the prediction of the LFL at high pressures, where data are scarce, based on the calculation of adiabatic flame temperatures for the mixtures H₂ + O₂ in CO₂ and N₂, between 1.0 and 300 bar and 288-348 K is presented. A group contribution equation of state (GC-EoS) has been selected to predict thermodynamic properties of the mixture, i.e. residual enthalpy, heat capacity and others, as well as phase equilibrium data, giving deviations lower than 10% at high pressures. The use of CO₂ as a diluent increases the operational margin from 4.5 mol% H₂ at 1 bar up to ca. 7.0-9.0 mol% H₂ at 200 bar due to the increase in the heat capacity. On the other hand, the use of nitrogen or air as a diluent only increases the margin from 5.2 mol% H₂ at 1 bar up to ca. 6.0 mol% H₂ at 200 bar.     

Group combustion of staggeringly arranged heptane droplets at various Reynolds numbers, oxygen mole-fractions, and separation distances

The group combustion of interacting heptanes liquid droplets are numerically simulated by solving two dimensional unsteady laminar Navier-Stokes equations. The unsteady computations for the time-varying vaporization of multi-droplets are carried out with parameters of the Reynolds number (Re), the separation distance (S) between the droplets, and the oxygen mole-fraction. The n-heptane droplets initially at T₀ = 300 K are in hot air of 10 atm at T_g = 1250 K. Multi-droplets are staggeringly arranged at a separation distance ranging from 4 to 15 droplet radius. The Reynolds number, based on the droplet diameter and free stream velocity, is varied from Re = 10 to 50. The oxygen mole-fraction of the surrounding air is changed from 15% to 90%. The time variations of the flame structure, the combustion characteristics, and the burning rates are presented and discussed. These results indicated that the staggered arrangement of the multi-droplets induced combustion characteristics distinct from those of a single droplet. The burning rate of the interacting droplets in the staggered arrangement exhibited a relatively strong dependence on the Re, S, and oxygen mole-fraction. The burning rate of the interacting multi-droplets, non-dimensionalized by that of a single droplet, was found as a function of S and Re.     

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.     

Fuel effects on diffusion flames at elevated pressures

This study addresses the influence of elevated pressures up to 1.6 MPa on the flame geometry and the flickering behavior of laminar diffusion flames and particular attention has been paid to the effect of fuel variability. It has been observed that the flame properties are very sensitive to the fuel type and pressure. The shape of the flame was observed to change dramatically with pressure. When the pressure increases, the visible flame diameter decreases. The height of a flame increases first with pressure and then reduces with the further increase of pressure. The cross-sectional area of the flame (A_cs) shows an average inverse dependence on pressure to the power of n (A_cs ∝ P⁻ⁿ), where n = 0.8 ± 0.2 for ethylene flame, n = 0.5 ± 0.1 for methane flame and n = 0.6 ± 0.1 for propane flame. It was observed that the region of stable combustion was markedly reduced as pressure was increased. High speed imaging and power spectra of the flame chemiluminescence reveal that an ethylene flame flickers with at least three dominant modes, each with corresponding harmonics at elevated pressures. In contrast methane flames flicker with one dominant frequency and as many as six harmonic modes at elevated pressures.     

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.     

Laminar burning velocities of three C3H6O isomers at atmospheric pressure

Laminar flames of three C₃H₆O isomers (propylene oxide, propionaldehyde and acetone), representative of cyclic ether, aldehyde and ketone species important as intermediates in oxygenated fuel combustion, have been studied experimentally and computationally. Most of these flames exhibited a non-linear dependency of flame speed upon stretch rate and two complementary independent techniques were adopted to provide the most reliable burning velocity data. Significant differences in burning velocity were noted for the three isomers: propylene oxide + air mixtures burned fastest, then propionaldehyde + air, with acetone + air flames being the slowest; the latter also required stronger ignition sources. Numerical modelling of these flames was based on the Konnov mechanism, enhanced with reactions specific to these oxygenated fuels. The chemical kinetics mechanism predicted flame velocities in qualitative rather than quantitative agreement with the measurements. Sensitivity analysis suggested that the calculated flame speeds had only a weak dependency upon parent fuel-specific reactions rates; however, consideration of possible break-up routes of the primary fuels has allowed identification of intermediate compounds, the chemistry of which requires a better definition.     

Speciation studies of methyl butanoate ignition

The current work presents the results of an experimental study of the intermediates formed during ignition of methyl butanoate (C₅H₁₀O₂) and air mixtures. A rapid-sampling system and the University of Michigan rapid compression facility were used to acquire gas samples at conditions of P = 10.2 atm and T = 985 K using mixtures of χ_mb = 0.96%, χ_O2 = 20.79%, χ_N2 = 52.89%, and χ_Ar = 25.25% (mole fraction, percent basis); corresponding to ? = 0.30 and an inert gas to O₂ molar ratio of 3.76. The samples were analyzed using gas chromatography. Quantitative measurements of mole fraction time-histories of methane, ethane, propane, ethene, propene, and 1-butene are compared with model predictions based on a reaction mechanism developed in previous work. The methane and ethene time-histories are in excellent agreement (within ∼20%), while propene and ethane are underpredicted by the model. Sensitivity analysis shows ignition is controlled primarily by competition between H₂O₂ and HO₂ kinetics at these conditions. Reaction path analysis shows the methyl butanoate fuel consumption is dominated by H-atom abstraction by OH.     

Flame stability and emission characteristics of turbulent LPG IDF in a backstep burner

The stability characteristics and emissions from turbulent LPG inverse diffusion flame (IDF) in a backstep burner are reported in this paper. The blow-off velocity of turbulent LPG IDF is observed to increase monotonically with fuel jet velocity. In contrast to normal diffusion flames (NDF), the flame in the present IDF burner gets blown out without getting lifted-off from the burner surface. The soot free length fraction, SFLF, defined as the ratio of visible premixing length, H_p, to visible flame length, H_f, is used for qualitative estimation of soot reduction in this IDF burner. The SFLF is found to increase with central air jet velocity indicating the occurrence of extended premixing zone in the vicinity of flame base. Interestingly, the soot free length fraction (SFLF) is found to be correlated well with the newly devised parameter, global momentum ratio. The peak value of EINO_X happens to occur closer to stoichiometric overall equivalence ratio.     

Study of the turbulent inverse diffusion flame in recessed backstep and coaxial burners

A comparative study of the turbulent Inverse Diffusion Flame (IDF) in recessed coaxial and backstep burners is carried out, based on visible flame appearance, flame length, flame stability, centerline temperature distribution, centerline oxygen concentration, and NO _x emissions. The backstep burner is observed to produce a compact flame shape with less luminosity at a higher air-fuel velocity ratio, as compared to the coaxial burner. Moreover, slightly better thermal characteristics and marginal reduction in NO _x emissions are provided by the backstep IDF, as compared to the recessed coaxial IDF. Besides this, the centerline oxygen concentration is marginally increased in the backstep IDF due to higher entrainment of ambient air. Interestingly, a lower flame stability limit is seen in the backstep burner than in the coaxial IDF, which can be attributed to its enhanced fuel-air mixing.     

Parametric study of combined premixed and non-premixed flame coal burner

A combined gas/air mixture–coal burner was developed to include heat recirculation by utilizing a radiative solid material with premixed flame jets impinging onto the downstream side to preheat the fuel/air jet on the upstream side. Providing the heat recirculation mechanism at different air staging degrees enhanced the destruction rates of the fuel nitrogen oxides. Concentric elliptical premixed gas/air and coal/air jets had a stronger preheating effect and a consequent increased NO_x reduction effectiveness as compared to concentric circular jets, where the inner elliptical jets enlarged the contact diffusion area and entrainment thus increasing the preheating time. The parametric variation in the feeding ports to the coal combustor affected the exhaust emissions, wherein the use of an inclined or shifted injection from the centre-line contributed to the NO_x reduction. Increasing the jet angle in the upstream direction reduced the CO concentrations, while the NO_x emissions varied depending on the degree of staging. The inverse/normal flame configuration was found more effective than the normal flame configuration with respect to NO_x reduction that was enhanced at higher heat input ratios. Utilizing inverse triple flames led to a further NO_x reduction since higher temperatures prevailed in the initial flame region with a five reaction zone structure. Finer particles produced less NO_x, which was further reduced by blending the coal with biomass.     

Stability of lean combustion in mini-scale porous media combustor with heat recuperation

In miniaturization of burners, it is very difficult to organize stable self-sustained combustion. A mini-scale porous media combustor with heat recuperation was set up to study the stability of lean combustion and its emission. The diameter of the porous media was only 20 mm and the burner was about 140 mm in length. Experimental results showed that when the mass flow rate of the premixed gas was 0.163 g/s, the extinction limit was extended to Φ = 0.40 in the methane combustion and Φ = 0.39 in the propane combustion. For most cases, the emission of CO was lower than 100 ppm in both methane and propane combustion. The maximal concentration of NO_x was 63 ppm in the methane combustion. The ultra-lean combustion was also predicted by a numerical simulation with a 2D two-temperature model. The heat recuperation efficiency η, as high as 40%, made the ultra-lean combustion extremely stable. Although the maximal flame temperature in the porous media reached above 2000 K, the exhausts temperature was lower than 900 K.     

Benzene addition to a fuel-stoichiometric methane/O2/N2 flat flame and to n-heptane/air mixtures under rapid compression machine

This article reports a detailed reaction mechanism able to predict our experimental data obtained for methane/benzene and n-heptane/benzene mixtures at low and high temperature. The high temperature data consist in mole fraction profiles of stable and reactive species measured in low-pressure (5.19×10⁻² bar) stoichiometric premixed CH₄/O₂/N₂ and CH₄/1.5%C₆H₆/O₂/N₂ flames by coupling MB/MS and GC/MS techniques. The low temperature data consist in cool flame and autoignition delay times of a 50%n-heptane/50%benzene/air mixture measured in a rapid compression machine in the ranges 620–885 K and 4.63–8.87 bar. Intermediate oxidation products were also analyzed. The proposed mechanism includes our previous GDF-Kin^®2.0 mechanism developed for natural gas oxidation, a low and high temperature literature submechanism of n-heptane oxidation, and a high temperature submechanism of benzene oxidation. Predicted mole fraction profiles measured in methane and methane/benzene flames are in reasonable agreement with experimental profiles for most molecular species. Reaction path analyses were performed to interpret the effect of benzene on the analyzed chemical species in the methane/benzene flame. The proposed mechanism reproduces well the evolution of both delay times versus temperature. Local sensitivity analyses show that, in the low temperature range, benzene plays a non-significant role in the reactivity of the n-heptane/benzene mixture and acts more as a diluent than as a chemical inhibitor.     

A comparative study on combustion synthesis of Ta-B compounds

A comparative study on the preparation of various tantalum borides (including Ta₂B, Ta₃B₂, TaB, Ta₅B₆, Ta₃B₄, and TaB₂) in the Ta-B system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of their corresponding stoichiometries. Both combustion temperature and reaction front velocity increased and then decreased with increasing boron content in the powder mixture. The fastest flame front with a reaction temperature of 1732 °C and a propagation rate of 11.2 mm/s was observed in the sample of Ta:B = 1:1. The combustion temperature (1205 °C) and flame-front velocity (3.82 mm/s) for the powder compact of Ta:B = 2:1 were the lowest. According to the XRD analysis, single-phase TaB and TaB₂ were produced from the samples of Ta:B = 1:1 and 1:2, respectively. However, multiphase products were synthesized from the samples of other stoichiometries. In the final products from Ta-rich samples of Ta:B = 2:1 and 3:2, two boride phases, Ta₂B and TaB, along with a large amount of residual Ta were detected. The products yielded from boron-rich reactants of Ta:B = 5:6 and 3:4 were composed of TaB, Ta₃B₄, and TaB₂. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with the formation of TaB and TaB₂ by solid state combustion were determined to be 131.1 and 181.4 kJ/mol, respectively.