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.     

Nitrogen dilution effect on flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The nitrogen dilution effect on flame stability was experimentally investigated in a lifted non-premixed turbulent hydrogen jet with coaxial air. Hydrogen gas was used as the fuel and coaxial air was injected to initiate flame liftoff. Hydrogen was injected into an axisymmetric inner nozzle (d_F = 3.65 mm) and coaxial air jetted from an axisymmetric outer nozzle (d_A = 14.1 mm). The fuel jet and coaxial air velocities were fixed at u_F = 200 m/s and u_A = 16 m/s, while the mole fraction of the nitrogen diluent gas varied from 0.0 to 0.2 with a 0.1 step. For the analysis of the flame structure and the flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF was performed. The stabilization point was in the region of the flame base with the most upstream region and was defined as the point where the turbulent flame propagation velocity was found to be balanced with the axial component of the local flow velocity. The turbulent flame propagation velocity increased as the nitrogen mixture fraction decreased. The nitrogen dilution makes the flame structure more premixed. That is, the stabilization mechanism shifts from edge flame propagation based mechanism toward premixed flame propagation based mechanism. We concluded that the turbulent flame propagation velocity was expressed as a function of the turbulent intensity and the axial strain rate, even though the mole fraction of the nitrogen diluent varied.     

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.     

Measurements of methane hydrate heat of dissociation using high pressure differential scanning calorimetry

The methane hydrate heat of decomposition was directly measured up to 20 MPa and 292 K using a high pressure differential scanning calorimeter (DSC). The methane hydrate sample was formed ex-situ using granular ice particles and subsequently transferred into the DSC cell under liquid nitrogen. The ice and water impurities in the hydrate sample were reduced by converting any dissociated hydrate into methane hydrate inside the DSC cell before performing the thermal properties measurements. The methane hydrate sample was dissociated by raising the temperature (0.5-1.0 K/min) above the hydrate equilibrium temperature at a constant pressure. The measured methane hydrate heat of dissociation (H→W+G), ΔH_d, remained constant at 54.44±1.45 kJ/mol gas (504.07±13.48 J/gm water or 438.54± 13.78 J/gm hydrate) for pressures up to 20 MPa. The measured ΔH_d is in agreement with the Clapeyron equation predictions at high pressures; however, the Clausius-Clapeyron equation predictions do not agree with the heat of dissociation data at high pressures. In conclusion, it is recommended that the Clapeyron equation should be used for hydrate heat of dissociation estimations at high pressures.     

A numerical and experimental study of counterflow syngas flames at different pressures

Synthesis gas or “Syngas” is being recognized as a viable energy source worldwide, particularly for stationary power generation due to its wide availability as a product of bio and fossil fuel gasification. There are, however, gaps in the fundamental understanding of syngas combustion and emissions characteristics, especially at elevated pressures that are relevant to practical combustors. This paper presents a numerical and experimental investigation of the combustion and NO_x characteristics of syngas fuel with varying composition, pressure and strain rate. Experiments were performed at atmospheric conditions, while the simulations considered different pressures. Both experiments and simulations indicate that stable non-premixed and partially premixed counterflow flames (PPFs) can be established for a wide range of syngas compositions and strain rates. Three chemical kinetic models, GRI 3.0, Davis et al., and Mueller et al. are examined. The Davis et al. mechanism is found to agree best with the experimental data, and hence used to simulate the PPF structure at different pressure and fuel composition. For the pressure range investigated, results indicate a typical double flame structure with a rich premixed reaction zone (RPZ) on the fuel side and a non-premixed reaction zone (NPZ) on the oxidizer side, with RPZ characterized by H₂ oxidation, and NPZ by both H₂ and CO oxidation. While thermal NO is found to be the dominant route for NO production, a reburn route, which consumes NO through NO + O + M→ NO₂ + M and H + NO + M → HNO + M reactions, becomes increasingly important at high pressures. The amount of NO formed in syngas PPFs first increases rapidly with pressure, but then levels off at higher pressures. At a given pressure, the peak NO mole fraction exhibits a non-monotonic variation with syngas composition, first decreasing to a minimum value, and then increasing as the amount of CO in syngas is increased. This implies the existence of an optimum syngas composition that yields the lowest amount of NO production in syngas PPFs, and can be attributed to the combined effects of thermal and reburn mechanisms.     

Laminar burning velocities of n-heptane, iso-octane, ethanol and their binary and tertiary mixtures

Measurements of the adiabatic laminar burning velocities of n-heptane, iso-octane, ethanol and their binary and tertiary mixtures are reported. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. The Heat Flux method was used to determine burning velocities under conditions when the net heat loss from the flame to the burner is zero. Initial temperatures of the gas mixtures with air were 298 and 338 K. Uncertainties of the measurements were analyzed and assessed experimentally. The overall accuracy of the burning velocities was estimated to be better than ±1 cm/s. These new measurements were compared with the literature data when available. Experimental results in lean ethanol + air mixtures are systematically higher than previous measurements under similar conditions. Good agreement for n-heptane + air flames and for iso-octane + air flames was found with the experiments performed in counter-flow twin flames with linear extrapolation to zero stretch.     

Till which pressures the fluid phase EOS models might stay reliable?

This paper examines the performance of three fluid phase equations of state in predicting the available very high-pressure data of n-pentane, n-hexane, cyclohexane, toluene, dichloromethane, chloroform and methanol. It is assumed that the key for success at such pressures is establishing the appropriate interrelation between the densities of saturated liquids and the imaginary infinity pressure states. The recently proposed EOS that combines SAFT with the cohesive term of cubic EOS (SAFT + Cubic) most likely satisfies this criterion. According to this model, the saturated liquid densities at T_r = 0.4 are approximately 2.1 ± 0.1 times smaller than the densities predicted at the infinity pressure. With this ratio SAFT + Cubic yields reliable density estimations as far as the substances remain liquid (stable or metastable) in all the considered cases. Its pressure limit for accurate predictions of the auxiliary properties such as sound velocities and bulk moduli appear to be lower, typically around 1 GPa.     

Electrochemical reduction of high pressure CO2 at a Cu electrode in cold methanol

The electrochemical reduction of high pressure CO₂ with a Cu electrode in cold methanol was investigated. A high pressure stainless steel vessel, with a divided H-type glass cell, was employed. The main products from CO₂ by the electrochemical reduction were methane, ethylene, carbon monoxide and formic acid. In the electrolysis of high pressure CO₂ at low temperature, the reduction products were formed in the order of carbon monoxide, methane, formic acid and ethylene. The best current efficiency of methane was of 20% at −3.0 V. The maximum partial current density for CO₂ reduction was approximately 15 mA cm⁻². The partial current density ratio of CO₂ reduction and hydrogen evolution, i(CO₂)/i(H₂), was more than 2.6 at potentials more positive than −3.0 V. This work can contribute to the large-scale manufacturing of fuel gases from readily available and inexpensive raw materials, CO₂-saturated methanol from industrial absorbers (the Rectisol process).     

Influence of low air pressure on combustion characteristics and flame pulsation frequency of pool fires

To investigate the effect of low air pressure on the combustion characteristics and puffing flame frequency of pool fires, ethanol and n-heptane pool fires were performed using 15 square burners of various size in both Lhasa (altitude: 3650 m; air pressure: 65 kPa) and Hefei (altitude: 24 m, air pressure: 100.8 kPa) fire laboratories. Comparison of the experimental results for pool fires of the same size in the two places shows that, firstly, the maximum rise in the centerline temperature in flames in Lhasa is generally larger than that in Hefei. Secondly, the dependence of the burning rate exponent n () on the air pressure varies with the equivalent diameter D of the burner, with n < 0 (D < 7 cm), (0-1) (7 cm < D < 10 cm), (1-1.45) (10 cm < D < 19 cm) and 1 (D > 19 cm). Thirdly, radiation fraction of the pool fire flames is smaller at low air pressure. Finally, the puffing frequency of the pool fire flames is higher at low air pressure. Compared with Hefei, in Lhasa, for the same size burner, a higher maximum temperature together with less radiation from the flame is observed. This shows that in low air pressure, the pool fire is more buoyant, which leads to stronger periodic oscillation and a higher flame puffing frequency.     

Free-radical propagation and termination kinetics of the butyl acrylate dimer studied by pulsed laser polymerization techniques

The propagation and termination rate coefficients for bulk polymerization of the butyl acrylate dimer (BA dimer) are determined by pulsed laser techniques. The rate coefficient for propagation, k_p, is deduced for temperatures from 20 to 90 °C via the pulsed laser polymerization-size exclusion chromatography (PLP-SEC) method at pulse repetition rates between 1 and 10 Hz. The Arrhenius parameters were found to be: E_A(k_p) = (34.2 ± 1.0) kJ mol⁻¹ and A(k_p)/L mol⁻¹ s⁻¹ = (1.08 ± 0.49) × 10⁷ L mol⁻¹ s⁻¹. The termination rate coefficient, k_t, has been measured via SP-PLP-ESR, single pulse-pulsed laser polymerization in conjunction with time-resolved electron spin resonance detection of radical concentration. The resulting Arrhenius parameters as deduced from the temperature range −15 to +30 °C are: E_A(〈k_t〉) = (22.8 ± 3.7) kJ mol⁻¹ and log(A/L mol⁻¹ s⁻¹) = 10.6 ± 1. The chain-length dependence of k_t was studied at 30 °C. For short chains a significant dependence was found which may be represented by an exponent α = 0.79 in the power-law expression k_t(i) = k_t⁰i^−α.     

Pulverized coal flame structures at elevated pressures. Part 1. Detailed operating conditions

The tests and simulations in this study characterize the chemical structure of pressurized pulverized coal flames, particularly (1) how the O₂ in simulated near-burner flame zones is apportioned among the various fuel components; and (2) the burner operating conditions and mechanisms that most strongly affect flame structure. CFD simulations resolved the structures of flames of a subbituminous and two hv bituminous coals for stoichiometric ratios (SR) from 0 to 1.8 for pressures from 1.0 to 3.0 MPa. The structures of all flames were largely determined by the accumulation of particles in the turbulent boundary layer on the flow tube wall. Gaseous fuel compounds always ignited first on the wall at the burner inlet, and this flame propagated toward the flow axis to form a 2D parabolic flame surface. Within the core, residual gaseous fuels, soot, and char may have eventually reached their ignition threshold and burned in a premixed mode. Residual CO, H₂, and char burned in the near-wall region after the volatiles flame had propagated deeper into the core.Whether or not the flame closed on the centerline was mainly determined by pressure and SR. Inlet conditions that formed closed flames at a lower test pressure eventually sustained open flames at progressively higher pressures. The impact of decreasing SR was qualitatively similar, due to the lower heat release rates for progressively lower SR. As the pressure is increased, flame ignition and, by association, flame stability will become more problematic due to the greater thermal capacitance of air streams at progressively higher pressures.     

Influence of burner-port geometry in hydrocarbon oxidation and NOx formation mechanisms in methane/air flames

The influence of burner-port geometry in the mechanisms of hydrocarbon oxidation and NO_x formation from a 50 kW industrial-type methane-fired burner was investigated experimentally. Imaging and tomographic reconstruction techniques were used to assess the effects of port geometry upon flame visible length and C₂ chemiluminescence distribution in the recirculation zone. C₂ emission of methane flames depicts that low fuel jet velocities allow very rich conditions at recirculation zone and lead methane oxidation through O₂-scarcity mechanism. Higher velocities imply that methane oxidises via a path including dissociation into free radicals. In-furnace measurements were performed from a refractory-lined vertical furnace. NO_x concentration results revealed that NO formation is closely connected with the dissociation process, suggesting that prompt-NO_x mechanism is more important than hitherto supposed.     

Experimental study on the hazards of the jet diffusion flame of liquefied dimethyl ether

Dimethyl ether (DME) has been considered as a substitute for diesel fuel because it has a low auto-ignition temperature and produces less NO_x, SO_x, and particulate matter. However, the introduction of DME vehicles needs widely available DME supply stations. Moreover, the preparation of safety regulations for DME supply stations is very important, and so safety data is needed. Therefore, the present paper reports the hazards of the DME jet diffusion flame, which is one of several hazardous properties of DME, by studying the results of leaking gas and liquid DME. DME jets were released horizontally from circular nozzles whose diameters were 0.2, 0.4, 0.8 and 2 mm, and the release pressure was varied from the saturated vapor pressure to 2 MPa. When gaseous DME was released at the saturated vapor pressure, the flame was blown out. However, when liquefied DME was released, the flame formed. We obtained the experimental equations for estimating the scale and thermal hazards of DME diffusion flames.     

Adiabatic laminar burning velocities of CH4 + H2 + air flames at low pressures

Experimental measurements of the adiabatic burning velocity in methane + hydrogen + air flames using the Heat Flux method are presented. The hydrogen content in the fuel was varied from 0 to 20%. Non-stretched flames were stabilized on a perforated plate burner from 20 to 100 kPa. The equivalence ratio was varied from 0.8 to 1.4. Adiabatic burning velocities of CH₄ + H₂ + air mixtures were found in good agreement with the literature results at atmospheric pressure. Also low-pressure measurements in CH₄ + air flames performed earlier were accurately reproduced. The effects of enrichment by hydrogen on the laminar burning velocity at low pressures have been studied for the first time. Calculated burning velocities using the Konnov mechanism are in satisfactory agreement with the experiments over the entire range of conditions. Pressure dependences of the burning velocities for the three fuels studied could be approximated by an empirical exponential correlation.     

Determination of the elastic constants of portlandite by Brillouin spectroscopy

The single crystal elastic constants C_ij and the shear and adiabatic bulk modulus of a natural portlandite (Ca(OH)₂) crystal were determined by Brillouin spectroscopy at ambient conditions. The elastic constants, expressed in GPa, are: C₁₁ = 102.0(± 2.0), C₁₂ = 32.1(± 1.0), C₁₃ = 8.4(± 0.4), C₁₄ = 4.5(± 0.2), C₃₃ = 33.6(± 0.7), C₄₄ = 12.0(± 0.3), C₆₆ = (C₁₁-C₁₂)/2 = 35.0(± 1.1), where the numbers in parentheses are 1σ standard deviations. The Reuss bounds of the adiabatic bulk and shear moduli are K_0S = 26.0(± 0.3) GPa and G₀ = 17.5(± 0.4) GPa, respectively, while the Voigt bounds of these moduli are K_0S = 37.3(± 0.4) GPa and G₀ = 24.4(± 0.3) GPa. The Reuss and Voigt bounds for the aggregate Young's modulus are 42.8(± 1.0) GPa and 60.0(± 0.8) GPa respectively, while the aggregate Poisson's ratio is equal to 0.23(± 0.01). Portlandite exhibits both large compressional elastic anisotropy with C₁₁/C₃₃ = 3.03(± 0.09) equivalent to that of the isostructural hydroxide brucite (Mg(OH)₂), and large shear anisotropy with C₆₆/C₄₄ = 2.92(± 0.12) which is 11% larger than brucite. The comparison between the bulk modulus of portlandite and that of lime (CaO) confirms a systematic linear relationship between the bulk moduli of brucite-type simple hydroxides and the corresponding NaCl-type oxides.     

The effect of temperature on the adiabatic laminar burning velocities of CH4−air and H2−air flames

The effect of temperature on the adiabatic laminar burning velocities of CH₄ + air and H₂ + air flames was analyzed. Available measurements were interpreted using correlation S_L = S_L0 (T/T₀)^α. Particular attention was paid to the variation of the power exponent α with equivalence ratio at fixed (atmospheric) pressure. Experimental data and proposed empirical expressions for α as a function of equivalence ratio were summarized. They were compared with predictions of detailed kinetic models in methane + air and hydrogen + air flames. Unexpected non-monotonic behavior of α was found in rich methane + air flames. Modeling results are further examined using sensitivity analysis to elucidate the reason of particular dependences of the power exponent α on equivalence ratio.     

The relationship of knock during controlled autoignition to temperature inhomogeneities and fuel reactivity

Experiments have been performed in a rapid compression machine (RCM), to investigate the conditions for and the origins of ‘knock’ in controlled autoignition (CAI), or homogeneous charge compression ignition (HCCI). Ignition in an RCM is the closest approach to that in a CAI engine without engendering the full complexity of reciprocating motion and fuel+air charge induction. As a representative fuel of intermediate reactivity, the combustion of n-pentane in air was studied at the compositions φ=1.0, 0.75 and 0.6 at end-of-compression pressures of 0.80-0.86 MPa (7.9-8.4 bar) and 1.4-1.5 MPa (13.8-14.8 bar), respectively, over the compressed gas temperature range 690-820 K. Autoignition is characterised by a two-stage development in these ranges of conditions, a ‘cool flame’ being followed by hot stage combustion.Filtered Rayleigh scattering from a planar laser sheet was used to characterise the temperature field that developed in the combustion chamber following rapid compression. High resolution pressure records, combined with image intensified, natural light output originating from chemiluminescence, were used to characterise the transition from non-knocking to knocking reaction and the evolution of the spatial development of chemical activity in this temperature field. It appears that knock originates from a localised development of the incandescent hot stage of ignition. Even though non-homogeneities prevail in the non-knocking reaction, it is associated with a relatively benign development, in which the cool flame is followed by a second stage, blue flame rather than the normal incandescent hot flame. The kinetics that may contribute to this distinction are discussed.     

Experiment study of oxygenates impact on n-heptane flames with tunable synchrotron vacuum UV photoionization

In order to determine the effects of oxygenates on the fuel combustion, the experiments reported here investigated the premixed n-heptane flame chemistry. Heptane typifies the large alkanes that comprise the bulk of most hydrocarbon fuels. The specific flames were low-pressure (25 Torr), laminar, premixed flames of n-heptane/oxygen/argon and n-heptane/oxygenate/oxygen/argon at an equivalence ratio of 1.0. Two different fuel oxygenates (i.e. MTBE and ethanol) were tested, these are the main oxygenates used to improve motor vehicle fuel properties. The experiment was performed with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry. Major species on the centerline of each flame were identified by measurements of the photoionization mass spectrum and photoionization efficiency (PIE) spectra. Mole fraction profiles of these species were derived at the selected photon energies near the ionization thresholds. A large amount of oxygenated intermediates was detected in the oxygenate containing flames. The species measurements indicated that MTBE and ethanol enhanced the heptane oxidation via different routes, and reduced the mole fractions of aromatics and cycloalkenes in varying degrees. The results are a useful databases for testing detailed chemical kinetic mechanism of fuel decomposition.     

The influence of axial pressure on relaxor properties of BaBi2Nb2O9 ceramics

On the basis of our studies it results that dielectric properties of BaBi₂Nb₂O₉ ceramics are sensitive to axial pressure applied. The pressure causes an increase of dispersion in the real part of dielectric permittivity ?′(T,f) and a rise in the temperature T_m at which the maximum in ?′(T,f) dependence occurs. The applied pressure induces in the ?′(T) dependence an additional step-like anomaly, which appears at the temperature T_A < T_m. The applied pressure shifts both T_m and T_A at the same rate, i.e. dT_A/dX = dT_m/dX = +14 °C/kbar at high axial pressure range, above the threshold pressure X_thresh. The Vogel–Fulcher relationship is employed to determine the axial pressure influence on relaxor properties of BBN ceramics. The simulated order parameter q takes non-zero values below Burn‘s temperature T_B, where the polar clusters appear on cooling. For pressures higher than 0.8 kbar, the T_B changes at the rate dT_B/dX = −200 °C/kbar. The decrease in the difference between Burn's T_B and the freezing T_f temperatures induced by the applied axial pressure is observed. This could be ascribed to the narrowing of temperature range of relaxor behavior.     

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.