H2 Diffusion Flames Research Articles

Combined experimental and numerical study on the extinction limits of non-premixed H2/CH4 counterflow flames with varying oxidizer composition

Chemical reaction mechanisms with detailed kinetics are an important topic in combustion science and an essential prerequisite for the accurate modeling of reactive flows in combustors. Besides isolating and studying individual reactions, the development of reaction mechanisms is often based on well-defined experimental observables, such as the laminar burning velocity and the ignition delay time. While many optimization targets are associated with premixed combustion, the extinction strain rate (ESR) of non-premixed flames in the counterflow configuration is another well-defined experimental observable which, however, often receives less attention. In order to reduce the scarcity of corresponding datasets for the emerging fuel hydrogen and its blends with methane, this work reports ESR measurements for H2, CH4/H2 and CH4 counterflow diffusion flames considering a variation of the oxygen content in the oxidizer stream between 14 % and 21 %. The experimental investigation is complemented by calculations with a 1D counterflow model utilizing a temperature-control continuation method in order to determine the extinction limits numerically. The simulations are performed with six different well-established chemical reaction mechanisms. It is shown from both, experimental and numerical results, that with the substitution of CH4 by H2 the ESR increases and further, that the ESR decreases with a reduction of the oxygen content in the oxidizer stream. In addition, decreasing flame temperatures are observed at extinction as the H2 content increases. Overall, all mechanisms are able to qualitatively recover the trends found for varying H2 contents, fuel mole fraction, and oxygen content in the oxidizer. However, significant quantitative deviations are observed between the numerical results regarding the ESR values and the deviations are larger than for other important flame characteristics, such as the laminar burning velocity. The results suggest that the ESR could be a useful optimization target for further improving chemical reaction mechanisms which underlines the importance of datasets such as the one presented in this work.

A parametric study of autoigniting hydrogen jets under compression-ignition engine conditions

This study examines the flame evolution of autoigniting H2 jets with high-speed schlieren and OH∗ chemiluminescence optical methods in a constant-volume combustion chamber over a wide range of simulated compression-ignition engine conditions. Parametric variations include the injector nozzle orifice diameter (0.31–0.83 mm), injection reservoir pressure (100–200 bar), ambient temperature (1000–1140 K), density (12.5–24 kg/m3) and O2 concentration (10–21 vol.%). The jet ignition delay was found to be highly sensitive to changes in ambient temperature while all other parameter variations resulted in minor ignition delay changes. Optical imaging reveals that in most cases, the reaction front of the H2 jet initiates from a localised kernel, before engulfing the entire jet volume downstream and recessing towards the nozzle. The flames attach to the nozzle, except at the lowest ambient oxygen condition of 10 vol.% O2 for which a lifted flame is observed. The H2 diffusion flame length shows a dependence on both the mass flow rate and the level of O2 entrainment that follows the same correlations as previously established for atmospheric H2 jet flames.

Experimental and numerical study on the quenching limits and mechanism of small-scale H2 diffusion flames

The quenching limits of small-scale H2 diffusion flame achieved by microtubes were studied experimentally and numerically. The experimental observation shows that the luminosity of small-scale H2 diffusion flame becomes ambiguous with decreasing fuel jet velocity. However, the OH radical in the flame can be detected clearly by using the planar laser-induced fluorescence (PLIF) measurement. The quenching limits of small-scale H2 diffusion flames at different inner tube diameters were obtained by OH-PLIF measurement and numerical computation with a detailed chemical mechanism. The computed and measured quenching limits show good agreement. The results show that the quenching mass flow rate decreases gradually with decreasing inner tube diameter. A power function relationship exists between the quenching mass flow rate and the inner tube diameter. The analysis of the computational results reveals that the quenching mass flow rate depends on the ratio between heat generation and heat loss, net fuel consumption ratio, the lower heating value of the fuel and total heat loss from the reaction-part of the flame. The decrease in the diameter of the reaction-part of the flame with decreasing inner tube diameter leads to a smaller quenching mass flow rate.

Stretch and Curvature Effects on NO Emission of H2/Air Diffusion Flames

Opposed jet H2 diffusion flames are simulated with different H2/O2 and NOx chemical kinetic mechanisms. The simulations with University of California, San Diego (UCSD) NOx and H2/O2 mechanisms are identified to have the best agreement with the experimental data. With these mechanisms, the pressure effect on NO emission of the opposed jet flame is studied and it is shown that higher pressure enhances combustion increasing the flame temperature and NO emissions. The curved opposed tubular H2 diffusion flames are simulated and compared with the opposed jet flames. It is shown that positive curvature enhances the preferential diffusion effect and increases flame temperature and NO emission; and vice versa for negative curvature. Flame curvature can change the NO emission as much as 5.4 times for the tested cases. A detailed flame structure comparison among curved flames and planar flames demonstrates that tubular flames are slightly narrower and their maximum stretch rates are slightly higher.

Simulation of NO Formation in Hydrogen-Air Counterflow Diffusion Flame

The combustion of hydrocarbon fuels produces large amounts of carbon dioxide. In order to cope with the challenge of greenhouse effect and global environmental protection. H2, as a cleaner and more energy-burning fuel, is being considered in many of the practical applications of combustion equipments. However, H2 fuel combustion will still produce pollutant NOx. Thus, the study of NOx emission is one of the most important topics in H2 combustion. With the classical counterflow burner, a numerical simulation of NO emission was carried out on the H2 diffusion flames. The stretch effects on flame temperature, NO concentration, and EINO are analyzed; the contribution of different NO generation routs are also quantified and analyzed. The major parameters influencing NO emission are flame temperature, radical concentration, and residence time, the observed relationship between stretch rate and NO emission is explained with the three major parameters.

Kinetic Ignition Enhancement of $\hbox{H}_{2}$ Versus Fuel-Blended Air Diffusion Flames Using Nonequilibrium Plasma

Kinetic ignition enhancement of H2 diffusion flames by a nonequilibrium plasma discharge of H2- and CH4-blended oxidizer was studied experimentally and numerically through the development of a well-defined counterflow system. Measurements of ignition temperatures and major species as well as computations of rates of production and sensitivity analyses were conducted to identify the important kinetic pathways. It was found that the competition between the catalytic effect of NOx and the inhibitive effects of H2O and CH4 governed the ignition processes in the system. With air as the oxidizer, ignition was enhanced from the plasma-produced NOx. With H2 addition to the oxidizer, H2O formation significantly increased the ignition temperature. However, with plasma activation, the inhibitive effect of H2O was significantly reduced because of the dominant role of NOx. With CH4 addition to the oxidizer, the ignition temperatures increased due to the radical quenching by H2O or CH4, depending upon the strain rate. The results showed that the inhibitive effects were significantly decreased with plasma activation. Unlike vitiated air ignition, plasma-enhanced ignition for fuel-air mixtures can suppress the inhibitive effects of H2O and CH4 because of the overwhelming catalytic NOx effect at low temperatures.

Unsteady analysis of acoustic pressure response in N2 diluted H2 and air diffusion flames

Acoustic pressure response of diluted hydrogen-air diffusion flames is investigated numerically by adopting a fully unsteady analysis of flame structures in a counterflow. Without the restriction of the previous quasi-steady approach, acoustic response to pressure oscillation is investigated in a wide range of frequencies and for two representative pressure regimes, namely, low- and medium-pressure regimes. Acoustic amplification/attenuation and their intensities are judged by the normalized amplification index derived from Rayleigh’s criterion. The numerical results show that acoustic pressure response has a complex pattern depending on acoustic frequency in each regime. In the low-pressure regime, the amplification index remains low and constant at low frequencies, which indicates a relatively weak acoustic amplification. As acoustic frequency increases, finite-rate chemistry is enhanced through a nonlinear accumulation of heat release rate, leading to a high amplification index. Finally, the flame responses decrease at high frequency because of the response lag of the transport zone. For a medium-pressure operation and low-frequency excitation, the amplification index is low and constant. The value is smaller than that found in the same frequency range for the low-pressure operation. It then decreases at moderate frequencies since the transport-zone response becomes dominant over the reaction-zone response. As frequency increases further, the amplification index increases appreciably because of an intense accumulation effect, and its peak value exceeds that found for low-pressure operation. The results predicted in the previous quasi-steady approach agree with acoustic responses to low-frequency pressure oscillations predicted in the present unsteady analysis. However, the accumulation effect occurring at high frequency, which could not be predicted in the quasi-steady approach, is here identified as a key amplification mechanism.

Numerical Prediction of Diffusion Flames Using Multicomponent Diffusion and the Accuracy Evaluation.

Numerical computations are made of axisymmetric laminar jet H2 diffusion flames, taking account of multicomponent diffusion and detailed chemical kinetics. Availability of multicomponent diffusion calculation is investigated by comparison between the computational results with exact multicomponent diffusion coefficients or effective binary diffusivity and experimental results. The effect of thermal diffusion is also investigated. Computational results with exact method are close to experimental results for major species. Consideration of thermal diffusion improves the accuracy of the prediction. When fuel is H2 diluted with N2, mass flux of major species calculated with effective binary diffusivity agrees with that with exact multicomponent diffusion coefficient except for N2 and the results of concentration agree with experimental results. In the pure H2 diffusion flame, concen-tration and mass flux of N2 and H2O obtained by the numerical method with effective binary diffusivity disagree with that by exact method or experimental results. Excessive diffusion of H2 is caused by the assumption that spatial gradient of mean molecular weight is neglected.

Finite Rate Chemistry and NO Molefraction in Non-Premixed Turbulent Flames

Finite rate chemistry is investigated in turbulent N2-diluted H2 diffusion flames by means of laser spectroscopic methods and a numerical model of combustion. The major species occurring in these hydrogen flames or the temperature are measured with SRS (Spontaneous Raman Spectroscopy) or Rayleigh spectroscopy. Some minor species, the radical OH and the pollutant NO, are simultaneously measured with linear LIF (Laser Induced Fluorescence). Multidimensional pdfs (probability density function) can be deduced from these measurements. Use is made of a numerical model with two principal variables, mixture fraction ξ and reaction progress variable η, as a basis for discussion of the experimental results. A k-ϵ-turbulence model together with a two-dimensional presumed pdf for the coupling of turbulence and chemistry are applied. So, experimental two-dimensional pdfs as well as mean values of ξ and η as functions of the position in the flame are deduced from the simultaneous measurements. The experimental and theoretical spatial maxima of the mean OH molefraction agree well in magnitude, despite the correlation coefficient between ξ and η of the measured pdf can be as high as 0.5. The neglect of this covariance for the calculation of the presumed pdf is quantified. It results in clear deviations for the OH molefraction. The experimental NO and OH molefractions are better simulated by flame calculations carried out with the presented combustion model than by the also shown calculations based on a single variable for description of chemistry.

Two-Dimensional LIF Approaches for the Accurate Determination of Radical Concentrations and Temperature in Combustion

Laser-induced fluorescence (LIF) techniques for the quantitative, point-wise measurement of radical concentrations and temperature were developed and applied in laboratory-scale combustion systems. Currently, the potential of similar measurement strategies for accurate two-dimensional, single-pulse experiments is being investigated. The sensitivity of typical LIF approaches for the determination of temperature and concentration to several parameters, especially to collision processes, is examined. Instantaneous two-dimensional two-species fluorescence imaging is demonstrated in a turbulent H2 diffusion flame. Implications for the quantitative interpretation of these results are discussed in view of single-pulse multi-species spontaneous Raman measurements under similar conditions.

プラズマジェットの着火・保炎機構に関する研究

Studies on the mechanism of ignition and flame-holding by upstream-injection plasma jets were conducted for H2 diffusion flames in high speed air streams. The plasma jets were operated with Ar gas involving active species such as O2, N2 and H2, in the range of relatively low power. It was observed that for ignition and flameholding, the concentration of active species in feedstock is more important than plasma jet power. This fact suggests the effectiveness of active species produced by plasma jets, rather than that of plasma-jet thermal energy, and also the importance of radical concentration supplied to the ignition-point of a fuel jet, compared to the total quantity of radicals produced. It was also found that the effectiveness of additives is in the order, O2≥N2>H2, and the results of numerical simulation on combustion-reaction promotion by the radicals added agreed qualitatively with the experimental ones.

UV raman spectroscopy of H2-air flames excited with a narrowband KrF laser

Raman spectra of H2 and H2O in flames excited by a narrowband KrF excimer laser are reported. Observations are made over a porous-plug, flat-flame burner reacting H2 in air, fuel-rich with nitrogen dilution to control the temperature, and with a H2 diffusion flame. Measurements made from UV Raman spectra show good agreement with measurements made by other means, both for gas temperature and relative major species concentrations. Laser-induced fluorescence interferences arising from OH and O2 are observed in emission near the Raman spectra. These interferences do not preclude Raman measurements, however.

Some Budgets of Turbulent Stresses in Round Jets and Diffusion Flames

Abstract Measurements are presented of budgets of shear stress, normal stresses and turbulent kinetic energy in a round air jet and in two H2 diffusion flames, all in a co-flowing air stream. The results are based on simultaneous two-component LDA and Mie-scattering measurements, and are presented in Favre averaged form. The flames show basically similar features to the cold flow, but overall magnitudes of the normalized terms are lower, and the advection contribution to turbulent kinetic energy is much reduced. The turbulence dissipation rate e is obtained from the budgets of turbulent kinetic energy by difference, and is used later to obtain values of the velocity pressure-gradient correlations u ∂p/∂x and ν ∂p/∂r, again by difference. The results are consistent with other published estimates of these terms; whilst they seem small in the turbulent kinetic energy balance, they dominate in the budgets for the stresses, and are largely balanced by production. A comparison with a well-publicized numerical m...

Measurement of Raman scattering in flames (Raman scattering of water vapor and carbon dioxide)

Vibrational Raman spectra of H2O and CO2 were theoretically obtained for the experimental use in temperature determination in flames. The variation of half-value width of H2O spectrum was used to evaluate the temperature; the relative intensity of the (0, 00, 0)⇾(1, 00, 0) transition scattering to the (0, 11, 0)⇾(1, 11, 0) was correlated to the temperature of CO2. These theoretical predictions are compared with the experimental measurements in H2 diffusion flames. Rotational Raman spectra of N2 and CO2 were also studied by the theoretical predictions based on the correlation of half-value width of spectra to gas temperature, and the results are discussed in comparison with the experimental observations.

Miniature High Temperature Laminar Diffusion Flames for Analytical Atomic Spectrometry

Miniature high temperature laminar diffusion flames are described for analytical atomic spectrometry. Fuel flow rates below 0.2 liters/min are used and temperatures in excess of 2500 K are maintained in a stable, fuel-rich combustion environment of low background emission. The internal atomizer chamber volume is 1.5 ml (below the burner top). A graphite cup atomizer is used for sample introduction and is “fired” inside the miniature burner head in the presence of “fuel only.” Oxidant diffuses into the fuel zone above the burner top. Compared to conventional electrothermal atomizers, the miniature N2O → H2 diffusion flame greatly reduced particulate light scattering in continuum excited atomic fluorescence spectrometry. In atomic absorption spectrometry, the hot miniature flame serves to effectively reduce the molecular spectral interference due to gaseous diatomic alkali halides. The enhanced gas phase concentration of atomic products presently results in a concentration sensitivity improvement of 30 times over conventional premixed flames used in Eimac continuum excited atomic fluorescence. The improvement in absolute weight sensitivity is 140 times for atomic fluorescence.

A New Optical System for Flame Spectroscopy with Special Reference to Thermally Assisted Anti-Stokes Fluorescence Applications

A new optical setup for increasing the intensity of atomic fluorescence flame spectrometry is presented. The arrangement consists of two concave spherical mirrors and an elliposidal mirror whose primary focus is located at the flame position, and secondary at the entrance slit of the monochromator. Approximately 10 times gain in the measured intensities was obtained. The system is also useful for flame emission work, for which about five times gain was observed. With a low background flame such as the air–H2 for the N2−H2 diffusion flame and 400 µ slits, calcium can be detected at the level of 0.005 µg/ml. Moreover, the observed thermally assisted anti-Stokes fluorescence of In and Ga can be of analytical importance since the detection limits with a total consumption burner and the air-H2 flame were found to be 0.015 and 0.02 µg/ml, respectively. Detection limits in atomic fluorescence for Cr, Tl, In, and Fe are also presented.