Oscillations and island evolution in radiating diffusion flames

We show how an island (isola) evolves out of the usual S-curve of steady states of diffusion flames when radiation losses are accounted for and how it eventually disappears when radiation increases further. At small activation temperatures there are never any islands. We show that stable oscillations evolve first out of perturbations of steady states on the S-curve at large Damköhler numbers. Only if the activation temperature is large enough do they also appear on the islands. The region of the stable oscillations grows larger as activation temperature decreases.     

Effect of pressure on structure and extinction of near-limit hydrogen counterflow diffusion flames

Results of measurements of critical conditions for extinction and of temperature profiles in counterflow diffusion flames are reported. The fuel was a hydrogen–nitrogen mixture with 14 mole percent hydrogen, and the oxidizer was air. Pressures ranged from 0.1 MPa to 1.5 MPa; measurements were made in a facility especially constructed for carrying out counterflow combustion experiments at high pressures. With increasing pressure, the strain rate at extinction first increases and then decreases, in qualitative agreement with predictions, but there are observable quantitative differences. Temperature profiles, obtained employing an R-type thermocouple at a fixed strain rate of 100/s, agree well with predictions, within experimental uncertainty. The results may help to improve knowledge of underlying chemical-kinetic and transport parameters at elevated pressures.     

Effect of H-atom concentration on soot formation in premixed ethylene/air flames

Soot formation is a major challenge in the development of clean and efficient combustion systems based on hydrocarbon fuels. Fundamental understanding of the reaction mechanism leading to soot formation can be obtained by investigating the role of key reactive species such as atomic hydrogen taking part in soot formation pathways. In this study, two-dimensional laser induced incandescence (LII) measurements using λ?=?1064?nm laser have been used to measure soot volume fraction (f_V) in a series of rich ethylene (C₂H₄)/air flames, stabilized over a McKenna burner fitted with a flame stabilizing metal disc. Moreover, a comparison of UV (λ?=?283?nm), visible (λ?=?532?nm) and IR (λ?=?1064?nm) laser excited LII measurements of soot is discussed. Recently developed, femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique has been applied for obtaining spatially resolved H-atom concentration ([H]) profiles under the same flame conditions. The structure of the flames has also been determined using hydroxyl radical (OH) planar laser induced fluorescence (PLIF) imaging. The results indicate an inverse dependence of f_V on [H] for a range of C₂H₄/air rich flames up to an equivalence ratio, Φ?=?3.0. Although an absolute relationship between [H] and f_V cannot be easily derived owing to the multiple steps involving H and other intermediate species in soot formation pathways, the present study demonstrates the feasibility to couple [H] and f_V obtained using advanced optical techniques for soot formation studies.     

Experimental and numerical investigation of microscale hydrogen diffusion flames

Characteristics of microscale hydrogen diffusion flames produced from sub-millimeter diameter (d = 0.2 and 0.48 mm) tubes are investigated using non-intrusive UV Raman scattering coupled with LIPF technique. Simultaneous, temporally and spatially resolved point measurements of temperature, major species concentrations (O₂, N₂, H₂O, and H₂), and absolute hydroxyl radical concentration (OH) are made in the microflames for the first time. The probe volume is 0.02 × 0.04 × 0.04 mm³. In addition, photographs and 2-D OH imaging techniques are employed to illustrate the flame shapes and reaction zones. Several important features are identified from the detailed measurements of microflames. Qualitative 2-D OH imaging indicates that a spherical flame is formed with a radius of about 1 mm as the tube diameter is reduced to 0.2 mm. Raman/LIPF measurements show that the coupled effect of ambient air leakage and pre-heating enhanced thermal diffusion of H₂ leads to lean-burn conditions for the flame. The calculated characteristic features and properties indicate that the buoyancy effect is minor while the flames are in the convection–diffusion controlled regime because of low Peclet number. Also, the effect of Peclet number on the flame shape is minor as the flame is in the convection–diffusion controlled regime. Comparisons between the predicted and measured data indicate that the trends of temperature, major species, and OH distributions are properly modeled. However, the code does not properly predict the air entrainment and pre-heating enhanced thermal-diffusive effects. Therefore, thermal diffusion for light species and different combustion models might need to be considered in the simulation of microflame structure.     

Modeled quenching limits of spherical hydrogen diffusion flames

Hydrogen–air diffusion flames were modeled with an emphasis on kinetic extinction. The flames were one-dimensional spherical laminar diffusion flames supported by adiabatic porous burners of various diameters. Behavior of normal (H₂ flowing into quiescent air) and inverse (air flowing into quiescent H₂) configurations were considered using detailed H₂/O₂ chemistry and transport properties with updated light component diffusivities. For the same heat release rate, inverse flames were found to be smaller and 290 K hotter than normal flames. The weakest normal flame that could be achieved before quenching has an overall heat release rate of 0.25 W, compared to 1.4 W for the weakest inverse flame. There is extensive leakage of the ambient reactant for both normal and inverse flames near extinction, which results in a premixed flame regime for diffusion flames except for the smallest burners with radii on the order of 1 μm. At high flow rates H + OH(+M) → H₂O(+M) contributes nearly 50% of the net heat release. However at flow rates approaching quenching limits, H + O₂(+M) → HO₂(+M) is the elementary reaction with the largest heat release rate.     

Effect of hydrogen concentration on superconductivity and clustering in palladium hydride

A method of preparing superconducting palladium hybride by electrolysis is described. The relationship of the superconducting transition temperature to hydrogen concentration and the behavior of the hydrogen clustering temperature at high concentrations are reported.     

Effect of Soret diffusion on lean hydrogen/air flames at normal and elevated pressure and temperature

The influence of Soret diffusion on lean premixed flames propagating in hydrogen/air mixtures is numerically investigated with a detailed chemical and transport models at normal and elevated pressure and temperature. The Soret diffusion influence on the one-dimensional (1D) flame mass burning rate and two-dimensional (2D) flame propagating characteristics is analysed, revealing a strong dependency on flame stretch rate, pressure and temperature. For 1D flames, at normal pressure and temperature, with an increase of Karlovitz number from 0 to 0.4, the mass burning rate is first reduced and then enhanced by Soret diffusion of H₂ while it is reduced by Soret diffusion of H. The influence of Soret diffusion of H₂ is enhanced by pressure and reduced by temperature. On the contrary, the influence of Soret diffusion of H is reduced by pressure and enhanced by temperature. For 2D flames, at normal pressure and temperature, during the early phase of flame evolution, flames with Soret diffusion display more curved flame cells. Pressure enhances this effect, while temperature reduces it. The influence of Soret diffusion of H₂ on the global consumption speed is enhanced at elevated pressure. The influence of Soret diffusion of H on the global consumption speed is enhanced at elevated temperature. The flame evolution is more affected by Soret diffusion in the early phase of propagation than in the long run due to the local enrichment of H₂ caused by flame curvature effects. The present study provides new insights into the Soret diffusion effect on the characteristics of lean hydrogen/air flames at conditions that are relevant to practical applications, e.g. gas engines and turbines.     

Comparison of exact and mean beam length results for a radiating hydrogen plasma

The purpose of this work is to investigate the accuracy of the mean beam length technique in high-temperature radiative heat transfer, combined with other modes of heat transfer. In order to study the validity of the mean beam length method, Nusselt numbers are presented for fully developed channel flow of a radiating nonisothermal hydrogen plasma. Black isothermal boundaries are considered. Numerical results obtained from the exact integrodifferential equation have been obtained previously for this problem. Linearized radiation and local thermodynamic equilibrium are assumed.The results show that the Nusselt numbers obtained by using the geometric mean beam length are in close agreement with results obtained by using the mean beam length. Therefore the complicated calculations needed to obtain the mean beam length are unnecessary.A comparison of the results obtained in the present work with previously reported work shows that the mean beam length technique is a better approximation to the exact solution than the optically-thick, or nongray differential approximation solutions.     

Effect of hydrogen on phase and microstructure of Ti600 alloy

Effects of hydrogen on microstructure characteristics and precipitation behavior have been investigated in Ti600 alloy hydrogenated at 750°C. Due to the precipitation of δ hydride and α″ martensite, X-ray diffraction (XRD) peaks of α phase shifted to lower angles and became broadened. XRD data showed that the addition of hydrogen expanded crystal lattice of α phase and lattice volume increased linearly in the range of 0?C _H?″ martensite and δ hydride. Compared to as-received one, microstructure of hydrogenated Ti600 alloy changed obviously. Equiaxed α phase decreased and even vanished with hydrogen contents increasing. Parallel fine lamellar structure appeared and the colonies were constructed by different orientation. Micro-hardness of hydrogenated Ti600 alloy increased with the increase of hydrogen concentration, and it was considered that interstitial solution strengthening, precipitation of δ hydride, and generation of defects were the major factors.     

Characteristic patterns of thermodiffusively unstable premixed lean hydrogen flames

Large-scale two-dimensional numerical simulations of thermodiffusively unstable, lean, premixed hydrogen flames have been performed using detailed finite rate chemistry to analyze flame intrinsic scales. The simulations feature a long integration time and large domain sizes to rule out effects of confinement on the dynamics of the flame front. For sufficiently large domain sizes, the total consumption speed of the flame is found to become independent of the domain size. An assessment of the characteristic scales of the flame front corrugation reveals the existence of a smallest and a largest flame intrinsic length scale. The smallest length manifests itself by local cusps, which lead to the formation of characteristic cells along the flame front. Their size is remarkably close to the most unstable wavelength predicted by a linear stability analysis of the flame front evolution in the linear regime. Independently of the domain size, a specific largest flame intrinsic structure, here referred to as flame finger, emerges from the interaction of multiple small-scale cusps. Thermodiffusively unstable flames are found to periodically form and destroy these flame fingers, but the formation of a global cusp that is known to emerge for purely hydrodynamically unstable flames is suppressed. The finite size of the largest scale fingers is explained by an instability in their movement. As they proceed towards the unburnt mixture, they tend to tilt and move laterally, thereby eventually being incorporated again by the rest of the flame. This behavior arises from the interaction of the flame fingers and the diverging velocity field ahead of them. Finally, the effect of equivalence ratio and unburnt gas temperature is investigated showing that flame fingers are found to develop only in case of a thermodiffusively unstable flame.     

Carbon dioxide concentration and temperature in flames by raman spectroscopy

The interpretation and prediction of high temperature Raman spectra for CO₂ in flames requires a more detailed treatment of the upper vibrational states than is required for room-temperature spectra. A method for calculating these upper state contributions is presented and is used to evaluate the results of a series of experiments. Laser Raman CO₂ spectra have been obtained for CH₄-air, CO₂ seeded CH₄-air, and CO-air flames. The results indicate that both the high and low temperature CO₂ spectra agree with the theoretical predictions. Comparison of the CO₂ temperature to the N₂ temperature, both measured at the same position in the flames, indicates a better prediction of temperature from the N₂ spectrum than from the CO₂ spectrum. Accurate values for the CO₂ concentrations, however, can be determined from a comparison of the CO₂ spectrum with the N₂ spectrum.     

Burning velocity of triple flames with gentle concentration gradient

We investigated the local flame speed of a two-dimensional, methane-air triple flame in a rectangular burner. The velocity fields and the concentration profiles were measured with particle image velocimetry and the Rayleigh scattering method, respectively. There was a requisite combination of initial velocity and initial concentration gradient for consistency of the local concentration gradient at the leading edge of the flame. In these cases, the flame curvatures were also consistent. Accordingly, the burning velocity, defined as local flow velocity at the triple point, was determined by the flame curvature. The burning velocity increased with increasing flame curvature, when the curvature was near zero. After that, the burning velocity decreased with increasing curvature. The peak value thus exceeded the adiabatic one-dimensional laminar burning velocity. Comparing the effects of the measured flame stretch rate on the flow strain κ_s and flame curvature κ_c, κ_s is larger and increases more rapidly than κ_c for flame curvatures satisfying 1/R_f < 250 m⁻¹ and then becomes constant while κ_c still increases for 250 m⁻¹ < 1/R_f, so that κ_c becomes much larger than κ_s. There is also a peak in burning velocity at roughly the transition in flame curvature specified above. Therefore, the burning velocity for a low concentration gradient correlates with the flame stretch rate.     

Oxygen concentration effects on laser-induced grating spectroscopy of toluene

The effects of variation in oxygen concentration on laser-induced grating spectroscopy (LIGS) signals from toluene vapour in nitrogen/oxygen mixtures is investigated. The modulation of LIGS signals arising from the interference of counter-propagating acoustic waves with a stationary density perturbation induced by pulsed excitation of toluene by frequency quadrupled radiation from a Nd:YAG laser has been measured as a function of oxygen partial pressure at total gas pressures up to 8?bar. The modulation depth or signal contrast is found to vary in an unexpected way with oxygen partial pressure and the behaviour is ascribed to energy transfer to excited singlet states of the oxygen molecule and subsequent collisional quenching. A simple model of the energy transfer dynamics is presented that reproduces the observed behaviour and the potential for using the signal contrast of LIGS signals as a measure of oxygen concentration is discussed.     

Explosive dynamics of bluff-body-stabilized lean premixed hydrogen flames at blow-off

Two-dimensional direct numerical simulation (DNS) databases of bluff-body-stabilized lean hydrogen flames representative of complicated reactive–diffusive system are analysed using the combined approach of computational singular perturbation (CSP) and tangential stretching rate (TSR) to investigate chemical characteristics in blow-off dynamics. To assess the diagnostic approaches in flame and blow-off dynamics, Damköhler number and TSR variables are applied and compared. Four cases are considered in this study showing different flame dynamics such as the steadily stable mode, local extinction by asymmetric vortex shedding, convective blow-off and lean blow-out. DNS data points in positive explosive eigenvalue conditions were subdivided into four different combinations in TSR and extended TSR space and categorized in four distinct characteristic regions, such as kinetically explosive or dissipative and transport-enhanced or dissipative dynamics. The TSR analysis clearly captures the local extinction point in the complicated vortex shedding and allows an improved understanding of the distinct chemistry-transport interactions occurring in convective blow-off and lean blow-out events.     

The influence of water on extinction and ignition of hydrogen and methane flames

The influence of water vapor on critical conditions of extinction and autoignition of premixed and nonpremixed flames is investigated. The fuels tested are hydrogen (H₂) and methane (CH₄). Studies on premixed systems are carried out by injecting a premixed reactant stream made up of fuel, oxygen (O₂), and nitrogen (N₂) from one duct, and an inert-gas stream of N₂ from the other duct. Critical conditions of extinction are measured for various amounts of water vapor added to the premixed reactant stream. The ratio of fuel to oxygen is maintained at a constant value, and the amounts of water vapor and nitrogen are so chosen that the adiabatic temperature remains the same. This ensures that the physical influence of water is the same for all cases. Therefore, changes in values for the critical conditions of extinction are attributed to the chemical influence of water vapor. Studies on nonpremixed systems are carried out by injecting a fuel stream made up of fuel and N₂ from one duct ,and an oxidizer stream made up of O₂ and N₂ from the other duct. Critical conditions of extinction are measured with water vapor added to the oxidizer stream. The concentrations of reactants are so chosen that the adiabatic temperature and the flame position stay the same for all cases. Critical conditions of autoignition are measured by preheating the oxidizer stream of the nonpremixed system. Water vapor is added to the oxidizer stream. Numerical calculations are performed using a detailed chemical-kinetic mechanism and compared with measurements. Experimental and numerical studies show that addition of water makes the premixed and nonpremixed flames easier to extinguish and harder to ignite. The chemical influence of water is attributed to its enhanced chaperon efficiency in three body reactions.     

Cellular instabilities of expanding hydrogen/propane spherical flames at elevated pressures: theory and experiment

An experimental and theoretical investigation of the onset of cellular instabilities on spherically expanding flames in mixtures of hydrogen and propane in air at elevated pressures was conducted. Critical conditions for the onset of instability were measured and mapped out over a range of pressures and mixture compositions. An asymptotic theory of hydrodynamic and diffusional-thermal cell development on flames in mixtures comprised of two scarce fuels burning in air was also formulated. Predicted values of Peclet number, defined as the flame radius at the onset of instability normalized by the flame thickness, were shown to compare favorably with the experimentally measured values.     

On polyhedral structures of lean methane/hydrogen Bunsen flames: Combined experimental and numerical analysis

In premixed flame propagation of lean hydrogen or hydrogen-enriched blends, both hydrodynamic and thermo-diffusive instabilities are governing the flame front shape and affect its propagation velocity. As a result, different types of cellular patterns can occur along the flame front in a laminar scenario. In this context, an interesting phenomenon is the formation of polyhedral flames which can be observed in a Bunsen burner. It is the objective of this work to systematically characterize the polyhedral structures of premixed methane/hydrogen Bunsen flames in a combined experimental and numerical study. A series of lean flames with hydrogen content varying between 20 and 85% at two equivalence ratios is investigated. The experiments encompass chemiluminescence imaging together with Planar Laser-induced Fluorescence (PLIF) measurements of the OH radical. Characteristic cell sizes are quantified from the experiments and related to the characteristic length scales obtained from a linear stability analysis. In the experiments, it is observed that the cell sizes at the base of the polyhedral Bunsen flames decrease almost linearly with hydrogen addition and only a weak dependence on the equivalence ratio is noted. These trends are well reflected in the numerical results and the length scale comparison further shows that the wavelength with the maximum growth rate predicted by the linear stability analysis is comparable to the cell size obtained from the experiment. The correlation between the experimental findings and the linear stability analysis is discussed from multiple perspectives considering the governing time and length scales, furthermore drawing relations to previous studies on cellular flames.