H2 Mass Fraction Research Articles

CFD Simulation of Turbulent Flows in Proto X-3 Bioenergy Micro Gas Turbine Combustor using STD k-ε and RNG k-ε Model for Green Building Application

This paper presents a numerical analysis of gas flow in the annular combustion chamber of a Proto X-3 Bioenergy micro gas turbine for green building applications. The computational fluids dynamics (CFD) simulation was conducted in two dimensions, turbulent flow and gas phase combustion, with the goal of comparing the effects of different models in real conditions. Two different turbulence models, standard (STD) k-e and renormalization group (RNG) k-e, were applied for simulations. The fuel used was biogas produced from animal waste. Fuel consumption was assumed to be 100 kJ/s for simulations. The results of the simulations were analyzed and compared for reference. The temperature and the mass fraction of CH4, H2, O2, and CO2 distributions gave almost the same results for both models; therefore, both models (STD k-e and RNG k-e) could be used to represent the combustion process phenomenon without many significant differences.

Scalar structure of turbulent stratified swirl flames conditioned on local equivalence ratio

In a recent paper (Kamal, et al., 2015), we reanalyzed single shot species and temperature measurements from non-swirling flames stabilized on the Cambridge/Sandia stratified burner by conditioning measurements on the local equivalence ratio, and found that the state space structure of the flames was closely approximated by that of a laminar flame at the given equivalence ratio. In the present communication, we show that the same state space relationships remain robust for species CH4, O2, H2O, and CO2 in premixed and stratified flames under high swirl. Conditioned mass fractions of CO and H2 in the stratified swirl flame show a greater effect of stratification than was observed in the non-swirling cases, and this is attributed to larger gradients in equivalence ratio that occur with the addition of swirl. Aside from this modest effect of stratification, major species mass fractions in the swirling flame are also closely approximated by laminar flame results at the local equivalence ratio.

Adaptive neuro-fuzzy system (ANFIS) based appraisal of accumulated heat from hydrogen-fueled engine

Hydrogen as an ecological fuel has called a great deal of attention and has been a dynamic field of studying interest for engine designers. The work presented herein is an investigation of the accumulated heat created in hydrogen fueled engine as affected by crank angle, equivalence ratio, penetration and H2 mass fraction. The capability of computational fluid dynamics (CFD) approach was applied to simulate the combustion process while the potential of a universal approximator, ANFIS (adaptive neuro-fuzzy inference system), was utilized to model the problem. Two statistical performance criteria of root mean square error (RMSE) and coefficient of determination (R2) were considered to further assess the modeling performance. The findings revealed that due to high diffusivity characteristics of hydrogen, the optimum level of equivalence ratio is affected by the addition of hydrogen. It was observed that hydrogen addition yields higher heat release since hydrogen and air can mix well at any standard scale. Higher engine speed than that of 3500 rpm resulted in a higher spray penetration at any specified crank-angle position. It was also concluded that an ANFIS structure with Trimf membership algorithm each at three membership functions yielded RMSE of 0.011527 and R2 equal to 0.999.

Molecular hydrogen emission in the interstellar medium of the Large Magellanic Cloud

We present the detection and analysis of molecular hydrogen emission towards ten interstellar regions in the Large Magellanic Cloud. We examined low-resolution infrared spectral maps of 12 regions obtained with the Spitzer infrared spectrograph (IRS). The pure rotational 0–0 transitions of H2 at 28.2 and 17.1 μm are detected in the IRS spectra for 10 regions. The higher level transitions are mostly upper limit measurements except for three regions, where a 3σ detection threshold is achieved for lines at 12.2 and 8.6 μm. The excitation diagrams of the detected H2 transitions are used to determine the warm H2 gas column density and temperature. The single-temperature fits through the lower transition lines give temperatures in the range 86–137 K. The bulk of the excited H2 gas is found at these temperatures and contributes ∼5–17 per cent to the total gas mass. We find a tight correlation of the H2 surface brightness with polycyclic aromatic hydrocarbon and total infrared emission, which is a clear indication of photoelectric heating in photodissociation regions. We find the excitation of H2 by this process is equally efficient in both atomic- and molecular-dominated regions. We also present the correlation of the warm H2 physical conditions with dust properties. The warm H2 mass fraction and excitation temperature show positive correlations with the average starlight intensity, again supporting H2 excitation in photodissociation regions.

H I-TO-H2TRANSITIONS AND H I COLUMN DENSITIES IN GALAXY STAR-FORMING REGIONS

We present new analytic theory and radiative transfer computations for the atomic to molecular (HI-to-H2) transitions, and the build-up of atomic-hydrogen (HI) gas columns, in optically thick interstellar clouds, irradiated by far-ultraviolet photodissociating radiation fields. We derive analytic expressions for the total HI column densities for (1D) planar slabs, for beamed or isotropic radiation fields, from the weak- to strong-field limits, for gradual or sharp atomic to molecular transitions, and for arbitrary metallicity. Our expressions may be used to evaluate the HI column densities as functions of the radiation field intensity and the H2-dust-limited dissociation flux, the hydrogen gas density, and the metallicity-dependent H2 formation rate-coefficient and far-UV dust-grain absorption cross-section. We make the distinction between "HI-dust" and "H2-dust" opacity, and we present computations for the "universal H2-dust-limited effective dissociation bandwidth". We validate our analytic formulae with Meudon PDR code computations for the HI-to-H2 density profiles, and total HI column densities. We show that our general 1D formulae predict HI columns and H2 mass fractions that are essentially identical to those found in more complicated (and approximate) spherical (shell/core) models. We apply our theory to compute H2 mass fractions and star-formation thresholds for individual clouds in self-regulated galaxy disks, for a wide range of metallicities. Our formulae for the HI columns and H2 mass fractions may be incorporated into hydrodynamics simulations for galaxy evolution.

Reprint of: Multi-fluid CFD modeling of biomass gasification in polydisperse fluidized-bed gasifiers

This work presents an Eulerian computational fluid dynamics (CFD) model of biomass gasification for use in fluidized-bed gasifier (FBG) simulations. The physical and chemical processes of particle gasification and their interaction with the reactive gas flow are modeled within a multi-fluid framework derived from kinetic theory of granular flows. The transport equations of continuous solid phases and species mass fraction of CO, CO2, CH4, H2, H2O, O2, N2, tar, char, and ash are coupled with chemical kinetic models, which describe moisture vaporization, particle devolatilization, homogeneous volatile reaction, heterogeneous char oxidation and char gasification. Continuously variable particle density due to volatilization of lighter components and chemical reactions is implemented to account for the evolution of reacting particles' physical properties. A time-splitting method is employed for decoupling the chemical source from convection. A time-step adaption and restart procedure is implemented to provide the solution stability for strong chemical reaction and to achieve numerical solution efficiency for energy reactor simulations in the time-splitting approach. The chemical source terms based on chemical kinetics are implemented with first-order accuracy in the finite-volume framework. The CFD model is used to simulate wood gasification using air as a fluidization agent in a lab-scale FBG. The simulation results provide detailed information on the dynamic particle processes, char elutriation, and gas composition at the reactor outlet. Operating conditions of different air/biomass mass flow ratio, reactor temperature, and biomass moisture content are simulated and analyzed as to their influence on gas composition and product yields at the gasifier outlet.

Multi-fluid CFD modeling of biomass gasification in polydisperse fluidized-bed gasifiers

This work presents an Eulerian computational fluid dynamics (CFD) model of biomass gasification for use in fluidized-bed gasifier (FBG) simulations. The physical and chemical processes of particle gasification and their interaction with the reactive gas flow are modeled within a multi-fluid framework derived from kinetic theory of granular flows. The transport equations of continuous solid phases and species mass fraction of CO, CO2, CH4, H2, H2O, O2, N2, tar, char, and ash are coupled with chemical kinetic models, which describe moisture vaporization, particle devolatilization, homogeneous volatile reaction, heterogeneous char oxidation and char gasification. Continuously variable particle density due to volatilization of lighter components and chemical reactions is implemented to account for the evolution of reacting particles' physical properties. A time-splitting method is employed for decoupling the chemical source from convection. A time-step adaption and restart procedure is implemented to provide the solution stability for strong chemical reaction and to achieve numerical solution efficiency for energy reactor simulations in the time-splitting approach. The chemical source terms based on chemical kinetics are implemented with first-order accuracy in the finite-volume framework. The CFD model is used to simulate wood gasification using air as a fluidization agent in a lab-scale FBG. The simulation results provide detailed information on the dynamic particle processes, char elutriation, and gas composition at the reactor outlet. Operating conditions of different air/biomass mass flow ratio, reactor temperature, and biomass moisture content are simulated and analyzed as to their influence on gas composition and product yields at the gasifier outlet.

Application of uncertainty quantification methods for coal devolatilization kinetics in gasifier modeling

The focus of this research is to study sensitivity of input parameters in terms of chemical reaction kinetics of coal devolatilization using non-intrusive uncertainty quantification (UQ) methods. The effects of heating rate, pressure, and temperature on coal devolatilization have been considered. Variations in coal devolatilization kinetics and product yields were captured via Carbonaceous Chemistry for Computational Modeling (C3M) for operating conditions similar to the transport gasifier using PC Coal Lab (PCCL) kinetic package. Temperature, pressure and heating rate were considered as three input parameters, while the quantities of interest or response variables were mass fractions of CO, CO2, H2, tar, H2O, and CH4 along with total volatile yield. A direct Monte Carlo-simulation-based approach was employed to perform the UQ analysis. The correlations among the response variables were investigated by computing a correlation matrix that supports the findings of yield of devolatilization reported by various experiments in the literature. Sensitivity study of the input parameters was analyzed by using the Sobol Total Indices methodology implemented in PSUADE, an open source UQ toolbox. These findings clearly demonstrate the pronounced effect of temperature on coal devolatilization product yields, and hence will be considered as a key parameter in future studies. The preliminary study presented in this paper paves a path for incorporating uncertainty caused by chemical reaction kinetics in computational fluid dynamics based modeling of coal gasifier systems and scale-up studies.

Direct Numerical Simulation Study of an Experimental Lifted H2/N2 Flame. Part 1: Validation and Flame Structure

Direct numerical simulation (DNS) is used to investigate an experimental lifted turbulent H2/N2 flame in a co-flow of hot products of lean H2/air combustion. The DNS code solves the fully compressible Navier–Stokes equations. A fourth-order explicit Runge–Kutta method for time integration and an eighth-order central differencing scheme for spatial discretization are used. A detailed 9 species and 19-step mechanism for hydrogen combustion is employed. The inflow turbulence is considered by adding a random velocity field to the mean flow. The Reynolds number based on the exit diameter and jet velocity is 23 600. In total, more than 285 million grids are used, and the grid spacing is sufficient to resolve all of the turbulence scales and the flame structure. The comparison of the DNS results and the measurements is carried out for various scalars, including the mixture fraction, temperature, and mass fractions of H2, O2, H2O, and OH. Good agreements are observed for the Favre mean and fluctuating components, which validate the present approach and code. The overall flame structure is also outlined. Two combustion modes are identified in the lifted flame, and the non-premixed combustion dominates.

THE IMPACT OF INTERACTIONS, BARS, BULGES, AND ACTIVE GALACTIC NUCLEI ON STAR FORMATION EFFICIENCY IN LOCAL MASSIVE GALAXIES

Using atomic and molecular gas observations from the GASS and COLD GASS surveys and complementary optical/UV data from the Sloan Digital Sky Survey and the Galaxy Evolution Explorer, we investigate the nature of the variations in the molecular gas depletion time observed across the local massive galaxy population. The large and unbiased COLD GASS sample allows us for the first time to statistically assess the relative importance of galaxy interactions, bar instabilities, morphologies, and the presence of active galactic nuclei (AGNs) in regulating star formation efficiency. We find that both the H2 mass fraction and depletion time vary as a function of the distance of a galaxy from the main sequence traced by star-forming galaxies in the SFR–M* plane. The longest gas depletion times are found in below-main-sequence bulge-dominated galaxies (μ* >5 × 108 M☉ kpc−2, C > 2.6) that are either gas-poor (/M* <1.5%) or else on average less efficient by a factor of ∼2 than disk-dominated galaxies at converting into stars any cold gas they may have. We find no link between the presence of AGNs and these long depletion times. In the regime where galaxies are disk-dominated and gas-rich, the galaxies undergoing mergers or showing signs of morphological disruptions have the shortest molecular gas depletion times, while those hosting strong stellar bars have only marginally higher global star formation efficiencies as compared to matched control samples. Our interpretation is that the molecular gas depletion time variations are caused by changes in the ratio between the gas mass traced by the CO(1–0) observations and the gas mass in high-density star-forming cores (as traced by observations of, e.g., HCN(1–0)). While interactions, mergers, and bar instabilities can locally increase pressure and raise the ratio of efficiently star-forming gas to CO-detected gas (therefore lowering the CO-based depletion time), massive bulges may prevent the formation of dense clumps by stabilizing gas disks against fragmentation, therefore producing the long depletion times. Building a sample representative of the local galaxy population with M* >1010 M☉, we derive a global Kennicutt–Schmidt star formation relation of slope 1.18 ± 0.24 and observe structure within the scatter around this relation, with galaxies having low (high) stellar mass surface densities lying systematically above (below) the mean relation, suggesting that is not the only parameter driving the global star formation ability of a galaxy.

Transport characteristic study of methane steam reforming coupling methane catalytic combustion for hydrogen production

Methane steam reforming coupling methane catalytic combustion for hydrogen production is an effective method of process intensification for this reaction with strong heat effect. In this paper, simulation of this coupling process was carried out. Effects of inlet parameters of methane steam reforming and methane catalytic combustion on reactor performance were investigated. Results showed that, methane conversion at steam reforming channel outlet nearly approached 100%, however, methane conversion was greatly influenced by inlet velocity in catalytic combustion channel; H2 and CO mass fraction was influenced by coupling of heat generation and consumption in the reactor. The temperature distribution in both steam reforming and combustion channels presented a T-shaped profile indicating a catalytic combustion controlling mechanism of the reactor. The maximum hot spot temperature difference variations in both channels expressed a half saddle-shaped. Local imbalance of strong heat effect of steam reforming and catalytic combustion led to a sharp hot spot near the reactor inlet. Simulation results of methane steam reforming and catalytic combustion coupling process indicate that the catalyst activity at reactor inlet needs to be redesigned and optimized.

Combustion simulation technique for reducing chemical mechanisms using look-up table of chemical equilibrium calculations: Application to CO–H2–air turbulent non-premixed flame

This paper presents a dimension reduction technique for turbulent non-premixed flames based on a chemical equilibrium method combined with a reaction calculation. An advantage of the proposed technique is that it simplifies the process of building a reduced chemical mechanism according to the accuracy requirement for the chemical species. The proposed technique reduces the computational time for the combustion simulation. In this technique, chemical species are roughly divided into two groups: The first includes fuels and intermediate species; the intermediate species include only those necessary for high accuracy. The second group comprises the rest of the species, and their mass fractions are obtained from a look-up table prepared using chemical equilibrium calculations conducted prior to the combustion simulation. The obtained results were compared with the experimental data of a CO–H2–air turbulent non-premixed flame and computational data obtained using the eddy dissipation concept (EDC) model. The major mass fractions of CO, H2, and H2O, as well as the temperature and mixture fraction obtained using the proposed technique, were in good agreement with these reference data. Furthermore, compared with the EDC model, the minor mass fractions of OH, O, and NO obtained using the proposed technique were close and the computational time was 75% lower.

Molecular hydrogen in Lyman alpha emitters

We present a physically motivated model to estimate the molecular hydrogen (H2) content of high-redshift (z~5.7,6.6) Lyman Alpha Emitters (LAEs) extracted from a suite of cosmological simulations. We find that the H2 mass fraction, (f_H2), depends on three main LAE physical properties: (a) star formation rate, (b) dust mass, and (c) cold neutral gas mass. At z~5.7, the value of f_H2 peaks and ranges between 0.5-0.9 for intermediate mass LAEs with stellar mass M_* ~ 10^{9-10} solar mass, decreasing for both smaller and larger galaxies. However, the largest value of the H2 mass is found in the most luminous LAEs. These trends also hold at z\sim6.6, although, due to a lower dust content, f_H2(z=6.6)\sim0.5 f_H2(z=5.7) when averaged over all LAEs; they arise due to the interplay between the H2 formation/shielding controlled by dust and the intensity of the ultraviolet (UV) Lyman-Werner photo-dissociating radiation produced by stars. We then predict the carbon monoxide (CO) luminosities for such LAEs and check that they are consistent with the upper limits found by Wagg et al. (2009) for two z>6 LAEs. At z\sim(5.7, 6.6), the lowest CO rotational transition observable for both samples with the actual capabilities of Atacama Large Millimeter Array (ALMA) is the CO(6-5). We find that at z\sim5.7, about 1-2% of LAEs, i.e., those with an observed Lyman Alpha luminosity larger than 10^{43.2} erg/s would be detectable with an integration time of 5-10 hours (S/N=5); at z\sim6.6 none of the LAEs would be detectable in CO, even with an ALMA integration time of 10 hours. We also build the CO `flux function', i.e., the number density of LAEs as a function of the line-integrated CO flux, S_CO, and show that it peaks at S_CO = 0.1 mJy at z\sim5.7, progressively shifting to lower values at higher redshifts. We end by discussing the model uncertainties.

Thermodynamic Analysis for the Pressure Effects on Reforming Reaction of Methane Wet-Air at Low Temperature in a Micro-Reactor

How to keep combustion stability and less carbon deposit in micro-channel for the methane wet-air auto-thermal reforming combustion technology is a hot research area all over the world now. Even if there are some literatures with respect to this area, but there are many problems to keep studying. In this paper, the effects on the carbon deposit rule, hydrogen and methane conversion characteristics under lean oxygen at pressure have been studied below 973K by thermodynamic analysis with constant feed gas mass flow. Results show that the carbon deposit increases with increasing the temperature first, and then, it decreases. At the same time, the carbon deposit always decreases with increasing reaction pressure. At the different reaction pressure, the temperature zone of carbon deposit is changed, but the upper temperature limit is around 850K. H2 mass fraction and CH4 conversion ratio are raised with increasing the temperature, decreased with increasing the pressure. Adding pressure helps to reduce carbon deposit and maintain a certain H2 mass fraction and CH4 conversion ratio. All results show the reasonable reaction pressure is 0.2MPa in micro reactor.

MOLECULAR GAS DISK STRUCTURES AROUND ACTIVE GALACTIC NUCLEI

We present new high resolution numerical simulations of the ISM in a central R ~32 parsecs region around a supermassive black hole at a galactic center. Three-dimensional hydrodynamic modeling of the ISM (Wada & Norman 2002) with the nuclear starburst now includes tracking of the formation of molecular hydrogen out of the neutral hydrogen phase. In a quasi equilibrium state, mass fraction of H2 is about 0.4 (total H2 mass is ~1.5 10^6 Msun) of the total gas mass for the uniform far UV (FUV) with G_0 = 10. The gas forms an inhomogeneous disk, whose scale-height becomes larger in the outer region. H2 forms a thin nuclear disk in the inner ~ 5 pc, which is surrounded by molecular clouds swelled up toward h < 10 pc. The velocity field of the disk is highly turbulent in the torus region, whose velocity dispersion is ~ 20 km/s on average. Average supernova rate (SNR) of ~ 5 10^-5/yr is large enough to energize these structures. Gas column densities toward the nucleus larger than 10^22 cm^-2 are observed if the viewing angle is smaller than \theta_v ~ 50 deg from the edge-on. However, the column densities are distributed over almost two orders of magnitude around the average for any given viewing angle due to the clumpy nature of the torus. For a stronger FUV (G_0 =100), the total H2 mass in an equillibrium is only slightly smaller (~ 0.35). Finally the morphology and kinematics of the circumnuclear molecular gas disks emerging from our models is similar to that revealed by recent near infrared observations using VLTI/Keck.

A two-dimensional simulation of developing laminar heat and mass transfer in fuel cell channels with uniform suction of O2 and H2

A numerical model was developed to simulate the momentum, heat, and mass transfer in the fuel cell channels at steady state and developing laminar flow with constant fluid properties. The continuity, momentum, energy, and species equations were solved for humidified air and humidified H2 with constant mass flux suction boundary condition for O2 in the cathode and H2 in the anode channels. A finite-volume method was used to solve the equations in two-dimensional cases. Friction coefficient was related to the flowrate, whereas Nusselt and Sherwood numbers were, respectively, related to the heat and mass transfer in the flow channels. The distribution of local Nu and Sh numbers and their average values were obtained on the porous walls of the channels (electrodes). The local Nu and Sh numbers were increased with increasing the pressure and Reynolds numbers, whereas the coefficient of friction and the average Nu and Sh numbers were decreased when the pressure and Re number are increased. The mass fraction of O2 and H2 were decreased along the channel, except at the entrance regions, whereas the mass fraction of water vapour increased along the channel, especially on the porous wall. Solving the species equations gave: (a) the distribution of mass concentration of O2 and H2 and other species in the channels and on the electrode surfaces for several stochiometric ratios and different current densities; (b) the local and average Sherwood numbers in the cathode channels for O2 at different stochiometric ratios, different inlet wall Reynolds numbers, and several inlet pressures; and (c) the water vapour mass fraction in both channels and the mass of liquid water if condensation occurs.

Radiative Shock–induced Collapse of Intergalactic Clouds

Accumulating observational evidence for a number of radio galaxies suggests an association between their jets and regions of active star formation. The standard picture is that shocks generated by the jet propagate through an inhomogeneous medium and trigger the collapse of overdense clouds, which then become active star-forming regions. In this contribution, we report on recent hydrodynamic simulations of radiative shock-cloud interactions using two different cooling models: an equilibrium cooling-curve model assuming solar metallicities and a nonequilibrium chemistry model appropriate for primordial gas clouds. We consider a range of initial cloud densities and shock speeds in order to quantify the role of cooling in the evolution. Our results indicate that for moderate cloud densities (≳1 cm-3) and shock Mach numbers (≲20), cooling processes can be highly efficient and result in more than 50% of the initial cloud mass cooling to below 100 K. We also use our results to estimate the final H2 mass fraction for the simulations that use the nonequilibrium chemistry package. This is an important measurement, since H2 is the dominant coolant for a primordial gas cloud. We find peak H2 mass fractions of ≳10-2 and total H2 mass fractions of ≳10-5 for the cloud gas, consistent with cosmological simulations of first star formation. Finally, we compare our results with the observations of jet-induced star formation in "Minkowski's Object," a small irregular starburst system associated with a radio jet in the nearby cluster of galaxies Abell 194. We conclude that its morphology, star formation rate (~0.3 M☉ yr-1) and stellar mass (~1.2 × 107 M☉) can be explained by the interaction of a ~9 × 104 km s-1 jet with an ensemble of moderately dense (~10 cm-3), warm (104 K) intergalactic clouds in the vicinity of its associated radio galaxy at the center of the galaxy cluster.

Composition of Volatile Gases and Thermochemical Properties of Wood for Modeling of Fixed or Fluidized Beds

The modeling of the conversion of solid fuel in combustion or gasification systems needs a description of the composition of the volatile gases that leave a fuel particle of typical size in a fixed or fluidized bed during devolatilization. Much work has been published on release of volatiles, but a general model of the composition of volatile gases and a comprehensive presentation of the related thermochemical properties of the fuel are still missing. Here, a simplified model is presented whose structure is valid for any solid fuel. The model consists of a heat and mass balance complemented by empirical data. The empirical coefficients have to be specified for certain classes of fuel: the fuels treated in this paper are one type of hardwood and one of softwood, having particle sizes in the range used in utility boilers. The species in the volatile gases are represented by the time mean mass fractions of CO2, CO, H2O, H2, light hydrocarbons, and heavy hydrocarbons. In addition, a comprehensive set of data...

Sensitivity analysis based reduction of complex reaction mechanisms in turbulent non-premixed combustion

A turbulent combustion model without simplifying assumptions about the chemical reactions is evaluated by means of a sensitivity analysis. The turbulence model is conventional and based on gradient transport assumptions and an eddy viscosity concept. In addition to the equations of the turbulent flow field the species conservation equations are solved. The chemical reactions are described by a detailed, elementary kinetics reaction mechanism. The source terms in the species conservation equations are treated by means of a presumed-shape pfd-closure relaxing the assumption of statistically independent variables. The sensitivity analysis for the turbulent combustion model is performed in terms of the local first order sensitivity coefficients. The turbulent combustion model and sensitivity analysis are adopted to a turbulent hydrogen air diffusion flame in a simple flow geometry. Temperature, major chemical species and OH radical concentrations are reasonable well predicted. The sensitivity analysis shows the relative importance of single elementary steps of the reaction mechanism compared with turbulent transport. Only few rate coefficients of the entire chemical mechanism influence the main response of the turbulent combustion model, viz. temperature and mass fractions of the major chemical components. From this simplifications of the closure for the mean chemical reaction rates are derived. Furthermore, for the turbulent hydrogen air flame under consideration sensitivity analysis shows control of turbulent transport throughout the largest part of the flame. Therefore, the substantial properties of the flame, viz. temperatures, mass fractions of H2, O2 and H2O are predicted reasonably by any reduced chemical model providing sufficiently large conversion rates of the fuel.

Piloted Diffusion Flames of Diluted Methane Near Extinction: Mean Structure from Raman/Rayleigh Fluorescence Measurements

Abstract Time- and space resolved measurements have been made, for the first time using simultaneous Raman, Rayleigh and LIF techniques in a turbulent jet diffusion flame of CH4, diluted alternatively with N2 and air. The spontaneous Raman/Rayleigh data are processed to yield instantaneous species mass fractions of CH4, O2, N2, H2O, H2, CO and CO2 as well as temperature, densily and mixture fraction. The LIF data is used to obtain the OH radical mass fraction. The effects of dilution on flame shape, soot interference, temperature and species mass fractions are reported for two cases each of N2 and air dilution. In all cases the flames are close to extinction so that finite chemistry effects are important. The results show that dilution shortens the flame and that air dilution markedly increases the blowoff jet velocity whilst N2 dilution reduces it. All visible (yellow) soot radiation disappears, and the interference by soot precursors with the Raman signals, a serious problem in undiluted CH4 flames, is ...