Mole Fractions Of Species Research Articles

Investigation of strut-ramp injector in a Scramjet combustor: Effect of strut geometry, fuel and jet diameter on mixing characteristics

The strut-based injector has been found to be one of the most promising injector designs for a supersonic combustor, offering enhanced mixing of fuel and air. The mixing and flow field characteristics of the straight (SS) and Tapered strut (TS), with fixed ramp angle and height at freestream Mach number 2 in conjunction with fuel injection at Mach 2.3 have been investigated numerically and reported. In the present investigation, hydrogen (H2) and ethylene (C2H4) are injected in oncoming supersonic flow from the back of the strut, where jet to freestream momentum ratio is maintained at 0.79 and 0.69 for H2 and C2H4, respectively. The predicted wall static pressure and species mole fractions at various downstream locations are compared with the experimental data for TS case with 0.6 mm jet diameter and found to be in good agreement. Further, the effect of jet diameter and strut geometry on the near field mixing in strut ramp configuration is discussed for both the fuels. The numerical results are assessed based on various parameters for the performance evaluation of different strut ramp configurations. The SS configuration for both the injectant has been found to be an optimum candidate; also it is observed that for higher jet diameter larger combustor length is required to achieve satisfactory near field mixing.

Coherent anti-Stokes Raman spectroscopy measurement of ethylene in combustion.

Width-increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) has been developed for spatially and temporally resolved simultaneous measurement of temperature and mole fraction of most major species in ethylene-air flames. This paper describes a method to infer coherent anti-Stokes Raman spectroscopy complex susceptibility distributions of the ν3 band of ethylene from WIDECARS spectra measured in heated mixtures of ethylene and air, and to use such distributions to fit experimental WIDECARS spectra in an ethylene-air flame. The method is used to measure mole fraction ethylene in a dual-mode supersonic combustor burning premixed ethylene and air with single-laser-shot precision (one standard deviation) of ±0.0025 (absolute).

Measurements of line strengths for some lines of ammonia in 6611-6618 cm-1

The target gas molecular absorption spectrum parameters especially line strengths are very important for measuring temperature and concentration with tunable diode laser absorption spectroscopy (TDLAS) technique. Usually, researchers use line strengths which come from spectrum database, like HITRAN and GEISA and HITEMP spectra database, but those database include values from the theoretical computation, as is well known, there is a big error between the theoretical value and the actual value. In order to modify the line strengths of the database, 20 ammonia spectrum absorption lines in a wavenumber range between 6611 and 6618 cm-1 are measured at different pressures by using direct tunable diode laser absorption spectroscopy (dTDLAS) technique. The measurement procedure is repeated at least 10 times at each pressure, and then average value is calculated. Voigt fitting is used to obtain all line integral area, and then the line integral area is obtained by linear fitting. The slope of the fitting straight line equals line strength. Uncertainty analysis is given for the measurements. The measured linestrength is a function of integrated area, temperature, pressure, species mole fraction and effective path length. So, the calculated linestrength uncertainties based on those parameters uncertainties, and the uncertainties of pressure, species mole fraction and effective path length are similar for all transitions with P =0.25%, =0.2%, and L =0.4%. The uncertainties of the integrated area and temperature are related to different lines, and their values come from the actual measurement. In the end, uncertainty propagation formula is used to calculate linestrength uncetainty. Uncertainties of our measured line strengths are in a 0.81%-3.33% range. Our measured line strength values are different from line strengths in the HITRAN 2012 database, and the deviations are in 0.51%-17.28% range.

Slow magnetic relaxations in a ladder-type Dy(iii) complex and its dinuclear analogue.

The complex {[Dy2(PDOA)3(H2O)6]·2H2O}n (1) (H2PDOA = 1,2-phenylenedioxydiacetic acid) was prepared from aqueous solution. Its crystal structure, built up of {-Dy-O-C-O-}n chains interlinked by PDOA ligands yielding a ladder-like arrangement, was determined at 173 K. 1 exhibits slow magnetic relaxation under a small magnetic field BDC = 0.2 T with two (LF and HF) relaxation channels. The LF relaxation time at BDC = 0.2 T and T = 1.85 K is as slow as τ(LF) = 46 ms whereas the HF channel is τ(HF) = 1.4 ms. The mole fraction of the LF species is xLF = 0.76 at 1.85 K and it escapes progressively on heating. In the dinuclear analogue [Dy2(PDOA)3(H2O)6]·3.5H2O (2) one PDOA ligand forms a bis(chelate) bridge between the two Dy(iii) atoms yielding a local structure analogous to that in 1; however its AC susceptibility data show slightly different quantitative characteristics of the single-molecule magnetic behaviour.

PRECIPITATION SIMULATION AND X-RAY ABSORPTION SPECTROSCOPY FOR HOT ROLLED V-HSLA STEEL

Aiming to the process optimisation while reducing or avoiding experimental work, the work has been carried out by a physically-based thermokinetic model proven by synchrotron X-ray absorption spectroscopy. The mole fraction and size distribution of different precipitate species as well as the consumption of the dissolved elements are of the main interest. The model considers the involved parameters in the hot rolling process, i.e., austenite grain size, disloca-tion density as a function of deformation, and thermal history during the process. Both grain boundary and dislocation are nucleation sites. The diffuse interface effect on the interfacial energy as well as a volumetric misfit of AlN at dislocations is also taken into account. The latter is because of its significant difference in the lattice parameter from the matrix. The X-Ray absorption spectroscopy (XAS) taken advantage from the synchrotron technology has been employed for the quantification of the precipitation of vanadium as well as its fraction in the solid solution. With very good agreement, V(C,N) precipitates significantly due to high dislocation and is not overridden by the competing AlN.

Development of a Reduced n-Tetradecane–Polycyclic Aromatic Hydrocarbon Mechanism for Application to Two-Stroke Marine Diesel Engines

Rigorous current regulation requires decrease emissions from marine engines. It is difficult to test all technologies for marine engines using experimental methods. It is important to develop simulated models for alternative fuels. A semireduced kinetic mechanism can be constructed to investigate the performance of the marine diesel engine, which consists of 341 elementary reactions and 74 species. Then, the Directed Relation Graph with Error Propagation (DRGEP) and the sensitivity analysis methods were used to reduce this mechanism. The reduced mechanism consists of 279 elementary reactions and 62 species. The ignition delay times with the semidetailed and reduced mechanisms were validated with the experimental data. At the same time, the results were compared with the data of published mechanisms. Good agreements of ignition delay times were obtained. It was also found that the trends of in-cylinder temperature, pressure, and main species mole fractions were almost the same as results of the published m...

Parametric study on catalytic tri-reforming of methane for syngas production

A two-dimensional numerical model for syngas production from tri-reforming of methane (TRM) in adiabatic tubular fixed-bed reactors was established. From the results obtained, it was found that reactant must be preheated to certain temperatures for TRM activation. Although the delay factor accounting for the varying catalytic bed activities produced different temperature and species mole fraction profiles in the reactor upstream, the reactor performance was delay factor independent if the reactor outlet results were used because nearly identical temperature and species mole fraction variations were obtained at the reactor downstream. The numerical results also indicated that reverse water-gas shift reaction plays an important role for H2 and CO yields. With higher O2 in reactant, high temperature resulted, leading to lower H2/CO ratio. The absence of H2O in the reactant caused dry reforming of methane as the dominant reaction, resulting in H2/CO ratio close to unity. With the absence of CO2 in the reactant, steam reforming of methane was the dominant reaction, resulting in H2/CO ratio close to 3. Using flue gas from combustion as TRM feedstock, it was found that H2/CO ratio was enhanced using lower CH4 amount in reactant. High-temperature flue gas was suggested for TRM for the activation requirement.

Effects of molecular transport in LES/PDF of piloted turbulent dimethyl ether/air jet flames

We report large-eddy simulation (LES)/probability density function (PDF) modeling of piloted turbulent dimethyl ether (DME)/air jet flames with a skeletal chemical mechanism. The modeled PDF transport equation with three different implementations of molecular transport in the interaction-by-exchange-with-the-mean (IEM) mixing model is solved using the NGA/HPDF code in order to assess the a posteriori performance of these LES/PDF methodologies applied to the DME flames D and F proposed in the TNF Workshop. The three implementations considered are the classical random-walk model (IEM-RW), the mean-drift model with a single molecular diffusivity (IEM-MD), and the mean-drift model with differential diffusion (IEM-DD). Better quantitative agreement with experimental data for the mean and root-mean-square profiles of mixture fraction, temperature and major species mole fractions and conditional means on mixture fraction are obtained with the IEM-MD model than with the IEM-RW model. In LES of the DME flame series, the molecular diffusivity dominates the turbulent diffusivity in the centerline region, so the accurate modeling of molecular transport plays a significant role in LES/PDF of these jet flames. Nevertheless, the IEM-DD model shows only minor effects on the model predictions compared to IEM-MD in these DME flames.

Experimental and numerical characterization of freely propagating ozone-activated dimethyl ether cool flames

Experimental flame characterization is necessary for the development and validation of chemical kinetics models. Low temperature oxidation produces a cool flame, which is a combustion phenomenon resulting from negative temperature coefficient (NTC) behavior. Kinematic stabilization of premixed, freely propagating, ozone-activated, cool flames of dimethyl ether (DME) has been investigated at sub-atmospheric pressure of 7.3kPa using a laminar flat flame Hencken burner. This platform permits estimation of laminar propagation speed, as well as spatially-resolved temperature and species mole fractions along the burner axis. Stability mapping for a range of equivalence ratios (ϕ) was performed to determine the range of ozone concentrations for which a cool flame can be sustained. Based on the results of stability mapping, an ozone concentration of 6.1% in oxygen was chosen to investigate the characteristics of DME cool flames over a range of equivalence ratios from ϕ=0.4 to 1.4. Two distinct cool flame stabilization modes were observed in experiments: a burner-stabilized mode at low reactant flow rates, and a freely propagating mode at higher flow rates. From the transition between the two modes, cool flame propagation speeds from ϕ=0.4 to 1.4 were determined. Flame temperatures were measured at these equivalence ratios, with maximum temperatures decreasing from 885K at ϕ=0.4 to 779K at ϕ=1.4. A single equivalence ratio of ϕ=0.6 was chosen for detailed investigation of the flame structure, including spatial measurements of temperature and species as a function of height above the burner. Experimental results were compared to numerical simulations of a ϕ=0.6 cool flame. Experimental propagation speed was found to be within 25% of the numerical value, and significant agreement between experimental and numerical species profiles was observed.

Development of a Reduced n-Decane/α-Methylnaphthalene/Polycyclic Aromatic Hydrocarbon Mechanism and Its Application for Combustion and Soot Prediction

In this work, a reduced n-decane/α-methylnaphthalene/polycyclic aromatic hydrocarbon (PAH) kinetic mechanism for calculation of combustion and soot behavior of diesel and its surrogate fuel was developed. This mechanism consists of the pyrolysis of n-decane (C10H22) and α-methylnaphthalene (C10H7CH3), C0–C3 core species reaction, and the formation of initial benzene and PAHs, including 77 species and 287 reactions. The pyrolysis reaction of fuel was obtained by pathway analysis from a detailed mechanism, while the reactions of core species and PAHs were reduced by a directed relation graph with error propagation, computational singular perturbation method, and direct sensitivity analysis, sequentially. The mechanism was validated by the mole fraction of main species and key PAH species in the ethylene premixed flame, ignition delay times of pure and mixed fuel in shock tubes, and the concentrations of major species in jet-stirred reactors. Finally, the new developed mechanism was coupled with a soot pheno...

Energy storage efficiency analyses of CO2 reforming of methane in metal foam solar thermochemical reactor

Solar driven CO2 reforming of methane can store extra 20% concentrated solar energy to chemically bonded energy. With the aim to evaluate key operational parameters on energy storage efficiency, a finite volume method (FVM) coupled with thermochemical kinetics was developed to analyze the solar driven CO2 methane reforming performance in a metallic foam thermochemical reactor. The LH kinetic model of CO2 reforming of methane was compiled by user defined functions (UDFs) to exactly predict the temperature distribution and mole fractions of gas species. The numerical results were validated with experimental results which were tested in Niigata University to guarantee the accuracy of the developed model. Effects of gas mixture inlet velocity, porosity of metal foam reactor, CH4/CO2 ratio and heat flux distribution on energy storage efficiency were analyzed for providing theoretical guideline to the application of solar driven CO2 methane reforming. The numerical results indicated that maximum energy storage efficiency of CO2 reforming of methane was found when the CH4/CO2 ratio is 0.67.

A three-dimensional mathematical model for the anode of a direct ethanol fuel cell

In this paper, we develop a mathematical model to analyze a direct ethanol fuel cell (DEFC). The three-dimensional model is able to predict the flow on all layers of the fuel cell and allow a better analysis of physical and chemical phenomena that occur inside it. In addition, the calculation of mole fraction of species allows one to observe that the diffusion layer has great influence on mass transfer of fuel between the input channel and the catalyst layer. Numerical simulation of reactive flow was made based on the central finite difference method. The equations were integrated in time using the simplified Runge-Kutta multistage scheme. The results obtained are in agreement with the experimental data found in the literature, for different feed concentrations of ethanol and for different operating temperatures of the cell. In this way, the paper contributes to the development of a model for direct ethanol fuel cells, taking into account all losses overpotentials at the anode and the cathode, providing a better understanding of the physical and chemical behavior inside the cell, and on the conversion of chemical energy into electrical energy.

Structure of premixed H2/O2/Ar flames at 1–5 atm studied by molecular beam mass spectrometry and numerical simulation

A lack of available experimental data for spatial distributions of the species mole fractions in the flames of hydrogen at the pressures higher than atmospheric, which are required for developing and validating reliable kinetic models for hydrogen combustion, motivated this study. Stoichiometric laminar premixed H2/O2/Ar flames stabilized on flat burners at pressures 1, 3 and 5atm were examined in this work by molecular beam mass spectrometry. Mole fraction profiles of all flame species (H2, O2, H2O, H2O2, H, O, OH, HO2) were measured. A decrease in the peak mole fractions of H, O, OH radicals and an increase in the peak mole fractions of HO2 and H2O2 with pressure was observed. Two detailed kinetic mechanisms proposed recently by Konnov (2008) and Burke et al. (2012) for hydrogen combustion were validated against new experimental data reported in this work. Both mechanisms reproduced well the mole fraction profiles of H2, O2, H2O and H, O, OH radicals in the flames. However, the mechanism of Burke et al. was found to be more adequate in predicting the mole fraction profiles of peroxy species in the flames. The observed changes in the flame structure with pressure were explained on the basis of a kinetic analysis of the model developed by Burke et al. The experimental data reported in this work can help in further development and improvement of the future kinetic models of hydrogen combustion at elevated pressures.

Equilibrium calculations of As, Pb, and Hg speciations during coal combustion in atmospheres 21O2/79N2 and 30O2/70CO2

ABSTRACTThe research of this paper investigated the influence of a coal combustion atmosphere, pressure, and temperature on the equilibrium speciation of As, Pb, and Hg. The input data for calculations in the FactSage were the results of coal from the mine “Bogdanka”. A change in the combustion atmosphere from air to 30%O2/70%CO2 results in an increase in the mole fractions of As, Pb, and Hg species that occur in the gaseous phase, i.e. AsO, AsO2, PbO, PbCl2, HgCl2, and Hg. At temperatures below 900°C, almost all arsenic moves to solid species in the form of salts derived from arsenic acid (V).

Investigation on Effect of Air Velocity in Turbulent Non-Premixed Flames

Abstract In this study, the turbulent non-premixed methane-air flame is simulated to determine the effect of air velocity on the length of flame, temperature distribution and mole fraction of species. The computational fluid dynamics (CFD) technique is used to perform this simulation. To solve the turbulence flow, k-ε model is used. In contrast to the previous works, in this study, in each one of simulations the properties of materials are taken variable and then the results are compared. The results show that at a certain flow rate of fuel, by increasing the air velocity, similar to when the properties are constant, the width of the flame becomes thinner and the maximum temperature is higher; the penetration of oxygen into the fuel as well as fuel consumption is also increased. It is noteworthy that most of the pollutants produced are NOx, which are strongly temperature dependent. The amount of these pollutants rises when the temperature is increased. As a solution, decreasing the air velocity can decrease the amount of these pollutants. Finally, comparing the result of this study and the other work, which considers constant properties, shows that the variable properties assumption leads to obtaining more exact solution but the trends of both results are similar.

Development of a combined reduced primary reference fuel-alcohols (methanol/ethanol/propanols/butanols/n-pentanol) mechanism for engine applications

A combined reduced primary reference fuel (PRF)-alcohols (methanol/ethanol/propanols/butanols/n-pentanol) combustion kinetic mechanism composed of 161 species and 622 reactions was developed for engine combustion simulations. The obtained reduced PRF-alcohols mechanism was constructed with a hierarchical structure. Minor adjustments were performed to ensure the predictive performance against experimental results. The reduced PRF-alcohols mechanism adequately predicted experimental ignition delays, laminar flame speeds, and species mole fraction profiles. New homogeneous charge compression ignition experiments fueled with 75% (mol.) n-propanol/25% n-heptane, 75% i-propanol/25% n-heptane, and 75% n-pentanol/25% n-heptane blends were also collected and served as further mechanism validations. By coupled with the toluene-polycyclic aromatic hydrocarbons sub-mechanism, the reduced PRF-alcohols mechanism was used for the three dimensional modeling studies to investigate the direct injection compression ignition (DICI) combustion fueled with diesel/alcohol blends at the 5% fuel oxygen content. Zero-dimensional modeling studies were also conducted. The modeling results indicated that in DICI combustion, it was the different physical mixing qualities incurred by the different fuel reactivity dominated the soot formation but not the different carbon chain chemical structures. The O atom of the fuel molecule was more efficient than the O2 molecule for the soot oxidation.

Model-based evaluation of an integrated autothermal biomass gasification and solid oxide fuel cell combined heat and power system

Numerical analysis of combined heat and power plant consisting of a solid oxide fuel cell and autothermal gasification system has been made for several cases of different composition of fuel relevant to air and steam blown biomass gasification process. Wet wood is fed to the fixed-bed downdraft gasifier and gaseous fuel is produced then after gas cleaning and conditioning can be used in solid oxide fuel cells. The integrated plant is investigated by thermodynamic modeling combining a one-dimensional model of direct internal intermediate planar type solid oxide fuel cell which allows monitoring the temperature gradients along the cell length in different operating conditions and a zero-dimensional autothermal gasifier. The solid oxide fuel cell mathematical model is developed based on gas species mass balances, energy balance, and an electrochemical model beside the kinetics describing internal reforming and water-gas shift reactions. Such a model can be integrated with adiabatic gasification modeling which includes atom balance conservation for assumed gas species and a modified thermodynamic equilibrium analysis. Both gasifier and solid oxide fuel cell models are verified against experimental and previous numerical data available in the literature. Two main parameters, namely modified equivalence ratio and air-to-steam ratio are investigated and the most important cycle parameters such as power, electric and combined heat and power efficiencies, temperature gradients along the cell length, and mole fractions of gaseous species of the produced fuel are analyzed. It has been revealed that any increase in air-to-steam ratio at fixed modified equivalence ratio leads to penalty on cold gas efficiency of the gasifier and both solid oxide fuel cell and combined heat and power plant electric efficiencies. Increased air-to-steam ratio at constant modified equivalence ratio produces a mixture with lower low heating value, higher steam-to-carbon ratio, rich in CO and lower in CH4 content. Under this condition the operating temperature of the cycle and solid oxide fuel cell increases and consequently improves the operating voltage of the cell and combined heat and power efficiency of the plant. On the other hand, results show that gasification with increased modified equivalence ratio at constant air-to-steam ratio produces mixtures richer in CH4 and CO, poorer in H2 with higher low heating value and cold gas efficiency, and lower steam-to-carbon ratio. Such condition improves the electric efficiency of the solid oxide fuel cell and the integrated plant, but the combined heat and power efficiency of the cycle decreases due to decreased operating temperature of the solid oxide fuel cell and the cycle.

On the effects of hydrogen addition in premixed formaldehyde flames

The influences of hydrogen addition on laminar premixed fuel-rich formaldehyde flames at the atmospheric pressure are studied by the detailed kinetic analysis. The chemical effects of hydrogen addition on flame structures, mole fractions of major species and free radicals are distinguished clearly from the dilution and thermal effects. The results show that the H2 chemical effects can promote the oxidation of formaldehyde and make the mole fraction profile of formaldehyde move toward the upstream side. The increase of H radical concentration is mainly due to the chemical effects of hydrogen addition. The reaction OH + H2 = H + H2O plays a major role in the productions of H and OH radicals. The productions of O, HCO and HO2 radicals are enhanced by the H2 chemical effects. However, the hydrogen dilution and thermal effects on decreasing these radicals' concentrations are more dominant. The mole fractions of CO and CO2 decrease with H2 addition. The H2 chemical effects facilitate the formation of CO, but suppress the production of CO2.

Experimental and kinetic modeling studies of low-pressure premixed laminar 2-methylfuran flames

Depletion and overconsumption of the traditional fossil fuels necessitate the searching of renewable energies and alternative fuels. With its similar properties to those of gasoline, 2-methylfuran (MF) can be used as an alternative fuel or fuel additive directly in the engines. In this work, low-pressure laminar premixed MF/O2/Ar flames with equivalence ratios of 0.80, 1.00, and 1.50 were studied to investigate the flame structure of MF. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used for the measurements of the mole fractions of the flame species. A revised MF mechanism based on that of Somers et al. was used to analyze the reaction pathways of MF, combining the formation and consumption reactions of some key species, such as 2-furylmethyl, propargyl, 1-oxo-1,3,4-pentatriene, 1-oxo-1,3-butadiene, acrolein, and methyl vinyl ketone, etc. The current mechanism may well predict the H-addition products of MF, i.e., 1-oxo-1,3-butadiene and furan, but under-estimate the H-abstraction products of MF, i.e., 2-furylmethyl and 1-oxo-1,3,4-pentatriene. As to the OH-addition products of MF, acrolein and methyl vinyl ketone, the former one is over-predicted. This can be improved by addressing the effect of the MF structure on the OH-addition reactions, taking into account both formation and consumption of these two species.

Application of a multi-zone model for the prediction of species concentrations in rapid compression machine experiments

A multi-zone model for simulating rapid compression machine (RCM) experiments is integrated with a chemistry solver for robust, computationally-efficient speciation predictions. The model builds upon previous work (Goldsborough et al., 2012) by simulating chemical kinetics in each temperature-varying main combustion chamber zone. The accuracies of the model predictions are evaluated by a comparison to results from computational fluid dynamics (CFD) simulations. The present model is shown to yield significant improvement over a homogeneous modeling approach in predicting chemical species mole fractions, as well as accurate predictions of temperature and pressure. Species mole fractions predicted by the present model are on average within ∼15% of CFD predictions, while the difference in predicted mole fractions between a homogeneous model and CFD are ∼90% on average. The improvement in species predictions is due to the fact that the present model simulates chemical reaction throughout the entire main reaction chamber in an RCM, while the homogeneous model utilizes a reactor model that simulates conditions inside the core of the reaction chamber only. The enhanced model will enable direct comparison to cylinder-averaged experimental speciation data that is obtained by expanding a reactive mixture to quench the reaction.