Burning Velocities of Stoichiometric Methane-Hydrogen-Air Flames at Gasturbine Like Conditions (original) (raw)
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Asymptotic Analysis of Methane-Hydrogen-Air Mixtures
2005
In this paper an asymptotic analysis of de Goey et al. concerning premixed stoichiometric methane-hydrogen-air flames is analyzed in depth. The analysis is performed with up to 50 mole percent of hydrogen in the fuel, at gas inlet temperatures ranging from 300 K to 650 K and pressures from 1 to 15 atm, which are about gasturbine conditions. The results of this analysis are compared with experimental and numerical data and give accurate predictions concerning the variation of the investigated parameters with increasing hydrogen content. Especially the flame thickness and laminar burning velocity give good results. * Corresponding author: r.t.e.hermanns@tue.nl Associated Web site: http://www.combustion.tue.nl Proceedings of the European Combustion Meeting 2005
Asymptotic Analysis of the Structure of Moderately Rich Methane-Air Flames
Combustion and Flame, 1998
The asymptotic structure of laminar, moderately rich, premixed methane flames is analyzed using a reduced chemical-kinetic mechanism comprising four global reactions. This reduced mechanism is different from those employed in previous asymptotic analyses of stoichiometric and lean flames, because a steady-state approximation is not introduced for CH 3 . The aim of the present analysis is to develop an asymptotic model for rich flames, which can predict the rapid decrease of the burning velocity with increasing equivalence ratio . In the analysis, the flame structure is presumed to consist of three zones: a preheat zone with a normalized thickness of the order of unity, a thin reaction zone, and a postflame zone. The preheat zone is presumed to be chemically inert, and in the postflame zone the products are in chemical equilibrium and the temperature is equal to the adiabatic flame temperature T b . In the reaction zone the chemical reactions are presumed to take place in two layers: the inner layer and the oxidation layer. The rate constants of these reactions are evaluated at T 0 , which is the characteristic temperature at the inner layer. In the inner layer the dominant reactions taking place are those between the fuel and radicals, and between CH 3 and the radicals. An important difference between the structure of the inner layer of rich flames and that of lean flames analyzed previously is the enhanced influence of the chain-breaking reaction CH 3 ϩ H ϩ (M) 3 CH 4 ϩ (M) in rich flames. Here M represents any third body. This reaction decreases the concentration of H radicals, which in turn decreases the values of the burning velocity. In the oxidation layer of rich flames, the reactive-diffusive balance of O 2 is considered. This differs from the structure of the oxidation layer of lean flames where the reactive-diffusive balance of H 2 and CO was of primary interest. The burning velocities calculated using the results of the asymptotic analysis agree reasonably well with the burning velocities calculated numerically using chemical-kinetic mechanisms made up of elementary reactions. The values of the characteristic temperature at the inner layer T 0 are found to increase with increasing values of the equivalence ratio and to approach T b at ϭ 1.36. When T 0 is very close to T b , the asymptotic analysis developed here is no longer valid and an alternative asymptotic analysis must be developed for even larger equivalence ratios.
Asymptotic structure of rich methane-air flames
Combustion and Flame, 2001
The asymptotic structure of unstrained, laminar, fuel-rich, premixed methane flame is analyzed by using a reduced chemical-kinetic mechanism made up of three global steps. Analysis is carried out for values of equivalence ratio greater than 1.3. The flame structure is presumed to comprise three zones: an inert preheat zone, a thin reaction zone, and a post-flame zone. In contrast to previous asymptotic analyses of lean flames and moderately rich flames, where the reaction zone of these flames was presumed to be made up of two layers, for rich flames analyzed here all chemical reaction are presumed to take place in one layer. The structure of the reaction zone of rich flames is obtained by integrating two second order ordinary differential equations, one giving the consumption of fuel and the other the consumption of oxygen. For values of equivalence ratio greater than 1.3, burning velocities obtained from the asymptotic analysis are found to agree reasonably well with those obtained using a chemical-kinetic mechanism made up of elementary reactions.
The asymptotic structure of premixed methane-air flames with slow CO oxidation
Combustion and Flame, 1992
The asymptotic structures of methane-air flames, for equivalence ratios from 0.5 to 1.4 and pressures from 1 to 70 arm, are analyzed on the basis of a reduced four-step chemical-kinetic mechanism that has previously predicted burning velocities with reasonable accuracy. The rates of these four steps are related to the rates of elementary reactions appearing in the Cj-chain mechanism for oxidation of methane. In the analysis, the overall flame structure is subdivided into four zones-a preheat zone with thickness of order unity, an inner fuel-consumption layer with thickness of order 8, a H2-oxidation layer with thickness of order e, and a CO-oxidation layer with thickness of order v. It is presumed here that 6 ,~ ~ ~ v < 1, contrary to previous investigations which treated 6 ,~ v ,~ e ,~ 1. The reason for introducing this modification is that recent estimates suggest that ~ < v, so that this opposite limit seems worthy of exploration. The inner layer is located between the preheat zone and the oxidation layers, and in this layer finite-rate reactions related to the consumption of the fuel are introduced by appropriate analysis essentially identical to that of Seshadri and Peters. In the H2-oxidation layer, the variations of the concentrations of CO and 0 2 are presumed to be negligible, leading to a new asymptotic analysis, and the H atoms are presumed to be in steady state. In the CO-oxidation layer, H e and H are both presumed to be in steady state, again requiring a new analysis. Analytical expressions are obtained for the burning velocity and for the characteristic temperature at which the hydrocarbon chemistry related to the fuel consumption occurs. The predictions are in good agreement with results of full numerical integrations and experiment for fuel-lean flames but give burning velocities somewhat too high for stoichiometric and rich flames.
Evaluation of limits for effective flame acceleration in hydrogen mixtures
Journal of Loss Prevention in The Process Industries, 2001
Results of experiments and data analysis on turbulent flame propagation in obstructed channels are presented. The data cover a wide range of mixtures: H2/air, H2/air/steam (from lean to rich) at normal and elevated initial temperatures (from 298 to 650 K) and pressures (from 1 to 3 bar); and stoichiometric H2/O2 mixtures diluted with N2, Ar, He and CO2 at normal initial conditions. The dataset chosen also covers a wide range of scales exceeding two orders of magnitude. It is shown that basic flame parameters, such as mixture expansion ratio σ, Zeldovich number β and Lewis number Le, can be used to estimate a priori a potential for effective flame acceleration for a given mixture. Critical conditions for effective flame acceleration are suggested in the form of correlations of critical expansion ratio σ∗ versus dimensionless effective activation energy. On this basis, limits for effective flame acceleration for hydrogen combustibles can be estimated. Uncertainties in determination of critical σ∗ values are discussed.
International Journal of Hydrogen Energy, 2009
Hydrogen Laminar burning velocity Markstein length Sensitivity analysis Flame structure a b s t r a c t An experimental and numerical study on laminar burning characteristics of the premixed methane-hydrogen-air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity and the Markstein length were obtained over a wide range of equivalence ratios and hydrogen fractions. Moreover, for further understanding of the effect of hydrogen addition on the laminar burning velocity, the sensitivity analysis and flame structure were performed. The results show that the unstretched laminar burning velocity is increased, and the peak value of the unstretched laminar burning velocity shifts to the richer mixture side with the increase of hydrogen fraction. Three regimes are identified depending on the hydrogen fraction in the fuel blend. They are: the methane-dominated combustion regime where hydrogen fraction is less than 60%; the transition regime where hydrogen fraction is between 60% and 80%; and the methane-inhibited hydrogen combustion regime where hydrogen fraction is larger than 80%. In both the methane-dominated combustion regime and the methane-inhibited hydrogen combustion regime, the laminar burning velocity increases linearly with the increase of hydrogen fraction. However, in the transition regime, the laminar burning velocity increases exponentially with the increase of hydrogen fraction in the fuel blends. The Markstein length is increased with the increase of equivalence ratio and is decreased with the increase of hydrogen fraction. Enhancement of chemical reaction with hydrogen addition is regarded as the increase of H, O and OH radical mole fractions in the flame. Strong correlation is found between the burning velocity and the maximum radical concentrations of H and OH in the reaction zone of the premixed flames.
2012
Rate-ratio asymptotic (RRA) analysis is carried out to elucidate the influence of carbon monoxide on the structure and critical conditions of extinction of nonpremixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams toward a stagnation plane is considered. One stream, called the fuel stream is made up of a mixture of methane (CH 4) and nitrogen (N 2). The other stream, called the oxidizer stream, is a mixture of oxygen (O 2), and N 2. Carbon monoxide (CO) is added either to the oxidizer stream or to the fuel stream. Chemical reactions, represented by four global steps, are presumed to take place in a thin reaction zone. To the leading order the reactants, CH 4 , O 2 , and CO are completely consumed in the reaction zone. On either side of this thin reaction zone, the flow field is inert. These inert regions represent the outer structure of the flame. The outer structures provide matching conditions required for predicting the structure of the reaction zone. In the reaction zone, chemical reactions are presumed to take place in two layers-the inner layer and the oxidation layer. The scalar dissipation
Combustion and Flame, 2010
Experimental measurements of burning rates, analysis of the key reactions and kinetic pathways, and modeling studies were performed for H 2 /CO/O 2 /diluent flames spanning a wide range of conditions: equivalence ratios from 0.85 to 2.5, flame temperatures from 1500 to 1800 K, pressures from 1 to 25 atm, CO fuel fractions from 0 to 0.9, and dilution concentrations of He up to 0.8, Ar up to 0.6, and CO 2 up to 0.4. The experimental data show negative pressure dependence of burning rate at high pressure, low flame temperature conditions for all equivalence ratios and CO fractions as high as 0.5. Dilution with CO 2 was observed to strengthen the pressure and temperature dependence compared to Ar-diluted flames of the same flame temperature. Simulations were performed to extend the experimentally studied conditions to conditions typical of gas turbine combustion in Integrated Gasification Combined Cycle processes, including preheated mixtures and other diluents such as N 2 and H 2 O.
Burning Velocities of Alternative Gaseous Fuels at Elevated Temperature and Pressure
AIAA Journal, 2010
This study has been undertaken to investigate turbulent burning velocities of alternative gaseous fuels at elevated temperature and pressure using the established Bunsen burner method. The experiments were conducted in the industrial scale High Pressure Optical Chamber (HPOC) in Gas Turbine Research Centre (GTRC) of Cardiff University. Five different gaseous fuels: methane, two methane-carbon dioxide mixtures and two methane-hydrogen mixtures were tested. Two different temperatures-473 K and 673 K, and two different pressures-3 bara and 7 bara, were chosen at which to conduct the experiments. Analysis of measurements made using pure methane indicated that the expected burning velocity trends occurred with temperature and pressure. When compared with the correlations proposed by Peters and Zimont et al. the results reported here showed good agreement, although the burning velocities recorded by Kobayashi et al. were somewhat higher. Hydrogen enrichment and carbon dioxide dilution of methane, show some expected trends, but others which require further consolidation and study. In particular it is seen that dilution of methane with carbon dioxide reduces the measured burning velocity. Increasing pressure and temperature in this case have competing effects with