Numerical and Experimental Investigations of CH4/H2 Mixtures: Ignition Delay Times, Laminar Burning Velocity and Extinction Limits (original) (raw)
Related papers
E3S Web of Conferences
By setting clear targets for reducing pollutant emissions, the researchers in the field of combustion are pushed lately to find new alternatives for cleaner combustion. The partial or total transition to other types of fuels, such as hydrogen, involves substantial changes in the combustion process and possible necessary constructive changes. In the study of the combustion of CH4-H2 mixtures, both numerically and experimentally, preliminary calculations are required, which will help to easily establish the parameters and working regimes and then to use for verifying the results. This paper aims to find an easier method of calculating these parameters, depending on the percentage of gas in the fuel mixture. The calculated values resulted this way will lead to some logical estimates of important aspects of combustion, such as flame field and temperature variation, related to the variation of the amount of hydrogen in the mixture. The method can be a useful tool in the preliminary desig...
Ignition and extinction of flames near surfaces: Combustion of CH4 in air
AIChE Journal, 1994
Ignition and extinction characteristics of homogeneous combustion of methane in air near inert surfaces are studied by numerical bifurcation theory for premixed methane/air gases impinging on planar surfaces with detailed chemistry involving 46 reversible reactions and 16 species. One-parameter bifurcation diagrams as functions of surface temperature and two-parameter bifurcation diagrams as functions of equivalence ratio and strain rate are constructed for both isothermal and adiabatic walls. Lean and rich composition limits for ignition and extinction, and energy production are determined from two parameter bifurcation diagrams. For a strain rate of 500 s-I, CH,/air mixtures exhibit hysteresis from -0.5% up to -12.5%
Ignition and extinction of flames near surfaces: Combustion of CH[sub 4] in air
AIChE Journal
Ignition and extinction characteristics of homogeneous combustion of methane in air near inert surfaces are studied by numerical bifurcation theory for premixed methane/air gases impinging on planar surfaces with detailed chemistry involving 46 reversible reactions and 16 species. One-parameter bifurcation diagrams as functions of surface temperature and two-parameter bifurcation diagrams as functions of equivalence ratio and strain rate are constructed for both isothermal and adiabatic walls. Lean and rich composition limits for ignition and extinction, and energy production are determined from two parameter bifurcation diagrams. For a strain rate of 500 s-I, CH,/air mixtures exhibit hysteresis from -0.5% up to -12.5%
Burning Behaviour of High-Pressure CH4-H2-Air Mixtures
Experimental characterization of the burning behavior of gaseous mixtures has been carried out, analyzing spherical expanding flames. Tests were performed in the Device for Hydrogen-Air Reaction Mode Analysis (DHARMA) laboratory of Istituto Motori-CNR. Based on a high-pressure, constant-volume bomb, the activity is aimed at populating a systematic database on the burning properties of CH 4 , H 2 and other species of interest, in conditions typical of internal combustion (i.c.) engines and gas turbines. High-speed shadowgraph is used to record the flame growth, allowing to infer the laminar burning parameters and the flame stability properties. Mixtures of CH 4 , H 2 and air have been analyzed at initial temperature 293÷305 K, initial pressure 3÷18 bar and equivalence ratio = 1.0. The amount of H 2 in the mixture was 0%, 20% and 30% (vol.). The effect of the initial pressure and of the Hydrogen content on the laminar burning velocity and the Markstein length has been evaluated: the relative weight and mutual interaction has been assessed of the two controlling parameters. Analysis has been carried out of the flame instability, expressed in terms of the critical radius for the onset of cellularity, as a function of the operating conditions.
Transported PDF modelling with detailed chemistry of pre- and auto-ignition in CH 4/air mixtures
Proceedings of The Combustion Institute, 2007
The pre-and auto-ignition behavior of methane under varying levels of preheat in a turbulent flow field has been studied through the combination of detailed chemistry with a transported PDF approach closed at the joint-scalar level. The study considers the Cabra Burner configuration, which consists of a central methane/air jet issuing into a vitiated co-flow. The aim of the work is to explore the detailed thermochemical flow structure and to substantially reduce uncertainties associated with the chemical kinetics. The applied chemistry features 44 solved species and 256 reactions and includes low temperature oxygen adducts. The mechanism has, in related work, been shown to reproduce the spontaneous temperature limit for methane and ethane along with ignition delays times at higher temperatures. Radiation is accounted for through the RADCAL method and the inclusion of enthalpy into the joint-scalar PDF. Molecular mixing is closed using the modified Curl's model and a set of time-scale ratios (C / = 2.3, 2.5, 3.0 and 4.0) have been used to explore the model sensitivity. The impact on predictions of variations in the pilot stream composition have been explored by varying concentrations of OH and H 2 over a wide range. A detailed analysis of the flame structure, focusing on the chemical processes occurring before and during the ignition, suggests that the burner conditions lead to a classical auto-ignition pattern with the early formation of HO 2 and CH 2 O prior to ignition. The work suggests that, under the current conditions, flame stabilization is dominated by turbulence-chemistry interactions rather than by specific modes of flame propagation. The work shows a significant sensitivity to the pilot stream composition and that residual H 2 acts as an ignition promoter. However, the sensitivity to the time-scale ratio C / is shown to be less than can be expected from studies of flame extinction using the same methodology.
Extinction limits and structure of counterflow nonpremixed H2O-laden CH4/air flames
Energy, 2015
In order to better understand combustion processes when large amounts of water (H 2 O) naturally incorporate into the fuel stream, e.g., the combustion of methane (CH 4) hydrates and H 2 O/fuel emulsions, the extinction limits and structure of counterflow nonpremixed flames of mixtures of H 2 O vapor and CH 4 and air were identified experimentally and computationally. With H 2 O vapor addition, the maximum flame temperature was experimentally determined, while the flame structure and extinction limits were computed using a detailed kinetic mechanism.
Effects of H2 enrichment on the propagation characteristics of CH4–air triple flames
Combustion and Flame, 2008
The effects of H 2 enrichment on the propagation of laminar CH 4 -air triple flames in axisymmetric coflowing jets are numerically investigated. A comprehensive, time-dependent computational model, which employs a detailed description of chemistry and transport, is used to simulate the transient ignition and flame propagation phenomena. Flames are ignited in a jet-mixing layer far downstream of the burner. Following ignition, a welldefined triple flame is formed that propagates upstream along the stoichiometric mixture fraction line with a nearly constant displacement velocity. As the flame approaches the burner, it transitions to a double flame, and subsequently to a burner-stabilized nonpremixed flame. Predictions are validated using measurements of the displacement flame velocity. As the H 2 concentration in the fuel blend is increased, the displacement flame velocity and local triple flame speed increase progressively due to the enhanced chemical reactivity, diffusivity, and preferential diffusion caused by H 2 addition. In addition, the flammability limits associated with the triple flames are progressively extended with the increase in H 2 concentration. The flame structure and flame dynamics are also markedly modified by H 2 enrichment, which substantially increases the flame curvature and mixture fraction gradient, as well as the hydrodynamic and curvature-induced stretch near the triple point. For all the H 2 -enriched methane-air flames investigated in this study, there is a negative correlation between flame speed and stretch, with the flame speed decreasing almost linearly with stretch, consistent with previous studies. The H 2 addition also modifies the flame sensitivity to stretch, as it decreases the Markstein number (Ma), implying an increased tendency toward diffusive-thermal instability (i.e. Ma → 0). These results are consistent with the previously reported experimental results for outwardly propagating spherical flames burning a mixture of natural gas and hydrogen.
Autoignition and structure of nonpremixed CH4/H2 flames: Detailed and reduced kinetic models
Combustion and Flame, 2006
The ignition dynamics and subsequent flame evolution of hydrogen-enriched methane mixtures are investigated numerically in a reacting vortex ring configuration. The CH 4 /H 2 combustion is studied using a detailed reaction mechanism (GRI-Mech v3.0) and two augmented reduced mechanisms (11-step and 12-step). The main objective of this study is to identify the extent that the current reduced mechanisms can go in replicating the dynamics of the ignition process and flame structure in an unsteady nonpremixed configuration. The parameters of the numerical simulations are adjusted such that flame ignition occurs during either the formation or the postformation of the ring. The quasi-steady state assumption for O in the 12-step reduced kinetic model leads to shorter ignition delay times than those in the other kinetic models. For formation-phase ignition runs, the flame structure near the stoichiometric region is captured well by the 12-step model compared to GRI-Mech 3.0. For postformation ignition runs, the 12-step model predicts larger heat release rates and main species mole fractions compared to GRI-Mech 3.0. The 11-step model predicts well the ignition delay time. At later times the fuel-rich side of the flame predicted by this reduced mechanism exhibits differences from the detailed model. Counterflow diffusion flame results are used to further compare the fuel-rich chemistry for the detailed and augmented reduced kinetic models.
The evaluation of safety parameters for binary and ternary mixtures of hydrogen and light hydrocarbons is essential for the definition of the combustion characteristics of bio-derived fuel-gas mixtures. Furthermore, with specific reference to laminar burning velocity, there is a strong need to define simple correlations, which would allow a fast prediction of global burning velocity of the mixtures starting from molar compositions and laminar burning velocity of the pure components, in analogy with Le Chatelier's rule for flammability limits. In this paper, a preliminary experimental and numerical study is performed for the assessment of safety parameters for mixtures of methane, propane and hydrogen with air at initial ambient pressure. Explosion tests have been conducted in a reinforced 5 liters steel vessel. The PREMIX module of the CHEMKIN package, coupled to the Marinov detailed reaction scheme, has been used to compute the un-stretched laminar burning velocity. For model validation, results have been compared to experimental data.
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.