Measurements of laminar burning velocities and onset of cellular instabilities of methane–hydrogen–air flames at elevated pressures and temperatures (original) (raw)
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International Journal of Hydrogen Energy, 2008
Measurements of laminar burning velocities and Markstein lengths of n-butanol-air premixed mixtures was made over a wide range of equivalence ratios at initial temperatures of 413, 443, and 473 K and initial pressures of 0.1 and 0.25 MPa using the high-speed schlieren photography and outwardly propagating flame. Effects of laminar flame thickness, thermal expansion ratio, and flame Lewis number on flame stability response were studied. Schlieren photos of flame propagation are recorded. The results show that laminar burning velocities of n-butanol-air premixed mixtures are increased with the increase of initial temperature, and they decrease with the increase of initial pressure. Markstein lengths are decreased with the increase of the equivalence ratio, and they decrease with the increase of initial temperature, and all these indicate that the flame instability is increased with the increase of equivalence ratio, the decrease of initial pressure, and the increase of initial temperature.
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.
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.
Proceedings of The Combustion Institute, 2005
The aim of the present work was to characterize both the effects of pressure and of hydrogen addition on methane/air premixed laminar flames. The experimental setup consists of a spherical combustion chamber coupled to a classical shadowgraphy system. Flame pictures are recorded by a high speed camera. Global equivalence ratios were varied from 0.7 to 1.2 for the initial pressure range from 0.1 to 0.5 MPa. The mole fraction of hydrogen in the methane + hydrogen mixture was varied from 0 to 0.2. Experimental results were compared to calculations using a detailed chemical kinetic scheme (GRIMECH 3.0). First, the results for atmospheric laminar CH4/air flames were compared to the literature. Very good agreements were obtained both for laminar burning velocities and for burned gas Markstein length. Then, increasing the hydrogen content in the mixture was found to be responsible for an increase in the laminar burning velocity and for a reduction of the flame dependence on stretch. Transport effects, through the reduction of the fuel Lewis number, play a role in reducing the sensitivity of the fundamental flame velocity to the stretch. Finally, when the pressure was increased, the laminar burning velocity decreased for all mixtures. The pressure domain was limited to 0.5 MPa due to the onset of instabilities at pressures above this value.
Effects of hydrogen concentration on premixed laminar flames of hydrogen-methane-air
2014
Methane Laminar burning velocity Markstein number Lewis number Flame instability a b s t r a c t The unstretched laminar burning velocities and Markstein numbers of spherically propagating hydrogenemethaneeair flames were studied at a mixture pressure of 0.10 MPa and a mixture temperature of 350 K. The fraction of hydrogen in the binary fuel was varied from 0 to 1.0 at equivalence ratios of 0.8, 1.0 and 1.2. The unstretched laminar burning velocity increased non-linearly with hydrogen fraction for all the equivalence ratios. The Markstein number varied non-monotonically at equivalence ratios of 0.8 and 1.0 and increased monotonically at equivalence ratio of 1.2 with increasing hydrogen fraction. Analytical evaluation of the Markstein number suggested that the trends could be due to the effective Lewis number, which varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0 and increased monotonically at 1.2. The propensity of flame instability varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0.
International Journal of Hydrogen Energy, 2011
An experimental and numerical study on laminar burning velocities of hydrogen/air flames was performed at low pressure, room temperature, and different equivalence ratios. Flames were generated using a small contoured slot-type nozzle burner (5 mm  13.8 mm). Measurements of laminar burning velocity were conducted using the angle method combined with Schlieren photography. Numerical calculations were also conducted using existing detailed reaction mechanisms and transport properties. Additionally, an analysis of the intrinsic flame instabilities of hydrogen/air flames at low pressure was performed. Results show that the behavior of the laminar burning velocity is not regular when decreasing pressure and that it depends on the equivalence ratio range. The behavior of the laminar burning velocity with decreasing pressure can be reasonably predicted using existing reaction mechanisms; however changes in the magnitude of the laminar burning velocity are underestimated. Finally, it has been found experimentally and proved analytically that the intrinsic flame instabilities are reduced when decreasing the pressure at sub-atmospheric conditions.
Laminar burning velocity and Markstein lengths of methane–air mixtures
Combustion and Flame, 2000
Flame propagation of premixed nitrogen diluted natural gas/hydrogen/air mixtures was studied in a constant volume combustion bomb under various initial pressures. Laminar burning velocities and Markstein lengths were obtained for the diluted stoichiometric fuel/ air mixtures with different hydrogen fractions and diluent ratios under various initial pressures. The results showed that both unstretched flame speed and unstretched burning velocity are reduced with the increase in initial pressure (except when the hydrogen fraction is 80%) as well as diluent ratio. The velocity reduction rate due to diluent addition is determined mainly by hydrogen fraction and diluent ratio, and the effect of initial pressure is negligible. Flame stability was studied by analyzing Markstein length. It was found that the increase of initial pressure and hydrogen fraction decreases flame stability and the flame tends to be more stable with the addition of diluent gas. Generally speaking, Markstein length of a fuel with low hydrogen fraction is more sensitive to the change of initial pressure than that of a one with high hydrogen fraction.
International Journal of Hydrogen Energy, 2009
Hydrogen Lean premixed flame Experimental study Numerical analysis a b s t r a c t Experimental and numerical study on the lean methane-hydrogen-air flames at elevated pressures and temperatures was conducted. The unstretched laminar burning velocities and Markstein lengths were obtained over a wide range of hydrogen fractions at elevated pressures and temperatures. The sensitivity analysis and flame structure were also analyzed. The results show good agreement between the computed results and experimental data. The unstretched laminar burning velocities are increased with the increase of initial temperature and hydrogen fraction, and they are decreased with the increase of initial pressure. With the increase of initial pressure and hydrogen fraction, Markstein lengths are decreased, indicating the increase of flame instability. Laminar burning velocity is depended on the competition between the main chain branching reaction and chain recombination reaction. The chain branching reaction is a temperature-sensitive reaction, while the recombination reaction is a temperature-insensitive reaction. Numerical study shows that the suppression (or enhancement) of overall chemical reaction with the increase of initial pressure (or temperature) is closely linking to the decrease (or increase) of H, O and OH mole fractions in the flames. Strong correlation is existed between burning velocity and maximum radical concentrations of H and OH radicals in the reaction zone of premixed flames.
Measurements of laminar burning velocities for natural gas–hydrogen–air mixtures
Combustion and Flame, 2006
Laminar flame characteristics of natural gas-hydrogen-air flames were studied in a constant-volume bomb at normal temperature and pressure. Laminar burning velocities and Markstein lengths were obtained at various ratios of hydrogen to natural gas (volume fraction from 0 to 100%) and equivalence ratios (φ from 0.6 to 1.4). The influence of stretch rate on flame was also analyzed. The results show that, for lean mixture combustion, the flame radius increases with time but the increasing rate decreases with flame expansion for natural gas and for mixtures with low hydrogen fractions, while at high hydrogen fractions, there exists a linear correlation between flame radius and time. For rich mixture combustion, the flame radius shows a slowly increasing rate at early stages of flame propagation and a quickly increasing rate at late stages of flame propagation for natural gas and for mixtures with low hydrogen fractions, and there also exists a linear correlation between flame radius and time for mixtures with high hydrogen fractions. Combustion at stoichiometric mixture demonstrates the linear relationship between flame radius and time for natural gas-air, hydrogen-air, and natural gas-hydrogen-air flames. Laminar burning velocities increase exponentially with the increase of hydrogen fraction in mixtures, while the Markstein length decreases and flame instability increases with the increase of hydrogen fractions in mixture. For a fixed hydrogen fraction, the Markstein number shows an increase and flame stability increases with the increase of equivalence ratios. Based on the experimental data, a formula for calculating the laminar burning velocities of natural gas-hydrogen-air flames is proposed.
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.