New insights into the peculiar behavior of laminar burning velocities of hydrogen–air flames according to pressure and equivalence ratio (original) (raw)
Related papers
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.
Laminar burning velocities of lean hydrogen–air mixtures at pressures up to 1.0 MPa
Combustion and Flame, 2007
Values of laminar burning velocity, u l , and the associated strain rate Markstein number, Ma sr , of H 2 -air mixtures have been obtained from measurements of flame speeds in a spherical explosion bomb with central ignition. Pressures ranged from 0.1 to 1.0 MPa, with values of equivalence ratio between 0.3 and 1.0. Many of the flames soon became unstable, with an accelerating flame speed, due to Darrieus-Landau and thermodiffusive instabilities. This effect increased with pressure. The flame wrinkling arising from the instabilities enhanced the flame speed. A method is described for allowing for this effect, based on measurements of the flame radii at which the instabilities increased the flame speed. This enabled u l and Ma sr to be obtained, devoid of the effects of instabilities. With increasing pressure, the time interval between the end of the ignition spark and the onset of flame instability, during which stable stretched flame propagation occurred, became increasingly small and very high camera speeds were necessary for accurate measurement. Eventually this time interval became so short that first Ma sr and then u l could not be measured. Such flame instabilities throw into question the utility of u l for high pressure, very unstable, flames. The measured values of u l are compared with those predicted by detailed chemical kinetic models of one-dimensional flames. (D. Bradley).
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.
The enhanced combustion properties of hydrogen as well as worldwide concerns due to global warming are acting together as the major driving force to a future hydrogen-based economy. However this will not be a step change and the blends of hydrogen and hydrocarbons must be looked as a transition solution to a purely hydrogen-based economy [1]. In the present study, numerical investigations of a wider range of fuel/air mixtures which includes H 2 -hydrocarbon blends have been carried out for the unstretched laminar burning velocities, S L0 at STP conditions using detailed reaction mechanisms. For this purpose, different reaction mechanisms are combined with the combustion simulation package Cosilab [2]. Comparative studies with experimental data from several independent investigations show that San Diego mechanism [3] predicts S L0 of H 2 /air mixtures very well while Konnov v0.5 [4] is the most valid mechanism for CH 4 /air mixtures. QMech [5] shows good agreement for C 3 H 8 /air mi...
Flammability limits and laminar flame speed of hydrogen–air mixtures at sub-atmospheric pressures
Hydrogen behavior at elevated pressures and temperatures was intensively studied by numerous investigators. Nevertheless, there is a lack of experimental data on hydrogen ignition and combustion at reduced sub-atmospheric pressures. Such conditions are related to the facilities operating under vacuum or sub-atmospheric conditions, for instance like ITER vacuum vessel. Main goal of current work was an experimental evaluation of such fundamental properties of hydrogen–air mixtures as flammability limits and laminar flame speed at sub-atmospheric pressures. A spherical explosion chamber with a volume of 8.2 dm3 was used in the experiments. A pressure method and high-speed camera combined with schlieren system for flame visualization were used in this work. Upper and lower flammability limits and laminar flame velocity have been experimentally evaluated in the range of 4–80% hydrogen in air at initial pressures 25–1000 mbar. An extraction of basic flame properties as Markstein length, overall reaction order and activation energy was done from experimental data on laminar burning velocity.
Studies of premixed and non-premixed hydrogen flames
Combustion and Flame, 2014
The hydrogen oxidation chemistry constitutes the foundation of the kinetics of all carbon- and hydrogen-containing fuels. The validation of rate constants of hydrogen-related reactions can be complicated by uncertainties associated with experimental data caused by the high reactivity and diffusivity of hydrogen. In the present investigation accurate experimental data on flame propagation and extinction were determined for premixed and non-premixed hydrogen flames at pressures between p = 1 and 7 atm. The experiments were designed to sensitize the three-body H + O2 + M → HO2 + M reaction, whose rate is subject to notable uncertainty. This was achieved by increasing the pressure and by adding to the reactants H2O and CO2 whose collision efficiencies are high compared to other species. In the present study, directly measured flame properties were compared against computed ones, in order to eliminate uncertainties associated with extrapolations, as is the case for laminar flame speeds. ...
Advances in Energy Research
The main purpose of this work is to test the validation of use of a four step reaction mechanism to simulate the laminar speed of hydrogen enriched methane flame. The laminar velocities of hydrogenmethane-air mixtures are very important in designing and predicting the progress of combustion and performance of combustion systems where hydrogen is used as fuel. In this work, laminar flame velocities of different composition of hydrogen-methane-air mixtures (from 0% to 40% hydrogen) have been calculated for variable equivalence ratios (from 0.5 to 1.5) using the flame propagation module (FSC) of the chemical kinetics software Chemkin 4.02. Our results were tested against an extended database of laminar flame speed measurements from the literature and good agreements were obtained especially for fuel lean and stoichiometric mixtures for the whole range of hydrogen blends. However, in the case of fuel rich mixtures, a slight overprediction (about 10%) is observed. Note that this overprediction decreases significantly with increasing hydrogen content. This research demonstrates that reduced chemical kinetics mechanisms can well reproduce the laminar burning velocity of methane-hydrogen-air mixtures at lean and stoichiometric mixture flame for hydrogen content in the fuel up to 40%. The use of such reduced mechanisms in complex combustion device can reduce the available computational resources and cost because the number of species is reduced.
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.
Proceedings of the Combustion Institute, 2017
Recognizing the need to improve the predictability of the foundational H 2 /O 2 chemical kinetic mechanism over extensive parametric ranges of pressure, adiabatic flame temperature and composition, laminar flame speeds extracted from constant-pressure expanding flames were experimentally determined for H 2 /O 2 /diluent flames spanning the parameter range of: equivalence ratio from ultra-lean to ultra-rich (0.3 and 2.5), flame temperature from 1,300 to 2,200K, pressure from sub-atmospheric to elevated values of 0.25 to 20atm, and dilution of He up to 65% and CO 2 up to 34%. A new kinetic model, HP-Mech, based on evaluation of the rate coefficients from recent high-level quantum chemistry calculations and shock tube measurements, was developed and subsequently used to investigate the controlling reaction steps of these flames. In particular, the overall reaction order was extracted, allowing for the pressure dependence of the adiabatic flame temperature. The results show that it can assume values not only less than zero at high pressures, as was reported before, but also values larger than 2 at lower pressures. Sensitivity and reaction path analyses show that these responses are controlled by the competition between two opposing pathways involving reaction R9: H+O 2 (+M)=HO 2 (+M).