Experimental and numerical study on laminar burning velocities and flame instabilities of hydrogen–air mixtures at elevated pressures and temperatures

Experimental and numerical study on hydrogen–air flames at elevated pressures and temperatures was conducted. Meanwhile, the calculation is extended to initial pressure and temperature up to 8.0 MPa and 950 K, respectively. Laminar burning velocities and Markstein lengths were obtained at the elevated pressures and temperatures. Sensitivity analysis and flame structure were also analyzed. The results show good agreement between the computed results and experimental data. The study shows that laminar burning velocities are increased with the increase of initial temperature, and they decrease with the increase of initial pressure. With the increase of initial pressure, advancement of the onset of cellular instability is presented and Markstein length is decreased, indicating an increase of flame instability with the increase of initial pressure. The study shows insensitivity of flame instability to initial temperature. Laminar burning velocity is depended on the competition between the main chain branching reactions and chain termination reaction. The chain branching reactions are the temperature-sensitive reaction, while the termination reaction is the temperature-insensitive reaction. Through the extraction of the overall reaction orders, it is demonstrated that with increasing pressure, the overall reaction orders give a decreasing trend and then increasing trend. This behavior suggests an analogy to three explosion limits of hydrogen/oxygen mixtures. Numerical study also 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. On the basis of the numerical data, an empirical formula for laminar burning velocity is correlated for the hydrogen–air premixed mixture at elevated pressures and temperatures. The correlated laminar burning velocities are in good agreement with the known experimental results and simulated results with CHEMKIN. The correlation can be used in the calculation of laminar burning velocities at evaluated pressures and temperatures.     

Combustion characteristics of methanol–air and methanol–air–diluent premixed mixtures at elevated temperatures and pressures

Combustion characteristics of the methanol–air premixed mixtures were studied in a constant volume bomb at different equivalence ratios, initial pressures and temperatures, and dilution ratios. The results show that the combustion pressure, the mass burning rate and the burned gas temperature get the maximum value at the equivalence ratio of 1.1 while the flame development duration and the combustion duration get the minimum value at the equivalence ratio of 1.1. The flame development duration, the combustion duration and the peak combustion pressure decrease with the increase of the initial temperature, while the maximum burned gas temperature increases with the increase of the initial temperature. The peak combustion pressure and temperature increase with the increase of the initial pressure. The flame development duration and combustion duration increase with the increase of the dilution ratio, while the peak combustion pressure and temperature decrease with the increase of the dilution ratio.     

Measurements of laminar burning velocities and onset of cellular instabilities of methane–hydrogen–air flames at elevated pressures and temperatures

An experimental study on laminar burning velocities and onset of cellular instabilities of the premixed methane–hydrogen–air flames was conducted in a constant volume combustion vessel at elevated pressures and temperatures. The unstretched laminar burning velocity and Markstein length were obtained over a wide range of hydrogen fractions. Besides, the effects of hydrogen addition, initial pressure and initial temperature on flame instabilities were analyzed. The results show that the unstretched flame propagation speed and the unstretched laminar burning velocity are increased with the increase of initial temperature and hydrogen fraction, and they are decreased with the increase of initial pressure. Early onset of cellular instability is presented and the critical radius and Markstein length are decreased with the increase of initial pressure, indicating the increase of hydrodynamic instability with the increase of initial pressure. Flame instability is insensitive to initial temperature compared to initial pressure. With the increase of hydrogen fraction, significant decrease in critical radius and Markstein length is presented, indicating the increase in both diffusional-thermal and hydrodynamic instabilities as hydrogen fraction is increased.     

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.     

Explosion characteristics of hydrogen–nitrogen–air mixtures at elevated pressures and temperatures

An experimental study on the combustion characteristics of nitrogen diluted hydrogen was conducted in a constant volume combustion vessel over a wide range of equivalence ratios and dilution ratios at elevated pressures and temperatures. The explosion characteristics such as the explosion pressure, the combustion duration, the maximum rate of pressure rise, the deflagration index and the normalized mass burning rate were derived. The result shows that a short combustion duration and higher normalized mass burning rate were presented with the increase of equivalence ratio. With the increase of initial temperature, the explosion pressure, the maximum rate of pressure rise and the deflagration index were decreased, and a shorter combustion duration and higher normalized mass burning rate were presented. With the increase of initial pressure, the explosion pressure, the maximum rate of pressure rise and the deflagration index increase, a shorter combustion duration and higher normalized mass burning rate were presented. Nitrogen dilution significantly reduces the normalized mass burning rate and the deflagration index and thus the potential of explosion hazards.     

Effects of hydrogen and carbon dioxide on the laminar burning velocities of methane–air mixtures

The effects of different mole fractions of hydrogen and carbon dioxide on the combustion characteristics of a premixed methane–air mixture are experimentally and numerically investigated. The laminar burning velocity of hydrogen-methane-carbon dioxide-air mixture was measured using the spherically expanding flame method at the initial temperature and pressure of 283 K and 0.1 MPa, respectively. Additionally, numerical analysis is conducted under steady 1D laminar flow conditions to investigate the adiabatic flame temperature, dominant elementary reactions, and NO formation. The measured velocities correspond with those estimated numerically. The results show that increasing the carbon dioxide mole fraction decreases the laminar burning velocity, attributed to the carbon dioxide dilution, which decreases the thermal diffusivity and flame temperature. Conversely, the velocity increases with the thermal diffusivity as the hydrogen mole fraction increases. Moreover, the hydrogen addition leads to chain-branching reactions that produce active H, O, and OH radicals via the oxidation of hydrocarbons, which is the rate-determining reaction. Furthermore, an increase in the mole fractions of hydrogen and carbon dioxide decreases the NO production amount.     

Turbulent burning velocity of hydrogen–air premixed propagating flames at elevated pressures

Lewis number represents the thermo-diffusive effects on laminar flames. That of hydrogen–air mixture varies extensively with the equivalence ratio due to the high molecular diffusivity of hydrogen. In this study, the influences of pressure and thermo-diffusive effects on spherically propagating premixed hydrogen–air turbulent flames were studied using a constant volume fan-stirred combustion vessel. It was noted that the ratio of the turbulent to unstretched laminar burning velocity increased with decreasing equivalence ratio and increasing mixture pressure. Turbulent burning velocity was dominated by three factors: (1) purely hydrodynamic factor, turbulence Reynolds number, (2) relative turbulence intensity to reaction speed, the ratio of turbulence intensity to unstretched laminar burning velocity, and (3) sensitivity of the flame to the stretch due to the thermo-diffusive effects, Lewis and Markstein numbers. A turbulent burning velocity correlation in terms of Reynolds and Lewis numbers is presented.     

Measurements of the laminar burning velocity of hydrogen–air premixed flames

Experimental and numerical studies on laminar burning velocities of hydrogen–air mixtures were performed at standard pressure and room temperature varying the equivalence ratio from 0.8 to 3.0. The flames were generated using a contoured slot-type nozzle burner (4 mm × 10 mm). Measurements of laminar burning velocity were conducted using particle tracking velocimetry (PTV) combined with Schlieren photography. This technique provides the information of instantaneous local burning velocities in the whole region of the flame front, and laminar burning velocities were determined using the mean value of local burning velocities in the region of non-stretch. Additionally, average laminar burning velocities were determined using the angle method and compared with the data obtained with the PTV method. Numerical calculations were also conducted using detailed reaction mechanisms and transport properties.     

Effects of dilution with carbon dioxide on the laminar burning velocity and flame stability of H2–CO mixtures at atmospheric condition

The objective of this investigation was to study the effect of dilution with CO₂ on the laminar burning velocity and flame stability of syngas fuel (50% H₂–50% CO by volume). Constant pressure spherically expanding flames generated in a 40 l chamber were used for determining unstretched burning velocity. Experimental and numerical studies were carried out at 0.1 MPa, 302 ± 3 K and ? = 0.6–3.0 using fuel-diluent and mixture-diluent approaches. For H₂–CO–CO₂–O₂–N₂ mixtures, the peak burning velocity shifts from ? = 2.0 for 0% CO₂ in fuel to ? = 1.6 for 30% CO₂ in fuel. For H₂–CO–O₂–CO₂ mixtures, the peak burning velocity occurred at ? = 1.0 unaffected by proportion of CO₂ in the mixture. If the mole fraction of combustibles in H₂–CO–O₂–CO₂ mixtures is less than 32%, then such mixtures are supporting unstable flames with respect to preferential diffusion. The analysis of measured unstretched laminar burning velocities of H₂–CO–O₂–CO₂ and H₂–CO–O₂–N₂ mixtures suggested that CO₂ has a stronger inhibiting effect on the laminar burning velocity than nitrogen. The enhanced dilution effect of CO₂ could be due to the active participation of CO₂ in the chemical reactions through the following intermediate reaction CO + OH ? CO₂ + H.     

Effects of hydrogen and steam addition on laminar burning velocity of methane–air premixed flame: Experimental and numerical analysis

Effects of hydrogen enrichment and steam addition on laminar burning velocity of methane–air premixed flame were studied both experimentally and numerically. Measurements were carried out using the slot burner method at 1 bar for fresh gases temperatures of 27 °C and 57 °C and for variable equivalence ratios going from 0.8 to 1.2. The hydrogen content in the fuel was varied from 0% to 30% in volume and the steam content in the air was varied from 0 to 112 g/kg (0–100% of relative humidity). Numerical calculations were performed using the COSILAB code with the GRI-Mech 3.0 mechanism for one-dimensional premixed flames. The calculations were implemented first at room temperature and pressure and then extended to higher temperatures (up to 917 K) and pressures (up to 50 bar). Measurements of laminar burning velocities of methane–hydrogen–air and methane–air–steam agree with the GRI-Mech calculations and previous measurements from literature obtained by different methods. Results show that enrichment by hydrogen increases of the laminar burning velocity and the adiabatic flame temperature. The addition of steam to a methane–air mixture noticeably decreases the burning velocity and the adiabatic flame temperature. Modeling shows that isentropic compression of fresh gases leads to the increase of laminar burning velocity.     

Development of a comprehensive laminar burning velocity and flame instability profile of refined producer gas (H2:CO:CH4) – Air mixtures at elevated pressures

Unstretched laminar burning velocity (LBV) and intrinsic instabilities of Refined producer gas (H₂:CO:CH₄)-Air mixtures were systematically investigated at 300 K, 1–4 bar and ? = 0.8–1.2 using freely expanding spherical flame method. In H₂/CO/CH₄ rich mixtures, LBV increased with increase in CO (and reduction in CH₄)/H₂ (and reduction in CH₄)/H₂ (and reduction in CO) at any given equivalence ratio, while peak LBV occurred at ? = (1.2 and remained at 1.2)/(1.2 and shifted to 1.1)/(1.1 and remained at 1.1). Computed unstretched LBV using GRI Mech 3.0 and FFCM mechanisms deviated from measurements with initial pressure. From the comprehensive susceptibility analysis (to instabilities), the composition H₂:CO:CH₄ = 0:1:1 had the highest resilience towards thermo-diffusive and hydrodynamic instabilities. Refined producer gas with higher mole fractions of H₂ were vulnerable to intrinsic instabilities, while increment in CH₄ suppressed the susceptibility to hydrodynamic instability and increment in CO suppressed the thermo-diffusive instability.     

A laminar flame speed correlation of hydrogen–methanol blends valid at engine-like conditions

The hydrogen addition is effective on improving the performance of spark-ignited (SI) methanol engines. However, there is still a lack of laminar flame speed data of hydrogen–methanol blends at engine-like conditions. This blocks the combustion modeling of hydrogen-enriched methanol engines. In this paper, a laminar flame speed correlation of hydrogen–methanol blends was proposed for the computational fluid dynamics (CFD) simulation. This correlation was derived through a self-developed calculation program according to the flame-temperature-based mixing rule. Wide ranges of hydrogen volume fractions (0–10%), equivalence ratios (0.6–1.5), unburned gas temperatures (400–2600 K), pressures (1–50 bar) and residual gas mass fractions (0–20%) were simultaneously considered in this correlation. The new correlation was implemented in the extended coherent flame model (ECFM) to evaluate its suitability for CFD simulation. Satisfying agreement between the experimental and calculated results was observed. This indicated that the new correlation was suitable for the CFD simulation of hydrogen-enriched methanol engines.     

Hydrogen addition effect on laminar burning velocity,flame temperature and flame stability of a planar and a curved CH4–H2–air premixed flame

The effects of hydrogen fraction on laminar burning velocity, flame stability (Markstein number) and flame temperature of methane–hydrogen–air flame at global equivalence ratios of 0.7, 1.0 and 1.2 have been investigated numerically based on the full chemistry and the detailed molecular species transport. The effect of stretch rate on combustion characteristics is examined using an opposed-flow planar flame model, while the effect of flame curvature is identified by comparing a tubular flame to the opposed-flow planar flame. The difference in response on hydrogen fraction between the planar and curved flames has been observed. The results show when hydrogen fraction increases, the flame temperature and laminar burning velocity increases, and this effect is more significant at a large stretch rate; while Markstein length decreases. At a fixed stretch rate of 400 s⁻¹, under which the flame approaches extinction limit, the flame temperature of the tubular flame is considerably higher than that of the planar opposed flow flame, which results most likely from the contribution of the positive flame curvature to the first Damkohler number.     

Laminar-burning velocities of hydrogen–air and hydrogen–methane–air mixtures: An experimental study

The laminar burning velocities of hydrogen–air and hydrogen–methane–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 hydrogen–air and different composition of hydrogen–methane–air mixtures (from 100% hydrogen to 100% methane) have been measured at ambient temperatures for variable equivalence ratios (ER=0.8–3.2$\mathrm{ER}=0.8–3.2$). A modified test rig has been developed from the former Cardiff University ‘Cloud Chamber’ for this experimental study. The rig comprises of a 250 mm length cylindrical stainless steel explosion bomb enclosed at one end with a stainless steel plug which houses an internal stirrer to allow mixing. The other end is sealed with a 120 mm diameter round quartz window. Optical access for filming flame propagation is afforded via two diametrically opposed quartz windows in both sides. Flame speeds are determined within the bomb using a high-speed Schlieren photographic technique. This method is an accurate way to determine the flame–speed and the burning velocities were then derived using a CHEMKIN computer model to provide the expansion ratio. The design of the test facility ensures the flame is laminar which results in a spherical flame which is not affected by buoyancy. The experimental study demonstrated that increasing the hydrogen percentage in the hydrogen–methane mixture brought about an increase in the resultant burning velocity and caused a widening of the flammability limits. This experiments also suggest that a hydrogen–methane mixture (i.e. 30% hydrogen+70% methane) could be a competitive alternative fuel for existing combustion plants.     

New insights into the peculiar behavior of laminar burning velocities of hydrogen–air flames according to pressure and equivalence ratio

Hydrogen is a clean and energetic fuel, and its oxidation mechanism is a subset of the oxidation mechanisms of all hydrocarbons. Therefore, the validation of the available kinetic schemes is of great importance. In the current study, experimental measurements of laminar flame speeds and modeling studies were performed for H₂–air premixed flames over a wide range of equivalence ratios (0.5–4.0) and pressures (0.2–3 bar). The large scale in mixture and thermodynamic conditions allows a better understanding of the peculiar behavior of hydrogen flame speeds with pressure. Two very recent detailed chemical kinetic mechanisms for hydrogen combustion were selected. Excellent agreement was observed between calculations and experimental results, confirming the validity of the kinetic schemes selected. The kinetic analyses performed allow proposing an explanation for the nonmonotonic variation of hydrogen/air flame speed with pressure observed in the experiments.     

Experimental and numerical study on laminar burning characteristics of premixed methane–hydrogen–air flames

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.     

Effect of N2/CO2 dilution on laminar burning velocity of H2–air mixtures at high temperatures

The laminar burning velocities of H₂–air mixtures diluted with N₂ or CO₂ gas at high temperatures were obtained from planar flames observed in externally heated diverging channels. Experiments were conducted for an equivalence ratio range of 0.8–1.3 and temperature range of 350–600 K with various dilution rates. In addition, computational predictions for burning velocities and their comparison with experimental results and detailed flame structures have been presented. Sensitivity analysis was carried out to identify important reactions and their contribution to the laminar burning velocity. The computational predictions are in reasonably good agreement with the present experimental data (especially for N₂ dilution case). The burning velocity maxima was observed for slightly rich mixtures and this maxima was found to shift to higher equivalence ratios (Ф) with a decrease in the dilution. The effect of CO₂ dilution was more profound than N₂ dilution in reducing the burning velocity of mixtures at higher temperatures.     

An experimental and kinetic modeling study of the autoignition of α-methylnaphthalene/air and α-methylnaphthalene/n-decane/air mixtures at elevated pressures

The autoignition of α-methylnaphthalene (AMN), the bicyclic aromatic reference compound for the cetane number (CN), and AMN/n-decane blends, potential diesel surrogate mixtures, was studied at elevated pressures for fuel/air mixtures in a heated high-pressure shock tube. Additionally, a comprehensive kinetic mechanism was developed to describe the oxidation of AMN and AMN/n-decane blends. Ignition delay times were measured in reflected shock experiments for Φ = 0.5, 1.0, and 1.5 AMN/air mixtures (CN = 0) for 1032-1445 K and 8-45 bar and for Φ = 1.0 30%-molar AMN/70%-molar n-decane/air (CN = 58) and 70%-molar AMN/30%-molar n-decane/air mixtures (CN = 28) for 848-1349 K and 14-62 bar. Kinetic simulations, based on the comprehensive AMN/n-decane mechanism, are in good agreement with measured ignition times, illustrating the emerging capability of comprehensive mechanisms for describing high molecular weight transportation fuels. Sensitivity and reaction flux analysis indicate the importance of reactions involving resonance stabilized phenylbenzyl radicals, the formation of which by H-atom abstractions with OH radicals has an important inhibiting effect on ignition.