Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures was investigated experimentally and numerically. The flame propagating photos of premixed combustion and direct-injection combustion was obtained by using a constant volume vessel and schlieren photographic technique. The pressure derived initial combustion durations were also obtained at different hydrogen fractions (from 0% to 40% in volumetric fraction) at overall equivalence ratio of 0.6 and 0.8, respectively. The laminar premixed methane–hydrogen–air flames were calculated with PREMIX code of CHEMKIN II program with GRI 3.0 mechanism. The results showed that the initial combustion process of lean burn natural gas–air mixtures was enhanced as hydrogen is added to natural gas in the case of both premixed combustion and direct-injection combustion. This phenomenon is more obvious at leaner mixture condition near the lean limit of natural gas. The mole fractions of OH and O are increased with the increase of hydrogen fraction and the position of maximum OH and O mole fractions move closing to the unburned mixture side. A monotonic correlation between initial combustion duration with the reciprocal maximum OH mole fraction in the flames is observed. The enhancement of the spark ignition of natural gas with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the flames.     

Effect of partially premixed and hydrogen addition on natural gas direct-injection lean combustion

Effect of partially premixed mixture and hydrogen addition on natural gas direct-injection lean combustion was studied experimentally using a constant volume vessel. Flame propagating photos and pressure derived combustion parameters were analysed at different premixed ratios (from 0% to 80%) and hydrogen fractions (from 0% to 40%) at overall equivalence ratio of 0.6, 0.8 and 1.0, respectively. The results show that the flame kernel is concentrated to the spark position with the increase of premixed ratio and/or hydrogen fraction. Flame propagating speed is decreased with the increase of premixed ratio while it increases as hydrogen is added to natural gas. Hydrogen addition has little effect on the partially direct-injection natural gas combustion at the stoichiometric fuel-air mixture condition and all premixed ratios. However, hydrogen addition significantly enhances the combustion rate of natural gas direct-injection combustion at lean mixture condition. Both the initial and main combustion durations are increased with the increase of premixed ratio, while they show the decreasing trend as hydrogen is added to natural gas at the lean mixture condition. Partially premixed direct-injection combustion combining with hydrogen addition can achieve the stable spark ignition and fast combustion at the lean mixture condition.     

Effects of hydrogen addition on the premixed laminar-flames of ethanol–air gaseous mixtures: An experimental study

The effects of hydrogen addition on the laminar premixed-flame characteristics of ethanol–air gaseous mixtures were investigated experimentally by using outwardly propagating spherical flames. The experiments were conducted in a constant-volume combustion vessel with a central ignition at an initial temperature of 383 K, a pressure of 0.1 MPa, a hydrogen fraction from 0% to 100%, and an equivalence ratio from 0.6 to 1.6, and the flame images were obtained by a high-speed schlieren camera system. The results show that the unstretched flame propagation speeds and burning velocities increase exponentially with the increase in hydrogen fraction for a constant equivalence ratio. When the hydrogen fraction is equal to or less than 60%, the burned gas Markstein length reduces with the increase of equivalence ratio, indicating a positive correlation between the flame instability and hydrogen fraction, while the opposite effect is observed when the hydrogen fraction is greater than 60%. At an equivalence ratio below 1.4, the Markstein length decreases with increased hydrogen fraction, indicating that the flame instability is exacerbated with hydrogen addition, while the reverse holds in the case of equivalence ratio above 1.4. Finally, an empirical formula is developed to estimate the laminar burning velocity of ethanol–hydrogen–air flames on the basis of present experimental data.     

An experimental analysis of hydrogen enrichment on combustion characteristics of a gasoline Wankel engine at full load and lean burn regime

In this paper, a gasoline Wankel engine was modified and equipped with self-developed hybrid electronic control unit to experimentally investigate the effect of hydrogen-enrichment level on combustion characteristics of a gasoline Wankel engine at wild open throttle position and lean burn regime. Testing were carried out under constant engine speed of 3000 rpm and the lean operating limit of the original gasoline engine. The spark timing was set at 15 °BTDC. The hydrogen energy fraction in the intake was gradually increased from 0% to 10%. The results showed that hydrogen enrichment was effective on improving the combustion process through the shortened of the flame development and the flame propagation periods, advancing the central heat release, increasing the HRRmax and reducing the cyclic variation proportionally to the amount of hydrogen added to the air fuel mixture. Furthermore, increasing hydrogen fraction in the intake improves the engine economy by reducing the cooling loss.     

Study of laminar combustion characteristics of gasoline surrogate fuel-hydrogen-air premixed flames

This paper investigated the effects of hydrogen addition to gasoline surrogates fuel-air mixture on the premixed spherical flame laminar combustion characteristics. The experiments were carried out by high speed Schlieren photography on a constant-volume combustion vessel. Combining with nonlinear fitting technique, the variation of flame propagation speed, laminar burning velocity, Markstein length, flame thickness, thermal expansion coefficient and mass burning flux were studied at various equivalence ratios (0.8–1.4) and hydrogen mixing ratios (0%–50%). The results suggested that the nonlinear fitting method had a better agreement with the experimental data in this paper and the flame propagation was strongly effected by stretch at low equivalence ratios. The stretched propagation speed increased with the increase of hydrogen fraction at the same equivalence ratio. For a given hydrogen fraction, Markstein length decreased with the increase of equivalence ratio; flame propagation speed and laminar burning velocity first increased and then decreased with the increase of equivalence ratio while the peaks of the burning velocity shifted toward the richer side with the increase of hydrogen fraction.     

Laminar burning velocities and flame instabilities of diluted H2/CO/air mixtures under different hydrogen fractions

An experimental study was conducted using outwardly propagating flame to evaluate the laminar burning velocity and flame intrinsic instability of diluted H₂/CO/air mixtures. The laminar burning velocity of H₂/CO/air mixtures diluted with CO₂ and N₂ was measured at lean equivalence ratios with different dilution fractions and hydrogen fractions at 0.1 MPa; two fitting formulas are proposed to express the laminar burning velocity in our experimental scope. The flame instability was evaluated for diluted H₂/CO/air mixtures under different hydrogen fractions at 0.3 MPa and room temperature. As the H₂ fraction in H₂/CO mixtures was more than 50%, the flame became more unstable with the decrease in equivalence ratio; however, the flame became more stable with the decrease in equivalence ratio when the hydrogen fraction was low. The flame instability of 70%H₂/30%CO premixed flames hardly changed with increasing dilution fraction. However, the flames became more stable with increasing dilution fraction for 30%H₂/70%CO premixed flames. The variation in cellular instability was analyzed, and the effects of hydrogen fraction, equivalence ratio, and dilution fraction on diffusive-thermal and hydrodynamic instabilities were discussed.     

Experimental and numerical investigation of low-pressure laminar premixed synthetic natural gas/O2/N2 and natural gas/H2/O2/N2 flames

The oxidation of laminar premixed natural gas flames has been studied experimentally and computationally with variable mole fractions of hydrogen (0, 20, and 60%) present in the fuel mixture. All flames were operated at low pressure (0.079 atm) and at variable overall equivalence ratios (0.74<?<1.0) with constant cold gas velocity. At the same global equivalence ratio, there is no significant effect of the replacement of natural gas by 20% of H₂. The small differences recorded for the intermediate species and combustion products are directly due to the decrease of the amount of initial carbon. However, in 60% H₂ flame, the reduction of hydrocarbon species is due both to kinetic effects and to the decrease of initial carbon mole fraction. The investigation of natural gas and natural gas/hydrogen flames at similar C/O enabled identification of the real effects of hydrogen. It was shown that the presence of hydrogen under lean conditions activated the H-abstraction reactions with H atoms rather than OH and O, as is customary in rich flames of neat hydrocarbons. It was also demonstrated that the presence of H₂ favors CO formation.     

Effects of initial pressure and hydrogen concentration on laminar combustion characteristics of diluted natural gas–hydrogen–air mixture

In this study, the experiment study about the laminar burning velocity and the flame stability of CO₂ diluted natural gas–hydrogen–air mixture was conducted in a constant volume combustion vessel by using the high-speed schlieren photography system. The unstretched laminar burning velocity and the Markstein length at different hydrogen fractions, dilution ratios and equivalence ratios and with different initial pressures were obtained. The flame stability was studied by analyzing the Markstein length, the flame thickness, the density ratio and the flame propagation schlieren photos. The results showed that the unstretched laminar burning velocity would be reduced with the increase of the initial pressure and dilution ratio and would be increased with the increase of the hydrogen fraction of the mixture. Meanwhile, the Markstein length would be increased with the increase of the equivalence ratio and the dilution ratio. Slight flaws occurred at the early stage. At a specific equivalence ratio, a higher initial pressure and hydrogen fraction would cause incomplete combustion.     

Experimental study of effect of hydrogen addition on combustion of low caloric value gas fuels

The effect of hydrogen on the premixed combustion of low calorific value gas in cylinder was carried out. The engine bench test system was built, and the effects of hydrogen on the pressure in cylinder, the characterization parameters of flame stability, flame shape, combustion cycle variation, flame surface structure and development were studied. The experimental results show that adding hydrogen to low calorific value gas fuels can increase laminar flame propagation rate, increase heat release rate, improve engine volumetric efficiency and reduce engine cycle variation. With the increase of hydrogen fraction, the maximum cylinder pressure of the engine rises, the corresponding crankshaft angle moves forward, and the flame development period and the rapid combustion period of the engine working cycle are reduced. The flame propagation rate of hydrogen blended low calorific value gas is larger, which enhances the strength of the vortex blob in cylinder to some extent. The increase of hydrogen fraction in mixture increases the flame propagation velocity, and enhances the effect of the combustion process on vortex intensity in cylinder, which increasing the effect of vortex blob on the flame surface structure. With the increase of hydrogen fraction, the vortices of the flow field in the cylinder increase, the time of which the flame surface reaches the same fold is in advance, and the flame development period is shortened.     

Numerical study of the effect of hydrogen addition on methane–air mixtures combustion

The stoichiometric methane–hydrogen–air freely propagated laminar premixed flames at normal temperature and pressure were calculated by using PREMIX code of CHEMKIN II program with GRI-Mech 3.0 mechanism. The mole fraction profiles and the rate of production of the dominant reactions contributing to the major species and some selected intermediate species in the flames of methane–hydrogen–air were obtained. The rate of production analysis was conducted and the effect of hydrogen addition on the reactions of methane–air mixtures combustion was analyzed by the dominant elementary reactions for specific species. The results showed that the mole fractions of major species CH₄, CO and CO₂ were decreased while their normalized values were increased as hydrogen is added. The rate of production of the dominant reactions contributing to CH₄, CO and CO₂ shows a remarkable increase as hydrogen is added. The role of H₂ in the flame will change from an intermediate species to a reactant when hydrogen fraction in the blends exceeds 20%. The enhancement of combustion with hydrogen addition can be ascribed to the significant increase of H, O and OH in the flame as hydrogen is presented. The decrease of the mole fractions of CH₂O and CH₃CHO with hydrogen addition suggests a potential in the reduction of aldehydes emissions of methane combustion as hydrogen is added. The methane oxidation reaction pathways will move toward the lower carbon reaction pathways when hydrogen is available and this has the potential in reducing the soot formation. Chemical kinetics effect of hydrogen addition has a little influence on NO formation for methane combustion with hydrogen addition.     

Laminar propagation of lean premixed flames ignited in stratified mixture

Combustion in stratified mixtures is envisaged in practical energy systems such as direct-injection spark-ignited (DISI) car engines, gas turbines, for reducing CO₂ and pollutant emissions while protecting their efficiency. The mixture gradients change the fundamental properties of the flame, especially by a difference in temperature and composition between the burnt gases and those of a flame consuming a homogeneous mixture. This paper presents an investigation of the properties of the flame propagating in a lean homogeneous mixture after ignition in a richer mixture according to the magnitude of the stratification. Three magnitudes of stratification are investigated. The local flame burning velocity is determined by an original PIV algorithm developed previously. The local equivalence ratio in the fresh gases is measured from anisole PLIF. From the simultaneous PIV–PLIF measurements, the flame burning velocities conditioned on the local stretch rate and equivalence ratio in fresh gases are measured. The flame propagating through the homogeneous lean mixture has properties depending on the ignition conditions in the stratified layer. The flame propagating in the lean mixture is back-supported longer for ignition under the richer condition. The change of stretch sensitivity and burning velocity of the flame in the lean mixture is measured over time for the three magnitudes of mixture stratification investigated. The ignition in richer mixtures compensates for the nonequidiffusion effect of lean propane flame and sustains its robustness to stretch. The flame propagation in the lean homogeneous mixture is enhanced by ignition in a richer stratified layer, as much by their robustness to stretch as by an increase in the flame speed or the burning velocity. The decay time of this influence of the stratification, called memory effect, is determined.     

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.     

Effects of non-equidiffusion on unsteady propagation of hydrogen-enriched methane/air premixed flames

A Large Eddy Simulation (LES) model was developed to simulate the unsteady propagation of hydrogen-enriched methane/air premixed flames around toroidal vortices. Although the LES model does not take into account the non-equidiffusive effects associated with the hydrogen presence (preferential diffusion and non-unity Lewis number), it gives good predictions of experimental data previously obtained for lean mixtures with hydrogen mole fraction in the fuel (hydrogen plus methane) varying from 0 to 0.5. In particular, for each fuel composition, size and velocity of the toroidal vortex generated ahead of the propagating flame front are well reproduced along with the evolution of the flame shape and structure resulting from the interaction with the vortex. The negligible role played by the non-equidiffusive effects has been attributed to the fact that, at the conditions investigated, the characteristic time of hydrogen diffusion is one order of magnitude higher than the characteristic time of flame roll-up around the vortex.     

Relationship of the combustion characteristics of natural gas-hydrogen/carbon dioxide mixtures with the ion current and pressure parameters

The characteristics of the ionization current, pressure, and flame schlieren photos during the combustion process of natural gas-hydrogen/CO₂ blends were studied using high-speed schlieren data acquisition technology and flame ionization analysis methods. According to the ion current waveform, pressure waveform, and flame propagation photos, the relationship of the characteristic parameters between the ion current and pressure were analysed using available experimental data. The ionization current waveform can reflect not only the entire combustion process (including ignition phase, the front flame phase and the post flame phase) but also the effect of the initial pressure, equivalence ratios, hydrogen fraction and CO₂ dilution ratios on the combustion process.This results of this study show that the amplitude and characteristic timing of the ion current can be used to predict the change of the chamber pressure. The ion current and pressure will increase with the increase of the initial pressure, equivalence ratio and hydrogen fractions, but the parameters will decrease with the increase of the CO₂ dilution ratio. The characteristic timing of the ion current and pressure peaks is advanced with the increase of the equivalence ratio and hydrogen fractions; however, the peaks are postponed by the increase in the initial pressure and CO₂ dilution ratio. The corresponding relationship of the characteristic timing between the ion current and pressure is basically unchanged. The results of this study provide a new approach for exactly controlling the combustion process.     

Optical study on the effects of the hydrogen injection timing on lean combustion characteristics using a natural gas/hydrogen dual-fuel injected spark-ignition engine

Lean combustion has the potential to achieve higher thermal efficiency for internal combustion (IC) engines. However, natural gas engines often suffer from slow burning rate and large cyclic variations when adopting lean combustion. In this study, using a dual-fuel optical engine with a high compression ratio, the effects of direct-injected hydrogen on lean combustion characteristics of natural gas engines was investigated, emphasizing the role of hydrogen injection timing. Synchronization measurement of in-cylinder pressure and high-speed photography was performed for combustion analysis. The results show that the direct-injected hydrogen exhibits great improvement in lean combustion instability and power capability of natural gas engines. Visual images and combustion phasing analysis indicate that the underlying reasons are ascribed to the fast flame propagation with hydrogen addition. Regarding the direct injection timings, it is found that late injection of direct-injected hydrogen can achieve higher thermal efficiency, manifesting advanced combustion phasing, and increased heat release rate. Specifically, the flame propagation speed is elevated by approximately 50% at ?100 CAD than that of ?250 CAD. Further analysis indicates that the improvement of engine performance is ascribed to the increased volumetric efficiency and in-cylinder turbulence intensity, manifesting distinct flame centroid pathways at different injection timings. The current study provides insights into the combustion optimization of natural gas engines under lean burning conditions.     

预燃室式天然气掺氢发动机燃烧及排放模拟

为探索掺氢对预燃室式大功率中速天然气发动机燃烧和排放的影响,采用计算流体动力学耦合化学动力学方法,在一台6ACD320型天然气发动机上,对氢气体积分数为0～ 30％的天然气-氢气混合燃料的燃烧过程进行了数值模拟.结果表明:在天然气中掺氢促使缸内产生了更多的0、OH等活性自由基,从而加速了缸内火焰传播,发动机的指示燃气消...     

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.     

Effects of hydrogen addition on combustion and flame propagation characteristics of laser ignited methane/air mixtures

In this study, effects of hydrogen addition on combustion and flame propagation characteristics of methane/air mixtures were investigated in a constant volume combustion chamber. Tested gas mixtures are 100% CH₄, 05% H₂ – 95% CH₄, 10% H₂ – 90% CH₄ and 15% H₂ – 85% CH₄, and such mixtures were ignited using a passively Q-switched Nd:YAG laser ignitor which has a pulse energy of 12.3 mJ, pulse duration of 2.4 ns and wave length of 1064 nm. A Schlieren setup coupled with a high-speed camera enabled evaluating flame propagation behavior, while pressure curve analysis provided necessary data for characterization of combustion properties. Additionally, lean flammability limits of gas mixtures were also determined at the test conditions. The unique properties of hydrogen (such as low density, high reactivity, high diffusivity etc) widened lean flammability limit. Rate of pressure rise and measured pressure values increased with hydrogen addition, regardless of the air-fuel equivalence ratio (λ). Lastly, hydrogen addition uniformly affected flame propagation characteristics and flame luminosity. Combustion process became more stable with hydrogen addition.     

Measurement of laminar burning velocities and Markstein lengths of diluted hydrogen-enriched natural gas

The laminar flame characteristics of natural gas–hydrogen–air–diluent gas (nitrogen/CO₂) mixtures were studied in a constant volume combustion bomb at various diluent ratios, hydrogen fractions and equivalence ratios. Both unstretched laminar burning velocity and Markstein length were obtained. The results showed that hydrogen fraction, diluent ratio and equivalence ratio have combined influence on laminar burning velocity and flame instability. The unstretched laminar burning velocity is reduced at a rate that is increased with the increase of the diluent ratio. The reduction effect of CO₂ diluent gas is stronger than that of nitrogen diluent gas. Hydrogen-enriched natural gas with high hydrogen fraction can tolerate more diluent gas than that with low hydrogen fraction. Markstein length can either increase or decrease with the increase of the diluent ratio, depending on the hydrogen fraction of the fuel.     

Effect of initial pressure on laminar combustion characteristics of hydrogen enriched natural gas

Flame propagation of premixed natural gas–hydrogen–air mixtures was studied in a constant volume combustion bomb. Laminar burning velocities and mass burning fluxes were obtained under various hydrogen fractions and equivalence ratios with various initial pressures, while flame stability and their influencing factors (Markstein length, density ratio and flame thickness) were obtained by analyzing the flame images at various hydrogen fractions, initial pressures and equivalence ratios. The results show that hydrogen fraction, initial pressure as well as equivalence ratio have combined influence on both unstretched laminar burning velocity and flame instability. Meanwhile, according to flame propagation pictures taken by the high speed camera, flame stability decreases with the increase of initial pressures; for given equivalence ratio and hydrogen fraction, flame thickness is more sensitive to the variation of the initial pressure than to that of the density ratio.