Laminar Burning Characteristics Research Articles

Laminar burning characteristics of 2-methylfuran and isooctane blend fuels

2-Methylfuran (MF) has become very attractive due to the recent breakthrough in its production method using the process of dehydration and hydrogenolysis of fructose. MF–gasoline blended fuel has been considered as a potential choice of alternative fuel pathway for spark ignition (SI) engines, as have other biofuel blends. Isooctane is used to represent gasoline in fundamental studies of gasoline blended fuels, however, little is known about the laminar burning characteristics of MF–isooctane blended fuels. In this study, high-speed schlieren photography is used to investigate the laminar burning characteristics of gaseous MF–isooctane at varying temperatures and equivalence ratios with an initial pressure of 0.1MPa in a constant-volume vessel. The outwardly spherical flame method is used to determine the stretched flame speeds. The un-stretched flame speeds, Markstein lengths, Markstein number, laminar burning velocities and laminar burning flux of MF20 (20% MF and 80% isooctane) and MF50 (50% MF and 50% isooctane) under different equivalence ratios and temperatures are then deduced and compared to MF and isooctane. The results show that the un-stretched flame speeds and laminar burning velocities of MF20 and MF50 are between those of MF and isooctane under all conditions. The peak un-stretched flame speeds of the blends occur in an equivalence ratio range of 1.1–1.2 at all temperatures, closer to the case of MF at higher temperatures. Both blended fuels have Markstein lengths closer to isooctane at an equivalence ratio lower than 1.2 at all temperatures. The burning velocities of MF50 are very close to the average values for MF and isooctane, particularly at 393K. MF in the blended fuel presents larger effects on burning velocities at higher temperatures.

F071003 火花点火過程における点火安定性の予測(【F07100】エンジン開発に関わる最新計測・CAE技術,エンジンシステム部門企画,先端技術フォーラム)

In order to estimate spark-ignitability in a fuel-air mixture, turbulent Karlovitz number Ka is used, which is calculated based on the local velocity and equivalence ratio. This estimation is applied some LES or RANS calculation results of a natural-gas jet with impingement on a cavity wall. The local Ka is estimated from flame thickness, laminar burning velocity and characteristics time-scale of turbulence. For appling the LES result, the turbulence time scale is calculated by Smagorinski's model. For appling the RANS result, that is calculated based on k and ε. The result shows that the spark-ignition at the point of Ka<50 cause a stable combustion.

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.

Laminar Flame Speeds of DMF/Iso-octane-Air-N2/CO2 Mixtures

Laminar combustion characteristics of 2,5-dimethylfuran (DMF)/iso-octane/air/diluent (15% N2/85% CO2) mixtures at atmospheric pressure and the initial temperature of 393 K were studied using the outwardly propagating spherical flame and high speed schlieren photography. Effects of DMF concentration in the fuel mixture and dilution ratio on flame properties were investigated, such as laminar flame speed, adiabatic temperature, effective Lewis number, and laminar flame thickness. Results show that laminar flame speed varied little, while the flame instability was slightly promoted as DMF concentration in the blends is increased. For all the fuels at fixed equivalence ratio, flame front morphology showed that the flame stability was enhanced with increasing dilution ratio. In addition, a linear relationship between the normalized laminar flame speed and the dilution ratio was found and an empirical formula was proposed for all the four different fuels at different equivalence ratios.

Laminar burning characteristics of 2,5-dimethylfuran and iso-octane blend at elevated temperatures and pressures

2,5-Dimethylfuran, known as DMF, is a promising second-generation biofuel candidate. The potential of 2,5-dimethylfuran as an additive in iso-octane (used as gasoline fuel substitute in this study) was studied. Using outwardly spherical flame method and high speed schlieren photography, laminar burning characteristics of 2,5-dimethylfuran/iso-octane (20%/80% vol., designated as D20)/air mixtures were experimentally investigated. Laminar flame speeds and Markstein lengths of D20–air mixtures were empirically organized as a function of initial pressures and temperatures. Onset of cellular structures at the flame front was observed at relatively higher initial pressures due to the combined effect of diffusional-thermal and hydrodynamic instabilities. Laminar flame speeds of D20–air mixtures are higher than those of iso-octane–air mixtures when the equivalence ratio is greater than 1.2.

Laminar Burning Characteristics of Diluted n-Butanol/Air Mixtures

Experimental and numerical studies on laminar burning characteristics of diluted n-butanol/air premixed mixtures were conducted. Experiments were conducted using the spherically expanding flames at different dilution ratios of nitrogen at the initial pressure of 0.1 MPa and temperature of 428 K. Both measured and computed results show that with the increase of dilution ratio of nitrogen, the laminar flame speed of n-butanol/air mixtures is decreased. This is attributed to the decrease of adiabatic flame temperature of the mixtures. In addition, effective Lewis numbers of n-butanol/air mixtures at varied dilution ratios were deduced, and flame instability was analyzed. Results show that in the case of equivalence ratio less than 1.4, flame tends to stabilize with nitrogen dilution for the n-butanol/air mixture. In contrast, in the case of equivalence ratio equals to and larger than 1.4, the flame tends to become unstabilized when mixtures are diluted with nitrogen. The measured laminar flame speeds are in good agreement with the computed laminar flame speeds at different nitrogen dilution ratios. Results show a linear relationship between the normalized laminar flame speed and nitrogen dilution ratio. Sensitivity analysis was conducted using the recently developed detailed kinetic mechanism. Results indicate that the three-body reactions are dominant at large dilution ratio.

Experimental determination of laminar burning velocities and Markstein lengths for 75% hydrous-ethanol, hydrogen and air gaseous mixtures

Hydrogen-rich mixtures generated by the on-board reforming of biomass-derived hydrous-ethanol can be used as a potential alternative fuel (i.e., reformed ethanol fuel, RE fuel). In this paper, outwardly propagating spherical flames were employed to observe the laminar flame characteristics of the gaseous mixtures composed of simulated RE fuel (mixture of 75% hydrous-ethanol and hydrogen) and air in a constant-volume combustion vessel at an initial temperature of 383 K, a pressure of 0.1 MPa, a hydrogen fraction from 0% to 80%, and an equivalence ratio from 0.6 to 1.6. The results show that the unstretched flame propagation speeds and burning velocities increase with increasing hydrogen fraction, especially when the fraction is above 40%. When the hydrogen fraction is less than 40%, the Markstein length and flame instability decrease and increase with the equivalence ratio, respectively, while the reverse holds when the hydrogen fraction is greater than 40%. At an equivalence ratio below 1.4, the Markstein length decreases with increasing hydrogen fraction, indicating a positive correlation between the flame instability and hydrogen fraction. At an equivalence ratio above 1.4, a negative relationship is observed. Finally, it is concluded that a hydrogen fraction of approximately 40% in simulated RE fuel is feasible for spark ignition engines by comparing the laminar burning characteristics of ethanol–air mixtures.

Study on Laminar Burning Characteristics of Premixed High-Octane Fuel−Air Mixtures at Elevated Pressures and Temperatures

Laminar burning characteristics of high-octane fuel−air premixed mixtures (ETBE, TBA, and ethanol) were studied in a constant-volume bomb at various equivalence ratios, initial temperatures, and initial pressures by using outwardly propagating spherical flames with a high-speed schlieren imaging system. The flame propagation speed, the laminar burning velocity, the Markstein length, the adiabatic flame temperature, the flame thickness, and the density ratio were obtained and the influence of equivalence ratio, initial temperature, and initial pressure on these parameters was analyzed. The experimental results show that both the unstretched flame propagation speed and unstretched laminar burning velocity increase with the increase of initial temperature and decrease with the increase of initial pressure. Thermal-diffusive instabilities (Markstein length) decrease at higher initial temperature and/or at lower initial pressure. Meanwhile, the unstretched flame propagation speed and the unstretched laminar bu...

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.

Laminar burning velocities and combustion characteristics of propane–hydrogen–air premixed flames

An experimental study on laminar burning characteristics of the spherically expanding premixed propane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, the laminar flame thickness, the Markstein number, the Zeldovich number and the global Lewis number were obtained over a range of equivalence ratios and hydrogen fractions. The influence of hydrogen addition on the laminar burning velocities and the flame front instabilities were analyzed. The results show that the unstretched laminar burning velocity increases, the laminar flame thickness decreases and the peak value of unstretched laminar burning velocity shifts to the richer mixture side with the increase of hydrogen fraction. When hydrogen fraction in the fuel is less than 60%, the Markstein number decreases with the increase of equivalence ratio, and the flame behavior is similar to that of propane–air flames. When hydrogen fraction is larger than 60%, the flame behavior is similar to that of hydrogen–air flames. At equivalence ratio less than 1.2, the Markstein number decreases with the increase of hydrogen fraction, indicating flame destabilization by hydrogen addition. At equivalence ratio larger than 1.2, the Markstein length increases with the increase of hydrogen fraction, indicating the stabilization of flame by hydrogen addition. In the case of lean mixture combustion, the Zeldovich number decreases with the increase of hydrogen addition, indicating the lowering of activation temperature of the mixture. The global Lewis number decreases with the increase of hydrogen fraction, and this indicates the increase of preferential-diffusion instabilities by hydrogen addition.

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.

Experimental Study on Premixed Combustion of Dimethyl Ether–Hydrogen–Air Mixtures

The laminar combustion characteristics of DME–hydrogen–air premixed mixture were studied at different equivalence ratios, ratios of hydrogen to DME plus hydrogen, and initial pressures by using a constant volume combustion bomb. The influences of the equivalence ratio, the hydrogen addition, and the initial pressure on flame speed and combustion characteristics are analyzed. The results show that the flame speed, the laminar burning velocity, and the mass burning rate increase with the increase of hydrogen addition. Increasing the ratio of hydrogen to DME plus hydrogen will shorten the combustion duration. In the case of small hydrogen addition, the Markstein length decreases with the increase of equivalence ratio, and this reveals that lean mixture has better flame front stability compared with that of the rich mixture. In the case of large hydrogen addition, the Markstein length increases with the increase of equivalence ratio, and this indicates that the rich mixture has better flame front stability th...

Numerical Studies on Combustion Characteristics of Interacting Pulverized Coal Particles at Various Oxygen Concentration

The two-dimensional laminar combustion characteristics of coal particles at various oxygen concentration levels of a surrounding gas have been numerically investigated. The numerical simulations, which use the two-step global reaction model to account for the surrounding gas effect, show the detailed interaction among the inter-spaced particles, undergoing devolatilization and subsequent char burning. Several parametric studies, which include the effects of gas temperature (1700 K), oxygen concentration, and variation in geometrical arrangement of the particles on the volatile release rate and the char burning rate, have been carried out. To address the change in the geometrical arrangement effect, multiple particles are located at various inter-spacings of 4–20 particle radii in both streamwise and spanwise directions. The results for the case of multiple particles are compared with those for the case of a single particle. The comparison indicates that the shift to the multiple particle arrangement resulted in the substantial change of the combustion characteristics and that the volatile release rate of the interacting coal particles exhibits a strong dependency on the particle spacing. The char combustion rate is controlled by the level of oxygen concentration and gas composition near particles during combustion. The char combustion rate is highly dependent on the particle spacing at all oxygen levels. The correlations of the volatile release rate and the change in total mass of particles are also found.