Laminar Burning Characteristics Research Articles

Experimental and numerical investigation of iC8H18 laminar combustion characteristics based on hydrogen – water complementary regulation

The present study examines using water dilution and hydrogen as control measures to regulate the laminar burning velocity of iso-octane/air mixtures in a constant volume combustion bomb. Experimental results are obtained by processing schlieren photographs and simulation results are calculated by Chemkin software. The water dilution rates of the mixtures were between 0 and 5% and the hydrogen addition ratio was between 5 and 55%. The initial pressure was set at 101.3 kPa with the preheating temperature ranging from 400 K to 500 K. According to the results, laminar burning velocity varied almost linearly with the dilution of water and the addition of hydrogen, respectively. This study also distinguishes the thermal and kinetic effects of water on the combustion processes. The thermodynamic effect of water vapour on laminar burning velocity was higher than the kinetic effect. In kinetics, O, H, and OH were the most affected free radicals by the addition of water vapour. The main free radical change affected the rate of the elementary reaction. Besides, the kinetic effect of water vapour promoted combustion. However, this effect was offset by the thermodynamic effect. The introduced hydrogens of premixed mixtures did not lead to measurable improvements in laminar burning velocity. Additionally, the change in the reaction rate of the elementary reaction was not obvious, which was inconsistent with the change of laminar burning velocity. Lastly, a laminar burning velocity model using hydrogen and water vapor content as control parameters was proposed. Accordingly, the designated laminar burning velocity of the iso-octane/air mixture was achieved by altering hydrogen and water vapour ratios in the mixture. BP (Back-Propagation) neural network algorithm (a multi-layer feed forward neural network trained by error back propagation algorithm) and empirical data were used to verify the model, and the model’s maximum error was less than 5%.

Effects of carbon dioxide on the combustion characteristics of the laminar premixed n-heptane/air flames at elevated pressures

In this paper, the laminar combustion characteristics of n-heptane/carbon dioxide/air flames are numerically investigated by using the Chemkin/Premix Code and LLNL v3.1 mechanism. The equivalence ratios vary from 0.8 to 1.4 at the initial temperature of 298 K. The initial pressure is set as 1, 2, 5, 10, 20 atm. Laminar burning velocity (LBV), adiabatic flame temperature (AFT), net heat release rate (NHRR) and the mole fractions of H, O, OH, CO at stoichiometric ratio are studied with carbon dioxide (CO2) addition. Meanwhile, temperature sensitivity and rate of production (ROP) are analyzed. In addition, the physical and chemical effects of CO2 are investigated by introducing fictitious carbon dioxide (FCO2).The results show that with the increase of initial pressures, LBV decreases, AFT and NHRR increase, the peaks of intermediate products such as H, O, OH and CO decrease. The promoting effect of elementary reactions on temperature is greatly enhanced, while the inhibiting effect is slightly enhanced. With the increase of CO2 doping ratios, LBV, AFT and NHRR decrease, while the peaks of H, O, OH, CO increase first and then decrease. The promoting and inhibiting effects of elementary reactions on temperature are enhanced. Both physical and chemical effects of CO2 inhibit the combustion of n-heptane, where physical effect dominates. The total ROPs of H, O, OH change from consuming to producing, while the total ROP of CO is opposite.

Laminar burning characteristics of ammonia/hydrogen/air mixtures with laser ignition

Ammonia, as a zero-carbon fuel, is drawing more and more attention. The major challenge of using ammonia as a fuel for the combustion engines lies in its low chemical reactivity, and therefore more fundamental researches on the combustion characteristics of ammonia are required to explore effective ways to burn ammonia in engines. In this study, the laminar burning characteristics of the premixed ammonia/hydrogen/air mixtures are investigated. In the experiment, the laser ignition was used to achieve stable ignition of the ammonia/air mixtures with an equivalence ratio range from 0.7 to 1.4. The propagating flame was recorded with the high-speed shadowgraphy. Three different processing methods were introduced to calculate the laminar burning velocity with a consideration of the flame structure characteristics induced by the laser ignition. The effects of initial pressure (0.1 MPa–0.5 MPa), equivalence ratio (0.7–1.4), hydrogen fraction (0–20%) on the laminar burning velocity were investigated under the initial ambient temperature of 360 K. The state-of-the-art kinetic models were used to calculate the laminar burning velocities in the CHEMKIN-pro software. Both the simulation and experimental results show that the laminar burning velocity of the ammonia mixtures increases at first, reaches the peak around ϕ of 1.1, and then decreases with the equivalence ratio increasing from 0.7 to 1.4. The peak laminar burning velocities of the ammonia mixture are lower than 9 cm/s and are remarkably lower than those of hydrocarbon fuels. The laminar burning velocity of the ammonia mixture decreases with the increase of the initial ambient pressure, and it can be drastically speeded up with the addition of hydrogen. While the models except for those by Miller and Bian can give reasonable predictions compared to the experimental results for the equivalence ratio from 0.7 to 1.1 in the ammonia (80%)/hydrogen (20%)/air mixtures, all the kinetic models overpredict the experiments for the richer mixtures, indicating further work necessary in this respect.

Effect of hydrogen enrichment on the laminar burning characteristics of dimethyl-ether/methane fuel: Experimental and modeling study

In this work, the new laminar flame speeds of hydrogen/dimethyl-ether/methane/air mixtures (H2/DME/CH4) covering wide ranges of equivalence ratios (0.6–1.6) and hydrogen enrichment ratios (0%–100%) were measured using spherical expansion flame method. Four DME chemical reaction kinetic mechanisms (Zhao mechanism, Wang mechanism, Sandiego mechanism and Chen mechanism) were used to calculate the laminar flame speeds and compared with the experimental measured data. Results showed that the experimental data agreed well with the calculated values using Zhao mechanism. The laminar flame speeds were observed to increase significantly with hydrogen enrichment ratio and they can be divided into three stages depending on the hydrogen enrichment ratio in the H2/DME/CH4 fuel. Furthermore, the laminar flame speeds at fuel-rich conditions were more sensitive to hydrogen enrichment than fuel-lean conditions. In the increasing hydrogen atmosphere, the diffusional-thermal instability was enhanced for fuel-lean conditions, while it was effectively suppressed for fuel-rich conditions. The hydrodynamic instability was enhanced for all cases, which was mainly due to the decrease of flame thickness. Numerical simulation revealed that with the enrichment of hydrogen, the laminar flame speeds have the strongest correlation with H + O radicals for the fuel-lean and fuel-rich conditions, while H radicals have the strongest correlation for the stoichiometric condition. Furthermore, the adiabatic flame temperature and thermal diffusivity can be enhanced with the enrichment of hydrogen.

Effect of Initial Temperature and Pressure on Laminar Combustion Velocity of ABE Mixtures with Different Component Contents

One-dimensional planar flame was numerically simulated to study the premixed laminar combustion characteristics of acetone-n-butanol-ethanol (ABE) mixtures under different component contents. The results showed that as the initial temperature was elevated, the laminar combustion velocity of ABE mixtures was gradually increased under different component contents, and the increase rate was not obviously changed. With the increase in the initial pressure, however, the laminar combustion velocity presented a declining trend, and the descent speed was gradually lowered.

Investigation of laminar combustion characteristics of ozonized methane-air mixture in a constant volume combustion bomb

The effect of ozone on the combustion performance of methane-air mixture was studied with a constant volume combustion bomb test bench under different initial pressures and equivalence ratios. The laminar combustion characteristic parameters were obtained according to the flame propagation images. The comparison of the laminar combustion characteristic parameters showed that the O-atom decomposed from ozone can promote the reaction activity of the mixture. Based on the Markstein length, it can be concluded that ozone can increase the flame instability of the mixture, also the flame thickness decreased with ozone addition due to the acceleration of chemical reaction. Both the laminar flame speed and laminar burning velocity increased with ozone addition, and the maximum relative increment of the parameters all appeared at the equivalence ratio of 0.6, which showed the ozone had a more notable enhancement effect for lean methane-air mixture. It was feasible to enhance the combustion performance for natural gas engine using lean burn combustion in principle.

Impact of dilution on laminar burning characteristics of premixed methane-dissociated methanol-air mixtures

A constant volume combustion vessel is employed to investigate the impact of different diluents and diluent gas fractions on the methane-dissociated methanol-diluent-air premixed flames at 0.3 MPa and 343 K. The diluent gas causes the adiabatic flame temperature and thermal diffusivity to decrease, which is the main reason for the decreased laminar burning velocity, and carbon dioxide exhibits a stronger inhibitory effect than nitrogen. With increasing diluent gas fraction, the diffusive-thermal instability is enhanced while the hydrodynamic instability is inhibited, these two factors are the main reason for the insensitivity of the flame instability to the diluent gas fraction. Further, flame instability is not sensitive to the diluent gas type. R38 is the dominant chain branching reaction, while R35 and R52 are the dominant chain termination reactions in the combustion process. With the increase in the diluent gas fraction, the rates of the dominant chain termination reactions shows greater inhibition of laminar burning velocity, a larger decrease in the chain branching reaction rate, and a smaller decrease in the chain termination reaction rate will significantly reduce the free radical concentration. The maximum reaction rate reduction when diluted by carbon dioxide is greater than when diluted by nitrogen.

Experimental and numerical study of the effects of oxygen-enriched air on the laminar burning characteristics of biomass-derived syngas

The laminar burning velocities (LBVs) of seven different composition stoichiometric mixtures (CO: 15%–20%, H2: 5%–20%, CH4: 5%–15%, CO2: 15%, N2: 50%) under different oxygen levels (O2: 21%–60%) were experimentally measured by spherical expansion flame method. Numerical calculations were conducted with the PREMIX code using seven detailed chemical reaction mechanisms (FFCM-1, GRI 3.0, USC-Ⅱ, San Diego + NOx, HP-mech, Li-2015, CRECK + NOx) and compared with experimental data. The CRECK mechanism provides an over-prediction of the LBV of CH4-free mixtures. San Diego is more prominent than CRECK with the addition of CH4, followed by GRI 3.0 and HP-mech. Three mechanisms—FFCM-1, Li-2015, USC-Ⅱ—that matched the experimental results well were chosen to perform the LBV calculations for a wider range of initial conditions (Φ: 0.6–1.6, Tu: 300 and 400 K, P0: 1 and 3 atm) for variable composition mixtures. The results show that for all cases, the LBV is considerably increased with increasing oxygen concentration. CH4-rich mixtures are more sensitive to oxygen concentrations and work to increase the LBVs mainly through thermal effects. An oxygen-rich environment makes the flammability limit wider even with high initial pressure. H2-rich mixtures accompanied by higher thermal diffusivity are more susceptible to the initial temperature, while chemical effects do not dominate in oxygen-rich environments. In oxygen-rich environments, diffusional-thermal instability is suppressed while the cellular structure of the flame is mainly affected by fuel composition and hydrodynamic instability. The formation of NO in flames of different composition mixtures in an oxygen-rich environment was analyzed.

A numerical and comparative study of the laminar combustion characteristics and kinetics of dimethyl ether and ethanol flames at elevated temperature

Ethanol and DME are the most promising gasoline and diesel alternative fuels among biofuels. In the present study, we conducted a numerical and comparative study of the laminar burning velocity, chemical kinetics and flame instability of DME and ethanol premixed flames at P = 1 bar, T = 408 K and equivalence ratio of 0.8–1.4. The laminar burning velocity and adiabatic flame temperature of DME are higher than ethanol at all equivalence ratios. In addition, acetaldehyde and formaldehyde are the dominant intermediate product of ethanol and DME, respectively. The peak mole fraction of H + OH + CH3 radicals correlates nearly linearly with the laminar burning velocity. The rate-of-production of DME and ethanol suggests that CH3 + O = CH2O + H is the main reaction of CH3 consumption in both DME and ethanol flames. In addition, the reverse reaction of CH3 + O = CH2O + H is enhanced to inhibit the consumption of CH3 due to the high mole fraction of CH2O in DME, which is one of the reasons that the mole fraction of CH3 in DME flame is higher than in ethanol flame. Furthermore, the hydrodynamic and diffusional-thermal instability of DME are both stronger than ethanol because of thinner flame thickness, higher thermal expansion ratio and smaller effective Lewis number.

A chemical kinetic investigation of laminar premixed burning characteristics for methane-hydrogen-air mixtures at elevated pressures

In this paper, three recently updated mechanisms were compared with experimental data, and FFCM 1.0 mechanism was chosen to perform chemical kinetic investigation of laminar burning velocity (LBV) for CH4/H2 mixtures. Results show that increasing hydrogen and pressure, LBV presents opposite results, which depends on the competition between promotion and inhibition effect at different hydrogen fractions and pressures. Based on growth rate of LBV, it is possible to determine three regimes for CH4/H2 mixtures at high pressures. When hydrogen fraction is less than 60%, the LBV of CH4/H2 mixtures at high pressures is not very sensitive to hydrogen, showing complex variation. Flame structure and radical concentration imply that the enhanced LBV by adding hydrogen, which is a combined result of chemical and thermal effects, is realized by improving the active radical production. Then, quantitative analysis of contribution value found that reaction OH+H2=H+H2O plays an important role in H production with increasing hydrogen fraction at high pressure. Through this work, it not only studied the interactive effect between CH4/H2 combustion system and varied conditions (such as pressure and hydrogen fraction), but also the influence mechanism was revealed in depth. All these have provided useful suggestions on fuel design for power machines.

Experimental and model analyses of laminar combustion characteristics of variable composition CO / H 2 / CH 4 mixtures at high N 2 and CO 2 concentrations

Experimental and model analyses of laminar combustion characteristics of variable composition <scp>CO</scp> / <scp> H ₂ </scp> / <scp> CH ₄ </scp> mixtures at high <scp> N ₂ </scp> and <scp> CO ₂ </scp> concentrations

Study on combustion and emission characteristics of a heavy-duty gas turbine combustor fueled with natural gas

In this paper, the ignition delay and laminar combustion characteristics of natural gas were tested in a chemical shock tube and a constant volume combustion reactor, respectively, and simulated with the Gri_3.0, USC_2.0 and NUI_Galway models, respectively. It is proved that the ignition delay and laminar combustion characteristics of natural gas simulated with the Gri_3.0 model are in better agreement with the experiment than the USC_2.0 and NUI_Galway models. Furthermore, the simulated variation trends of the mole fractions of main species, especially NO and NO2 with temperature by the Gri_3.0 model are in better agreement with the experimental data in the oxidation of natural gas. The combustion process and pollutants generation characteristics in a single flame tube of a heavy-duty gas turbine burning natural gas were simulated by coupling the Gri_3.0 model with the computational fluid dynamics. It is shown that the simulated average temperature and the NOx concentration at the outlet of the flame tube are slightly higher than the experimental value by 8.06% and 5.08%, respectively. Because the NOx emission is higher than the designed value, fuel flows in the annular, main, and pilot combustion zones of the flame tube were reassigned and effects of the fuel redistributions on the NOx emission characteristics were investigated. It is shown that the NOx concentration at the outlet of the flame tube increases from 157.1 mg/m3 to 175.5 mg/m3 when the fuel flow in the pilot combustion zone is redistributed to the main combustion zone, and decreases from 157.1 mg/m3 to 139.51 mg/m3 when the fuel flow in the pilot combustion zone is redistributed to the annular combustion zone. The research results will provide theoretical basis and technical support for the optimization of this gas turbine operation.

Laminar Combustion Characteristics of Premixed Shale Gas and Air Flames

AbstractThe laminar combustion characteristics of premixed shale gas/air flames were studied by spherical expansion flame experiments and chemical kinetics simulations. The influence of the initial...

Experimental and numerical studies of laminar flame characteristics of ethyl acetate with or without hydrogen addition

Hydrogen (H2) is an effective additive to improve the issue of low laminar burning velocity of some biofuels. In order to better understand the laminar burning characteristics of ethyl acetate (EA) with or without H2 addition, experimental investigations of laminar burning characteristics were carried out by using the high-speed Schlieren photography technique in a constant volume combustion chamber. Tests were conducted under various equivalence ratios ranging from 0.5 to 1.4 with an initial temperature of 358 K, an initial pressure of 0.1 MPa and a H2/air proportion of 0%, 4%, 8% and 12% by volume. Laminar burning velocities, together with other parameters such as laminar burning flux, flame thickness, Markstein length and Markstein number, were calculated and discussed. In addition, the experimental data were compared with numerical simulations based on the Dayma model. Results showed that the laminar burning velocity of EA was enhanced with the increase of H2 addition, and the maximum value reached 95.09 cm/s at φ = 0.6 with 12% H2, a value more than twice as fast as that of pure EA (39.3 cm/s). Moreover, H2 was found to extend the lower flammability limit of EA. The laminar burning velocities simulated with the Dayma model agreed well with the experimental results of EA at various H2 additions.

Determination of laminar burning characteristics of a surrogate for a pyrolysis fuel using constant volume method

A fast pyrolysis biofuel, mainly consisting to 98% of ethanol, ethyl acetate, diethyl ether, acetone and 2-butanone with mass ratios of 9:6:2:1:1, was catalytically produced from rice husk. A preliminary engine test demonstrated this biofuel has the possibility of being a gasoline blending stock. However, its fundamental burning features are not fully understood. This work presents the experimental investigations of the spherical propagating flame of a surrogate fuel representing the biofuel in a constant volume combustion chamber (CVCC). Tests were conducted at initial pressures of 0.1–0.4 MPa, initial temperatures of 358–418 K, and equivalence ratios of 0.7–1.4. Employing the constant volume method (CVM) allows determining laminar burning speeds (Su) of this surrogate at conditions far beyond the initial conditions (0.1–0.8 MPa, 358–490 K). Power law fitting correlations between Su and pressure were obtained via the constant volume method (CVM). Cellularity appears when pressure or temperature is high, and cellular burning speed was calculated by CVM as well. Su determined via the constant pressure method (CPM) were compared with those from the CVM. Discrepancies between the results from the CVM and the CPM are within 15%, except at the conditions where flame cellularity appeared. Additionally, an explicit correlation of Su was obtained from the experimental results.

A new surrogate fuel for emulating the physical and chemical properties of RP-3 kerosene

A new surrogate fuel was developed to emulate both the physical and chemical properties of RP-3 kerosene. Six property targets were chosen for the new surrogate fuel formulation, which are hydrogen/carbon ratio, average molecular weight, cetane number, low calorific value, density and viscosity. The surrogate fuel was developed with only those species readily available in existing kinetic mechanisms, that are n-decane, n-dodecane, iso-cetane, methylcyclohexane and toluene. The mole fractions of the above components are 14%, 10%, 30%, 36%, 10%, respectively, which determined by an optimizer based on the error of the property targets between the surrogate fuel and RP-3 kerosene. Comparisons with RP-3 kerosene show that the newly developed surrogate fuel reproduces the six property targets of RP-3 kerosene. Further experimental investigations were performed in a chemical shock tube and constant volume bomb to obtain the ignition delay and laminar combustion characteristics of the new surrogate fuel and RP-3 kerosene. The results show that the ignition delay times and laminar combustion velocities of the surrogate fuel are in good agreement with that of RP-3 kerosene.

Measurements of laminar flame speeds and flame instability analysis of E30-air premixed flames at elevated temperatures and pressures

Ethanol is regarded as one of the most promising alternative renewable fuels and as well as an oxygenate blending component in gasoline fuels, with widespread usage in many countries around the world. Laminar flame speeds can have strong influence on the stability and operability of Spark-Ignition combustion in certain operating regimes, and so the effects of different initial conditions on laminar combustion characteristics of E30 (gasoline blended with ethanol of 30% liquid volume) were analyzed in a constant-volume combustion vessel using the high-speed Schlieren method. This work presents results for equivalence ratios of 0.7–1.4, dilution ratios of 0%, 5%, 10%, and at different initial temperatures (408, 453 and 498 K) and initial pressures (1, 2 and 3 bar). It can be concluded that the laminar burning velocity has a positive correlation with initial temperature, but negative correlation with initial pressure and dilution ratio. The laminar burning velocity always reaches its maximum value at an equivalence ratio of 1.1 and does not change with varying initial conditions' the adiabatic flame temperature displays a similar variation with the initial conditions. The flame instability of E30-air mixture is enhanced as the initial pressure increases. Flame stability at lean and rich mixtures are exactly opposite at different initial temperature and dilution ratio. The laminar burning velocity was significantly promoted relative to gasoline and E10 by the addition of higher volume fractions of ethanol, highlighting one of the benefits of ethanol’s use as a blending component in gasoline fuels.

The Effect of Initial Conditions on the Laminar Burning Characteristics of Natural Gas Diluted by CO2

The initial conditions such as temperature, pressure and dilution rate can have an effect on the laminar burning velocity of natural gas. It is acknowledged that there is an equivalent effect on the laminar burning velocity between any two initial conditions. The effects of initial temperatures (323 K–423 K), initial pressures (0.1 MPa–0.3 MPa) and dilution rate (0–16%, CO2 as diluent gas) on the laminar burning velocity and the flame instability were investigated at a series of equivalence ratios (0.7–1.2) in a constant volume chamber. A chemical kinetic simulation was also conducted to calculate the laminar burning velocity and essential radicals’ concentrations under the same initial conditions. The results show that the laminar burning velocity of natural gas increases with initial temperature but decreases with initial pressure and dilution rate. The maximum concentrations of H, O and OH increase with initial temperature but decrease with initial pressure and dilution rate. Laminar burning velocity is highly correlated with the sum of the maximum concentration of H and OH.

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 characteristics of premixed methane-dissociated methanol-diluent mixtures

The influence of dissociated methanol (DM) and diluent (CO2 and N2) addition on methane was investigated in a constant volume chamber under initial conditions of 3 bar and 343 K. CO was also added in separate proportions instead of DM under the same conditions to assess its effect. The laminar burning velocity, Markstein length and flame instability were analyzed systematically under various equivalence ratios (0.8–1.4), dissociated methanol gas ratios (40 and 80%), CO ratios (40 and 80%) and dilution ratios (0–15%). Furthermore, the flame speed of the fuel mixture and the production rate of key reactants were analyzed based on the calculation results of the Aramco Mech 2.0 mechanism to determine the influence principles of dilution. The results show that dissociated methanol gas increases the flame speed of the mixtures and promotes instability of the flame, and H2 is the dominant component in enhancing the combustion process. Within the dilution ratio range of this study, the diluents decrease the laminar burning velocity of the mixtures since the addition of diluent gas decreases the concentration of key reactants, such as H and OH. The addition of diluent gas can inhibit the flame instability, but the effect is not clear. Compared with N2, the effect of CO2 is more significant.