Adiabatic burning velocity of H2–O2 mixtures diluted with CO2/N2/Ar

Global warming due to CO₂ emissions has led to the projection of hydrogen as an important fuel for future. A lot of research has been going on to design combustion appliances for hydrogen as fuel. This has necessitated fundamental research on combustion characteristics of hydrogen fuel. In this work, a combination of experiments and computational simulations was employed to study the effects of diluents (CO₂, N₂, and Ar) on the laminar burning velocity of premixed hydrogen/oxygen flames using the heat flux method. The experiments were conducted to measure laminar burning velocity for a range of equivalence ratios at atmospheric pressure and temperature (300 K) with reactant mixtures containing varying concentrations of CO₂, N₂, and Ar as diluents. Measured burning velocities were compared with computed results obtained from one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics and good agreement was obtained. The effectiveness of diluents in reduction of laminar burning velocity for a given diluent concentration is in the increasing order of argon, nitrogen, carbon dioxide. This may be due to increased capabilities either to quench the reaction zone by increased specific heat or due to reduced transport rates. The lean and stoichiometric H₂/O₂/CO₂ flames with 65% CO₂ dilution exhibited cellular flame structures. Detailed three-dimensional simulation was performed to understand lean H₂/O₂/CO₂ cellular flame structure and cell count from computed flame matched well with the experimental cellular flame.     

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

Experimental investigation of stabilization and emission characteristics of ammonia/air premixed flames in a swirl combustor

Ammonia is a possible candidate for use as a hydrogen energy carrier as well as a carbon-free fuel. In this study, flame stability and emission characteristics of swirl stabilized ammonia/air premixed flames were experimentally investigated. Results showed that ammonia/air premixed flame could be stabilized for various equivalence ratios and inlet flow velocity conditions in a swirl burner without any additives to enhance the reaction of ammonia even though the laminar burning velocity of ammonia is very slow. The lean and rich blowoff limits were found to be close to the flammability limits of the ammonia flame. In addition, emission characteristics were investigated using an FTIR gas analyzer. The NO concentration decreased and ammonia concentration increased under rich conditions. Moreover, it was found that there is an equivalence ratio in rich condition in which NO and ammonia emission are in the same order.     

Experimental and numerical study on lean premixed methane–hydrogen–air flames at elevated pressures and temperatures

Experimental and numerical study on the lean methane–hydrogen–air flames at elevated pressures and temperatures was conducted. The unstretched laminar burning velocities and Markstein lengths were obtained over a wide range of hydrogen fractions at elevated pressures and temperatures. The sensitivity analysis and flame structure were also analyzed. The results show good agreement between the computed results and experimental data. The unstretched laminar burning velocities are increased with the increase of initial temperature and hydrogen fraction, and they are decreased with the increase of initial pressure. With the increase of initial pressure and hydrogen fraction, Markstein lengths are decreased, indicating the increase of flame instability. Laminar burning velocity is depended on the competition between the main chain branching reaction and chain recombination reaction. The chain branching reaction is a temperature-sensitive reaction, while the recombination reaction is a temperature-insensitive reaction. Numerical study 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.     

Burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methane–air flames

The burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methane–air flames were experimentally studied by laser tomography visualization method using a V-shaped flame configuration. Turbulent burning velocity was measured and the variation of flame surface characteristics due to hydrogen addition was analyzed. The results show that hydrogen addition causes an increase in turbulent burning velocity for lean premixed CH₄–air mixtures when turbulent level in unburned mixture is not changed. Moreover, the increase of turbulent burning velocity is faster than that of the corresponding laminar burning velocity at constant equivalence ratio, suggesting that the kinetics effect is not the sole factor that results in the increase in turbulent burning velocity when hydrogen is added. The further analysis of flame surface characteristics and brush thickness indicates that hydrogen addition slightly decreases local flame surface density, but increases total flame surface area because of the increased flame brush thickness. The increase in flame brush thickness that results in the increase in total surface area may contribute to the faster increase in turbulent burning velocity, when hydrogen is added. Besides, the stretched local laminar burning velocity may be enhanced with the addition of hydrogen, which may also contribute to the faster increase rate of turbulent burning velocity. Both the variation in flame brush thickness and the enhancement in stretched local laminar burning velocity are due to the decreased fuel Lewis number when hydrogen is added. Therefore, the effects of fuel Lewis number and stretch should be taken into account in correlating burning velocity of turbulent premixed flames.     

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.     

Hydrogen addition effect on NO formation in methane/air lean-premixed flames at elevated pressure

The present study investigates freely propagating methane/hydrogen lean-premixed laminar flames at elevated pressures to understand the hydrogen addition effect of natural gas on the NO formation under the conditions of industrial gas turbine combustors. The detailed chemical kinetic model which was used in the previous study on the NO formation in high pressure methane/air premixed flames was adopted for the present study to analyze NO formation of methane/hydrogen premixed flames. The present mechanism shows good agreement with experimental data for methane/hydrogen mixtures, including ignition delay times, laminar burning velocities, and NO concentration in premixed flames. Hydrogen addition to methane/air mixtures with maintaining methane content leads to the increase of NO concentration in laminar premixed flames due to the higher flame temperature. Methane/hydrogen/argon/air premixed flames are simulated to avoid the flame temperature effect on NO formation over a pressure range of 1–20atm and equivalence ratio of 0.55. Kinetic analyses shows that the N₂O mechanism is important on NO formation for lean flames between the reaction zone and postflame region, and thermal NO is dominant in the postflame zone. The hydrogen addition leads to the increase of NO formation from prompt NO and NNH mechanisms, while NO formation from thermal and N₂O mechanisms are decreased. Additionally, the NO formation in the postflame zone has positive pressure dependencies for thermal NO with an exponent of 0.5. Sensitivity analysis results identify that the initiation reaction step for the thermal NO and the N₂O mechanism related reactions are sensitive to NO formation near the reaction zone.     

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.     

On the similitude between lifted and burner-stabilized triple flames: a numerical and experimental investigation

We have investigated lifted triple flames and addressed issues related to flame stabilization. The stabilization of nonpremixed flames has been argued to result due to the existence of a premixing zone of sufficient reactivity, which causes propagating premixed reaction zones to anchor a nonpremixed zone. We first validate our simulations with detailed measurements in more tractable methane–air burner-stabilized flames. Thereafter, we simulate lifted flames without significantly modifying the boundary conditions used for investigating the burner-stabilized flames. The similarities and differences between the structures of lifted and burner-stabilized flames are elucidated, and the role of the laminar flame speed in the stabilization of lifted triple flames is characterized. The reaction zone topography in the flame is as follows. The flame consists of an outer lean premixed reaction zone, an inner rich premixed reaction zone, and a nonpremixed reaction zone where partially oxidized fuel and oxidizer (from the rich and lean premixed reaction zones, respectively) mix in stoichiometric proportion and thereafter burn. The region with the highest temperatures lies between the inner premixed and the central nonpremixed reaction zone. The heat released in the reaction zones is transported both upstream (by diffusion) and downstream to other portions of the flame. Measured and simulated species concentration profiles of reactant (O₂, CH₄) consumption, intermediate (CO, H₂) formation followed by intermediate consumption and product (CO₂, H₂O) formation are presented. A lifted flame is simulated by conceptualizing a splitter wall of infinitesimal thickness. The flame liftoff increases the height of the inner premixed reaction zone due to the modification of the upstream flow field. However, both the lifted and burner-stabilized flames exhibit remarkable similarity with respect to the shapes and separation distances regarding the three reaction zones. The heat-release distribution and the scalar profiles are also virtually identical for the lifted and burner-stabilized flames in mixture fraction space and attest to the similitude between the burner-stabilized and lifted flames. In the lifted flame, the velocity field diverges upstream of the flame, causing the velocity to reach a minimum value at the triple point. The streamwise velocity at the triple point is ≈0.45 m s⁻¹ (in accord with the propagation speed for stoichiometric methane–air flame), whereas the velocity upstream of the triple point equals 0.7 m s⁻¹, which is in excess of the unstretched flame propagation speed. This is in agreement with measurements reported by other investigators. In future work we will address the behavior of this velocity as the equivalence ratio, the inlet velocity profile, and inlet mixture fraction are changed.     

Hydrogen enrichment for improved lean flame stability

The stability characteristics of a premixed, swirl-stabilized flame were studied to determine the effects of hydrogen addition on flame stability under fuel-lean conditions. The burner configuration consisted of a centerbody with an annular, premixed methane/air jet introduced through five, 45° swirl vanes. Flame stability was studied over a range of operating conditions. Under fuel-rich conditions the flame was lifted from the burner surface due to the mixing with entrained ambient air that was needed to form a flammable mixture. As the fuel/air mixture ratio was decreased toward stoichiometric, the resulting increase in flame speed allowed the flame to propagate upstream through the low-velocity wake region and attach to the centerbody face. The maximum blowout velocity occurred at stoichiometric conditions, and decreased as the mixture became leaner. OH PLIF measurements were used to study the behavior of OH mole fraction as the lean stability limit was approached. Near the lean stability limit the overall OH mole fraction decreased, the flame decreased in size and the high OH region took on a more shredded appearance. The addition of up to 20% hydrogen to the methane/air mixture resulted in a significant increase in the OH concentration and extended the lean stability limits of the burner.     

Laser optical investigation of turbulent transport of temperature ahead of the preheat zone in a premixed flame

This experimental study investigates temperature profiles upstream of localized thin reaction zones in a turbulent premixed flame stabilized on a low-swirl-burner. A simultaneous dual-sheet Rayleigh/OH-LIPF measurement technique was applied using two parallel laser light sheets to measure the temperature in the preheat zone for three lean (ϕ=0.7) turbulent methane/air flames. The ratio of the turbulence intensity to the laminar burning velocity v′/s_L was varied from 3.5 to 9.2 and to 18.7. The first of these flames lies on the borderline between the “corrugated flamelets” regime and the “thin reaction zones” regime, while the other two are in the “thin reaction zones” regime. The results confirm the occurrence of strong temperature fluctuations ahead of the preheat zone for flames in the “thin reaction zones” regime. Single temperature images show a significant temperature increase ahead of the preheat zones of up to 700 K for flames at the highest turbulence intensity. For statistical analysis conditional mean temperature profiles and probability density functions conditioned on the distance from the flame contour were calculated from the experimental data. Only those portions of the flame front were included which were found to be approximately normal to the two laser sheets. The resulting probability density functions show that the effect of temperature rise ahead of the preheat zone becomes significant only for flames in the “thin reaction zones” regime. The mean temperature profiles show a much smaller temperature rise which, however, increases with increasing velocity ratios v′/s_L.     

Experimental investigation on the self-acceleration of 10%H2/90%CO/air turbulent expanding premixed flame

The self-acceleration characteristics of a syngas/air mixture turbulent premixed flame were experimentally evaluated using a 10% H₂/90% CO/air mixture turbulent premixed flame by varying the turbulence intensity and equivalence ratio at atmospheric pressure and temperature. The propagation characteristics of the turbulent premixed flame including the variation in the flame propagation speed and turbulent burning velocity of the syngas/air mixture turbulent premixed flame were evaluated. In addition, the effect of the self-acceleration characteristics of the turbulent premixed flame was also evaluated. The results show that turbulence gradually changes the radius of the premixed flame from linear growth to nonlinear growth. With the increase of turbulence intensity, the formation of a cellular structure of the flame front accelerated, increasing the flame propagation speed and burning speed. In the transition stage, the acceleration exponent and fractal excess of the turbulent premixed flame decreased with increasing equivalence ratio and increased with increasing turbulence intensity at an equivalence ratio of 0.6. The acceleration exponent was always greater than 1.5.     

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 investigation of three-dimensional flame-front structure in premixed turbulent combustion—I: hydrocarbon/air bunsen flames

OH concentrations and three-dimensional gradients of the reaction progress variable have been measured in turbulent liquefied petroleum gas/air and compressed natural gas/air premixed flames stabilized on a Bunsen-type burner with a combined two-sheet Rayleigh scattering and planar LIF-OH imaging technique. The progress variable is observed to undergo a transition from lamella-like to non-flamelet front structure with increasing turbulence. This is consistent with the recently proposed change of the combustion regimes from complex-strain to turbulent flame front regime on a recently proposed premixed combustion diagram. The anisotropy of local flame-front orientation in three-dimensional space is explained by the forward propagation ability of the planar turbulent flame brush. Weighting functions have thus been derived for the isotropic pdf distributions of the in-plane and out-of-plane orientation angles to agree better with the experimental data. A linear scaling is found between the overall flame surface area and the turbulence intensity normalized by the laminar burning velocity. However, flames with excess backward-facing flame fronts do not comply with this linear relationship, showing enhanced flame surface folding. The thin-flame assumption breaks down when non-flamelet broadening effects become important, although the pdf’s of the progress variable are still bimodal-like. Non-unity Lewis-number combined curvature effects are evident for LPG/air flames of weak turbulence, in that the conditional mean scalar dissipation increases steadily from the unburnt to burnt side across the flame brush. A consistent correlation exists between the Favre-averaged scalar dissipation and progress variable variance. This implies that small-scale scalar dissipation of local flame-fronts is linked to large-scale scalar fluctuations. Sub- or super-flamelet OH concentration is found in lean LPG/air or CNG/air premixed flames, respectively, and occurs in line with a positive or negative correlation between OH concentrations and magnitudes of the progress variable gradient.     

Hydrogen addition effects in a confined swirl-stabilized methane-air flame

The effect of hydrogen addition in methane-air premixed flames has been examined from a swirl-stabilized combustor under confined conditions. The effect of hydrogen addition in methane-air flame has been examined over a range of conditions using a laboratory-scale premixed combustor operated at 5.81 kW. Different swirlers have been investigated to identify the role of swirl strength to the incoming mixture. The flame stability was examined for the effect of amount of hydrogen addition, combustion air flow rates and swirl strengths. This was carried out by comparing adiabatic flame temperatures at the lean flame limit. The combustion characteristics of hydrogen-enriched methane flames at constant heat load but different swirl strengths have been examined using particle image velocimetry (PIV), micro-thermocouples and OH chemiluminescence diagnostics that provided information on velocity, thermal field, and combustion generated OH species concentration in the flame, respectively. Gas analyzer was used to obtain NO_x and CO concentration at the combustor exit. The results show that the lean stability limit is extended by hydrogen addition. The stability limit can reduce at higher swirl intensity to the fuel-air mixture operating at lower adiabatic flame temperatures. The addition of hydrogen increases the NO_x emission; however, this effect can be reduced by increasing either the excess air or swirl intensity. The emissions of NO_x and CO from the premixed flame were also compared with a diffusion flame type combustor. The NO_x emissions of hydrogen-enriched methane premixed flame were found to be lower than the corresponding diffusion flame under same operating conditions for the fuel-lean case.     

Ultra-lean combustion through the backflow of burned gas in rotating counterflow twin premixed flames

Rotating counterflow twin premixed flames of methane–air were numerically simulated with detailed chemistry based on a similarity solution to explore the leanest extinction limit without preheating and to elucidate the mechanism of “ultra-lean” combustion. We focused on high rotation rate cases in which unrealistic backflow from infinity is allowed to occur since ultra-lean combustion was found to be realized in such a situation. It was found that the reaction zone is in the backflow zone, where the flame’s apparent burning velocity is negative, and that the flame zone width is much smaller than that of a 1-D planar premixed flame due to an inversion of the convexity directions of the profiles of temperature and main species concentrations. The decrease of the flame width seems to make the flame less extinguishable. The equivalence ratio of the leanest flame obtained neglecting radiative heat loss is 0.32, while that obtained with an optically thin radiation model is 0.42, which is still much leaner than the ratio of 0.49 for a 1-D planar premixed flame generated using the same radiation model.     

A mechanistic study of Soret diffusion in hydrogen–air flames

The separate and combined effects of Soret diffusion of the hydrogen molecule (H₂) and radical (H) on the structure and propagation speed of the freely-propagating planar premixed flames, and the strain-induced extinction response of premixed and nonpremixed counterflow flames, were computationally studied for hydrogen–air mixtures using a detailed reaction mechanism and transport properties. Results show that, except for the conservative freely-propagating planar flame, Soret diffusion of H₂ increases the fuel concentration entering the flame structure and as such modifies the mixture stoichiometry and flame temperature, which could lead to substantial increase (decrease) of the flame speed for the lean (rich) mixtures respectively. On the other hand, Soret diffusion of H actively modifies its concentration and distribution in the reaction zone, which in turn affects the individual reaction rates. In particular, the reaction rates of the symmetric, twin, counterflow premixed flames, especially at near-extinction states, can be increased for lean flames but decreased for rich flames, whose active reaction regions are respectively located at, and away from, the stagnation surface. However, such a difference is eliminated for the single counterflow flame stabilized by an opposing cold nitrogen stream, as the active reaction zone up to the state of extinction is always located away from the stagnation surface. Finally, the reaction rate is increased in general for diffusion flames because the bell-shaped temperature distribution localizes the H concentration to the reaction region which has the maximum temperature.     

Experimental and simulation study of premixed syngas-air deflagration dynamics with elevated temperature and CO2 addition

Experimental and dynamic analyses of the deflagration characteristics of laminar premixed syngas-air at different preheating temperatures and with different CO₂ volume fractions were carried out in a rectangular half-open pipe. The effects of CO₂ concentration and different initial temperatures on the flame structure evolution, flame structure profile and reaction rate of critical radicals, flame propagation speed, overpressure dynamics and hydrodynamic instability of syngas-air mixture were studied. The FFCM-1 mechanism was used to predict the laminar burning velocity of syngas-air under relevant conditions. The results revealed that the addition of CO₂ inhibited the flame propagation and reduced the concentration of H, OH and O, thus reduced the laminar burning velocity. The increase in temperature promotes the chemical effect of CO₂, and the interaction between the flame front and the pressure wave is more pronounced, prolonging the duration of the " tulip " flame. Adding CO₂ reduces the flame front speed and overpressure, decreases the oscillation amplitude in late flame propagation, and inhibits the explosion intensity. Meanwhile, the temperature increase accelerates the flame propagation in the spherical and finger stages, and the maximum flame propagation speed and peak pressure appear earlier. In addition, as CO₂ content and temperature rise, flame hydrodynamic instability is difficult to ignore. However, there is a lack of data from studies of syngas deflagration dynamics at higher temperatures and with higher CO₂ additions. This suggests a focus on studies at higher temperatures as well as with higher CO₂ additions to enable the development of accurate kinetic models for wide range of syngas combustion. Also, the higher the initial temperature, the longer the time required for heating.     

Effects of CO addition on the characteristics of laminar premixed CH4/air opposed-jet flames

The effects of CO addition on the characteristics of premixed CH₄/air opposed-jet flames are investigated experimentally and numerically. Experimental measurements and numerical simulations of the flame front position, temperature, and velocity are performed in stoichiometric CH₄/CO/air opposed-jet flames with various CO contents in the fuel. Thermocouple is used for the determination of flame temperature, velocity measurement is made using particle image velocimetry (PIV), and the flame front position is measured by direct photograph as well as with laser-induced predissociative fluorescence (LIPF) of OH imaging techniques. The laminar burning velocity is calculated using the PREMIX code of Chemkin collection 3.5. The flame structures of the premixed stoichiometric CH₄/CO/air opposed-jet flames are simulated using the OPPDIF package with GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. The measured flame front position, temperature, and velocity of the stoichiometric CH₄/CO/air flames are closely predicted by the numerical calculations. Detailed analysis of the calculated chemical kinetic structures reveals that as the CO content in the fuel is increased from 0% to 80%, CO oxidation (R99) increases significantly and contributes to a significant level of heat-release rate. It is also shown that the laminar burning velocity reaches a maximum value (57.5 cm/s) at the condition of 80% of CO in the fuel. Based on the results of sensitivity analysis, the chemistry of CO consumption shifts to the dry oxidation kinetics when CO content is further increased over 80%. Comparison between the results of computed laminar burning velocity, flame temperature, CO consumption rate, and sensitivity analysis reveals that the effect of CO addition on the laminar burning velocity of the stoichiometric CH₄/CO/air flames is due mostly to the transition of the dominant chemical kinetic steps.     

A computational study and correlation of premixed isooctane air laminar reaction fronts diluted with EGR

The current work investigates the propagation of premixed laminar reaction fronts for mixtures of isooctane–air and recirculated combustion products (or EGR) under high pressure and temperature conditions. The work uses a transient one-dimensional flame simulation with a skeletal 215 species chemical kinetic mechanism to generate laminar burning velocity and front thickness predictions. The simulation was exercised over fuel–air equivalence ratios, unburned gas temperatures, pressures and EGR levels ranging from 0.1 to 1.0, 400 to 1000 K, 1 to 250 bar, and 0% to 60% (by mass) respectively, a range extending beyond that of previous researchers. Steady reaction fronts with burning velocities in excess of 5 cm/s could not be established under all of these conditions, especially when burned gas temperatures were below 1450 K and/or when characteristic reaction front propagation times were on the order of the unburned gas ignition delay. For a given pressure, T_u and T_b, the burning velocity of an EGR dilute mixture was found to be lower than that of an air dilute mixture, with the decrease in burning velocity attributed primarily to the reduced oxygen concentration’s effect on chemistry. Steady premixed laminar burning velocities were correlated using a modified two-equation form based on the asymptotic structure of a laminar flame, which produced an average error of 3.4% between the simulated and correlated laminar burning velocities, with a standard deviation of 4.3%, while additional correlations were constructed for reaction front thickness and adiabatic flame temperature. Correlations are presented based on a non-product equivalence ratio φ and a fraction of stoichiometric combustion products X_SCP. Conversion factors are provided to facilitate application to modern direct injection internal combustion engines with inherent charge stratification where the local global Φ is different from the global Φ of the residual gas.