Mixture Equivalence Ratio Research Articles

Experimental study of the influence of the swirl number on lean premixed combustion regimes

Lean premixed turbulent swirling combustion is known for significantly reducing pollutants emissions, when compared to non-premixed turbulent flames. Characterizing the swirl number influence is therefore essential to comparing different combustion chamber operational regimes. The present study is devoted to the experimental characterization of a new family of swirlers currently being developed. The associated combustion process is found to exhibit different flame topologies, or combustion regimes, which are functions of the mixture equivalence ratio and flow rate. The flame topologies and the boundaries between the observed combustion regimes are evidenced by means of OH$$^*$$ chemiluminescence and overall flame chemiluminescence.

Dynamics of 2D, V-shaped Bluff-body Stabilized Turbulent Premixed Flames near and Away from Blowoff with Different Gaseous Fuels

ABSTRACT V-shaped premixed flames of propane, methane, and ethylene-air mixtures were stabilized on a triangular-shaped flame holder under three different free stream turbulence levels; 4%, 18%, and 34% rms at a nominal burner exit velocity of 10 m/s. Flames were studied at strongly burning and near blowoff conditions to observe the changes in the overall flame dynamics. Flame behavior at different upstream flow turbulence conditions was visualized by simultaneously capturing the end and side views of flame chemiluminescence using high speed (4 kHz) imaging. A gradual shift from symmetric to asymmetric flame structures was observed with reduction of equivalence ratio approaching blowoff. Particle image velocity measurements confirmed the gradual switch of vortex shedding from symmetric varicose mode to the asymmetric sinuous mode as flame blowoff is approached by reduction of the mixture equivalence ratio. It is found that average flame front position crosses the shear layer from its outer location at strongly burning conditions to within the shear layers bounding the recirculation zone as flame blowoff is approached. Transition of flame behavior from “varicose” to “sinuous” oscillations is documented by way of a symmetry index as a function of equivalence ratio and turbulence intensity for the three fuels studied.

Experimental Study of Continuous H2/Air Rotating Detonations

ABSTRACT An experimental study of a lab scale rotating detonation combustor (RDC) has been conducted to identify and classify its modes of operation under different feeding conditions. The chamber uses air as oxidizer and hydrogen as fuel. The stability diagram of the RDC has been determined based on the detonation chamber Reynolds number and reactants mixture equivalence ratio values. The Reynolds number is based on the air volume flow rate, chamber annulus size, and the reactants mixture feeding slot area. Hence the effects of geometrical parameters and operational parameters on the detonation wave stability can be presented in a single stability map. This diagram identifies different zones in terms of the stability, propagation characteristics, and the number of detonation waves. Absence of a stable detonation wave at low Reynolds numbers and observation of different detonation modes with increasing Reynolds number, in different ranges of equivalence ratios, have been established in this research. This paper describes the various modes of rotating detonation wave propagation in the RDC. These modes include the single-wave detonation mode, the dual-waves unidirectional detonation mode, the dual-waves colliding detonation mode, extinguishing and re-initiating detonation mode, and the deflagration mode

Prechamber ignition: An exploratory 2-D DNS study of the effects of initial temperature and main chamber composition

A numerical investigation was conducted to study ignition and the initial phases of methane combustion in a two-dimensional setup consisting of a pre- (PC) and main (MC) chamber. The effect of the wall thermal boundary condition (isothermal Tw=500 K or adiabatic), initial mixture temperature (Tu=300 and 800 K) and equivalence ratio in the main chamber (ϕMC=0.5 and 1.0) was studied. A stoichiometric mixture was used in the PC and the mixtures were initially quiescent in all cases. Flame propagation in the prechamber is affected mainly by the initial temperature, while the thermal state of the wall plays a minor role. The transient jet that is generated at the exit of the orifice connecting the two chambers creates an intense flow field with vortical structures that, depending on the initial temperature, persist for a long time or dissipate quickly affecting combustion in the main chamber. Depending on the local flow and mixing conditions close to the orifice exit, the hot jet can be broken into small kernels at low Tu or forms quickly a flame torch at hot conditions, strongly affecting ignition of the MC mixture and flame propagation in the main chamber. In the lean MC cases, the intense mixing with the stoichiometric PC mixture creates local compositions that are more favorable for ignition by the hot turbulent reactive jet that subsequently exits from the PC at a temperature that is significantly lower than the adiabatic flame temperature of the corresponding mixture. Despite the short residence time, the reactive state of the mixture is affected as it flows through the nozzle. The flame structures in the MC are described in terms of the progress variable and mixture fraction and compared to flamelet-type calculations. The local flame structure differs strongly from that of the 1-D unstrained premixed flame, particularly for the low Tu cases. The flamelet-type calculations show that ignition of the most reactive mixture is enhanced by the radicals in the hot reactive jet, while scalar dissipation rate accelerates the ignition of the whole mixture. The 2-D simulations show that ignition is significantly longer than what is predicted by the flamelet calculations.

Experimental Investigations on Laminar Burning Velocities of n-Heptane + Air Mixtures at Higher Mixture Temperatures Using Externally Heated Diverging Channel Method

Experimental measurements of laminar burning velocity are presented for mixture temperatures above the autoignition temperature of premixed n-heptane + air mixtures using an externally heated mesoscale diverging channel method. Direct measurements of laminar burning velocity are carried out at 1 atm of pressure for an unburnt mixture temperature range (350–600 K) with the mixture equivalence ratio varying from ϕ = 0.6 to 1.5. Nonlinear power-law correlation is proposed to delineate the effect of change in the mixture temperature on the burning velocity variation at various equivalence ratios. A minimum value for the temperature exponent is observed for stoichiometric n-heptane + air mixtures. The maximum value of laminar burning velocity is measured for ϕ ≈ 1.1 at all mixture temperatures. The reported data sets are compared with the available experimental results and the predictions of various detailed kinetic models, i.e., LLNL V3.1 (2011), JetSurF 2.0 (2010), and Poli Mi (2014). Good agreement of the present data is observed with the predictions of the LLNL V3.1 reaction mechanism at all unburnt mixture temperatures. The predictions of other kinetic models show slight under-prediction at higher mixture temperatures. Sensitivity analysis using the LLNL V3.1 mechanism is reported to highlight the contribution of key reactions enhancing/reducing the laminar burning velocity at 470 and 600 K mixture temperatures. With an increase in the mixture temperature from 470 to 600 K, the influence of the chain branching reaction (H + O₂ ↔ O + OH) on laminar burning velocity increases nearly 63.5%. Formation of a vinyloxy radical (CH₂CHO) from the oxidation of a vinyl radical (C₂H₃) enhances the burning velocity at a 470 K temperature. Oxidation of C₂H₃ (at 600 K) becomes crucial in governing engine knocking in low-temperature oxidation of n-heptane. It is deduced from the reaction pathway diagram that the production of a highly reactive compound, C₂H₂, at 600 K plays a significant role and governs the overall reaction rate of the mixture.

Experimental Investigation of Flame Characteristics of H2/CO/CH4/CO2 Synthetic Gas Mixtures

ABSTRACT Chemical composition of synthetic gas and concentration of its components depend on gasification process and feedstock. Synthetic gas mainly consists of hydrogen (H2) and carbon monoxide (CO), and may contain trace amounts of carbon dioxide (CO2) and methane (CH4). In this study, combustion and emission characteristics of H2/CO/CH4/CO2 blending syngas mixtures with high H2/CO ratio were experimentally investigated in a swirl-stabilized premixed combustor. Furthermore, stable operating ranges (flashback and blowout equivalence ratios) of such mixtures and flame response of 67.5%H2-22.5%CO-5%CO2-5%CH4 mixture to acoustic forcing were also detected. During the experiments, CH4 amount in tested gas mixtures varied between 5% and 20% by volume (at intervals of 5%), and H2/CO ratio and CO2 concentration were kept constant at the values of 3 and 5%, respectively. As a consequence, mixtures of 67.5%H2-22.5%CO-5%CO2-5%CH4, 63.75%H2-21.25%CO-5%CO2-10%CH4, 60%H2-20%CO-5%CO2-15%CH4 and 56.25%H2-18.75%CO-05%CO2-20%CH4 were derived and tested under the same boundary and physical conditions. All experiments were conducted at 0.4 equivalence ratio (Φ) and 0.2 geometric swirl number (SN). Preliminarily, stable operating ranges of respective gas mixtures were determined to find an equivalence ratio at which all gas mixtures could be tested without causing any flame instability and then, CH4 addition effects on combustion and emission characteristics of such mixtures were evaluated via utilizing axial and radial temperature, CO and O2 profiles. Lastly, flame behavior of 67.5%H2-22.5%CO-5%CO2-5%CH4 mixture was evaluated under externally modified acoustic conditions. Acoustic field in the combustor was mechanically altered by using side-mounted loudspeakers in the frequency range of 30–200 Hz. To identify flame instabilities and to evaluate the degree of coupling between heat release and pressure oscillations, pressure sensors and photodiodes installed on combustion chamber and burner were utilized. Results of this study showed that CH4 addition to synthetic gas mixtures improves rich flammability limits, and increases flame temperature, flame height and emissions. Under externally modified acoustic conditions, flame behavior slightly alters (flashback and blowout does not occur). Additionally, pollutant emissions improve in an environment-friendly manner under such conditions.

Towards the development of a high power density, high efficiency, micro power generator

An innovative method to use combustion-based source for portable power generation for Microelectromechanical systems with significant improvement in power density and conversion efficiency has been reported in this study. A triple microcombustor configuration with backward facing steps and associated provision for efficient heat recirculation is proposed for flame stabilization. The presence of three parallel combustion zones increases the combustion intensity compared to a single combustor configuration and enhances the overall thermal performance. A novel technique to extract maximum heat from the hot combustion products for thermoelectric power generation is implemented. The thermoelectric modules are mounted to facilitate direct contact of the module surface with the hot combustion products. A significant improvement in heat transfer rate from the hot combustion gases to the thermoelectric modules (~16 times) is achieved. An electric power output of 4.9 W with overall conversion efficieny of 5.09% at a mixture velocity of 10 m/s and a mixture equivalence ratio of 0.9 is achieved. This signifies an improvement of ~10–20% in overall conversion efficiency as compared to previously reported systems.

Analysis of Equivalence Ratio in an Oxygen Acetylene Mixture

The paper presents an exclusive approach to analyze the oxygen and acetylene mixture in a Pulse Detonation Engine setup at PEC. The experiment was carried out by varying the amount of mass flow of acetylene and oxygen into the tube at different pressures in the settling chamber. The equivalence ratio of the acetylene-oxygen mixture with respect to the valve opening time of acetylene and oxygen was plotted. The analysis of equivalence ratio also defines the procedure which involved the cycle of valves and pressure supply to attain different values of filling fraction of the tube. The results experimentally predicted the feasible range of operation at different pressures in the settling chamber of the pulse detonation engine.

Effects of non-thermal plasma on the lean blowout limits and CO/NOx emissions in swirl-stabilized turbulent lean-premixed flames of methane/air

This study investigates experimentally the effects of non-thermal plasma (NTP) induced by a dielectric barrier discharge (DBD) reactor on the characteristics of swirl-stabilized turbulent lean-premixed methane/air flames in a laboratory scale combustor by systematically varying the applied AC voltage, VAC, and frequency, fAC. Especially, it is elucidated how the NTP influences the lean blowout (LBO) limits and the characteristics of CO/NOx emissions depending on flame configuration. Without applying the NTP as the mixture equivalence ratio, ϕ, decreases from the stoichiometry to an LBO limit, the flame configuration changes from an M-flame (Regime I) to a conical flame (Regime II) and to a columnar flame (Regime III) for the whole range of the mixture nozzle exit velocity, U0, (4–10 m/s). With the NTP, however, it exhibits only Regimes I and II at relatively-low U0 range (4–6 m/s), while all three regimes at relatively-high U0 range (7–10 m/s). For both velocity ranges, the LBO limits are significantly extended by the NTP enhancing the flame stability. Under the relatively-low U0 range, streamers induced by the DBD reactor play a critical role in stabilizing the flames such that the degree of extension of the LBO limit depends linearly on VAC and fAC. Under the relatively-high U0 range, however, ozone generated by the DBD reactor in Regime III is found to be a major reason in extending the LBO limit, which is substantiated by another flame regime diagram with ozone addition only, and hence, the extension of LBO limit minimally depends on fAC. Simultaneously, the NTP considerably reduces CO emission, while slightly increases NOx emission near the LBO limits due to the enhanced combustion by ozone.

Laminar Flame Speed Experiments of Alternative Liquid Fuels

Abstract New laminar flame speed experiments have been collected for two alternative liquid fuels. Understanding the combustion characteristics of these synthetic fuels is an important step in developing new chemical kinetics mechanisms that can be applied to real fuels. Included in this study are two synthetic Jet fuels: Syntroleum S-8 and Shell GTL. The precise composition of these fuels is known to change from sample to sample. Since these are low-vapor pressure fuels, there are additional uncertainties in their introduction into gas-phase mixtures, leading to uncertainty in the mixture equivalence ratio. An in-situ laser absorption technique was implemented to verify the procedure for filling the vessel and to minimize and quantify the uncertainty in the experimental equivalence ratio. The diagnostic utilized a 3.39-μm HeNe laser in conjunction with Beer's law. The resulting spherically expanding, laminar flame experiments were conducted over a range of equivalence ratios from φ = 0.7 to φ = 1.5 at initial conditions of 1 atm and 403 K in the high-temperature, high-pressure (HTHP) constant-volume vessel at Texas A&M University. The experimental results show that both fuels have similar flame speeds with a peak value just under 60 cm/s. However, it is shown that when comparing the results from different datasets for these real fuels, equivalence ratio may not be the best parameter to use. Fuel mole fraction may be a better parameter to use as it is independent of the average fuel molecule or fuel surrogate used to calculate equivalence ratio in these real fuel/air mixtures.

Coefficient of Rayleigh Index based performance evaluation of radial micro jet injection technique for thermo-acoustic instability control

This study proposes a coefficient of Rayleigh Index (CRI) obtained by normalizing the acquired data which can circumvent the problem of arbitrary scale and unit while quantifying the Rayleigh Index. CRI is estimated from the simultaneous measurements of luminescence of a flame as an indicator of unsteady heat release and pressure fluctuation near the burner head. Experiments were carried out for six different mixture equivalence ratios ranging from 0.75 to 1.0 and a burner positioned at x/L = 0.2. The control technique of micro jets is found to completely suppress the thermo-acoustics by decoupling the pressure waves and unsteady heat release waves as indicated by increasing phase difference between them. The coefficient of Rayleigh Index is found to reduce to nearly zero with the use of radial micro jets for suppressing the thermo-acoustic instability. In such case, the flame luminescence was observed to significantly reduce. The proposed coefficient of Rayleigh index is very useful in quantifying the effectiveness of control technique for the thermo-acoustic instability.

Numerical Studies of the Pilot Flame Effect on a Piloted Jet Flame

ABSTRACT In a piloted jet flame, the pilot flame has an effect of stabilizing the main flame. Detailed mechanisms of pilot flame/main flame interaction are however not well studied. It is expected that the pilot flame affects the main flame through the following mechanisms: (a) the pilot flame provides the heat and radicals to the reaction zone of the main flame, (b) the pilot flame prevents the cold ambient air from being entrained into the main flame, and (c) the pilot flame modifies the stretch rate of the main flame. In this paper, detailed numerical simulations of piloted laminar methane/air jet flames are carried out to elucidate the effect of pilot flame on the structure and burning velocity of the main jet flame. One-dimensional (1D) freely propagating flame is also simulated to investigate the effect of hot gas mixing with the unburned fuel/air mixture, and 1D counter-flow flame is simulated to study the diffusion of the hot gas from the pilot flame to the reaction zone of the main flame and the effect of flame stretch. The results showed that heat transfer from the pilot flame to the main flame has a more significant effect on the structures and propagation of the main flame than the mass transfer from the pilot flame to the main flame. The heat and mass transfer from the pilot flame affects the local equivalence ratio and temperature of the unburned mixture, which gives rise to a significant enhancement of burning velocity. When the hot gas from the pilot flame is at sufficiently high temperatures, an ultra-lean fuel/air mixture can burn at equivalence ratio below the flammability limit. The reaction rate and burning velocity of ultra fuel-lean flames are enhanced by the strain rate, whereas for a main flame with the equivalence ratio closer to that of pilot flame, the reactivity and burning velocity of the main flame decrease with increasing strain rate.

Numerical analysis of flow, mixture formation and combustion in a direct injection natural gas engine

Numerical simulation of flow, mixture formation and combustion in a direct injection natural gas engine was conducted using the large-eddy simulation method. The dynamic thickened flame model coupling with a skeletal methane reaction mechanism was implemented and applied for the combustion simulation. In Situ Adaptive Tabulation (ISAT) method was used for the efficient chemistry solving. It was found that in-cylinder large-scale vortices are principally formed as the fuel jet impinges on the cylinder wall, and they enter the combustion chamber due to the effect of the squish flow at the end of the compression stoke. Most of in-cylinder combustible mixtures begin to be produced after the jet-wall impingement. Under the stoichiometric condition, with the delay of the fuel injection timing, the distribution of in-cylinder equivalence ratio is wider at the ignition timing, and the inhomogeneity of mixtures increases. The fuel injection timing determines the in-cylinder dominant combustion mode and the flame displacement speed at the initial stage of flame propagation by affecting the distribution of mixture equivalence ratio at the ignition timing, thereby affecting the increase of the flame area. Mass fractions of CO, H2, H-atom and N2O in the rich-burn region are higher than those in the lean-burn region. Mass fractions of O-atom, OH, NCN, HCN and NNH in the lean-burn region are higher than those in the rich-burn region. In the region with temperature below 1800 K, NO is principally produced by NO2 conversion, while in the region with temperature above 1800 K, NO is mainly produced by the thermal pathway.

Flame dynamics in a stepped micro-combustor for non-adiabatic wall conditions

Flame stabilization in micro scale combustors has generated significant research interest to develop combustion-based power generators for microelectromechanical (MEMS) systems. Flame stabilization in such systems is significantly impacted by increased susceptibility to quenching due to increased heat-loss to walls and flame stability is governed by thermal-wall coupling. The present work is carried out to understand the role of thermal-wall coupling on flame stabilization, and unsteady flame dynamics in a backward step micro-combustor. The effect of various operating parameters such as mixture equivalence ratio and flow velocities on flame dynamics is reported for a range of non-adiabatic conditions by changing the geometrical parameters such as the diameter and length of the heat recirculating cup around the micro-combustor. The role of these parameters on flame stabilization and flame dynamics is delineated through flame regime diagrams. It is observed that flame stability limits are significantly enhanced due to a change (increase) in the dimensions of the heat recirculation cup, which enhances the heat recirculation through combustor walls. Steady and unsteady flame propagation modes are observed for the range of conditions investigated. The role of various parameters leading to the formation of stable and unstable flame propagation modes is experimentally investigated. The flame stabilization is observed to depend on heat recirculation through combustor walls and heat recirculation cup. Various interesting aspects of rotating and pulsating flame propagation modes are presented. The unstable flame modes lead to a significant decrease in combustion and thereby reducing combustion efficiency.

The effects of differential diffusion in counter-flow premixed flames with dilution and hydrogen enrichment

The effects of differential diffusion on local flamelet velocities, turbulent burning rates, and structure of lean turbulent premixed flames in the thin reaction zone regime are investigated using aerodynamically stabilized flames in a counter-flow apparatus. Various fuel–oxidizer–inert mixtures with different transport properties, representative of distinct effective Lewis numbers, are studied. In order to minimize the effects of mixture reactivity in these experiments, unstretched laminar flame speed is kept constant during mixture dilution, and hydrogen enrichment of hydrocarbon flames, through changing the mixture equivalence ratio. Furthermore, bulk-flow properties and stagnation surface temperature are kept constant; hence, the study focuses on the effects of differential diffusion, which is the change in transport properties of the mixture, i.e., fuel and heat diffusivities, in the context of fuel flexibility. Highly strained laminar flame measurements are also reported as a reference of comparison. Local instantaneous statistics of various flame parameters within the imaged plane, such as flame location, flame velocity, and flame-front topology, are quantified using high-speed particle image velocimetry (PIV) and Mie scattering flame tomography at two levels of turbulence intensity. These parameters are presented as probability density functions using sufficiently large data sets to ensure statistical accuracy. The results for various flame-parameter statistics, which are measured over a wide range of Lewis numbers, show that the effects of differential diffusion are important in turbulent flames in the thin reaction zone regime. At constant turbulence intensity, differential diffusion increases the burning rates of turbulent flames in thermo-diffusively unstable mixtures through two main mechanisms: (1) increasing local flamelet displacement velocity, and (2) increasing flame-surface area. The relative contribution of these two parameters in increasing turbulent burning rates is approximately 76% and 24%, respectively, which is not dependent on the fuel, oxidizing-gas mixture, or turbulence intensity, and the results overlap over a wide range of Lewis numbers.

Shock tube investigation of methyl tert butyl ether and methyl tetrahydrofuran high‐temperature kinetics

AbstractThe autoignition and pyrolysis of two C5 ethers, methyl tert butyl ether (MTBE) and 2‐methyltetrahydrofuran (2‐MTHF), are investigated using the shock tube reactor. The experiments are carried out at pressures of 3.5 and 12 atm at temperatures above 1000 K with argon as a diluent gas. By means of direct laser absorption, carbon monoxide time histories and associated chemical kinetic timescales are also determined. It is observed that the competition between ignition and pyrolysis times depends on the temperature and equivalence ratio of the ignition mixture, such that there is a temperature above which pyrolysis predominates oxidative kinetics. This crossover temperature shifts toward higher temperatures for reactive systems with a fixed fuel concentration but higher oxygen content. The resulting experimental observations are also compared with predictions of existing chemical kinetic models from the literature. The results point to differences in chemical reactivity, such that in pyrolysis conditions, the reactivity of the cyclic ether, 2‐MTHF, is generally higher than that of the aliphatic ether, MTBE. While agreement between experimental observations and model predictions is observed under certain conditions, significant variance between observations and predictions is observed under other conditions. With respect to prediction of the pyrolysis time used to capture the global kinetics of pyrolysis, it is observed that the relation of this time to the time needed to attain 90% of the equilibrium CO concentration varies greatly with the result that the models used in this work generally predict a faster initial formation of CO but a much slower approach to the equilibrium concentration. This is thought to arise from the slow transformation of intermediate CH2O and CH2CO to CO. The chemical kinetic models considered in this work are therefore not capable of predicting the CO time histories during pyrolysis.

Mathematical Modeling of Aluminum Diboride Combustion in an Air Flow

A mathematical model and the results of calculating the ignition and combustion of energetic condensed systems based on mono- and polydispersed aluminum diboride particles in air flows in constant-cross-section channels are reported. The kinetic characteristics of the transformations that separate aluminum diboride particles formed by gasification of energetic condensed systems undergo in a high-temperature oxidizing medium were determined using the dependences of the ignition induction period and combustion time on the air temperature and diameter and initial temperature of the particles. These dependences, in turn, were calculated using the model of parallel chemical reactions. The range of combustion conditions corresponding to the initial air temperatures from 300 to 2000 K and Mach numbers in the channel from 0.1 to 1.5 was considered. The influence of the aluminum diboride particle size and of the rate and initial temperature of the air flow on the combustion efficiency was demonstrated. The relationships between the combustion completeness factor of aluminum diboride particles at various initial parameters of the air flow and gasification products of energetic condensed systems at various fuel mixture equivalence ratios, corresponding to the diffusion and kinetic combustion, were determined. The conditions of the transition between the diffusion and kinetic control modes were found.

Ethanol Kinetic Model Development and Validation at Wide Ranges of Mixture Temperatures, Pressures, and Equivalence Ratios

A new detailed kinetic model of ethanol with 107 species and 1795 reactions was developed using a reaction mechanism generator and extensively validated against measured ignition delay times, lamin...

Expansion of low-load operating range by mixture stratification in a natural gas-diesel dual-fuel premixed charge compression ignition engine

Dual-fuel premixed charge compression ignition (DF-PCCI) combustion can achieve low nitrogen oxides (NOX) and particulate matter (PM) emissions for wide ranges of engine operations. However, the deterioration in thermal efficiency, and hydrocarbon (HC) and carbon monoxide (CO) emissions at low loads were recognized as the barriers for expanding the low-load operating range. In this study, the causes of the barriers were investigated and a mixture preparation strategy was suggested for overcoming the barriers in a natural gas (NG)-diesel DF-PCCI engine. Combustion and energy balance analysis was conducted to evaluate the strategy. Baseline DF-PCCI was determined by combinations of diesel start of injection (SOI) and NG substitution ratio (SR) at low loads from 0.3 to 0.6 MPa indicated mean effective pressure (IMEP). An increase in the homogeneity of a fuel-air mixture in the baseline DF-PCCI effectively reduced the NOX and PM emissions but increased the HC and CO emissions in each low-load operation. As the engine load was decreased, the formation of an overly-lean mixture intensified the effects of the mixture homogeneity. Therefore, the thermal efficiency, and HC and CO emissions deteriorated at 0.3 MPa IMEP. A mixture stratification strategy was established to increase the local equivalence ratio and reactivity of the fuel-air mixture. The strategy was realized by a retarded diesel SOI, a lower NG SR, and a higher exhaust gas recirculation rate. The strategy increased the degree of constant volume combustion by enhancing the combustion performance. The enhanced combustion reduced the combustion loss, and thus, improved the thermal efficiency. The HC and CO emissions also decreased mainly due to the improved combustion and the reduced mass flow rates of the NG.

Laminar burning velocities of H2/CO/CH4/CO2/N2 -air mixtures at elevated temperatures

The laminar burning velocity of nine different syngas compositions has been measured at elevated temperatures (350–650 K) for various equivalence ratios using an externally heated diverging channel method. The measured values are compared with the predictions of two detailed kinetic models (GRI 3.0 and FFCM-1). The predictions of FFCM-1 for various syngas (H2/CO/CH4/CO2/N2) mixtures accurately capture the fundamental combustion characteristics, and GRI 3.0 mechanism overpredicts the laminar burning velocity for fuel rich syngas compositions. Temperature exponents mildly decrease with increase in equivalence ratio for mixtures investigated in the present work.Two correlations, one using linear regression, and another, a linear equation model is proposed to predict the laminar burning velocity accurately for various syngas compositional modifications. The developed linear model can be used for direct calculation of laminar burning velocities of different syngas compositions. Similarly, a temperature exponent correlation and its comparison with present measurements and simulations are also presented. The efficacy of the proposed models is evaluated through a detailed comparison with the literature. The model provides an excellent matching for following syngas compositions; 0.05 <XH2<0.50, 0.08 < XCO < 0.50, 0 <XCH4< 0.13, 0 <XCO2< 0.50, 0 <XN2< 0.65, for an equivalence ratio of ϕ = 0.7–1.25, and mixture temperature from 300 to 650 K.