Laminar Premixed Flames Research Articles

Phenalenyl growth reactions and implications for prenucleation chemistry of aromatics in flames.

The energetics and kinetics of phenalene and phenalenyl growth reactions were studied theoretically. Rate constants of phenalene and phenalenyl H-abstraction and C2H2 addition to the formed radicals were evaluated through quantum-chemical and rate-theory calculations. The obtained values, assigned to all π radicals, were tested in deterministic and kinetic Monte Carlo simulations of aromatics growth under conditions of laminar premixed flames. Kekulé and non-Kekulé structures of the polycyclic aromatic hydrocarbons (PAHs) evolving in the stochastic simulations were identified by on-the-fly constrained optimization. The numerical results demonstrated an increased PAH growth and qualitatively reproduced experimental observations of Homann and co-workers of non-decaying PAH concentrations with nearly equal abundances of even and odd carbon-atom PAHs. The analysis revealed that the PAH growth proceeds via alternating and sterically diverse acetylene and methyl HACA additions. The rapid and diverse spreading in the PAH population supports a nucleation model as PAH dimerization, assisted by the non-equilibrium phenomena, forming planar aromatics first and then transitioning to the PAH-PAH stacking with size.

Determination method of Markstein number based on wavenumber measurement of cellular flames at the onset of parametric instability of downward propagating flames

This work proposes a new method to estimate the Markstein number by investigating the thermoacoustic parametric instability of laminar premixed flames propagating downwards in an open-closed tube. Methane-air flames were propagated in a combustion tube to capture the flame evolution in transition to parametric instability. By synchronized high-speed imaging filming both lateral and longitudinal views of flame propagation, cellular flame wavenumbers are measured from the clear transition of flat flames to cellular flames visualized in the present approach. The Markstein number is indirectly determined from wavenumber measurements at the onset of parametric instability by employing a laminar flame model under acoustic excitation treating the Markstein number as a free parameter. Acoustic velocity fluctuation amplitudes obtained from pressure fluctuation measurements were determined to further evaluate the validity of wavenumber-derived Markstein numbers. Markstein numbers obtained in the present work are compared with Markstein numbers relative to fresh gases in the literature from indirect and computational methods.

Temperature measurement in methane-ammonia co-fired laminar premixed flame using mid-infrared laser absorption tomography

Temperature measurement in methane-ammonia co-fired laminar premixed flame using mid-infrared laser absorption tomography

Thermodiffusively unstable laminar hydrogen flame in a sufficiently large 3D computational domain – Part I: Characteristic patterns

Thermodiffusive instabilities can have a leading order effect on flame propagation for lean premixed hydrogen flames. Many simulation studies have been performed to study this effect, but almost exclusively in two-dimensional (2D) or domain sizes too small to support the characteristic large-scale features of the instability. The main purpose of this study is to quantify the differences of 3D and 2D flames using simulations on sufficiently large domains. To this end, direct numerical simulations (DNS) of thermodiffusively unstable laminar premixed hydrogen flames stabilized in 3D domains were performed. The effects of confinement on the flame dynamics are rigorously investigated by varying the domain size. The flame burning velocity shows a strong dependence on the domain size when the lateral domain width is less than 50 laminar flame thicknesses. The characteristic patterns of the thermodiffusively unstable flame are analyzed in detail, including the instantaneous flame structure, global burning velocity, flame surface area, stretch factor, curvature distribution, and the cell size. The effects of the computational setup (2D vs. 3D) on the local flame front curvature and the distributions of the thermo-chemical quantities are quantified through a conditional analysis. The formation and destruction mechanisms of the distinct cellular structures observed in the 3D domain are analyzed focusing on the interactions between the flame dynamics and the flow field, and the contributions of the production of flame surface density, kinematic restoration, and curvature dissipation are quantified. Compared with the corresponding 2D simulation, the flame surface area in the cut plane of the 3D configuration is similar, yet the burning velocity and the stretch factor are increased. The reason for this is the difference in curvature statistics in 2D and 3D. In 3D, larger extreme curvature values are obtained, leading to quantitatively different distributions of the thermo-chemical variables. For example, the peak H radical concentration in the 3D simulation is about twice higher than in the 2D simulation.Novelty and significance statementThe novelty of the present work includes the following aspects:(i) First large-scale 3D DNS of laminar lean premixed hydrogen flame on domain large enough to obtain domain-independent results.(ii) First quantitative assessment of characteristic patterns of 3D thermodiffusively unstable premixed hydrogen flames.(iii) First quantitative assessment of differences between 2D and 3D thermodiffusively unstable premixed hydrogen flames for domain-independent results.

Safety aspects of Organic Rankine Cycles (ORC) with combustible working fluid and sub-ambient condenser pressure

ORCs are often designed with the lower pressure limited by the ambient pressure, to avoid air leaking into the working fluid. Safely lowering this limit will be beneficial for power-production efficiency. First, cycle operations with air inleak are investigated. Exemplary cases with n-pentane and benzene are used. The investigation combines tools from thermodynamics of cycle working fluids, pump operation and analysis and combustion science. It appears that pumps customarily used in ORC will forward only liquid, and air mixing into the working fluid at the high-pressure side will not be sufficient for flammable conditions. Next, combustion properties of the air/working fluid mixture in the condenser are evaluated. Transient zero-dimensional and one-dimensional steady state laminar premixed flames are applied for the modelling. Detailed chemical mechanisms from the CRECK and TDTVT groups are used for benzene and n-pentane. It is found that some substances, like pentane, will not make flammable mixtures within the conditions of the power cycle. However, fluids that at the relevant temperatures have saturation pressure much lower than the ambient, might give flammable mixtures with air inleak. For potential sub-ambient use of these, ignition energy sources and quenching geometries have to be considered carefully.

Effects of hydrogen addition on the laminar premixed flames and emissions of methane and propane

In this work, the laminar premixed flames for methane and propane at atmospheric pressure were numerically investigated with respect to different hydrogen concentrations, ranging from 5 % to 25 % (by vol.). Based on the two-dimensional (2D) flame predictions, hydrogen decreased the maximum temperatures and species concentrations such as formaldehyde (CH2O), acetylene (C2H2), carbon monoxide (CO) and nitric oxide (NO) for methane and propane. However, the CH2O mole fractions were raised by a maximum deviation of 10 % when hydrogen was added into the fuel lean and stoichiometric propane mixtures. The decrease in C2H2 eventually lowered the soot volume fractions and number densities. Besides, the axial locations of the maximum species concentrations, soot volume fractions and number densities were also affected by hydrogen. Meanwhile, the addition of hydrogen increased the flame temperatures and laminar burning velocities for the one-dimensional (1D) premixed flames of methane and propane. This trend was identically predicted for the formation of intermediate species such as CH2O, hydroperoxyl (HO2) and hydroxyl (OH) whereas C2H2, CO and NO were lowered, as justified by the sensitivity analyses. For propane at an equivalence ratio (φ) = 1.3, the rates of production (ROPs) of C2H2 were increased linearly with hydrogen, which resulted in higher C2H2 concentrations.

Inhibition of Laminar Premixed Flames of Dimethyl Ether by a Phosphorus-Containing Compound

ABSTRACT The present paper reports experimental and modeling results on the laminar burning velocity and structure of stoichiometric and fuel-rich premixed laminar flames of DME with and without TMP additives. The spatial variations of the mole fractions of H, OH, PO, PO2, HOPO, HOPO2 and some intermediate hydrocarbons in the one-dimensional burner-stabilized flames with various equivalence ratios are measured by the flame-sampling molecular beam mass spectrometry. The novel measurement data for the DME flames are used to validate a kinetic mechanism available in the literature for flame inhibition by organophosphorus compounds. The TMP inhibition effectiveness of DME flames has been determined and revealed to be lower than that of CH4/air flame. The speeds of CH4/air and DME/air flames were found to be sensitive to the same reactions involving phosphorus compounds, while the sensitivity of the methane flame speed to inhibition reactions is higher than that of DME/air flame. The observed differences were explained by the higher concentration of the chain carriers H, OH, and O in the DME flame. Based on the results obtained, we conclude that the phosphorus-containing inhibitors are promising candidates for reducing the flammability and ensuring the fire safety of DME mixtures with air.

Effect of ferrocene as a combustion catalyst on the premixed combustion flame characteristics of Jatropha biodiesel

Fuel additives are widely used to optimize the combustion process, reduce the harmful emissions, and improve the performance of engines. Among the metal-based additives, ferrocene is soluble in most fuels, including biodiesel. Furthermore, it is easily available and has high stability, low toxicity, and high lipophilicity. It can evenly disperse in biodiesel without causing any change in the physicochemical properties. Therefore, it can serve as an effective combustion catalyst for biodiesel. In this study, the effect of ferrocene as a combustion catalyst on the pyrolysis and combustion performance of Jatropha biodiesel is investigated. The coupled technique of thermogravimetry-Fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS) is used to analyze the evolution characteristics of multicomponent gaseous products generated during the pyrolysis and high-temperature oxidation of Jatropha biodiesel (JB100) and Jatropha biodiesel + ferrocene (JBF). The laminar premixed combustion flame and pollutant emissions of JB100 and JBF are examined using a variety of techniques, including OH planar laser-induced fluorescence (OH-PLIF), flue gas analysis, spectral analysis, transmission electron microscopy, etc. The results reveal that after adding ferrocene, the activation energy for the pyrolysis reaction of JB100 decreases by 4.66 kJ/mol and the complete weight loss temperature decreases by 10 ℃. Further, the addition of ferrocene reduces the endothermic heat by 0.38 mW/mg during the initial stage of the high-temperature oxidation reaction of JB100, and the amounts of final products CO2 and H2O are increased. Compared with JB100, JBF shows an enhanced OH group intensity in the combustion flame and a lower soot spectral peak. Furthermore, compared with JB100, under JBF combustion, the pollutant PM2.5 emissions decrease by 13.3 %, PM10 emissions decrease by 13.2 %, PM> 10 particulate matter emissions decrease by 19.3 %, CO emissions decrease by 6.8 %, NOx emissions increase by 21.2 %, and the size of soot particles decreases by 20.8 %.

Large Eddy Simulation of Soot Formation in a Real Aero-Engine Combustor Using Tabulated Chemistry and a Quadrature-Based Method of Moments

Abstract Considering the increasingly stringent targets for aircraft emissions, computational fluid dynamics (CFD) is becoming a viable tool for improving future aero-engine combustors. However, predicting pollutant formation remains challenging. In particular, directly solving the evolution of soot particles is numerically expensive. To reduce the computational cost but retain detailed physical modeling, quadrature-based moments methods can be efficiently employed to approximate the particle number density function (NDF). An example is the recently developed split-based extended quadrature method of moments (S-EQMOM), which enables a continuous description of the soot particles' NDF, essential to consider particle oxidation accurately. This model has shown promising results in laminar premixed flames up to turbulent laboratory scale configurations. However, the application to large-scale applications are still scarce. In this work, the S-EQMOM model is applied to the Rolls-Royce BR710 aero-engine combustor to investigate the soot evolution process in practically relevant configurations. For this, the soot model is embedded into a high-fidelity simulation framework, consisting of large eddy simulation for the turbulent flow and mixing and the flamelet-generated manifold method for chemistry reduction. An additional transport equation for polycyclic aromatic hydrocarbons is solved to model their slow chemistry and the transition from the gaseous phase to the solid phase. Simulations are performed for different operating conditions (idle, approach, climb, takeoff) to validate the model using experimental data. Subsequently, the results are analyzed to provide insights into the complex interactions of hydrodynamics, mixing, chemistry, and soot formation.

Effects of unburnt reaction progress on stretch flame dynamics under elevated temperatures

Flames in practical combustors and engines are inevitably subject to the effects of upstream chemical reaction progress and stretch, induced by elevated thermodynamic conditions and flow non-uniformities, respectively. Recent shock tube experiments and simulation studies on flame propagation have shown that flame speed under engine-relevant conditions can be enhanced with non-negligible upstream chemical reaction progress, especially when low-temperature heat release is involved in the unburnt mixture. On the other hand, depending on the mixture equivalence ratio and diluents, nonequidiffusion (including the non-unity Lewis number effect and the preferential diffusion effect) can couple with the flame stretch to fundamentally affect the flame propagation, which can manifest as either facilitation or suppression. Depending on the transport property of a reacting mixture, there hence can be either inhibition or promotion from unburnt reaction progress and stretch effects on flame propagation. In the current work, through one-dimensional numerical simulations of transient planar and spherical flames of n-heptane/air under elevated thermodynamic conditions, the combined effects of upstream chemical reaction progress and stretch on flame propagation are investigated. Results show that for both lean and rich n-heptane/air mixtures, flame speed can be substantially promoted with reaction progress, while the rich mixtures can exhibit opposite stretch dependence after first-stage ignition. Different definitions of effective Lewis number are adopted to explain the change in Markstein length for spherical flame in lean and rich mixtures reformed by the low temperature chemistry. This work fills an important gap in laminar premixed flame research relevant to practical combustion systems and can provide useful insight into local turbulent flame behaviors and phi-sensitivity in engine combustion.

A combustion mechanism reduction method based on entropy production analysis in fuel auto-ignition and laminar flames

This study provides a chemical mechanism reduction strategy based on entropy production analyses in both auto-ignition and laminar flames, which enhances the predictive accuracy for laminar burning velocities. In addition to chemical reactions, other irreversible sources causing entropy generation, such as mass diffusion and heat conduction, are considered in the modified approach. Specifically, initial skeletal mechanisms are first generated based on important reactions that contribute to entropy production in auto-ignition processes. Mechanism patches are then constructed to include important species and reactions, which contribute to entropy production from mass diffusion and heat conduction in laminar premixed flames beyond the pre-defined thresholds, respectively. Finally, the initial skeletal mechanisms and mechanism patches are combined to establish the final skeletal mechanisms. In this way, two final skeletal mechanisms for n-dodecane, consisting of 162 species and 2276 reactions, and 160 species and 1916 reactions, respectively, are developed from the detailed POLIMI mechanism with 451 species and 17,848 reactions. The two final skeletal mechanisms are proven to accurately predict laminar burning velocities and entropy production in n-dodecane flames with insignificant variations in the simulation results compared to the detailed mechanism, while their accuracy in predicting ignition delay times relies on the initial skeletal mechanisms. Specifically, the reduced mechanism with 160 species and 1916 reactions exhibits less satisfactory performance in predicting ignition delay compared to that with 162 species and 2276 reactions, indicating that a lower threshold is required to generate the initial skeletal mechanism through entropy production analysis of homogeneous auto-ignition processes. Additionally, compared with the reduced mechanisms with similar sizes obtained with other mechanism reduction strategies, the two final skeletal mechanisms accurately capture the characteristics of laminar burning velocities and ignition delay times, with similar calculation time required.

Evolution of Flame Displacement Speed Within Flame Front in Different Regimes of Premixed Turbulent Combustion

A transport equation for the flame displacement speed evolution in premixed flames is derived from first principles, and the mean behaviours of the terms of this equation are analysed based on a Direct Numerical Simulation database of statistically planar turbulent premixed flames with a range of different Karlovitz numbers. It is found that the regime of combustion (or Karlovitz number) affects the statistical behaviour of the mean contributions of the terms of the displacement speed transport equation which are associated with the normal strain rate and curvature dependence of displacement speed. The contributions arising from molecular diffusion and flame curvature play leading order roles in all combustion regimes, whereas the terms arising from the flame normal straining and reactive scalar gradient become leading order contributors only for the flames with high Karlovitz number values representing the thin reaction zones regime. The mean behaviours of the terms of the displacement speed transport equation indicate that the effects arising from fluid-dynamic normal straining, reactive scalar gradient and flame curvature play key roles in the evolution of displacement speed. The mean characteristics of the various terms of the displacement speed transport equation are explained in detail and their qualitative behaviours can be expounded based on the behaviours of the corresponding terms in the case of 1D steady laminar premixed flames. This implies that the flamelet assumption has the potential to be utilised for the purpose of any future modelling of the unclosed terms of the displacement speed transport equation even in the thin reaction zones regime for moderate values of Karlovitz number.

Stochastic Modeling of Partially Stirred Reactor (PaSR) for the Investigation of the Turbulence-Chemistry Interaction for the Ammonia-Air Combustion

The Partially Stirred Reactor (PaSR) model is carried out for the ammonia-air combustion system by means of stochastic modeling, namely by solving the transport equation for the joint Probability Density Function (PDF). The turbulent mixing is accounted for by the Linear Mean-Square Estimation (LMSE) mixing model. Notwithstanding the simplified nature of the PaSR modeling, the transported-PDF method enables capturing the effect of mixing frequency on the combustion system, especially the NOx emission. Since the chemical source term is in a closed form in the transported-PDF method, it allows us to apply different chemical mechanisms to explore, whether the set of elementary reactions that are identified as important for the prediction of NOx in the PaSR model is sensitive to the choice of chemical mechanisms. Furthermore, the effect of the residence time in the PaSR model has also been studied, and compared with those in the Perfectly Stirred Reactor (PSR) model (infinite large mixing frequency). Moreover, since the ammonia under oxygen enrichment shows some similar combustion behaviors in terms of e.g. laminar burning velocity as the ammonia under hydrogen enrichment, how large the difference of thermo-kinetic states (e.g. temperature and NOx emission) predicted by PaSR models and in laminar premixed flame configuration is also investigated. A further discussion focuses on the effect of thermal radiation, where the radiative heat loss roles in the prediction of NOx for the turbulent simulation is examined. By using the optically thin approximation model, it is shown that the thermal radiation exhibits little effect on the considered combustion systems within a typical turbulent time-scale.

A numerical study on the laminar diffusion and premixed flames for methane−air and methane−ethanol with ozone at atmospheric and elevated pressures

This work elucidated the laminar diffusion and premixed flames for methane–air and methane–ethanol (20 % and 40 %) with ozone concentrations of 100 ppm and 5000 ppm from 1 bar to 10 bar. The predicted diffusion flames were sensitive to ozone but depended on the fuels and pressures. At 1 bar, ozone induced higher peak flame temperatures and acetylene (C2H2) but with lower flame heights and soot. At higher pressures, the soot yields predicted for methane were increased by ozone whereas those for methane–ethanol were varied non-linearly. Meanwhile, C2H2 and soot formation were raised with ethanol while the maximum flame temperatures behaved non-monotonically. Besides, ethanol extended the flames with ozone at 1 bar but shortened these flames from 2 bar onwards whereas this trend was opposite for the flames without ozone. Owing to the higher pressures, the diffusion flame heights, temperatures and soot were raised. C2H2 for methane was unchanged while those for methane–ethanol flames were declined by 30 %. For the laminar premixed flames, ozone encouraged fuel decomposition and hence increased the laminar burning velocities for methane–air and methane–ethanol flames at all pressures by 11 % (maximum deviation). At 1 bar, the consumption of intermediate species concentrations i.e. formaldehyde (CH2O), hydroxyl (OH) and the formation of benzene (A1) and naphthalene (A2) were raised by ozone. At 2 bar and above, ozone generally produced lower intermediate species whereas inconsistent trends were found for the soot/polycyclic aromatic hydrocarbon (PAH) species. Separately, the effect of ethanol was only predicted at 1 bar, where the intermediate and soot/PAH species were promoted. As pressure increased from 1 bar to 10 bar, the laminar burning velocities and intermediate species for methane and methane–ethanol were decreased but with higher soot/PAH species.

Measurements and simulations on effects of elevated pressure and strain rate on NOx emissions in laminar premixed NH3/CH4/air and NH3/H2/air flames

Efforts to reduce NOx emissions by utilizing ammonia-fired combustors are made through various methods; this work investigates the possibility of combining the effects of pressure and stretch rate to reduce NOxemissions in laminar premixed flames. NOx emissions, including NO, NO2, N2O, NH3,and HCN, from NH3/CH4/air and NH3/H2/air unstrained and strained laminar premixed flames were separately measured and simulated with a high-pressure heat flux burner and CHEMKIN software. The key points are that increasing the pressure can decrease most NOx emissions, including NO, N2O, and HCN. However, it slightly increases the unburnt ammonia emissions and largely increases the NO2 formation. Medium XNH3 conditions are the most favorable for reducing NO emissions by increasing pressure. High strain rates can reduce NO emissions, but only in stoichiometric and rich conditions; they reversely increase NO emissions for lean equivalence ratios before the NO peak equivalence ratio (ϕ). The reason is explained by kinetic and reaction pathway analyses, which showed that in high-strain flames, more fuel-N species participated in the reaction to form N2 instead of NO for higher ϕ, while O and OH increased for lower ϕ, promotingNO formation. The combination of increasing pressure and strain rate can efficiently reduce NO emissions but ensure full ammonia oxidation for the medium ammonia content at ϕ = 0.9 ∼ 1.1. This sheds light on the utilization of highly strained ammonia-blend flames in more practical high-pressure burners to reduce NOx emissions.

Flash-back, blow-off, and symmetry breaking of premixed conical flames

Hydrodynamic and thermal-diffusive effects subject premixed flames to intrinsic instabilities that strongly influence their shape and propagation/stabilization characteristics. However, the interaction and coupling of intrinsic flame dynamics with background flow gradients and boundary conditions remain poorly understood. This paper presents a global nonlinear bifurcation analysis of burner-stabilized laminar premixed conical flames with varying reaction rates and reactant diffusivities, respectively parameterized by the Damköhler number Da and the Lewis number Le. Using a dimensionless formulation of the reacting, weakly-compressible Navier–Stokes equations, the dynamics of flash-back, blow-off, and cellular instability are explored in a fully-coupled framework. Our analysis identifies steady conical flame states over a finite range of Da and Le, limited by saddle–node bifurcations corresponding to spontaneous blow-off of the axisymmetric flame below a Le-dependent lower critical Da value and spontaneous boundary-layer flash-back beyond a higher critical Da value. Furthermore, the analysis reveals that the conical flame loses its axisymmetry via circle–pitchfork bifurcations as Le or Da decrease or increase beyond respective critical values. These bifurcations are shown to correspond to stationary three-dimensional global modes describing steady polyhedral or tilted flame structures, each associated with distinct azimuthal periodicities. Statement of SignificanceThis work presents a bifurcation analysis of laminar premixed conical flames within a fully-coupled flame/flow model. By considering variations in reaction rate and reactant diffusivity, the analysis identifies saddle–node bifurcations corresponding to spontaneous flash-back and blow-off of the axisymmetric flame. It also identifies axisymmetry breaking bifurcations associated with transitions to steady three-dimensional polyhedral and tilted flame states. These global instabilities indicate that hydrodynamic and thermal-diffusive effects strongly influence a flame’s steady structure in the azimuthal dimension even when no instabilities appear in the plane of symmetry. As such, future analyses of flame dynamics should rule out symmetry-breaking behaviors before adopting a two-dimensional computational framework.

Parametric instability in propagating flames and the impact of hydrogen enrichment on the onset of secondary instability

A self-excited acoustic instability of laminar premixed flames propagating in an open-ended tube with a length of 700 mm and a radius of 10 mm was simulated by solving the reacting unsteady compressible Navier–Stokes equations, to understand the way of massive acoustic generation and its onset behaviors. Four fuel–air mixtures with an equivalence ratio of 1.2 were considered, namely, methane–air and methane–hydrogen–air mixtures, to identify the role of hydrogen in rich methane–air mixture. Parametric instability, which generated huge acoustic disturbance and violent flame pulsations, was observed only for a particular methane–hydrogen–air mixture with RH = 0.2, consistent with previously reported experimental observations. For the investigation of the reinforcement mechanism of acoustic instability under parametric instability, the flame surface area modulation was examined. It was found that violent subharmonic flame front pulsations could strongly modulate the flame surface area in the fundamental mode, resulting in a fluctuating heat release rate and increased thermoacoustic coupling. When hydrogen addition was small, attaining a higher level of primary instability, which is the precursor of the parametric instability, was more dominant than increasing the threshold level for the onset of the parametric instability. With larger hydrogen addition, the increase in the threshold level was more dominant than attaining a higher level of the primary instability. In particular, as the flame propagation time decreased, the level of the primary instability was saturated in larger hydrogen addition. This study elucidates the mechanism for the acoustic generation of propagating flames under the parametric instability, and the effects of hydrogen enrichment within rich methane–air mixtures.

Effect of oxidation on the combustion flame characteristics of Jatropha biodiesel

This study investigated the effect of oxidation on the pyrolysis and combustion flame performance of Jatropha biodiesel. To analyze the escape characteristics of multi-component gas products during the pyrolysis and high-temperature oxidation processes of fresh and oxidized Jatropha biodiesel, the TG-FTIR-MS combined system was employed. In addition, various analytical equipment such as an OH-PLIF system, a flue gas analyzer, a spectral analyzer, and a transmission electron microscope were employed to examine the laminar premixed combustion flame and pollutant emissions of Jatropha biodiesel before and after oxidation. The results demonstrate that oxidation exhibits a nonlinear effect on the thermal decomposition of Jatropha biodiesel. Notably, the reaction activation energy exhibits an initial increase and followed by a decrease. Moreover, an increased oxidation degree led to a reduction in the activation energy of the high-temperature oxidation reaction of Jatropha biodiesel, without considerably affecting the total weight loss temperature. Furthermore, the peak temperatures of DTG and DSC exhibited a minor decrease. The production of C2, CH, and OH free radicals in the combustion flame of oxidized Jatropha biodiesel increased, along with an elevation in the spectral intensity of carbon smoke. With an increase in the degree of oxidation, the CO emission concentration from the combustion of Jatropha biodiesel decreased gradually from 75 mg/m3 to 36 mg/m3 after 6 h of oxidation. Additionally, NOx emissions gradually increased from 131 mg/m3 to 186 mg/m3 after 6 h of oxidation, and the particle size of carbon smoke exhibited a reduction of 28%.

A novel diffusion flux modeling for laminar premixed flame simulation with OpenFOAM

In this study, we developed a mass diffusion coefficient model for premixed combustion to reduce the computational time, in mixture-averaged diffusivities to serve as a microcomputer resource. In the development, a preliminary theory was derived to construct the associated variables and corrected using a benchmark model as the mixture-averaged model based on Chapman–Enskog binary diffusion. In the validation, the calculation results obtained using the proposed model with a benchmark case, regardless of thermal diffusion and for several fuels at various pressures, temperatures, and initial mixture compositions, showed good agreement. Validation was also performed with the two-dimensional premixed and partially premixed combustion models by implementing the novel diffusion coefficient model in OpenFOAM software. The OpenFOAM solver results were in good agreement with the experimental results. This solver showed good simulation performance at both micro and macro scales. In addition, the processing time was reduced by approximately 70% in all the simulated premixed flame models. Based on these findings, we conclude that the developed model allows researchers to work with more complex premixed combustion modeling with less computational time.

Effects of ferrocene addition on soot formation characteristics in laminar premixed burner-stabilized stagnation ethylene flames

The influence of ferrocene addition (∼60 ppm on a molar basis) on soot formation in laminar premixed burner-stabilized stagnation ethylene flames (with an equivalence ratio of 2.0) is investigated both experimentally and computationally. The results show that ferrocene addition has a very small effect on both flame temperature and concentrations of small soot precursors from kinetic modeling, but apparently promotes soot inception and particle size growth from particle size distribution measurements, and decreases soot maturity from elemental analysis. Transmission electron microscope (TEM) images and energy dispersive spectrometry (EDS) results show that distinct iron cores are present in large soot aggregates, consistent with the kinetic modeling results that iron predominates over iron oxide in ferrocene-doped flames. Iron in the incipient soot may be too dispersed or form too small cores to be observed by TEM; and as the soot particles coagulate, iron in the soot coalesces into large cores. Since iron is not exposed on the soot surface, and the concentrations of soot precursors are also not significantly increased by ferrocene, ferrocene promotes soot size growth mainly through enhanced coagulation rather than surface growth.