Non-premixed Counterflow Flames Research Articles

A novel swiss-roll counterflow micro-combustor: Experimental investigation of methane-oxygen flame behavior over time

This study introduces a novel Swiss-roll micro-combustor design that employs a non-premixed counterflow flame for efficient and stable micro-combustion. This design addresses practical system challenges by (1) stabilizing the flame, (2) enhancing heat recovery and gas preheating, (3) aligning gas momentum with the exhaust, and (4) minimizing disruptive fluid motion within the channels. This research investigates the dynamic behavior and combustion characteristics of a methane-oxygen flame within the combustor across a range of equivalence ratios (1.50–3.38). A synergistic approach combining flame dynamics analysis, spectroscopy, and RGB image processing explores the interplay between flame temperature, strain rate, and equivalence ratio. The results demonstrate the effectiveness of the RGB method as a cost-efficient tool for predicting variations in C2∗ and CH∗ radicals and light-intensity radiations. While flame thickness and length exhibit complex dependencies on the fuel-to-oxygen ratio and wall temperature, specific radical emissions vary dynamically with the equivalence ratio. Notably, equivalence ratios of 1.94 and 2.26 exhibit minimal temporal variations in flame properties, suggesting optimal stability and predictability for practical applications. The investigated flame is classified as a “cool flame,” and efficiency declines with richer mixtures due to increased exhaust heat loss and incomplete combustion. This work paves the way for further micro-combustor design optimization and unlocks the potential of non-premixed counterflow flames in diverse micro-scale applications.

Comprehensive Assessment of STGSA Generated Skeletal Mechanism for the Application in Flame-Wall Interaction and Flame-Flow Interaction

In this study, we conduct a thorough evaluation of the STGSA-generated skeletal mechanism for C2H4/air. Two STGSA-reduced mechanisms are taken into account, incorporating basic combustion models such as the homogeneous reactor model, one-dimensional flat premixed flame, and non-premixed counterflow flame. Subsequently, these models are applied to more complex combustion systems, considering factors like flame-flow interaction and flame-wall interaction. These considerations take into account additional physical parameters and processes such as mixing frequency and quenching. The results indicate that the skeletal mechanism adeptly captures the behavior of these complex combustion systems. However, it is suggested to incorporate strain rate considerations in generating the skeletal mechanism, especially when the combustion system operates under high turbulent intensity.

Kinetic effects of methanol addition on benzene formation in methyl decanoate/air counterflow nonpremixed flames

In this study, counterflow nonpremixed flame modeling was conducted to examine the effects of methanol addition on benzene (A1) formation in methyl decanoate (MD) /air flames. Methanol was added to either the fuel side or oxidant side of the counterflow flames, to explore the chemistry difference in A1 formation when different methanol addition strategies were adopted. The formation routes of A1 were emphatically analyzed, and some important polycyclic aromatic hydrocarbons (PAHs), such as naphthalene (A2) and phenanthrene (A3) were also evaluated in terms of emission index. Results show that most of A1 is produced at the downstream of MD pyrolysis zone, through the recombination reactions of C3 radicals, while methanol is primarily to produce methyl groups within a relatively higher temperature zone. Both the air-side and fuel-side additions of methanol can decrease A1 concentration, but the former shows a more significant inhibiting effect. Methanol addition featured both dilution and chemical inhibitory effects on the formation of A1 precursors, and the dilution effect was superior to the chemical effects. The chemical inhibition on A1 formation was mainly achieved through the enhancement in the generation trend of active groups and the H-addition reaction of methyl groups. The air-side addition of methanol featured a minimal effect on A1 formation, and the reduced A1 concentration within the flames was mainly attributed to the increased radial leakages of reactive species. Besides, both addition strategies of methanol show the inhibitory effect on A2 and A3 formation, but the change in flames with the air-side addition is relatively weaker than that of the fuel-side addition flames.

A generalized mixture fraction formulation for nonpremixed ammonia–hydrogen flames

For nonpremixed combustion, the mixture fraction variables are widely used as a conserved scalar to map out the flame structure in a unified manner. For ammonia–hydrogen flames, however, a proper definition of the mixture fraction variable is difficult due to the presence of fuel-bound nitrogen in ammonia. This work proposes a revised mathematical definition of the mixture fraction as a modified form of the Bilger’s formula in order to describe the characteristics of nonpremixed ammonia–hydrogen blend flames at general fuel mixture ratios. The newly defined mixture fraction properly accounts for differential diffusion effects in the presence of hydrogen, and thus serves as a convenient invariant coordinate for the flame manifolds. Counterflow nonpremixed flames with different ammonia–hydrogen ratios were computed and the validity of the new formulation is demonstrated. The analysis of the flame structure revealed the presence of double reaction zones, one dominated by the hydrogen oxidation and the other by ammonia oxidation. The role of differential diffusion is crucial in determining the existence of double reactive layers and its impact is evaluated by varying the strain rate. Detailed reaction analysis revealed that the hydrogen flame is related to HO2 reactions, while the ammonia flame to N, NH2 and NH3 reaction groups. As the strain rate is increased, OH reactions are responsible for shifting dominant heat release rate from the hydrogen flame to the ammonia flames. The scalar dissipation rate profile shows two distinct peaks, suggesting that a different functional description is needed in the flamelet modeling framework.

Experimental and kinetic modeling of soot formation in counterflow non-premixed flames of surrogate fuel components: n-dodecane and iso-dodecane

Normal dodecane (n-C12) and iso-dodecane (i-C12) are often used as components for surrogate mixtures of aviation and diesel fuels. Although studies have been performed to understand the combustion and spray behavior of dodecane isomers, the soot formation from the combustion of n- and i-C12 in non-premixed flames is not well studied. In this work, soot volume fraction (SVF) profiles of n- and i-C12 were measured across a wide range of strain rates and mixture conditions in a counterflow burner facility at the University of Connecticut. Neat and binary mixtures of n- and i-C12 were investigated to study the influence of alkane branching on soot formation. A soot model was developed and validated in this study by extending the detailed kinetic model developed at the Lawrence Livermore National Laboratory (LLNL) for the formation of polycyclic aromatic hydrocarbons (PAHs) to simulate soot formation and growth based on the discrete sectional method. Additional gas phase reactions forming pyrene and ring enlargement reactions were added to the existing PAH model. The LLNL kinetic model was validated with SVF data obtained in this work for n-C12 and i-C12 flames along with data available in literature for ethylene, iso-butene, n-heptane, and iso-octane. The soot precursor reactions added in this work were found to play a critical role in simulating the experimentally observed non-linear trend of the peak SVF with increased branched alkanes. Reaction path analysis was conducted to illustrate the fuel structure effects on soot formation pathways in n- and i-C12 mixtures. In view of the satisfactory agreement between modeling results and experimental data, as well as capturing the non-linear variation in peak SVF with the alkane branching for the first time, further investigation within the framework of the soot model is discussed and critical insights are provided into the reaction pathways which require further attention.

Chemical insights into ethyl acetate flames from experiment and kinetic modeling: Laminar burning velocity, speciation and NOx emission

Oxygenated fuels, such as alcohols, ethers, and esters, are promising alternatives to conventional fuels. These fuels can help reduce detrimental emissions like carbon monoxide and unburned hydrocarbons and enhance octane ratings. Among these oxygenates, ethyl acetate (EA), a small alkyl ester sourced from biomass, emerges as a clean, promising energy carrier. It serves as a surrogate fuel to facilitate investigations into the combustion behaviours of biodiesel. Despite its importance, the literature knowledge of EA combustion characteristics is limited. Therefore, this study aims to broaden the knowledge of the combustion behaviour of this type of oxygenated fuel compound. In this study, we measured the laminar burning velocities of EA by employing a heat flux burner and a closed combustion vessel over the equivalence ratios of 0.7 – 1.7, pressures of 1 – 10 bar and temperatures ranging from 353 – 423 K. Further, we also measured the NOx emissions in exhaust gas of the premixed flames fueled by EA/air for the first time over the equivalence ratio of 0.8 – 1.2. Additionally, we employed a non-premixed counterflow flame setup for extensive characterisation of species and their concentration under diverse conditions encompassing various strain rates and oxygen concentrations. Finally, we utilized these newly measured data to construct and validate a detailed kinetic model developed as part of this work. The newly developed model will help characterize the combustion properties of EA.

Direct Numerical Simulation of a Reacting Turbulent Hydrogen/Ammonia/Nitrogen Jet in an Air Crossflow at 5 Bar

The article aims to analyze the fluid dynamics and combustion characteristics of a non-premixed flame burning a fuel mixture derived from ammonia partial decomposition injected in an air crossflow. Nominal pressure (5 bar) and inlet air temperature (750 K) conditions are typical of micro-gas turbines. The effects of strain on the maximum flame temperature and NO generation in laminar non-premixed counter-flow flames are initially explored. Then, the whole three-dimensional fluid dynamic problem is investigated by setting up a numerical experiment: it consists of a Direct Numerical Simulation, based on accurate transport, chemical, and numerical models. The flow topology of the specific reacting jet in crossflow configuration is described in terms of its main turbulent structures, like shear layers, ring, and horse-shoe vortices, as well as of its leeward recirculation region anchoring the flame. The reacting region is characterized by providing instantaneous spatial distributions of temperature, heat release, and some transported chemical species, including NO, and calculating the Flame Index to identify non-premixed and premixed combustion local conditions. The latter is quantified by looking at the distribution of the volume fraction associated with a certain Flame Index versus the Flame Index and at the distribution of the average values of both the Heat Release Rate and NO versus the Flame Index and the mixture fraction.

Near-extinction transient behaviors of counterflow nonpremixed flames in a porous spherical burner at very low stretch rate

Near extinction transient behaviors of CH4/N2 versus O2/N2 counterflow nonpremixed flames were investigated by adopting a porous spherical burner with a large radius of curvature to achieve a very low buoyant stretch rate. By varying the oxygen concentrations in the oxidizer stream, four regimes were identified by three limits of sooting, instability, and extinction. The sooting limit was sensitive to flame temperature, and was characterized by two indexes of fuel injection velocity uF and oxygen concentration XO2. When decreasing uF, the sooting limit was different from high stretch flames because of appreciable heat loss toward the burner surface. In the instability regime, periodic holes were observed whose frequency decreased with XO2. Periodic holes were also found to occur with N2 dilution in the fuel stream, which were more regular with larger frequencies compared with the periodic holes in the diluted oxidizer streams. The edge propagation behaviors for periodic holes in diluted fuel streams were analyzed. The statistical average of the propagation speed of advancing edge (hole closing) Se decreased with increasing fuel concentration gradient, and the ratio of Se with the stoichiometric laminar burning velocity SLst of Se/SLst was larger than unity. Also, the stretch rate significantly affected the propagation speed of advancing edge flames. Determination of the propagation speed of the retreating edge (hole opening) had intrinsic difficulty in estimating the transient radial flow velocity, however, the approximated |Se/SLst| was larger than unity and decreased with the fuel mole fraction. Nitrogen dilution in the oxidizer stream was proven to be more effective than in the fuel stream for flame suppression of methane. This study could enrich the understanding of very low-stretch nonpremixed flames in a counterflow burner and provide information regarding low-stretch flame suppression, which may be relevant to microgravity flame behaviors.

Structure of Low Stretched Non-Premixed Counterflow Flames Stabilized in Planar Channel: Mass Spectrometric Study and Numerical Simulation

ABSTRACT In this paper, a methodology for experimental investigation of chemical and thermal structure of weakly stretched counterflow flames under terrestrial conditions is proposed. The non-premixed counterflow flames are stabilized in a narrow channel between quartz plates, and the flame gases are sampled by a microprobe inserted via a thin slit in one of the quartz plates. The gas samples are then analyzed by mass spectrometry. The results of the measurements of mole fraction profiles of the main components of methane-air non-premixed flames, namely, CH4, O2, H2O, CO2, and CO, are presented. The temperature profiles are also reconstructed using the measurement data for the gas composition. The experimental profiles are compared to the results of the 3D numerical simulations undertaken within the global reaction mechanisms (one- and four-step) and a detailed reaction mechanism (San-Diego). The results of this work clearly show that the thermal and chemical structures of the counterflow flame in the planar channel are essentially non-one-dimensional, and 3D-calculations are necessary to correctly predict the flame behavior in this configuration. The methodology demonstrated in the current work can be successfully employed for the investigation of the low stretched counterflow flames of various hydrocarbon fuels and fuel blends. This will allow verifying and developing the reaction kinetic models capable of quantitative prediction of weakly stretched flame behavior in confined conditions.

Effects of n-decane substitution on structure and extinction limits of formic acid diffusion flames

Formic acid (FA) is a low-carbon fuel that can be produced from renewable hydrogen (H2) and carbon dioxide (CO2). However, transitioning completely to low carbon fuels might be lengthy and challenging, so it makes sense to gradually introduce low carbon fuels into the energy sector in conjunction with existing fuels. One practical approach could be blending FA with existing fossil fuels, such as kerosene in aviation. This study investigates the behavior of flames comprising FA and its blends with n-decane. The extinction limits of pure FA and FA/n-decane mixtures were measured in a counterflow non-premixed laminar flame setup at various blending ratios. Additionally, the flame structure was studied by measuring temperature and species distribution using probe-based sampling. The results indicate that replacing FA with n-decane in the FA-N2 flame greatly enhances the flame's reactivity. Blending the fuels also led to a decrease in CO2 production in the flame, while increasing the flame temperature and the concentration of H2 and CO species. Experimental results were modeled using an updated FA and n-decane model. The kinetic model agrees with the experimental trends, but slightly overpredicts H2, CH4 species, and flame temperature. The kinetic model was also used to investigate the kinetic coupling between FA and n-decane in non-premixed laminar flames. Kinetic analyses indicate that HOCO and OCHO intermediates play a crucial role in pure FA flames by reacting with active radicals like H and OH to produce CO2 and CO, while in n-decane blended flames, fuel decomposition proceeds through two other pathways leading to H2 and H2O2.

Structure and extinction of methane and propane nonpremixed counterflow flames at extremely low stretch rates in buoyant flowfields

This study experimentally investigated the structures and extinction behaviors of methane-air and propane-air counterflow nonpremixed flames influenced by individually controlled fuel injection velocity uF at extremely low stretch rates, which were induced by natural convection alone. A sooting limit fuel injection velocity (SLFIV) index was introduced, which increased with stretch rate in the low stretch rate region. Methane had higher SLFIV than propane. Quasi-one-dimensional blue flames appeared for both methane and propane within a certain uF range. The range for methane was higher than that for propane. The steady flame standoff distances yf had a relationship with uF and low stretch rate ab of yf∝uFabλ∞1/2, which differed from high-stretch flames. Different flame instabilities occurred for methane and propane by changing uF only without fuel or oxidizer dilutions. At low uF near extinction limit of methane, periodic flame holes appeared, which was due to large heat loss. This instability finally caused flame extinction by keeping retreating edges. At high uF, simultaneous cellular and wave flame instability appeared for propane, which differed from cellular flames occurred alone in highly diluted oxy-fuel flames. By modulating uF alone in this experiment, the cellular/wave flame was due to Rayleigh-Taylor and hydrodynamic instabilities. Flame extinction at the lowest uF for propane was through outer edge shrinking, where the displacement speed increased with stretch rate. Flammability maps of limit uF versus low ab were established for both methane and propane. Both maps were divided into four regions by sooting, instability and extinction limits. The three limits uF all increased with ab. This changing tendency was opposite with high-stretch flames reported previously. For propane, the instability limit uF approached infinity at higher ab. This work provided knowledge of extremely low stretch flame combustion, which can be applied to microgravity flames, because the combustion characteristics of microgravity forced low stretch flames were identical to those of low stretch flame induced by natural convection alone.

Electric fields in a counterflow nonpremixed flame: measurement and simulation

In electric field modified flames, the electric body force on fluid elements can play a role in modifying the flow field, affecting flame characteristics by this modified flow motion. Numerical studies have developed ion kinetic mechanisms and appropriate transport models for charged species, validating them with a voltage-current trend in 1D premixed flames. Recent experimental approaches have measured the electric field by adopting the Electric Field Induced Second Harmonic generation (EFISH) technique. However, the quantification has turned out very challenging due to the inherent distortion in the EFISH signal, as well as inhomogeneous temperature and concentration fields in the combustion field. Here, we propose measurement and calibration schemes to quantify the EFISH signal in a laminar counterflow nonpremixed flame and present comparison with numerical results using an in-house multi-physics CFD (Computational Fluid Dynamics) code. Overall, the quantified electric fields agreed well with those from numerical simulation, specifically capturing null electric fields near the flame in the sub-saturated regime due to the electric field screening effect. In the saturated regime, notable discrepancy was found in a fuel stream when electrons moved through it: experiment indicated a significant number of negative ions in the fuel stream, whereas numerical results predicted negligible negative ions, due to the implemented ion-mechanism. This suggested that the experimentally obtained electric fields may serve as validation data for modeling studies to improve transport models and ion-mechanism. In-situ measurement of charged species in the presence of external electric fields should be a future work.

Combining flamelet-generated manifold and machine learning models in simulation of a non-premixed diffusion flame

Flamelet Generated Manifold (FGM) is an example of a chemistry tabulation or a flamelet method that is under attention because of its accuracy and speed in predicting combustion characteristics. However, the main problem in applying the model is a large amount of memory required. One way to solve this problem is to apply machine learning (ML) to replace the stored tabulated data. Four different machine learning methods, including two Artificial Neural Networks (ANNs), a Random Forest (RF), and a Gradient Boosted Trees (GBT), are trained, validated, and compared in terms of various performance measures. The progress variable source term and transport properties are replaced with the ML models. Particular attention was paid to the progress variable source term due to its high gradient and wide range of its value in the control variables space. Data preprocessing is shown to play an essential role in improving the performance of the models. Two ensemble models, namely RF and GBT, exhibit high training efficiency and acceptable accuracy. On the other hand, the ANN models have lower training errors and take longer to train. The four models are then combined with a one-dimensional combustion code to simulate a counterflow non-premixed diffusion flame in engine-relevant conditions. The predictions of the ML-FGM models are compared with detailed chemical simulations and the original FGM model for key combustion properties and representative species profiles.

Combustion regime identification and characteristics of nitrogen-diluted ammonia with hot air oxidizer: Non-premixed counterflow flame

In this study, a numerical investigation was done to study the characteristics of N2-diluted ammonia combustion in a counterflow diffusion configuration with an elevated oxidizer temperature. The flame structures have been characterized in a steady laminar, unidimensional configuration in which hot oxidant and diluted fuel are representative of recirculating flow required to achieve High Temperature Air Combustion (HiTAC) and moderately or intense low-oxygen dilution (MILD) combustion. Investigations were done from a combustion-regime viewpoint to find the effect of each regime on the ammonia flame structure and emission. The specific working conditions of each regime were also identified. The heat release rate typology and temperature rise were analyzed with respect to external parameters such as dilution, oxidizer temperature, and thermal power to determine the boundaries of each regime of ammonia combustion. The results suggest the desired fuel dilution and oxidizer temperature to achieve a favorable combustion regime. It has been found that a high dilution of 0.8 is at least required for the MILD combustion of ammonia to occur. An enhancement has been found in the heat release rate, temperature, and main intermediate and final species in correspondence with combustion regime transition. It is shown that ammonia combustion characteristics in HiTAC and MILD regimes can be favorable in terms of stability and Emissions.

Extinction and NO formation of ammonia-hydrogen and air non-premixed counterflow flames

Green ammonia, produced using renewable energy, is a promising carbon-free energy vector and fuel. This work studies combustion of ammonia-hydrogen fuel mixtures with air in counterflow diffusion flame experiments and provides an improved kinetic mechanism for modeling ammonia combustion. The extinction strain rate is measured for a range of 0 to 15% hydrogen in the fuel blend. The flame structure is also investigated with quantitative laser-induced fluorescence (LIF) measurements of nitric oxide (NO) for the same hydrogen concentrations and strain rate range from 26 to 134 s−1. For these conditions, NO concentration increases with both strain rate and fuel hydrogen content. The previously published kinetic model developed by the authors is used to perform one-dimensional flame simulations of the experimental setup and conditions, and results are compared to three other recently published ammonia mechanisms. None of the selected models satisfactorily predict both the measured extinction strain rate and flame NO concentration. The models mainly fail to predict extinction strain rate at higher H2 fraction and NO formation at the highest experimental strain rates and H2 fraction. The reaction rate parameters for some of the key reactions in the published model developed by authors were updated to improve agreement with experimental results. The updated model results are closely aligned with extinction strain rate measurements, and have improved prediction of flame NO concentration. The model reveals that the reactions from the NH2 and NH sub-mechanism are sensitive in predicting the extinction strain rate as well as NO. In particular, the reaction NH+NO=N2O+H had significant impact on NO predictions.

Reaction diffusion manifolds (REDIMs) applied to soot formation in ethylene counterflow non-premixed flames: an uncoupled methodology

Reaction diffusion manifolds (REDIMs) applied to soot formation in ethylene counterflow non-premixed flames: an uncoupled methodology

Reduced modeling of the NOx formation based on the reaction-diffusion manifolds method for counterflow diffusion flames

The accurate prediction of the NOx formation has gained great attention in view of clean combustion. In this direction, a reliable reduction technique for the chemical kinetics is important to capture the NOx formation accurately with reduced computational costs for the practical turbulent combustion processes. This work focuses on the hierarchical construction of Reaction-Diffusion Manifolds (REDIM) to include the NOx chemistry, which is well-known to be governed by very slow chemical reactions. Based on the hierarchical structure of the REDIM method, a two-dimensional (2D) and a three-dimensional (3D) REDIM model are generated, without any principle extension of the REDIM method. Sample calculations of NOx formation in methane/air non-premixed counterflow flames verify the REDIM method for both steady and transient processes.

Effects of methanol addition on the combustion characteristics of bio-diesel based counterflow nonpremixed flames

With the improvement of combustion technology and the prominence of emission-related environmental concerns, the combined application of macromolecular unconventional fuels (bio-diesel) and alcohol ether fuels in engines is being considered. In this study, the effect of methanol addition on the combustion characteristics of methyl decanoate (MD)/air counterflow nonpremixed flames were investigated using numerical simulation. Two addition strategies of methanol were employed: premixing methanol into MD on the fuel side, and partially premixing of methanol on the air side. The corresponding flames are respectively double-flames with a rich premixed reaction zone and a nonpremixed reaction zone, or triple-flames with an additional lean reaction zone close to the air side. Results showed that, with the increased methanol addition, the global parameters of the double-flames and triple-flames were altered to different degrees. The both strategies of methanol addition can widen the flame thickness, and comparatively, the broadened flames with air-side addition of methanol were more obvious. The introduction of methanol showed little direct effects on the oxidation and pyrolysis of MD, and the key kinetic roles on the production of the active intermediates, such as OH and H in the flame domain. For both premixing strategies, the integral production of OH and H in the flames were increased, and the flames with the air-side methanol addition present the more significant production.

Real-fluid thermophysicalModels: An OpenFOAM-based library for reacting flow simulations at high pressure

Although OpenFOAM is a widely-used open source computational fluid dynamics (CFD) tool, it is limited to numerical simulations of multi-dimensional reacting/nonreacting flows at relatively-low pressures. This is not only because real-fluid models that can evaluate thermophysical properties at high pressures are not available in the thermophysicalModels library of OpenFOAM, but also because the existing mixing model cannot handle various mixing rules of real-fluid models. In the present study, we develop a novel algorithm applicable for a mixture model incorporating various mixing rules in OpenFOAM. Based on the new algorithm, we update the thermophysicalModels library of OpenFOAM 6.0 by implementing a set of real-fluid models such as the Soave-Redlich-Kwong/Peng-Robinson equation of state, Chung's model for dynamic viscosity and thermal conductivity, mixture averaged model for mass diffusivity using Takahashi's correction for binary diffusion coefficients at high pressure. The new library is validated against experimental data and is further assessed for compressible reacting flows by performing two-dimensional numerical simulations of axisymmetric laminar non-premixed counterflow flames and one-dimensional numerical simulations of premixed CH4/air flames at high pressures. The developed library can be used for any reacting flow solvers in OpenFOAM 6.0 that adopt a set of implemented real-fluid models. Program summaryProgram title: Real-fluid thermophysicalModelsCPC Library link to program files:https://doi.org/10.17632/n8zb2wpjp6.1Developer's repository link:https://github.com/danhnam11/realFluidThermophysicalModels-6Licensing provisions: GPLv3Programming language: C++Nature of problem: The mixing rules required for evaluating the real-fluid based thermophysical properties of mixtures are relatively complicated and different from each other, while the existing thermophysicalModels library offers only a mixing rule based on the mass fraction weighted average of each species, for which overloading operator functions (“*” and “+=”) are utilized to update parameters for a mixture. However, this approach is not applicable for complicated mixing rules in the real-fluid models.Solution method: To build a mixture class containing different mixing rules in different models such as the equation of states and thermodynamics/transport properties in OpenFOAM, we adopt void-type functions (parameter updating functions) to update parameters of a mixture instead of the original overloading operator functions in OpenFOAM. The parameters in the mixing rules that depend on pressure and temperature are handled by decomposition such that they can be updated in the mixture class.

Effects of the electric field on soot formation in combustion: A coupled charged particle PBE-CFD framework

In this article, a coupled PBE-CFD framework has been proposed to study counterflow non-premixed flames and soot formation under an external electric field. This framework integrates the population balance equation (PBE) for nanoparticle dynamics into an in-house CFD solver for the multicomponent reactive flows. Different electric properties have been considered in this model. An ion mechanism used in both fuel-rich and fuel-lean combustion is combined with a detailed chemistry for neutral gaseous species and small-size aromatics to retain the full chemistry. In order to model soot particles carrying charges and the movement of the reacting fluid medium in the electric field, a second PBE for the production and transport of charges on soot particles is introduced for the first time and incorporated into the original PBE for the number density of particles. Also, the electric force for the gas mixture is included in the momentum equations. The electric drift velocities for ions and soot particles are also considered in the transport equations of ions and the PBE of soot particles, respectively. The simulations have shown that the presence of the electric field modifies the stagnation plane of the counterflow flames and reduces the soot formation in both rich-fuel and lean-fuel conditions in agreement with experimental observations. The application of the soot particle charging model, accompanied by a proper electric correction factor on the nanoparticle processes of nucleation and surface growth, significantly improves the stability of the flame structure. The introduction of the electric correction factor reveals that the suppression of soot formation in an electric field is mainly caused by the inhibited chemical reactions of the PAH nucleation and particle surface growth, which is more important than the electric drift of the charged particles. Reducing the critical size of the particle charging process enhances the electric drift of nascent soot, thus lessening its subsequent evolution.