Well-stirred Reactor Model Research Articles

On the use of flamelet model approach in analyzing the mixing-controlled spray flame dynamics

This research examines the application of unsteady flamelet modeling to consider the impact of turbulent-chemistry interaction (TCI) on a mixing-controlled spray flame. The model incorporates the Representative Interactive Flamelets (RIF) approach to represent the sub-grid scaled heterogeneous mixture along with chemical reactions. This is combined with an Eulerian Particle Flamelet Model (EPFM), which uses multiple flamelets to account for the history of unsteady flames. The results are compared with those of a first-order moment method known as the Well-Stirred Reactor (WSR) model. The numerical simulations were carried out using a Reynolds Averaged Navier–Stokes (RANS) solver incorporated in two different CFD platforms, a commercially available CFD code, CONVERGE, and C++ based open-source CFD code, OpenFOAM. The test conditions were employed based on the Engine Combustion Network (ECN) Spray-A setup. The simulation results obtained interchangeably using both CFD software are investigated to address essential aspects of the RIF model. In order to ensure accurate predictions in the mixing field, adjustments were made to the original OpenFOAM code to enhance the treatment of time-stepping for spray source terms. This refinement allows for an adequate resolution of spray-induced mixing. While there are slight variations in the implementation of the RIF model between OpenFOAM and CONVERGE, both CFD codes effectively reproduce the physics of mixing-controlled combustion. This includes accurately representing important phenomena like mixing-controlled turbulent spray flame and combustion recession.

Investigation of the fire mass loss rate in confined and mechanically ventilated enclosures on the basis of a large-scale under-ventilated fire test

This study concerns the under-ventilated combustion regimes of a fire scenario in mechanically ventilated enclosures. Using the theoretical approach of the well-stirred reactor model and the data-base of large-scale pool fire tests, the study analyses the mass loss rate as a function of the environmental conditions. The novelties lie in the analysis of a large set of realistic scenarios involving complex geometries (several compartment connected) and new ventilation configurations and the applicability of the well-stirred reactor model to interpret these scenarios. The variables of interest are the combustion regimes (stationary, transient or with rapid quenching), the duration of the combustion phase and the mass loss rate. The results show a satisfactory prediction of the well-stirred reactor model to interpret the experimental results of complex and realistic fire scenario and the robustness of this model to deal with the under-ventilated regimes. The analysis also highlights the interest of two new parameters rarely used in the literature, the ventilation factor and the mass factor, to characterize under-ventilated scenarios. The analysis points out the importance of the relationship expressing the burning rate as a function of environmental conditions (oxygen level and external flux) and extinction conditions.

High-Fidelity Energy Deposition Ignition Model Coupled With Flame Propagation Models at Engine-Like Flow Conditions

Abstract With the heightened pressure on car manufacturers to increase the efficiency and reduce the carbon emissions of their fleets, more challenging engine operation has become a viable option. Highly dilute, boosted, and stratified charge, among others, promise engine efficiency gains and emissions reductions. At such demanding engine conditions, the spark-ignition process is a key factor for the flame initiation propagation and the combustion event. From a computational standpoint, there exist multiple spark-ignition models that perform well under conventional conditions but are not truly predictive under strenuous engine operation modes, where the underlying physics needs to be expanded. In this paper, a hybrid Lagrangian–Eulerian spark-ignition (LESI) model is coupled with different turbulence models, grid sizes, and combustion models. The ignition model, previously developed, relies on coupling Eulerian energy deposition with a Lagrangian particle evolution of the spark channel, at every time-step. The spark channel is attached to the electrodes and allowed to elongate at a speed derived from the flow velocity. The LESI model is used to simulate spark ignition in a nonquiescent crossflow environment at engine-like conditions, using converge commercial computational fluid dynamics (CFD) solver. The results highlight the consistency, robustness, and versatility of the model in a range of engine-like setups, from typical with Reynolds-averaged Navier–Stokes (RANS) and a larger grid size to high fidelity with large-eddy simulation (LES) and a finer grid size. The flame kernel growth is then evaluated against Schlieren images from an optical constant volume ignition chamber with a focus on the performance of flame propagation models, such as G-equation and thickened flame model, versus the baseline well-stirred reactor model. Finally, future development details are discussed.

Laser-driven ionization mechanisms of aluminum for single particle aerosol mass spectrometry

Single particle aerosol mass spectrometry (SPAMS), an analytical technique for measuring the size and composition of individual micron-scale particles, is capable of analyzing atmospheric pollutants and bioaerosols much more efficiently and with more detail than conventional methods which require the collection of particles onto filters for analysis in the laboratory. Despite SPAMS’ demonstrated capabilities, the primary mechanisms of ionization are not fully understood, which creates challenges in optimizing and interpreting SPAMS signals. In this paper, we present a well-stirred reactor model for the reactions involved with the laser-induced vaporization and ionization of an individual particle. The SPAMS conditions modeled in this paper include a 248 nm laser which is pulsed for 8 ns to vaporize and ionize each particle in vacuum. The ionization of 1 μm, spherical Al particles was studied by approximating them with a 0-dimensional plasma chemistry model. The primary mechanism of absorption of the 248 nm photons was pressure-broadened direct photoexcitation to Al(y2D). Atoms in this highly excited state then undergo superelastic collisions with electrons, heating the electrons and populating the lower energy excited states. We found that the primary ionization mechanism is electron impact ionization of various excited state Al atoms, especially Al(y2D). Because the gas expands rapidly into vacuum, its temperature decreases rapidly. The rate of three-body recombination (e− + e− + Al+ → Al + e−) increases at low temperature, and most of the electrons and ions produced recombine within several μs of the laser pulse. The importance of the direct photoexcitation indicates that the relative peak heights of different elements in SPAMS mass spectra may be sensitive to the available photoexcitation transitions. The effects of laser intensity, particle diameter, and expansion dynamics are also discussed.

Ignition of hexane-air mixtures by highly under-expanded hot jets

We report an experimental study of ignition of flammable mixtures by highly unexpanded, supersonic hot jets. The high-pressure, hot-gas reservoir supplying the jet is created by impacting a projectile on a plunger to rapidly compress and ignite a rich n-hexane/air mixture, resulting in a peak reservoir pressure of more than 20 MPa. A locking mechanism was used to prevent the plunger from rebounding and the jet was created by rupturing a diaphragm covering a nozzle with an exit diameter between 0.25 and 1 mm. The jet development and ignition processes in the main chamber filled with hexane-air mixture were visualized using high-speed schlieren and OH* chemiluminescence imaging. The ignition threshold was determined as a function of composition in the jet and main chamber, the nozzle diameter, and the initial pressure in the main chamber. Unlike the case of subsonic jets in which ignition occurs at the shear layer near the nozzle exit, ignition of combustion in the main chamber was found to take place downstream of the Mach disk terminating the supersonic expansion and within the turbulent mixing region created by the startup of the supersonic jet. The results are interpreted using a constant-pressure, well-stirred reactor model simulating the mixing between the hot jet and cold ambient gas. The critical conditions for ignition are determined by the competition between energy release due to chemical reactions initiated by the hot jet gas and cooling due to mixing with the cold chamber atmosphere. The critical value (maximum for which ignition occurs) of the mixing rate was computed using a detailed chemical reaction model and found to be a useful qualitative guide to our observations.

A Mapping Approach for Efficient CFD Simulation of Low-Speed Large-Bore Marine Engine with Pre-Chamber and Dual-Fuel Operation

A natural-gas-diesel dual-fuel marine engine with a pre-chamber is a promising solution for ocean transportation to meet the International Maritime Organization (IMO) emission regulations. This engine system employs a pre-chamber with direct injection of diesel to ignite premixed natural gas due to its higher ignition energy, which can enable lower lean limit and higher thermal efficiency. The dual-fuel pre-chamber marine engine presents complex multi-regime combustion characteristics in- and outside the pre-chamber, thus posing challenges in its numerical simulation in a cost-effective manner. Therefore, this paper presents a three-dimensional modeling study for the multi-regime combustion in a large-bore two-stroke marine dual-fuel engine, proposing a novel mapping approach, which couples the well-stirred reactor (WSR) model with the G-equation model to achieve high computational accuracy and efficiency simultaneously. In-depth analysis is performed using representative exothermic reaction (RXR) analysis and premixed turbulent combustion fundamentals to better understand the combustion process and to provide guidance in the selection of mapping timing. The results show that the use of mapping to switch from the WSR to the G-equation model can effectively reduce the runtime significantly by 71.5%, meanwhile maintaining similar accuracies in predictions of in-cylinder pressure traces, HRR and NOx emissions, compared to using WSR all along. Additionally, the choice of mapping timing based on several parameters is preliminarily discussed.

CFD modeling of pre-spark heat release in a boosted direct-injection spark-ignition engine

Accurate predictions of low-temperature heat release (LTHR) are critical for modeling auto-ignition processes in internal combustion engines. While LTHR is typically obscured by deflagration, extremely late ignition phasing can lead to LTHR prior to the spark, a behavior known as pre-spark heat release (PSHR). In this research, PSHR in a boosted direct-injection spark-ignition engine was studied using 3-D computational fluid dynamics (CFD) and detailed chemical kinetics. The turbulent combustion was modeled via a hybrid approach that incorporates the G-equation model for tracking the turbulent flame front, and the well-stirred reactor model with detailed chemistry for assessing the low-temperature reactions in unburnt gas. Simulations were conducted using Co-Optima alkylate and E30 fuels at operating conditions characterized by different PSHR intensities. The predicted in-cylinder pressure and heat release rate were found to agree well with experiments. It was found the estimate of previous-cycle trapped residuals is of utmost importance for capturing PSHR correctly. A simulation best practice was developed which keeps the detailed chemistry solver active throughout the entire simulation, allowing to track the evolution of intermediate species from one cycle to the next. Following the validation, the dynamics of PSHR were analyzed in detail employing the pressure-temperature (P-T) trajectory framework. It was shown that PSHR correlated with the first-stage ignition delay of the fuel, hence showing close relation to the in-cylinder P-T trajectory and the chemical kinetics. Besides, it was indicated that LTHR is a self-limiting process that has the effect of attenuating the thermal stratification in the combustion chamber. Furthermore, it was observed the occurrence of PSHR caused the P-T trajectory of end-gas to overlap with the negative temperature coefficient region of the fuel’s ignition-delay maps. This effect was more significant in the fuel-rich regions where engine knock tendency would be generally higher, with potential implications on knock control and mitigation.

Numerical Study of the Influence of Turbulence–Chemistry Interaction on URANS Simulations of Diesel Spray Flame Structures under Marine Engine-like Conditions

The present work performs unsteady Reynolds-averaged Navier-Stokes simulations to study the effect of turbulence-chemistry interaction (TCI) on diesel spray flames. Three nozzle diameters (d0) of 100, 180, and 363 μm are considered in the present study. The Eulerian stochastic fields (ESF) method (with the TCI effect) and well-stirred reactor (WSR) model (without the TCI effect) are considered in the present work. The model evaluation is carried out for ambient gas densities (ρam) of 30.0 and 58.5 kg/m3. The ESF method is demonstrated to be able to reproduce the ignition delay time (IDT) and lift-off length (LOL) with an improved accuracy than that from the WSR method. Furthermore, TCI has relatively more influence on LOL than on IDT. A normalized LOL (LOL*) is introduced, which considers the effect of d0, and its subsequent effect on the fuel-richness in the rich premixed core region is analyzed. The RO2 distribution is less influenced by the TCI effect as ambient density increases. The ESF model generally predicts a longer and wider CH2O distribution. The difference in the spatial distribution of CH2O between the ESF and WSR model diminishes as d0 increases. At ρam = 30.0 kg/m3, the ESF method results in a broader region of OH with lower peak OH values than in the WSR case. However, at ρam = 58.5 kg/m3, the variation of the peak OH value is less susceptible to the increase in d0 and the presence of the TCI model. Furthermore, the influence of TCI on the total OH mass decreases as d0 increases. The total NOx mass qualitatively follows the same trend as the total OH mass. This present work clearly shows that the influence of TCI on the global spray and combustion characteristics becomes less prominent when d0 increases. (Less)

Assessment of Turbulent Combustion Models for Simulating Prechamber Ignition in a Natural Gas Engine

Abstract Lean combustion is a promising strategy to increase thermal efficiency in an internal combustion engine, by exploiting a favorable specific heat ratio of the fresh mixture while simultaneously suppressing the heat losses to the cylinder wall. However, unstable ignition and slow flame propagation at fuel-lean conditions lead to large cycle-to-cycle variability and limit the high-efficiency engine operating range. Prechamber ignition is considered an effective concept to extend the lean operating limit, by providing spatially distributed ignition with multiple turbulent flame-jets and enabling a faster combustion rate compared to the conventional spark ignition approach. From a numerical modeling standpoint to date science base and available simulation tools are inadequate to properly understand and predict the combustion processes in prechamber ignited engines. In this paper, conceptually different Reynolds-averaged Navier–Stokes (RANS) combustion models widely adopted in the engine modeling community are used to simulate the ignition and combustion processes in a medium-duty natural gas engine with a prechamber spark-ignition system. A flamelet-based turbulent combustion model, i.e., G-equation, and a multizone well-stirred reactor model are employed for this modeling study. Simulation results are compared with experimental data in terms of in-cylinder pressure and heat release rate. Finally, the analysis of the performance of the two models is carried out to highlight the strengths and limitations of the two evaluated approaches.

A conservative and consistent scalar filtered mass density function method for supersonic flows

A novel scalar filtered mass density function (SFMDF) method is developed for high-speed flows, especially for supersonic reactive flows. The total energy is proposed as the energy form for SFMDF, instead of the commonly used enthalpy or sensible enthalpy. Such an energy form is entirely consistent with the one typically used in large eddy simulation (LES) for fully compressible flows, so that the exact/modeled energy equations in both LES and SFMDF are readily identical. Moreover, the total energy can formulate the SFMDF energy transport equation in such a way that the high-speed source term is strictly conservative. Following the conservative formulation, numerically robust conservative schemes are readily available for flows with discontinuities. Tests in one-dimensional Euler equations show that the temperature redundantly obtained based on the total energy (with conservative high-speed source terms) shows better agreement with the analytical result than the one based on the enthalpy. The proposed LES-SFMDF method is further tested in a shock tube interacting with an isotropic turbulent flow, a compressible two-dimensional non-reactive temporally developing mixing layer, and a supersonic three-dimensional reactive temporally developing mixing layer. Results show that SFMDF with the total energy can considerably improve the temperature distribution in both non-reactive and reactive flows. The proposed LES-SFMDF method with the total energy predicts the turbulence–chemistry interaction better than LES-SFMDF with the enthalpy as well as LES with the well-stirred reactor model in supersonic combustion. This conservative and consistent SFMDF method can be readily extended to more sophisticated probability density function methods in high-speed flows.

Numerical Investigation of a Central Fuel Property Hypothesis Under Boosted Spark-Ignition Conditions

AbstractIn the present work, a central fuel property hypothesis (CFPH), which states that fuel properties are sufficient to provide an indication of a fuel’s performance irrespective of its chemical composition, was numerically investigated. In particular, the objective of the study was to determine whether Research Octane Number (RON) and Motor Octane Number (MON), as fuel properties, are sufficient to describe a fuel’s knock-limited performance under boosted spark-ignition (SI) conditions within the framework of CFPH. To this end, four TPRF-bioblendstock surrogates having different compositions but matched RON (=98) and MON (=90), were first generated using a non-linear regression model based on artificial neural network (ANN). Three unconventional bioblendstocks were included in the analysis: di-isobutylene (DIB), isobutanol, and Anisole. Skeletal reaction mechanisms were generated for the TPRF-DIB, TPRF-isobutanol, and TPRF-anisole blends from a detailed kinetic mechanism. Thereafter, numerical simulations were performed for the fuel surrogates using the skeletal mechanisms and a virtual cooperative fuel research (CFR) engine model, under a representative boosted operating condition. In the computational fluid dynamics (CFD) model, the G-equation approach was employed to track the turbulent flame front and the well-stirred reactor model combined with the multi-zone binning strategy was used to capture auto-ignition in the end-gas. In addition, laminar flame speed (LFS) was tabulated for each blend as a function of pressure, temperature, and equivalence ratio a priori, and the lookup tables were used to prescribe laminar flame speed as an input to the G-equation model. Parametric spark timing sweeps were performed for each fuel blend to determine the corresponding knock-limited spark advance (KLSA) and 50% burn point (CA50) at the respective KLSA timing. It was observed that despite same RON, MON, and engine operating conditions, the TPRF-anisole blend exhibited markedly different knock-limited performance from the other three blends. This deviation from the octane index (OI) expectation was shown to be caused by differences in laminar flame speed. However, it was found that relatively large fuel-specific differences in LFS (>20%) would have to be present to cause any appreciable deviation from the OI framework. Otherwise, RON and MON would still be robust enough to predict a fuel’s knock-limited performance.

Numerical Investigation of Fuel Property Effects on Mixed-Mode Combustion in a Spark-Ignition Engine

Abstract In this study, lean mixed-mode combustion is numerically investigated using computational fluid dynamics (CFD) in a spark-ignition engine. A new E30 fuel surrogate is developed using a neural network model with matched octane numbers. A skeletal mechanism is also developed by automated mechanism reduction and by incorporating a NOx submechanism. A hybrid approach that couples the G-equation model and the well-stirred reactor model is employed for turbulent combustion modeling. The developed CFD model is shown to well predict pressure and apparent heat release rate (AHRR) traces compared with experiment. Two types of combustion cycles (deflagration-only and mixed-mode cycles) are observed. The mixed-mode cycles feature early flame propagation and subsequent end-gas auto-ignition, leading to two distinctive AHRR peaks. The validated CFD model is then employed to investigate the effects of NOx chemistry. The NOx chemistry is found to promote auto-ignition through the residual gas, while the deflagration phase remains largely unaffected. Sensitivity analysis is finally performed to understand effects of fuel properties, including heat of vaporization (HoV) and laminar flame speed (SL). An increased HoV tends to suppress auto-ignition through charge cooling, while the impact of HoV on flame propagation is insignificant. In contrast, an increased SL is found to significantly promote both flame propagation and end-gas auto-ignition. The promoting effect of SL on auto-ignition is not a direct chemical effect; it is rather caused by an advancement of the combustion phasing, which increases compression heating of the end-gas.

Multi-scale analysis of the under-ventilated combustion regime for the case of a fire event in a confined and mechanically ventilated compartment

This work deals with under-ventilated pool fires in confined and mechanically ventilated enclosures. The objective is to demonstrate the suitability of reduced-scale experiments to investigate under-ventilated combustion regimes in the case of a mechanically ventilated enclosure, by analysing combustion regimes at two different scales. The fire scenario consists in a dodecane liquid pool fire located in a mechanically ventilated room. Reduced-scale experiments concern a 0.031 m2 pool size inside a 1.875 m3 enclosure and large-scale experiments concern a 0.4 m2 pool size inside a 120 m3 enclosure. The varying parameter is the ventilation flow rate, which allows the fire behaviour to be investigated, from free-burning to confined situations. At both scales, results show that the burning rate decreases with the ventilation flow rate and a linear relationship between oxygen concentration and fuel mass loss rate is revealed. Comparison between the well-stirred reactor model and the experimental results shows good agreement for both scales. The work demonstrates the suitability of the reduced-scale approach to reproduce the combustion process in a vitiated environment, provided that appropriate dimensionless parameters are used: a reduced heat-release rate, a thermal ratio parameter and the global equivalence ratio.

Effect of hydrogen addition on NOx formation mechanism and pathways in MILD combustion of H2-rich low calorific value fuels

Hydrogen is a major component of low heating value (LHV) fuels like syngas, coke oven gas, and blast furnace gas. These fuels can potentially replace methane in the industrial process under MILD combustion conditions. NOx formation mechanism in MILD combustion regime is different from conventional air combustion due to uniform temperature distribution and low oxygen concentration. Hence, it is important to investigate the effect of hydrogen addition inmethane (CH4) on different NOx formation mechanism and pathways under MILD combustion conditions. A well-stirred reactor (WSR) model with atmospheric pressure and long residence time was employed to simulate the ideal MILD combustion conditions with Chemkin Pro software and GRI Mech 2.11. H2 and CH4 blends ranging from 0% H2 (100% CH4) to 60% H2 (B60 = 40%CH4/60%H2) were selected as fuels to emulate MILD combustion of LHV fuels. The oxygen mass fraction was kept from 3% to 9%, and the fuel blends were altered. The results showed that hydrogen addition enhanced NNH route while decreasing prompt route and N2O intermediate route whereas thermal NOx route was insignificant for lower O2 mass fraction of 3% and 6%. It was observed that H2 addition reduced NO-reburning which led to more NOx emission. Moreover, the sensitivity analysis was conducted to analyze the pathway of NOx formation and to quantitively separate the relative contribution of the individual route on the total NOx formation, which was beneficial to control the NOx emission of LHV fuels during MILD combustion.

Assessment of the validity of RANS knock prediction using the resonance theory

Following the resonance theory by Bradley and co-workers, engine knock is a consequence of an auto-ignition in the developing detonation regime. Their detonation diagram was developed using direct numerical simulations and was applied in the literature to engine knock assessment using large eddy simulations. In this work, it is analyzed if the detonation diagram can be applied for post-processing and evaluation of predicted auto-ignitions in Reynolds-averaged Navier–Stokes simulations even though the Reynolds-averaged Navier–Stokes approach cannot resolve the fine structures resolved in direct numerical simulations and large eddy simulations that lead to the prediction of a developing detonation. For this purpose, an engine operating point at the knock limit spark advance is simulated using Reynolds-averaged Navier–Stokes and large eddy simulations. The combustion is predicted using the G-equation and the well-stirred reactor model in the unburnt gases based on a detailed gasoline surrogate reaction scheme. All the predicted ignition kernels are evaluated using the resonance theory in a post-processing step. According to the different turbulence models, the predicted pressure rise rates and gradients differ. However, the predicted ignition kernel sizes and imposed gas velocities by the auto-ignition event are similar, which suggests that the auto-ignitions predicted by Reynolds-averaged Navier–Stokes simulations can be given a meaningful interpretation within the detonation diagram.

Investigation of NO2 Formation Kinetics in Dual-Fuel Engines With Lean Premixed Methane–Air Charge

A dual fuel engine concept with lean premixed methane–air charge ignited by a diesel pilot flame is highly promising for reducing NOx and soot emissions. One drawback of this combustion method, however, is the high nitric dioxide (NO2) emissions observed at certain operating points. The conditions leading to increased NO2 formation have been investigated using a batch reactor model in cantera. It has been found that the high emission levels of NO2 can be traced back to the mixing of small amounts of quenched CH4 with NO from the hot combustion zones followed by postoxidation in the presence of O2, requiring that the temperatures are within a certain range. NO2 formation in the exhaust duct of a test engine has been modeled and compared to the experimental results. The well-stirred reactor model has been used that calculates the steady-state of a uniform composition for a certain residence time. An appropriate reaction mechanism that considers the effect of NO/NO2 on methane oxidation at low temperature levels has been used. The numerical results of NO–NO2 conversion in the duct at low temperature levels show good agreement with the experimental results. The partial oxidation of CH4 can be predicted well. It can be shown that methane oxidation in the presence of NO/NO2 at low temperature levels is enhanced via the reaction steps CH3+NO2⇌CH3O+NO and CH3O2+NO⇌CH3O+NO2. In addition, the elementary reaction HO2+NO⇌NO2+OH is the important NO oxidizing step.

Numerical analysis of laser-pulse transient ignition of oxygen/methane mixtures in rocket-like combustion chamber

This paper describes a numerical study of the laser-pulse ignition sequence occurring in the combustion chamber of the M3 facility at DLR-Lampoldshausen. Our methodological approach relies on a variable fidelity modeling of the main phenomena of interest. In-house tools are fed with spatially homogeneous, isochoric, forced ignition problems, to derive a simplified methane/oxygen kinetic mechanism. The kernel formation after the laser pulse energy deposition is firstly studied in a one-dimensional framework, so as to test the simplified kinetic mechanism ability to reproduce the ignition behavior. The kernel initiation is also simulated in a two-dimensional mixing layer, to assess the ability of the simplified mechanism to accurately describe both the kernel initiation and its spatial propagation. Both the one- and two-dimensional simulations are carried out by means of a wavelet-based CFD library. Next, a lumped analysis of the events connected with the ignition sequence is carried out by adopting a well-stirred reactor model for the M3 chamber. The numerical results are compared to experiments carried out by DLR-Lampoldshausen, and the discrepancies are discussed. Finally, the whole M3 geometry and ignition sequence are simulated by means of an Unsteady Reynolds-averaged Navier-Stokes (URANS) model under the axi-symmetric flow approximation. The URANS results are compared with the experimental data, showing that URANS axi-symmetric calculation are able to provide a rather accurate picture of the ignition transients. Nevertheless, some issues on the quantitative accuracy of the URANS predictions are found and discussed in detail.

CFD and kinetic modelling study of methane MILD combustion in O2/N2, O2/CO2 and O2/H2O atmospheres

To deepen the understanding of combining moderate or intense low-oxygen dilution (MILD) combustion with oxy-fuel combustion for enhancing flame stability while realizing carbon capturing and storage, this paper presents a numerical study of methane MILD combustion in three atmospheres, i.e.: O2/N2, O2/CO2 and O2/H2O, with both computational fluid dynamics (CFD) and kinetic calculation approaches. Firstly, CFD predictions for the three conditions were performed following a systematic validation of the numerical method against experimental measurement from methane/air MILD combustion in a laboratory-scale closed furnace. Subsequently, kinetic calculations with a well-stirred reactor model was used to quantitatively identify the operating ranges of MILD combustion in the three atmospheres for methane. Moreover, the kinetic calculation provided additional insight into the fuel oxidation pathway. The results reveal that replacing N2 with either CO2 or H2O would help to establish MILD combustion mode from the viewpoint of lower temperature increase, due to both physical and chemical property discrepancies among the diluents. Specifically, the chemical effect and physical effect are responsible to the lower temperature rise for CO2-diluted case and H2O-diluted case, respectively. Inside the MILD combustion furnace, the negative heat release region disappears in regardless of atmospheres, indicating the eliminated fuel pyrolysis process under MILD combustion mode. Detailed analysis of the flame structure suggests that the combustion regimes inside the furnace in the three atmospheres are all in well-stirred combustion regime, and CO2-diluted case has the most extended reaction zone. Kinetic calculation indicates that CO2 or H2O dilution would result in a wider MILD combustion operating range compared to N2 dilution, while it is more pronounced for CO2. These observations all imply that MILD combustion will be more easily established with CO2 dilution than N2 or H2O dilution. However, higher CO formation is obtained in O2/CO2, forcing more attention to be paid on CO emission under CO2-diluted MILD combustion. Furthermore, the hydrocarbon recombination route is negligible under MILD combustion in spite of the atmospheres, implying lower sooting tendency as compared with conventional combustion.

Investigation of turbulence–chemistry interactions in a heavy-duty diesel engine with a representative interactive linear eddy model

Simulations of a heavy-duty diesel engine operated at high-load and low-load conditions were compared to each other, and experimental data in order to evaluate the influence of turbulence–chemistry interactions on heat release, pressure development, flame structure, and temperature development are quantified. A recently developed new combustion model for turbulent diffusion flames called representative interactive linear eddy model which features turbulence–chemistry interaction was compared to a well-stirred reactor model which neglects the influence of turbulent fluctuations on the mean reaction rate. All other aspects regarding the spray combustion simulation like spray break-up, chemical mechanism, and boundary conditions within the combustion chamber were kept the same in both simulations. In this article, representative interactive linear eddy model is extended with a progress variable, which enables the model to account for a flame lift-off and split injection, when it is used for diffusion combustion. In addition, the extended version of representative interactive linear eddy model offers the potential to treat partially premixed and premixed combustion as well. The well-stirred reactor model was tuned to match the experimental results, thus computed pressure and apparent heat release are in close agreement with the experimental data. Representative interactive linear eddy model was not tuned specifically for the case and thus the computed results for pressure and heat release are in reasonable agreement with experimental data. The computational results show that the interaction of the turbulent flow field and the chemistry reduce the peak temperatures and broaden up the turbulent flame structure. Since this is the first study of a real combustion engine (metal engine) with the newly developed model, representative interactive linear eddy model appears as a promising candidate for predictions of spray combustion in engines, especially in combustion regimes where turbulence–chemistry interaction plays an even more important role like, example given, in low-temperature combustion or combustion with local extinction and re-ignition.

Soot and spectral radiation modeling for high-pressure turbulent spray flames

A transported probability density function (PDF) method and a photon Monte Carlo/line-by-line (PMC/LBL) spectral model are exercised to generate physical insight into soot processes and spectral radiation characteristics in transient high-pressure turbulent n-dodecane spray flames, under conditions that are relevant for compression-ignition piston engines. PDF model results are compared with experimental measurements and with results from a locally well-stirred reactor (WSR) model that neglects unresolved turbulent fluctuations in composition and temperature. Computed total soot mass and soot spatial distributions are highly sensitive to the modeling of unresolved turbulent fluctuations. To achieve reasonable agreement between model and experiment and to capture the highly intermittent nature of soot in the turbulent flame, it is necessary to accurately represent mixing and the low diffusivity of soot particles. This is accomplished in the PDF framework using a mixing model that enforces locality in the gas-phase composition space, while not mixing the transported soot variables. The results suggest that mixing is at least as important as kinetics in controlling soot formation and evolution in high-pressure turbulent flames. Regarding radiation, radiant fractions and global influences of radiation in these flames are relatively small. Nevertheless, an examination of spectral radiative heat transfer provides valuable insight into the nature and modeling of radiation in high-pressure turbulent combustion systems. There are complex spectral interactions that are revealed using PMC/LBL. CO2 dominates the total radiative emission and reabsorption, but most of the emitted CO2 radiation is reabsorbed before reaching the walls. On the other hand, most of the emitted soot radiation reaches the walls, but soot radiation is a small contribution overall; H2O dominates the radiation that reaches the walls. Global turbulence–radiation interactions (TRI) effects are small, but radiative emission from soot increases by approximately a factor two when TRI are considered. Radiative transfer contributes both to energy redistribution in the vessel and to wall heat losses. The results suggest that a simple model that considers soot radiation and the principal CO2 and H2O spectral bands might be sufficient to capture the key influences of radiation in engine CFD. It is expected that these findings will contribute to the development of truly predictive CFD models for engines and other high-pressure turbulent combustion systems.