Abstract

This work presents a newly developed model for the oxidation of methanol and ethanol and their fuel interaction with NOx (NO and NO2) chemistry in jet-stirred and flow reactors, freely propagating and burner-stabilized premixed flames, as well as shock tubes. This work takes into account a very recent study on NO formation in pure methanol and ethanol burner-stabilized flames (Combust. Flame, 194, 363–375, 2018), augmented with so far unpublished experimental data. The paper mainly focuses on fuel interaction with nitrogen chemistry and NO formation in laminar premixed flames but also considers the formation and reduction of nitrogen oxides depending on the conditions of the surrounding gas phase. In agreement with previous experimental work, we find that doping of fuel blends with NO shifts the onset of fuel oxidation to lower temperatures depending on the gas conditions. The model suggests that the reactivity promoting effect of NO is mainly due to the net increase of OH radical concentrations, which causes increased fuel oxidation via the NO/NO2 interconversion reaction channel, NO+HO2⇋NO2+OH, NO2+H⇋NO+OH, NO2+HO2⇋HONO+O2, followed by the thermal decomposition of HONO. In burner-stabilized premixed ethanol flames, NO is mainly formed via a NCN pathway for all equivalence ratios, while for methanol flames the NCN pathway is only favored at rich conditions and the N2O pathway is favored at lean conditions.

Abstract

The prediction of local heat transfer and thermal stratification in the zero-dimensional stochastic reactor model is compared to direct numerical simulation published by Schmitt et al. in 2015. Direct numerical simulation solves the Navier–Stokes equations without incorporating model assumptions for turbulence and wall heat transfer. Therefore, it can be considered as numerical experiment and is suitable to validate approximations in low-dimensional models. The stochastic reactor model incorporates a modified version of the Euclidean Minimum Spanning Tree mixing model proposed by Subramaniam et al. in 1998. To capture the thermal stratification of the direct numerical simulation, the total enthalpy (H) is used as the only mixing limiting scalar within the newly proposed H-Euclidean-Minimum-Spanning-Tree. Furthermore, a stochastic heat transfer model is incorporated to mimic turbulence effects on local heat transfer distribution to the walls. By adjusting the Cϕ mixing time and Ch stochastic heat transfer parameter, the stochastic reactor model predicts accurately the thermal stratification of the direct numerical simulation. Comparing the Woschni, Hohenberg and Heinle heat transfer model shows that the modified Heinle model matches accurately the direct numerical simulation results. Thereby, the Heinle model accounts for the influence of turbulent kinetic energy on the characteristic velocity in the heat transfer coefficient calculation. This highlights the importance of incorporating turbulence effects in low-dimensional heat transfer models. Overall, the zero-dimensional stochastic reactor model with the H-Euclidean-Minimum-Spanning-Tree mixing model, the stochastic heat transfer model and the modified Heinle correlation have proven successfully the prediction of mean quantities like temperature and heat transfer and thermal stratification of the direct numerical simulation.

Abstract

This work introduces a newly developed reaction mechanism for the oxidation of ammonia in freely propagating and burner stabilized premixed flames as well as in shock tubes, jet stirred reactors and plug flow reactors experiments. The paper mainly focuses on pure ammonia and ammonia-hydrogen fuel blends. The reaction mechanism also considers the formation of nitrogen oxides, as well as the reduction of nitrogen oxides depending on the conditions of the surrounding gas phase. Doping of the fuel blend with NO2 can result in acceleration of H2 autoignition via the reaction NO2+HO2⇋HONO+O2 followed by the thermal decomposition of HONO, or in deceleration of H2 oxidation via NO2+OH⇋NO+HO2. The concentration of HO2 is decisive for the active reaction pathway. The formation of NO in burner stabilized premixed flames is shown to demonstrate the capability of the mechanism to be integrated into mechanisms for hydrocarbon oxidation.

Abstract

The ability of a mechanism describing the oxidation kinetics of toluene reference fuel (TRF)/n-butanol mixtures to predict the impact of n-butanol blending at 20% by volume on the autoignition and knock properties of gasoline has been investigated under conditions of a strongly supercharged spark ignition (SI) engine. Simulations were performed using the LOGEengine code for stoichiometric fuel/air mixtures at intake temperature and pressure conditions of 320 K and 1.6 bar, respectively, for a range of spark timings. At the later spark timing of 6 °CA bTDC, the predicted knock onsets for a gasoline surrogate (toluene reference fuel, TRF) and the TRF/n-butanol blend are higher compared to the measurements, which is consistent with an earlier study of ignition delay times predicted in a rapid compression machine (RCM, Agbro et al., Fuel, 2017, 187:211-219). The discrepancy between the predicted and measured knock onsets is however quite small at higher pressure and temperature conditions (spark timing of 8 °CA bTDC) and can be improved by updating a key reaction related to the toluene chemistry. The ability of the scheme to predict the influence of n-butanol blending on knock onsets requires improvement at later spark timings. The simulations highlighted that the low-intermediate temperature chemistry within the SI engine end gas, represented by the presence of a cool flame and negative temperature coefficient (NTC) phase, plays an important role in influencing the high temperature heat release and consequently the overall knock onset. This is due to its sensitisation effect (increasing of temperature and pressure) on the end gas and reduction of the time required for the high temperature heat release to occur. Therefore, accurate representation of the low-intermediate temperature chemistry is crucial for predicting knock. The engine simulations provide temperature, heat release and species profiles that link conditions in practical devices and ignition delay times predicted in an RCM. This facilitates a better understanding of the chemical processes affecting knock onsets predicted within the engine and the main reactions governing them.

Abstract

We have developed a new model utilizing our existing kinetic gas phase models to simulate experimental particle size distributions emerging in dry supersaturated H2SO4 vapor homogeneously produced by rapid oxidation of SO2 through stabilized Criegee-Intermediates from 2-butene ozonolysis. We use a sectional method for simulating the particle dynamics. The particle treatment in the model is based on first principles and takes into account the transition from the kinetic to the diffusion-limited regime. It captures the temporal evolution of size distributions at the end of the ozonolysis experiment well, noting a slight underrepresentation of coagulation effects for larger particle sizes. The model correctly predicts the shape and the modes of the experimentally observed particle size distributions. The predicted modes show an extremely high sensitivity to the H2SO4 evaporation rates of the initially formed H2SO4 clusters (dimer to pentamer), which were arbitrarily restricted to decrease exponentially with increasing cluster size. In future, the analysis presented in this work can be extended to allow a direct validation of quantum chemically predicted stabilities of small H2SO4 clusters, which are believed to initiate a significant fraction of atmospheric new particle formation events. We discuss the prospects and possible limitations of the here presented approach.

Abstract

Double-imaging photoelectron/photoion coincidence (i2PEPICO) spectroscopy using a multiplexing, time-efficient, fixed-photon-energy approach offers important opportunities of gas-phase analysis. Building on successful applications in combustion systems that have demonstrated the discriminative power of this technique, we attempt here to push the limits of its application further to more chemically complex combustion examples. The present investigation is devoted to identifying and potentially quantifying compounds featuring five heavy atoms in laminar, premixed low-pressure flames of hydrocarbon and oxygenated fuels and their mixtures. In these combustion examples from flames of cyclopentene, iso-pentane, iso-pentane blended with dimethyl ether (DME), and diethyl ether (DEE), we focus on the unambiguous assignment and quantitative detection of species with the sum formulae C5H6, C5H7, C5H8, C5H10, and C4H8O in the respective isomer mixtures, attempting to provide answers to specific chemical questions for each of these examples. To analyze the obtained i2PEPICO results from these combustion situations, photoelectron spectra (PES) from pure reference compounds, including several examples previously unavailable in the literature, were recorded with the same experimental setup as used in the flame measurements. In addition, PES of two species where reference spectra have not been obtained, namely 2-methyl-1-butene (C5H10) and the 2-cyclopentenyl radical (C5H7), were calculated on the basis of high-level ab initio calculations and Franck-Condon (FC) simulations. These reference measurements and quantum chemical calculations support the early fuel decomposition scheme in the cyclopentene flame towards 2-cyclopentenyl as the dominant fuel radical as well as the prevalence of branched intermediates in the early fuel destruction reactions in the iso-pentane flame, with only minor influences from DME addition. Furthermore, the presence of ethyl vinyl ether (EVE) in DEE flames that was predicted by a recent DEE combustion mechanism could be confirmed unambiguously. While combustion measurements using i2PEPICO can be readily obtained in isomer-rich situations, we wish to highlight the crucial need for high-quality reference information to assign and evaluate the obtained spectra.
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Abstract

Within this thesis, a detailed multicomponent gasoline surrogate reaction scheme was developed and reduced to a four component scheme of skeletal size. The main target is to cover the most important features for typical spark ignited (SI) combustion - flame propagation, emission formation and the tendency to auto ignite and subsequently cause engine knock. To achieve this a variable mechanism concept was developed to include sub models for different fuels as needed. Using this approach a detailed mechanism describing the oxidation of n-heptane, iso-octane, toluene and ethanol was compiled and compared against various experiments published in literature. Furthermore, correlations were developed to suggest four component gasoline surrogates based on typical fuel data sheets. The correlation method is validated against measurements in Cooperative Fuel Research (CFR) engine from various groups and further compared against correlations between octane numbers (ON) and predicted 0D ignition delay times. These correlations are used to identify and discuss the impact of the uncertainty of two reactions on ignition delay time of a multicomponent fuel. To be able to reduce the detailed scheme in a time efficient way existing reduction concepts where improved and applied to different schemes and targets. Since various reduction techniques are available, an optimal sequence of those was worked out. Using this sequence of reduction steps two multicomponent schemes were compiled: one scheme for the prediction of laminar flame speeds and one for the prediction of major emissions and auto-ignition. To underline that the suggested reduction procedure is universal it was also applied to n-heptane as single fuel surrogate for diesel fuel and to a large two component fuel from another work group.

Abstract

The present work is concerned with the simulation of combustion, emission formation and fuel effects in Diesel engines. The simulation process is built around a zero-dimensional (0D) direct injection stochastic reactor model (DI-SRM), which is based on a probability density function (PDF) approach. An emphasis is put on the modelling of mixing time to improve the representation of turbulence-chemistry interactions in the 0D DI-SRM. The mixing time model describes the intensity of mixing in the gas-phase for scalars such as enthalpy and species mass fraction. On a crank angle basis, it governs the composition of the gas mixture that is described by PDF distributions for the scalars. The derivation of the mixing time is based on an extended heat release analysis that has been fully automated using a genetic algorithm. The predictive nature of simulations is achieved through the parametrisation of the mixing time model with known engine operating parameters such as speed, load and fuel injection strategy. It is shown that crank angle dependency of the mixing time improves the modelling of local inhomogeneity in the gas-phase for species mass fraction and temperature. In combination with an exact treatment of the non-linearity of reaction kinetics, it enables an accurate prediction of the rate of heat release, in-cylinder pressure and exhaust emissions, such as nitrogen oxides, unburned hydrocarbons and soot, from differently composed fuels. The method developed is particularly tailored for computationally efficient applications that focus on the details of reaction kinetics and the locality of combustion and emission formation in Diesel engines.

Abstract

Three different methods for Reynolds-averaged navier-Stokes computational fluid dynamics modeling of non-premixed ignition and combustion using tabulated chemsitry have been developed. All methods make use of flamelet libraries, where the flamelet auto-ignition process is parameterized using a progress variable. the progress variable parametrization of the autoignition chemsitry allows for using arbitrarily large chemical mechanisms, at constant computational costs and for modeling of turbulence-chemsitry interactions.

In the first method a coordinate transform from time and space to a space described by mixture-fraction and a progress variable is made. the method was shown to be capabale of predicting the response of injection pressure and nozzle diameter on lift-off length. It was shown that it was possible to apply the method for use in computational fluid dynamics simulations of compression-ignited engine combustion.

In the second method, the transient flamelet libraries were directly used in an interactive flamelet setting. It was investigated if it was possible to generate tables by computing homogeneous adiabatic constant-pressure reactors instead of igniting flamelets. It was found that omitting the effect of scalar dissipation rate during the tabulation process leads to an error in prediction of ignition delay.

In the third method a simplified conditional moment closure approach was developed. By using tabulated chemsitry, and by making the conditional moment closure for the progress variable only, it was possible to use the same computational grid as used by the flow solver for the spatial transport of the conditionally averaged scalars. This method was tested for a simple autoigniting spray configuration and it was found that it was able of capturing the response of the ignition dlay and lift-off length due to changed ambient oxygen level. Software technical improvements from the transient flamelet library based approaches were carried over the stationary flamelet library based on soot source term model, and further model updates yielded a model capable of predicting soot emissions for a light-duty diesel engine.

Abstract

The virtual development of future Spark Ignition (SI) engine combustion processes in three-dimensional Computational Fluid Dynamics (3D-CFD) demands for the integration of detailed chemsitry, enable - additionally to the 3D-CFD modeling of flow and mixture formation - the prediction of fuel-dependent SI engine combustion in all its complexity. the conflict of goal arising in coupling 3D-CFD calculations with detailed chemistry is to keep computational costs low while achieving accurate results.

This work presents an approach which constitutes a coupled solution for flame propagation, autoignition and emission formation modeling incorporating detailed chemsitry, while exhibiting low computational costs.

For modeling the regular flame propagation, a laminar flamelet approach, the G-equation is used. This approach describes the flame propagation based on the turbulent flame speed, which is determined by the turbulence and the fuel-specific laminar flame speed. The latter one is incorporated using an adequate fitting function.

Auto-ignition phenomena are addressed using an integrated flamelet approach, which bases on the tabulation of fuel-dependant reaction kinetics. By introducing a progress variable for the autoignition - the Ignition progress Variable (IPV) - detailed chemistry is integrated in 3D-CFD. the tabulation approach only demands for the soltuion of the IPV transport equation, thus keeping the computational demand low, while allowing the consideration of local effects on auotigntion chemsitry on cell level.

The modeling of emissions formation bases on an interactively coupled flamelet approach, the Transient interactive Flamelet model. By transforming the species balance equations into a one-dimensional form, the numerical effort incorporated with the solution of small chemical time-scales is separated from the 3D-CFD flow field solution. Thus, the emission formation is calculated under representative boundary conditions. the description of the soot formation bases on a detailed soot model, and the properties of the soot Particle Size Dsitribution Function are calculated using the method of moments.

The coupling between the G-equation, integrated flamelet, and interactive flamelet models is done based on the IPV. The functionality of the combined approach to model the variety of SI enigne combustion phenomena is prooved first in terms of fundamentals and standalone sub-model functionality studies. For standalone and model coupling functionality studies, a simplified test case is introduced, representing an adiabatic pressure vessel without moving meshes. the vessel is initialised homogeneously, allowing the selective investigation of different parameters on combutsion process and direct comparison with direct numerical solution of the detailed chemistry in 0D homogeneous reactor calculations. Following the basic functionality studies, the standalone and combined sub-model functionalities are investigated in adequate engine test cases.