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[ES] La actual crisis climática ha instado a la comunidad investigadora y a los fabricantes a brindar soluciones para hacer que el sector del transporte sea más sostenible. De entre las diversas tecnologías propuestas, la combustión a baja temperatura ha sido objeto de una extensa investigación. La combustión premezclada dual-fuel es uno de los conceptos que abordan el compromiso de NOx-hollín en motores de encendido por compresión manteniendo alta eficiencia térmica. Esta combustión hace uso de dos combustibles con diferentes reactividades para mejorar la controlabilidad de este modo de combustión en un amplio rango de funcionamiento. De manera similar a todos los modos de combustión premezclados, esta combustión es sensible a las condiciones de operación y suele estar sujeta a variabilidad cíclica con gradientes de presión significativos. En consecuencia, se requieren estrategias de control avanzadas para garantizar un funcionamiento seguro y preciso del motor. El control en bucle cerrado es una herramienta eficaz para abordar los desafíos que plantea la combustión premezclada dual-fuel. En este tipo de control, para mantener el funcionamiento deseado, las acciones de control se adaptan y corrigen a partir de una retroalimentación con las señales de salida del motor. Esta tesis presenta estrategias de control basadas en la medición de la señal de presión en el cilindro, aplicadas a motores de combustión premezclada dual-fuel. En ella se resuelven diversos aspectos del funcionamiento del motor mediante el diseño de controladores dedicados, haciéndose especial énfasis en analizar e implementar estas soluciones a los diferentes niveles de estratificación de mezcla considerados en estos motores (es decir, totalmente, altamente y parcialmente premezclada). Inicialmente, se diseñan estrategias de control basadas en el procesamiento de la señal de presión en el cilindro y se seleccionan acciones proporcionales-integrales para asegurar el rendimiento deseado del motor sin, [CA] L'actual crisi climàtica ha instat a la comunitat investigadora i als fabricants a brindar solucions per a fer que el sector del transport siga més sostenible. D'entre les diverses tecnologies proposades, la combustió a baixa temperatura ha sigut objecte d'una extensa investigació. La combustió premesclada dual-fuel és un dels conceptes que aborden el compromís de NOx-sutge en motors d'encesa per compressió mantenint alta eficiència tèrmica. Aquesta combustió fa ús de dos combustibles amb diferents reactivitats per a millorar la controlabilitat d'aquest tipus de combustió en un ampli rang de funcionament. De manera similar a tots els tipus de combustió premesclada, aquesta combustió és sensible a les condicions d'operació i sol estar subjecta a variabilitat cíclica amb gradients de pressió significatius. En conseqüència, es requereixen estratègies de control avançades per a garantir un funcionament segur i precís del motor. El control en bucle tancat és una eina eficaç per a abordar els desafiaments que planteja la combustió premesclada dual-fuel. En aquesta mena de control, per a mantindre el funcionament desitjat, les accions de control s'adapten i corregeixen a partir d'una retroalimentació amb els senyals d'eixida del motor. Aquesta tesi presenta estratègies de control basades en el mesurament del senyal de pressió en el cilindre, aplicades a motors de combustió premesclada dual-fuel. En ella es resolen diversos aspectes del funcionament del motor mitjançant el disseny de controladors dedicats, fent-se especial èmfasi a analitzar i implementar aquestes solucions als diferents nivells d'estratificació de mescla considerats en aquests motors (és a dir, totalment, altament i parcialment premesclada). Inicialment, es dissenyen estratègies de control basades en el processament del senyal de pressió en el cilindre i se seleccionen accions proporcionals-integrals per a assegurar el rendiment desitjat del motor sense excedir les limitacions mecàniques del motor. Ta, [EN] The current climate crisis has urged the research community and manufacturers to provide solutions to make the transportation sector cleaner. Among the various technologies proposed, low temperature combustion has undergone extensive investigation. Premixed dual-fuel combustion is one of the concepts addressing the NOx-soot trade-off in compression ignited engines, while maintaining high thermal efficiency. This combustion makes use of two fuels with different reactivities in order to improve the controllability of this combustion mode over a wide range of operation. Similarly to all premixed combustion modes, this combustion is nevertheless sensitive to the operating conditions and traditionally exhibits cycle-to-cycle variability with significant pressure gradients. Consequently, advanced control strategies to ensure a safe and accurate operation of the engine are required. Feedback control is a powerful approach to address the challenges raised by the premixed dual-fuel combustion. By measuring the output signals from the engine, strategies can be developed to adapt and correct the control actions to maintain the desired operation. This thesis presents control strategies, based on the in-cylinder pressure signal measurement, applied to premixed dual-fuel combustion engines. Various objectives were addressed by designing dedicated controllers, where a special emphasis was made towards analyzing and implementing these solutions to the different levels of mixture stratification considered in these engines (i.e., fully, highly and partially premixed). At first, feedback control strategies based on the in-cylinder pressure signal processing were designed. Proportional-integral actions were selected to ensure the desired engine performance without exceeding the mechanical constraints of the engine. Extremum seeking was evaluated to track efficient combustion phasing and NOx emissions reduction. The in-cylinder pressure resonance was then analyzed and a knock-like

This paper provides a numerical study on n-dodecane flames using Large-Eddy Simula- tions (LES) along with the Flamelet Generated Manifold (FGM) method for combustion modeling. The computational setup follows the Engine Combustion Network Spray A operating condition, which consists of a single-hole spray injection into a constant volume vessel. Herein we propose a novel approach for the coupling of the energy equation with the FGM database for spray combustion simulations. Namely, the energy equation is solved in terms of the sensible enthalpy, while the heat of combustion is calculated from the FGM database. This approach decreases the computational cost of the simulation because it does not require a precise computation of the entire composition of the mixture. The flamelet database is generated by simulating a series of counterflow diffusion flames with two popu- lar chemical kinetics mechanisms for n-dodecane. Further, the secondary breakup of the droplet is taken into account by a recently developed modified version of the Taylor Anal- ogy Breakup model. The numerical results show that the proposed methodology captures accurately the main characteristics of the reacting spray, such as mixture formation, igni- tion delay time, and flame lift-off. Additionally, it captures the “cool flame" between the flame lift-off and the injection nozzle. Overall, the simulations show differences between the two kinetics mechanisms regarding the ignition characteristics, while similar flame structures are observed once the flame is stabilised at the lift-off distance.

This thesis consists of a detailed study of spray combustion of diesel and biodiesel fuels. Our study is focused on the complex physical and chemical phenomena that take place in such flows and consists of three parts. In the first part, we investigate atomization and droplet breakup of a non-reactive fuel spray via Large-Eddy Simulations (LES). To this end, we compare three popular droplet-breakup models, the Taylor Analogy Breakup (TAB), Reitz-Diwakar and Pilch-Erdman models. Further, we propose a modification of the original TAB model and assess its predictive capacity. According to our numerical tests, the proposed modification leads to improved accuracy in the computation of important properties of the spray, such as the liquid- and vapor-penetration distance. In the second part, we present a numerical study of n-dodecane spray flames via LES. With regard to combustion modelling and turbulence-chemistry interaction, we employ the Flamelet-Generated Manifolds (FGM). Herein, we propose a novel approach for the coupling of the energy equation with the FGM database that offers considerable computational savings. The efficiency of this approach is assessed via comparisons with experimental data and earlier numerical results for n-dodecane spray flames. In the last part, we present an LES study of spray combustion of Karanja Oil Methyl Ester. First, we examine the influence of the initial oxygen concentration level on the spray ignition and the flame topology. Further, we compare the combustion processes of the biodiesel surrogate with the corresponding diesel surrogate (n-dodecane). Finally, we assess the impact of the thermophysical properties of the biodiesel on the evolution of the spray and the overall combustion process., (SC - Sciences) -- UCL, 2022

Industrial gas turbines constitute an integral part of today’s electricalpower production infrastructure. Reacting gas flows in these machines arevery interesting and complex in nature since they exhibit highly turbulentbehaviour which is strongly coupled to the chemical reaction dynamics.Thus developing accurate CFD models for such flows while keeping thecomputational expense reasonable is a non-trivial task. In this studyLarge-Eddy simulations of hydrogen-methane fuel mixture combustion inthe SGT800’s (Siemens Energy Gas turbine 800) burner, in atmosphericconditions are performed using the CFD code Starccm+ (versions 16.04and 16.06).

Industrial gas turbines constitute an integral part of today’s electricalpower production infrastructure. Reacting gas flows in these machines arevery interesting and complex in nature since they exhibit highly turbulentbehaviour which is strongly coupled to the chemical reaction dynamics.Thus developing accurate CFD models for such flows while keeping thecomputational expense reasonable is a non-trivial task. In this studyLarge-Eddy simulations of hydrogen-methane fuel mixture combustion inthe SGT800’s (Siemens Energy Gas turbine 800) burner, in atmosphericconditions are performed using the CFD code Starccm+ (versions 16.04and 16.06).

In this chapter, the research dedicated to moderate or intense low-oxygen dilution (MILD) combustion (also called flameless combustion) that relied on direct numerical simulations (DNS) is summarized. In particular, the various DNS carried out are detailed and three different configurations are considered: the autoigniting mixing layer between fuel and hot and diluted oxidizer, the premixed MILD combustion resulting from internal exhaust gas recirculation, and the nonpremixed MILD combustion with internal exhaust gas recirculation. Focus is placed here on different aspects of MILD combustion. First, works that relate to the onset of MILD combustion and the apparition of the initial ignition kernels are discussed, in particular, a summary is provided on the findings that show the particular physics of MILD combustion, where the initial ignition kernels are mostly related to the distribution of mixture fraction and recirculating radicals. Subsequently, the identified physical mechanisms involved in the development of those ignition kernels are summarized. In particular, focus is placed on the balance between ignition and deflagrative mechanisms. Using different analysis methods, the works summarized here show that, while there is a coexistence between ignition and deflagration, ignition is the main contributor to the overall heat release. Finally, the implications of these findings on the modeling of MILD combustion are discussed through various studies that assessed a priori different modeling frameworks for MILD combustion. In those, models that capture this essential and dominant ignition behavior of MILD combustion were shown to be more accurate., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Aerodynamics

Computational fluid dynamics analysis represents a useful approach to design and develop new engine concepts and investigate advanced combustion modes. Large chemical mechanisms are required for a correct description of the combustion process, especially for the prediction of pollutant emissions. Tabulated chemistry models allow to reduce significantly the computational cost, maintaining a good accuracy. In the present work, an investigation of tabulated approaches, based on flamelet assumptions, is carried out to simulate turbulent Diesel combustion in the Spray A framework. The Approximated Diffusion Flamelet is tested under different ambient conditions and compared with Flamelet Generated Manifold, and both models are validated with Engine Combustion Network experimental data. Flame structure, combustion process and soot formation were analyzed in this work. Computed results confirm the impact of the turbulent–chemistry interaction on the ignition event. Therefore, a new look-up table concept Five-Dimensional-Flamelet Generated Manifold, that accounts for an additional dimension (strain rate), has been developed and tested, giving promising results.

Computational fluid dynamics analysis represents a useful approach to design and develop new engine concepts and investigate advanced combustion modes. Large chemical mechanisms are required for a correct description of the combustion process, especially for the prediction of pollutant emissions. Tabulated chemistry models allow to reduce significantly the computational cost, maintaining a good accuracy. In the present work, an investigation of tabulated approaches, based on flamelet assumptions, is carried out to simulate turbulent Diesel combustion in the Spray A framework. The Approximated Diffusion Flamelet is tested under different ambient conditions and compared with Flamelet Generated Manifold, and both models are validated with Engine Combustion Network experimental data. Flame structure, combustion process and soot formation were analyzed in this work. Computed results confirm the impact of the turbulent–chemistry interaction on the ignition event. Therefore, a new look-up table concept Five-Dimensional-Flamelet Generated Manifold, that accounts for an additional dimension (strain rate), has been developed and tested, giving promising results.

[EN] An implementation and validation of the coupled Sigma-gamma ADF model is presented in this work for reacting Diesel spray CFD simulations under a RANS turbulence modeling approach. An Approximated Diffusion Flamelet (ADF) model Michel et al. (2008) implemented in the OpenFOAM CFD open-source library by Winklinger (2014)15 fed with the spray description, i.e. mixing formation process, provided by the Sigma-gamma Eulerian atomization model Garcia-Oliver et al. (2013). In the present investigation, the Engine Combustion Network Spray A reference configuration is used for validation. Specifically, the model can provide accurate predictions of typical reacting spray metrics, such as the ignition delay and the lift-off length. Moreover, the internal structure is also fairly reproduced in terms of quasi-steady spatial distribution of formaldehyde and OH, related with low and high temperature reactions respectively. Additionally, modeling results have been compared to recent Particle image velocimetry (PIV) measurements Garcia-Oliver et al. (2017) under both inert and reacting conditions. Flow response to heat release is quantitatively predicted by the model, both in terms of local velocity increase as well as radial dilation. The model has been used to understand combustion-induced reduction in entrainment, in particular around the lift-off length location. Flow confinement does not seem to influence the global flame behaviour, even though some changes in the local flow hint can be observed when moving from an open to a closed domain. (C) 2017 Elsevier Ltd. All rights reserved.

[EN] An implementation and validation of the coupled Sigma-gamma ADF model is presented in this work for reacting Diesel spray CFD simulations under a RANS turbulence modeling approach. An Approximated Diffusion Flamelet (ADF) model Michel et al. (2008) implemented in the OpenFOAM CFD open-source library by Winklinger (2014)15 fed with the spray description, i.e. mixing formation process, provided by the Sigma-gamma Eulerian atomization model Garcia-Oliver et al. (2013). In the present investigation, the Engine Combustion Network Spray A reference configuration is used for validation. Specifically, the model can provide accurate predictions of typical reacting spray metrics, such as the ignition delay and the lift-off length. Moreover, the internal structure is also fairly reproduced in terms of quasi-steady spatial distribution of formaldehyde and OH, related with low and high temperature reactions respectively. Additionally, modeling results have been compared to recent Particle image velocimetry (PIV) measurements Garcia-Oliver et al. (2017) under both inert and reacting conditions. Flow response to heat release is quantitatively predicted by the model, both in terms of local velocity increase as well as radial dilation. The model has been used to understand combustion-induced reduction in entrainment, in particular around the lift-off length location. Flow confinement does not seem to influence the global flame behaviour, even though some changes in the local flow hint can be observed when moving from an open to a closed domain. (C) 2017 Elsevier Ltd. All rights reserved.

[EN] The ability of flamelet models to reproduce turbulent combustion in devices such as diesel engines or gas turbines has enhanced the usage of these approaches in Computational Fluid Dynamics (CFD) simulations. The models based on turbulent look-up tables gener- ated from counterflow laminar diffusion flames (DF model) permit drastic reduction of the computational cost of the CFD calculation. Nevertheless, for complex molecular fuels, such as n-heptane, the oxidation process involves hundreds of species and the calculation of the transport equations together with the ODE system that models the chemical kinetics for the DF solution becomes unaffordable for industrial devices where hundreds of flamelets are required. In this context, new hypotheses have to be introduced in order to reduce the computational cost maintaining the coherence of the combustion process. Recently, a new model known as Approximated Diffusion Flamelet (ADF) has been proposed with the aim of solving the turbulent combustion for complex fuels in a reduced time. However, the validity of this model is still an open question and has to be verified in order to justify subsequent CFD calculations. This work assesses the ADF model and its ability to repro- duce accurately the combustion process and its main parameters for three fuels with dif- ferent chemical complexity and boundary conditions by its comparison with the DF model. Results show that although some discrepancies arise, the ADF model has the ability to cor- rectly describe the ignition delay and the combustion structure in the auto-ignition zone that is the most relevant one for industrial processes.

[EN] The ability of a computational fluid dynamics (CFD) simulation to reproduce the diesel-like reacting spray ignition process and its corresponding flame structure strongly depends on both the fidelity of the chemical mechanism for reproducing the oxidation of the fuel and also on how the turbulence-chemistry interaction (TCI) is modeled. Therefore, investigating the performance of different chemical mechanisms not only in perfect stirred reactors but directly in the diesel-like spray itself is of great interest in order to evaluate their suitability for being further applied to CFD engine simulations. This research work focuses on applying a presumed probability density function (PDF) unsteady flamelet combustion model to the well-known spray A from the Engine Combustion Network (ECN), using three chemical mechanisms widely accepted by the scientific community as a way to figure out the influence of chemistry in the key characteristics of the combustion process in the frame of diesel-like spray simulations. Results confirm that in spite of providing all of them correct trends for ignition delays (ID) and lift-off lengths (LOL), when comparing with experimental results, the structure of the flame presents noticeable differences, especially in the low and intermediate temperatures and high equivalence ratio regions. Consequently, the selection of the chemical mechanism has an impact on the zones of influence of key species as observed in both spatial coordinates and also in the equivalence ratio-temperature maps. These differences are expected to be relevant considering the implications when coupling pollutant emissions models. The analysis of temperature and oxygen concentration parametric studies evidences how the observed differences are consistent and moderately dependent on the ambient conditions.

[EN] The ability of flamelet models to reproduce turbulent combustion in devices such as diesel engines or gas turbines has enhanced the usage of these approaches in Computational Fluid Dynamics (CFD) simulations. The models based on turbulent look-up tables gener- ated from counterflow laminar diffusion flames (DF model) permit drastic reduction of the computational cost of the CFD calculation. Nevertheless, for complex molecular fuels, such as n-heptane, the oxidation process involves hundreds of species and the calculation of the transport equations together with the ODE system that models the chemical kinetics for the DF solution becomes unaffordable for industrial devices where hundreds of flamelets are required. In this context, new hypotheses have to be introduced in order to reduce the computational cost maintaining the coherence of the combustion process. Recently, a new model known as Approximated Diffusion Flamelet (ADF) has been proposed with the aim of solving the turbulent combustion for complex fuels in a reduced time. However, the validity of this model is still an open question and has to be verified in order to justify subsequent CFD calculations. This work assesses the ADF model and its ability to repro- duce accurately the combustion process and its main parameters for three fuels with dif- ferent chemical complexity and boundary conditions by its comparison with the DF model. Results show that although some discrepancies arise, the ADF model has the ability to cor- rectly describe the ignition delay and the combustion structure in the auto-ignition zone that is the most relevant one for industrial processes.

[EN] In the present investigation the Spray A reference configuration defined in the framework of the Engine Combustion Network (ECN) has been modeled by means of an Unsteady Flamelet Model (USFM) including detailed parametric studies to evaluate the impact of ambient temperature, oxygen concentration and density. The study focuses on the analysis of the spray ignition delay, the flame lift-off length and the internal structure of the spray and flame according to the experimental information nowadays available for validating the results provided by the model. Promising results are obtained for the nominal case and also for the parametric variations (temperature, oxygen...) in terms of liquid and vapor penetration, ignition delay (ID) and lift-off length (LOL). The model permits to predict the ID and the LOL which constitute two parameters of key importance for describing the characteristics of transient reacting sprays. Valuable insight on the details of the combustion process is obtained from the analysis of formaldehyde (CH2O), acetylene (C2H2) and hydroxide (OH) species in spatial coordinates and also in the so-called phi-T maps. Important differences arise in the inner structure of the flame in the quasi-steady regime, which is closely linked to soot formation, when varying the ambient boundary conditions. Additionally, the auto-ignition process is investigated in order to describe in detail the spatial onset and propagation of combustion. Results confirm the impact of the ambient conditions on the regions of the spray where start of combustion takes place, so the relation between the local scalar dissipation rate and mixture fraction variance is also discussed. This investigation provides an insight of the potential of the USFM combustion model to describe the physical and chemical processes involved in transient spray combustion.

[EN] The ability of a computational fluid dynamics (CFD) simulation to reproduce the diesel-like reacting spray ignition process and its corresponding flame structure strongly depends on both the fidelity of the chemical mechanism for reproducing the oxidation of the fuel and also on how the turbulence-chemistry interaction (TCI) is modeled. Therefore, investigating the performance of different chemical mechanisms not only in perfect stirred reactors but directly in the diesel-like spray itself is of great interest in order to evaluate their suitability for being further applied to CFD engine simulations. This research work focuses on applying a presumed probability density function (PDF) unsteady flamelet combustion model to the well-known spray A from the Engine Combustion Network (ECN), using three chemical mechanisms widely accepted by the scientific community as a way to figure out the influence of chemistry in the key characteristics of the combustion process in the frame of diesel-like spray simulations. Results confirm that in spite of providing all of them correct trends for ignition delays (ID) and lift-off lengths (LOL), when comparing with experimental results, the structure of the flame presents noticeable differences, especially in the low and intermediate temperatures and high equivalence ratio regions. Consequently, the selection of the chemical mechanism has an impact on the zones of influence of key species as observed in both spatial coordinates and also in the equivalence ratio-temperature maps. These differences are expected to be relevant considering the implications when coupling pollutant emissions models. The analysis of temperature and oxygen concentration parametric studies evidences how the observed differences are consistent and moderately dependent on the ambient conditions.

[EN] The ability of a computational fluid dynamics (CFD) simulation to reproduce the diesel-like reacting spray ignition process and its corresponding flame structure strongly depends on both the fidelity of the chemical mechanism for reproducing the oxidation of the fuel and also on how the turbulence-chemistry interaction (TCI) is modeled. Therefore, investigating the performance of different chemical mechanisms not only in perfect stirred reactors but directly in the diesel-like spray itself is of great interest in order to evaluate their suitability for being further applied to CFD engine simulations. This research work focuses on applying a presumed probability density function (PDF) unsteady flamelet combustion model to the well-known spray A from the Engine Combustion Network (ECN), using three chemical mechanisms widely accepted by the scientific community as a way to figure out the influence of chemistry in the key characteristics of the combustion process in the frame of diesel-like spray simulations. Results confirm that in spite of providing all of them correct trends for ignition delays (ID) and lift-off lengths (LOL), when comparing with experimental results, the structure of the flame presents noticeable differences, especially in the low and intermediate temperatures and high equivalence ratio regions. Consequently, the selection of the chemical mechanism has an impact on the zones of influence of key species as observed in both spatial coordinates and also in the equivalence ratio-temperature maps. These differences are expected to be relevant considering the implications when coupling pollutant emissions models. The analysis of temperature and oxygen concentration parametric studies evidences how the observed differences are consistent and moderately dependent on the ambient conditions.

[EN] The ability of a computational fluid dynamics (CFD) simulation to reproduce the diesel-like reacting spray ignition process and its corresponding flame structure strongly depends on both the fidelity of the chemical mechanism for reproducing the oxidation of the fuel and also on how the turbulence-chemistry interaction (TCI) is modeled. Therefore, investigating the performance of different chemical mechanisms not only in perfect stirred reactors but directly in the diesel-like spray itself is of great interest in order to evaluate their suitability for being further applied to CFD engine simulations. This research work focuses on applying a presumed probability density function (PDF) unsteady flamelet combustion model to the well-known spray A from the Engine Combustion Network (ECN), using three chemical mechanisms widely accepted by the scientific community as a way to figure out the influence of chemistry in the key characteristics of the combustion process in the frame of diesel-like spray simulations. Results confirm that in spite of providing all of them correct trends for ignition delays (ID) and lift-off lengths (LOL), when comparing with experimental results, the structure of the flame presents noticeable differences, especially in the low and intermediate temperatures and high equivalence ratio regions. Consequently, the selection of the chemical mechanism has an impact on the zones of influence of key species as observed in both spatial coordinates and also in the equivalence ratio-temperature maps. These differences are expected to be relevant considering the implications when coupling pollutant emissions models. The analysis of temperature and oxygen concentration parametric studies evidences how the observed differences are consistent and moderately dependent on the ambient conditions.

[EN] In the present investigation the Spray A reference configuration defined in the framework of the Engine Combustion Network (ECN) has been modeled by means of an Unsteady Flamelet Model (USFM) including detailed parametric studies to evaluate the impact of ambient temperature, oxygen concentration and density. The study focuses on the analysis of the spray ignition delay, the flame lift-off length and the internal structure of the spray and flame according to the experimental information nowadays available for validating the results provided by the model. Promising results are obtained for the nominal case and also for the parametric variations (temperature, oxygen...) in terms of liquid and vapor penetration, ignition delay (ID) and lift-off length (LOL). The model permits to predict the ID and the LOL which constitute two parameters of key importance for describing the characteristics of transient reacting sprays. Valuable insight on the details of the combustion process is obtained from the analysis of formaldehyde (CH2O), acetylene (C2H2) and hydroxide (OH) species in spatial coordinates and also in the so-called phi-T maps. Important differences arise in the inner structure of the flame in the quasi-steady regime, which is closely linked to soot formation, when varying the ambient boundary conditions. Additionally, the auto-ignition process is investigated in order to describe in detail the spatial onset and propagation of combustion. Results confirm the impact of the ambient conditions on the regions of the spray where start of combustion takes place, so the relation between the local scalar dissipation rate and mixture fraction variance is also discussed. This investigation provides an insight of the potential of the USFM combustion model to describe the physical and chemical processes involved in transient spray combustion.

[EN] The ability of a computational fluid dynamics (CFD) simulation to reproduce the diesel-like reacting spray ignition process and its corresponding flame structure strongly depends on both the fidelity of the chemical mechanism for reproducing the oxidation of the fuel and also on how the turbulence-chemistry interaction (TCI) is modeled. Therefore, investigating the performance of different chemical mechanisms not only in perfect stirred reactors but directly in the diesel-like spray itself is of great interest in order to evaluate their suitability for being further applied to CFD engine simulations. This research work focuses on applying a presumed probability density function (PDF) unsteady flamelet combustion model to the well-known spray A from the Engine Combustion Network (ECN), using three chemical mechanisms widely accepted by the scientific community as a way to figure out the influence of chemistry in the key characteristics of the combustion process in the frame of diesel-like spray simulations. Results confirm that in spite of providing all of them correct trends for ignition delays (ID) and lift-off lengths (LOL), when comparing with experimental results, the structure of the flame presents noticeable differences, especially in the low and intermediate temperatures and high equivalence ratio regions. Consequently, the selection of the chemical mechanism has an impact on the zones of influence of key species as observed in both spatial coordinates and also in the equivalence ratio-temperature maps. These differences are expected to be relevant considering the implications when coupling pollutant emissions models. The analysis of temperature and oxygen concentration parametric studies evidences how the observed differences are consistent and moderately dependent on the ambient conditions.

El modelado CFD se ha convertido en una herramienta aceptada y ampliamente utilizada en el ámbito del diseño de motores de combustión interna alternativos. Los modelos de combustión avanzados ayudan a comprender los fenómenos complejos químicos y físicos del proceso de combustión y aportan información detallada que no se puede obtener con experimentos. Indudablemente, el modelado del proceso de combustión turbulenta parcialmente premezclada característico de los chorros Diesel es particularmente difícil y por lo tanto es un tema de gran interés para la comunidad científica. Los retos más importantes del modelado de este tipo de llamas son la predicción del proceso del auto-encendido, caracterizado por el tiempo de retraso, y la estructura de la llama cuasi-estacionaria con su característica longitud de lift-off. Estos dos parámetros globales de los chorros Diesel son importantes por varios aspectos. Primero, es relativamente sencillo medir estos dos parámetros y por lo tanto utilizarlos para la validación de modelos y segundo, son factores determinantes en el proceso de la combustión en un motor. El auto-encendido marca el inicio de la tasa de liberación de calor y la longitud de lift-off desempeña un papel fundamental en la formación de hollín. El mecanismo de estabilización de la llama en la zona del lift-off todavía no es bien conocido aunque existen diferentes teorías en la literatura, por lo que su modelado es en la actualidad un reto no resuelto. De acuerdo con el contexto descrito previamente, en este trabajo se pretende implementar un modelo de combustión integrado en un solver RANS utilizando la plataforma CFD OpenFOAM de código abierto. El modelo propuesto está basado en el concepto de flamelets usando una química detallada combinado con funciones de probabilidad determinadas a priori (presumed-PDF) para considerar el efecto de interacción entre la química y las características del flujo turbulento, que implica hipótesis importantes. En primer lugar, con e

[EN[ This study is focused on a non-premixed combustion model based on the laminar flames, called flamelets, concept. Solving these flamelets can be very expensive in terms of computational cost so new assumptions must be considered. In this study two models are compared, a complete model called Diffusion Flamelet (DF) against a simplified model called Approximated Diffusion Flamelet (ADF) in order to determine its validity. For this purpose, the steady solutions are evaluated with both models with the S-curves and the unsteday solutions are compared in terms of the ignition delay, the solution profiles in the mixture fraction space and the values of the transient term for several fuels, [ES] Este estudio está centrado en un modelo de combustión no premezclada basado en el concepto de llamas laminares, denominadas flamelets. Resolver estas flamelets puede ser muy costoso computacionalmente por lo que nuevas hipótesis deben ser consideradas. En este estudio dos modelos son comparados, un modelo completo denominado Diffusion Flamelet (DF) y un modelo simplificado denominado Approximated Diffusion Flamelet (ADF) con el fin de determinar su validez. Para ello, las soluciones permanentes son evaluadas con ambos modelos con las curvas S y las soluciones transitorias son comparadas en términos de tiempo de retraso, perfiles solución en el espacio fracción de mezcla y valores del término transitorio para distintos combustibles

[EN[ This study is focused on a non-premixed combustion model based on the laminar flames, called flamelets, concept. Solving these flamelets can be very expensive in terms of computational cost so new assumptions must be considered. In this study two models are compared, a complete model called Diffusion Flamelet (DF) against a simplified model called Approximated Diffusion Flamelet (ADF) in order to determine its validity. For this purpose, the steady solutions are evaluated with both models with the S-curves and the unsteday solutions are compared in terms of the ignition delay, the solution profiles in the mixture fraction space and the values of the transient term for several fuels, [ES] Este estudio está centrado en un modelo de combustión no premezclada basado en el concepto de llamas laminares, denominadas flamelets. Resolver estas flamelets puede ser muy costoso computacionalmente por lo que nuevas hipótesis deben ser consideradas. En este estudio dos modelos son comparados, un modelo completo denominado Diffusion Flamelet (DF) y un modelo simplificado denominado Approximated Diffusion Flamelet (ADF) con el fin de determinar su validez. Para ello, las soluciones permanentes son evaluadas con ambos modelos con las curvas S y las soluciones transitorias son comparadas en términos de tiempo de retraso, perfiles solución en el espacio fracción de mezcla y valores del término transitorio para distintos combustibles

The oxidation and pyrolysis of a variety of fuels of relevance to air breathing propulsion were studied experimentally behind reflected shock waves, in flow reactors, as well as in laminar and turbulent flames. The measurements included ignition delays and species time evolutions in shock tubes, species profiles in flow reactors, and propagation speeds and ignition/extinction limits of laminar flames. Additionally, a facility was constructed that allows for the study of turbulent flames at very high Re numbers and fuels as heavy as n-dodecane. The main goal of the completed program was to identify the important kinetic pathways of jet fuel pyrolysis and oxidation under conditions that mimic those encountered in jet engines, and to develop appropriate models for real fuels based on the experimental data. This was a collaborative three-year research effort between the following universities and investigators: University of Southern California: F.N. Egolfopoulos (PI), H. Wang (co-PI); Drexel University: N.P. Cernansky (co-PI), D.L. Miller (co-PI); Princeton University: C.K. Law (co-PI); Stanford University: C.T. Bowman (co-PI), R.K. Hanson (co-PI), H. Wang (co-PI)., Prepared in cooperation with Drexel University, Princeton University, and Stanford University. The original document contains color images.

Emission and fuel consumption are among the key parameters when designing a combustion system. Combustion CFD can assist in this task only if good enough accuracy is achieved regarding combustion and emission predictions. The aim of this master thesis is to evaluate the use of detailed reaction mechanisms (as a substitute for standard combustion model) in terms of computational time and result accuracy. Several mechanisms for n-heptane are tested. Lund University optical engine experimental case is used for this evaluation.Results showed that detailed chemistry can predict ignition accurately but differences are observed in the peak cylinder pressure. The computational time also increased significantly as size and complexity of the mechanism increased. Recommendations are given to improve predictions using detailed chemistry approach which is found to be an interesting approach especially for lift-off length predictions.

Emission and fuel consumption are among the key parameters when designing a combustion system. Combustion CFD can assist in this task only if good enough accuracy is achieved regarding combustion and emission predictions. The aim of this master thesis is to evaluate the use of detailed reaction mechanisms (as a substitute for standard combustion model) in terms of computational time and result accuracy. Several mechanisms for n-heptane are tested. Lund University optical engine experimental case is used for this evaluation.Results showed that detailed chemistry can predict ignition accurately but differences are observed in the peak cylinder pressure. The computational time also increased significantly as size and complexity of the mechanism increased. Recommendations are given to improve predictions using detailed chemistry approach which is found to be an interesting approach especially for lift-off length predictions.

Chemistry tabulation techniques such as flamelet models are a popular way to account for detailed chemistry effects in numerical simulation. These techniques are based on the identification of a low dimensional manifold in chemical space that accurately represents chemical evolutions associated to a specific combustion regime. During the last years, several authors used the Principal Component Analysis (PCA) to identify low dimensional manifold for combustion problems. However, full coupling between this manifold and flow solver has not yet been performed to the authors knowledge. The present paper introduces a new approach called Manifold Generated by a Local PCA or MG-L-PCA, which fully couple the manifold identified by a PCA and a DNS flow solver. The first part of the paper presents the PCA approach. Then, the coupling between this manifold and a DNS solver is presented. The MG-L-PCA approach is finally validated against a DNS simulation of flame vortex interaction using both detailed mechanism and a FPI manifold. Unlike FPI, the MG-L-PCA reproduces the dispersion in the chemical space induced by the flame-vortex interaction both for the species and the source terms., SCOPUS: ar.j, info:eu-repo/semantics/published

Chemistry tabulation techniques such as flamelet models are a popular way to account for detailed chemistry effects in numerical simulation. These techniques are based on the identification of a low dimensional manifold in chemical space that accurately represents chemical evolutions associated to a specific combustion regime. During the last years, several authors used the Principal Component Analysis (PCA) to identify low dimensional manifold for combustion problems. However, full coupling between this manifold and flow solver has not yet been performed to the authors knowledge. The present paper introduces a new approach called Manifold Generated by a Local PCA or MG-L-PCA, which fully couple the manifold identified by a PCA and a DNS flow solver. The first part of the paper presents the PCA approach. Then, the coupling between this manifold and a DNS solver is presented. The MG-L-PCA approach is finally validated against a DNS simulation of flame vortex interaction using both detailed mechanism and a FPI manifold. Unlike FPI, the MG-L-PCA reproduces the dispersion in the chemical space induced by the flame-vortex interaction both for the species and the source terms., SCOPUS: ar.j, info:eu-repo/semantics/published

The goals of this project were the development of new sub-filter models for large eddy simulation of turbulent combustion and of chemical mechanisms for jet fuel surrogates. The sub-filter modeling work focuses on the development of a framework for describing multiple combustion regimes using the flamelet approach, on describing the scalar dissipation rate in turbulent non-premixed combustion, and on modeling strain effects in turbulent premixed combustion. The chemistry work proposes a method for defining jet fuel surrogates, describes how different sub-mechanisms can be incorporated into the definition, and finally creates and validates a mechanism that serves as an accurate surrogate for jet fuel behavior., The original document contains color images.

Long-duration, kHz-framerate OH-PLIF measurements were analyzed to determine the physical mechanism responsible for a previously observed phenomenon wherein isolated pockets of high OH-concentration fluid suddenly appear in regions of unburned gas away from the contiguous flame zone in a gas turbine model combustor. Prior research on the burners indicate these isolated flame-kernels may result either from auto-ignition of hot, unburned reactants upstream of the primary flame or from transport of reacting fluid into the field of view from beyond the imaging plane. An image-processing routine was developed to autonomously identify and statistically characterize these flame-kernels. Phase sorting of the kernel centroids with respect to the dominant fluid-dynamic structure of the combustors (a helical precessing vortex core or thermo-acoustically forced shear-layer vortices) indicate through-plane transport of reacting fluid best explains their sudden appearance the PLIF images. The concentration of flame-kernel events around the periphery of the mean location of the precessing vortex core (PVC) indicates they are likely the result of wrinkling and break-up of the primary flame sheet associated with the passage of the PVC as it circumscribes the burner centerline. The prevailing through-plane velocity of the swirling flow-field transports these fragments into the imaging plane of the OH-PLIF system. The lack of flame-kernel events near the center of the PVC, where conditions are more conducive to auto-ignition (lower strain, longer fluid dynamic residence time), indicates auto-ignition is not a viable explanation for these flame-kernels in a majority of cases. The lack of flame-kernel centroid variation in the case of the TM-Burner 'Quiet flame' (which has no PVC) further supports this explanation., The original document contains color images.

Diplomová práce se zabývá legislativními požadavky na analýzy rizik v průmyslových podnicích, zpracovává jejich metody. Jako podporu analýz a havarijních plánů zpracovává přehled metod na modelování dopadů havárií. Na praktickém případě aplikuje vybrané analýzy s podporu modelu dopadů rozsáhlého požáru ve skladu pneumatik pomocí software Terex., This thesis deals with the legislative requirements for risk analysis in industry, processes their methods. As a supporting of analysis and emergency plan dealing out an overview of methods to model the impact of accidents. The practical part applying the selected analysis model to support large-scale effects of fire in the tire store software by Terex., Ústav elektroniky a měření, obhájeno

Aim of this report is to present the physical model describing LiH combustion, and to discuss by means of some approximated models the main topics associated with thermal fluxes, diffusion rate, lithium vaporization and combustion. To give a rough numerical estimate of the characteristic times involved in a flight configuration was investigated. The analysis performed has shown that LiH, when used as fuel, offer performance comparable with those of commonly used fuels such as LCH4 and LH2., The original document contains color images.

Rezultati numeričkih modeliranja prezentovani u ovom radu, deo su istraživanja laboratorije za sagorevanje Mašinskog fakulteta u Beogradu, sprovedenog u okviru FP6 projekta 'FlexHEAT', koji su zajedno realizovale institucije iz šest evropskih zemalja a koji je finansiran od strane Evropske unije (www.flexheat.uni-erlangen.de). U radu su prezentovani uporedni rezultati numeričkih proračuna kojima se simulira sagorevanje goriva na bazi metana (prirodni gas, srpski gas, biogas i sl), a koji su dobijeni korišćenjem eksperimentalno proverenog, kompletnog hemijskog mehanizma GRI 3.0 koji čini 325 hemijskih reakcija i odgovarajućeg veoma redukovanog hemijskog mehanizama, koga čine 2 hemijske reakcije (od kojih je jedna povratna) razvijenog na bazi dvostepenog C.K. Westbrook & F.L. Dryer (WD) modela. Rezultati su dobijeni upotrebom programskog paketa 'ChemKin' i determinišu oblast i mogućnost primene ovako značajno redukovanih hemijskih mehanizama u CFD proračunima, u situacijama kada kompleksnost samog strujanja sa hemijskim reakcijama kroz složene geometrije gorionika i peći ne dozvoljava upotrebu složenijih hemijskih mehanizama., The paper presents comparison of the results obtained in numerical research conducted, with the aim to simulate combustion of methane-based fuels like natural gas, Serbian gas, biogas, etc. Presented results were obtained using experimentally verified, full chemical mechanism GRI 3.0, with 325 chemical reactions, and using selected, very reduced chemical mechanism, with only two chemical reactions, based on two-step C.K. Westbrook & F.L. Dryer (WD) model. 'ChemKin' software was used for all numerical simulations of chemical kinetics and 'Fluent' for CFD analysis. The goal was to determine possibilities and application fields of this very reduced chemical mechanism in CFD calculations, especially in cases when flows with chemical reactions through complex geometry of burners and furnaces, disable usage of full mechanisms.

This licentiate thesis concerns the modeling of spark ignition engine combustion for use in one dimensional simulation tools. Modeling of knock is of particular interest when modeling turbocharged engines since knock usually limits the possible engine output at high load. The knocking sound is an acoustic phenomenon with pressure oscillations triggered by autoignition of the unburned charge ahead of the propagating flame front and it is potentially damaging to the engine. To be able to predict knock it is essential to predict the temperature and pressure in the unburned charge ahead of the flame front. Hence, an adequate combustion model is needed. The combustion model presented here is based on established correlations of laminar burning velocity which are used to predict changes in combustion duration relative to a base operating condition. Turbulence influence is captured in empirical correlations to the engine operating parameters spark advance and engine speed. This approach makes the combustion model predictive in terms of changes in gas properties such as mixture strength, residual gas content, pressure and temperature. However, a base operating condition and calibration of the turbulence correlations is still needed when using this combustion model. The empirical models presented in this thesis are based on extensive measurements on a turbocharged four cylinder passenger car engine. The knock model is simply a calibration of the Arrhenius type equation for ignition delay in the widely used Livengood-Wu knock integral to the particular fuel and engine used in this work., QC 20101209

In the first part of this work, instability is correctly predicted for a Rijke tube with a new two-term acoustic forcing term derived from a one-dimensional flame dynamics model. The new two-term acoustic forcing term, which is comprised of the summation of chemical heat release rate and heat transfer due to convection, correctly predicts instability where older models of acoustic forcing based solely on chemical heat release rate incorrectly predicted stability. This stability analysis correctly predicts the inlet conditions of the instability in addition to the frequency of instability. In the second part of this work, networks of dynamic well-stirred reactors are used to model qualitative behavior observed in turbulent combustion. First a model of dynamic well-stirred reactor is derived, and then several reactors are coupled together by recirculation. The dynamics of the various models are computed and assessed. The models exhibit interesting behavior that has been viewed experimentally including hysteresis and peaking in the dynamic response.