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A joint experimental and theoretical project has been initiated at Virginia Tech to study the effects of dual-mode combustion at high pressures for a two-dimensional Wolfhard-Parker burner. This thesis is the first stage of the theoretical part of the project, and contains a numerical study of laminar coflow diffusion flames stabilized on a confined Wolfhard-Parker burner. A global finite difference method is used where the nonlinear equations written on a stream function-vorticity formulation are solved with a flame sheet approach. The pseudotransient, approximative factorization method is utilized to solve the coupled system of equations. Adaptive gridding, numerical evaluation of Jacobians and iterations within time step are implemented for computational efficiency. Numerical results have been obtained for different fuels under different conditions. Comparison with measured data by Smyth et al. (1985) for a buoyancy dominated methane-air flame is made. The location of the flame front is accurately predicted. The temperature is over predicted in the fuel rich zone since pyrolysis and radiation effects have not been accounted for in the numerical model. Good agreement is observed for major species and velocities. As expected, large velocity increase and horizontal inflow of nitrogen and combustion products associated with buoyancy occur in the lower region of the flame. / Master of Science

This dissertation presents a detailed account of the study undertaken on the subject of fouling of Exhaust Gas Recirculator (EGR) coolers. The fouling process in EGR coolers is identified to be due to two primary reasons — deposition of fine soot particles and condensation of hydrocarbons known as dry soot and wet soot fouling, respectively. Several numerical simulations are performed to study the fouling process. Preliminary analysis of the particle forces for representative conditions reveal that drag, thermophoresis and Brownian forces are the significant transport mechanisms and among them, the deposition process is dominated by thermophoresis. Soot deposition in a representative turbulent plain channel shows a direct relationship of the amount of deposition with the near-wall temperature gradient. Subsequently, periodic and developing flow simulations are performed on a wavy channel geometry, a common EGR design for various Reynolds numbers and thermal boundary conditions. Constant heat flux boundary condition is used in the periodic fully-developed calculations, which assist in establishing various deposition trends. The wavy nature of the walls is noted to affect the fouling process, resulting in specific deposition patterns. For the lower Reynolds number flows, significantly higher deposition is observed due to the higher particle residence times. On the other hand, the developing flow calculations facilitate the use of wall temperature distributions that typically exist in EGR coolers. The linear dependence of the amount of deposition on the near-wall temperature gradient or in other words, the heat flux, is ascertained. It is also observed in all the calculations, that for the sub-micron soot particles considered, the deposition process is almost independent of the particle size. In addition, the nature of the flow and heat transfer characteristics and the transition to turbulence in a developing wavy channel are studied in considerable detail. Finally, a study on the condensation of heavy hydrocarbons is undertaken as a post-processing step, which facilitates the prediction of the spatial distribution and time-growth of the combined fouling layer. From the calculations, the maximum thickness of the dry soot layer is observed to be near the entrance, whereas for the wet soot layer, the peak is found to be towards the exit of the EGR cooler. Further, parametric studies are carried out to investigate the effect of various physical properties and inlet conditions on the process of fouling. / Master of Science

[ES] Hoy en día, las regulaciones sobre emisiones de los automóviles se están haciendo más estrictas. Además de los ciclos de homologación estándar, actualmente se están empezando a considerar nuevos métodos de homologación que tienen en cuenta las condiciones reales que se dan en la carretera. Los sistemas de Recirculación de Gases de Escape (EGR) son estrategias que han demostrado ser efectivas durante estacionarios y que también pueden ser usadas en ese tipo de ciclos dinámicos que corresponden a condiciones reales de conducción. Esta tesis se centra en la implementación de diferentes sistemas EGR para su uso en condiciones dinámicas en motores diésel turbosobrealimentados. En primer lugar, se lleva a cabo un análisis del ciclo de conducción para identificar las operaciones específicas de tipo transitorio más frecuentes en los ciclos dinámicos como WLTC y RDE. Los resultados muestran que la frecuencia en la que se producen fuertes transitorios en carga es mayor que en la que se producen transitorios de velocidad. Entre ellos, el número de operaciones de tipo Tip-out es superior a las de tipo Tip-Ins, especialmente en el rango de 1250-2000 rpm. Estos fuertes transitorios se repiten en el banco de ensayos de motor equipado con analizadores de gas de alta frecuencia, de forma que se registran la concentración instantánea de CO2 y NOx. También se ha realizado un estudio paramétrico de la actuación de la válvula de EGR durante la operación de varios transitorios fuertes, cuantificando el retraso en el transporte, la concentración de NOx y las partículas. El lazo de EGR de baja presión, LPEGR, ha resultado ser más efectivo cuando se operaba a plena carga, así como durante los transitorios, comparado con el lazo de EGR de alta presión, HPEGR. De esta forma, se propone la válvula de control más adecuada para LPEGR, lo que puede ser útil para la calibración de los transitorios de los motores diésel turbosobrealimentados. Además de ello, se señala el compromiso entre rendimiento y emisiones durante los transitorios de EGR. Al implementar la recirculación de los gases de escape a lo largo de todo el mapa del motor se minimiza la aparición de picos inesperados de emisión de NOx. Concretamente, las estrategias LPEGR consiguen reducir alrededor de un 20-60% los NOx emitidos durante los primeros pocos segundos con menos de un 5% de penalización en el rendimiento. Adicionalmente, en el documento también se presentan las simulaciones que se han realizado de los modelos unidimensionales de los transitorios. El control de la turbina de geometría variable juega un papel importante a la hora de calibrar el modelo para transitorios de EGR. Además de ello, se lleva a cabo una optimización de la separación de EGR para varios puntos estacionarios por medio de simulaciones que están basadas en el compromiso entre rendimiento y emisiones. Además, se propone un algoritmo para optimizar la separación de EGR, reduciendo en alrededor de un 80% el tiempo de cálculo de un DOE o un método de algoritmo genético. Finalmente, se crea un modelo simple de NOx 3D cuasi-estacionario para predecir las emisiones durante el transitorio en condiciones de conducción reales. La tasa de EGR, como tercera entrada del modelo, muestra una mejora significativa a la hora de predecir el transitorio de NOx con respecto al modelo 2D. / [EN] The automotive emission regulations are getting more stringent these days. New methods of homologation are being considered other than standard cycles considering the real driving behavior on road. The EGR system is one of the proven and well tested strategies in steady state which can be used on those dynamic real driving conditions too. This dissertation focuses on implementation of different EGR systems during dynamic operations of turbocharged diesel engine. Firstly, a driving cycle analysis is carried out to identify the specific and frequent transient operations on dynamic cycles like WLTC and RDE. The results show that, the frequency of harsh load transients is higher than speed transients. Among them, the number of Tip-Out operations outnumber the Tip-Ins with higher density in 1250-2000 RPM range. Therefore, these harsh transients are repeated separately on the dynamic engine test bench equipped with high frequency gas analyzers to track the instantaneous CO2 and NOx concentration. A parametric study is carried out with EGR valve actuation during various severe load transients, quantifying the transportation delays, NOx concentration and particulate matter. The LPEGR is found to be more effective at the full load as well as during transient operations compared to HPEGR. The best suited LPEGR valve control is proposed, which can be helpful for transient calibration of a turbocharged diesel engine. Moreover, the trade-off between the performance and emission during EGR transients is also pointed out. The implementation of EGR all over the engine map minimizes the unexpected NOx peaks during transients. Specifically, LPEGR strategies manages to reduce around 20-60% of NOx in first few seconds with less than 5% of penalty in performance. Additionally, 1D model simulation results of load transient operations are presented in the document. The VGT control plays important role to calibrate the model for transient operations with EGR. Apart from this, the EGR split optimization on various steady points is carried out by simulations following the trade-off between performance and emissions. Furthermore, an algorithm to search the optimum split is proposed, reducing around 80% of the calculation time consumed by DOE or genetic algorithm method. Finally, a simple 3D quasi steady NOx model is created to predict the transient emissions in real driving conditions. EGR rate, as 3rd input in model shows significant improvement in prediction of transient NOx over the 2D model. / [CA] En els darrers temps, les regulacions sobre emissions contaminants dels vehicles s'han fet més estrictes. A més dels cicles d'homologació estàndards, actualment s'estan començant a considerar nous mètodes d'homologació que tinguen en compte les condicions reals que es donen en la carretera. Els sistemes de Recirculació de Gasos d'Escapament (EGR) són estratègies que s'han demostrat com a efectives durant condicions estacionàries i que també poden ser emprades en aquest tipus de cicles dinàmics, que corresponen a condicions reals de conducció. Aquesta tesi està centrada en la implementació de diferents sistemes EGR per al seu ús en condicions dinàmiques en motors dièsel turbosobrealimentats. En primer lloc, es du a terme un anàlisi del cicle de conducció per a identificar les operacions específiques de tipus transitori més freqüents en els cicles dinàmics WLTC i RDE. Els resultats mostren que la freqüència a la que s'obtenen forts transitoris de càrrega és major que en aquella en la que es produeixen transitoris de velocitat. Entre aquestos, el nombre d'operacions de tipus Tip-out és superior a les del tipus Tip-ins, especialment en l'interval de 1250-2000 rpm. Aquestos forts transitoris es repeteixen en el banc d'assajos de motor equipat amb analitzadors de gasos d'alta freqüència, de manera que es registren les concentracions de CO2 i NOx. També s'ha realitzat un estudi paramètric de l'actuació de la vàlvula d'EGR durant l'operació de diversos transitoris forts, quantificant el retard en el transport, la concentració de NOx i les partícules. El llaç d'EGR de baixa pressió, LPEGR, ha resultat ser més efectiu quan s'operava a plena càrrega, així com durant els transitoris, comparat amb el llaç d'EGR d'alta pressió, HPEGR. D'aquesta forma, es proposa la vàlvula de control més adequada per a LPEGR, el que pot resultar útil per a la calibratge dels transitoris dels motors dièsel turbosobrealimentats. A banda d'això, s'ha assenyalat el compromís entre rendiment i emissions durant els transitoris d'EGR. Al implementar la recirculació dels gasos d'escapament a tot arreu del mapa del motor es minimitza l'aparició de pics inesperats d'emissió de NOx. Més concretament, les estratègies LPEGR aconsegueixen reduir al voltant d'un 20-60% els NOx emesos durant els primers pocs segons amb menys d'un 5% de penalització en el rendiment. Addicionalment, en el document també es presenten les simulacions que s'han realitzat dels models unidimensionals dels transitoris. El control de la turbina de geometria variable juga un paper important a l'hora de calibrar el model per a transitoris d'EGR. A més d'això, s'ha dut a terme una optimització de la separació d'EGR en diversos punts estacionaris per mitjà de simulacions que estan basades en el compromís entre rendiment i emissions. També es proposa un algoritme per a optimitzar la separació d'EGR, reduint al voltant d'un 80\% el temps de càlcul d'un DOE o un mètode d'algoritme genètic. Finalment, es crea un model simple de NOx 3D quasi-estacionari per a predir les emissions durant el transitori en condicions de conducció real. La taxa d'EGR, com a tercera entrada del model, mostra una millora significativa a l'hora de predir el transitori de NOx respecte al model 2D. / Patil, CY. (2020). Effects of EGR transient operation on emissions and performance of automotive engines during RDE cycles [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/149498

This study addresses the influence of elevated pressures, fuel type, fuel flow rate and co-flow air on the flame structure and flickering behaviour of laminar oscillating diffusion flames. Photomultipliers, high speed photography and schlieren, accompanied with digital image processing techniques have been used to study the flame dynamics. Furthermore, the effects of pressure on the flame geometry and two-dimensional soot temperature distribution in a laminar stable diffusion flame have been investigated, utilising narrow band photography and two-colour pyrometry technique in the near infra-red region. This study provides a broad dataset on the diffusion (sooty) flame properties under pressures from atmospheric to 16 bar for three gaseous hydrocarbon fuels (methane, ethylene and propane) in a co-flow burner facility.It has been observed that the flame properties are very sensitive to the fuel type and flow rate at elevated pressures. The cross-sectional area of the stable flame shows an average inverse dependence on pressure to the power of n, where n was found to be 0.8±0.2 for ethylene flame, 0.5±0.1 for methane flame and 0.6±0.1 for propane flame. The height of a flame increases firstly with pressure and then decreases with further increase of pressure. It is observed that the region of stable combustion was markedly reduced as pressure was increased. An ethylene flame flickers with at least three dominant modes, each with corresponding harmonics at elevated pressures. In contrast, methane flames flicker with one dominant frequency and as many as six harmonic modes at elevated pressures. The increase in fuel flow rate was observed to increase the magnitude of oscillation. The flickering frequency, however, remains almost constant at each pressure. The dominant flickering frequency of a methane diffusion flame shows a power-law dependence on chamber pressure.It has been observed that the flame dynamics and stability are also strongly affected by the co-flow air velocity. When the co-flow velocity reached a certain value, the buoyancy driven flame oscillation was completely suppressed. The schlieren imaging has revealed that the co-flow of air is able to push the initiation point of outer toroidal vortices beyond the visible flame to create a very stable flame. The oscillation frequency was observed to increase linearly with the air co-flow rate. The soot temperature results obtained by applying the two-colour method in the near infra-red region shows that in diffusion flames the overall temperatures decrease with increasing pressure. It is shown that the rate of temperature drop is greater for a pressure increase at lower pressures in comparison with higher pressures.

<p dir="ltr">Soot is a byproduct formed during incomplete combustion of hydrocarbon fuels. Atmospheric soot from aircraft emissions increases local air temperatures, drives cloud formation, and decreases albedo on snow and ice: three factors that promote global warming. It is also potentially harmful to humans and has been associated with negative effects on heart and lung health. Operationally, soot formation indicates an inefficiency in combustion and can cause deterioration in aircraft engines. Modeling soot formation in complex flow fields is difficult and has been largely unsuccessful. In-situ soot measurements at relevant conditions can inform the design and operation of aircraft engines with reduced soot emissions. Laser-induced incandescence (LII) is a diagnostic that allows for non-intrusive measurements of soot volume fraction and primarily particle size in combustion environments. It involves laser-heating soot particles to temperatures at which they incandescence and measuring the radiated signal. The strong absorption capabilities and high sublimation temperature of soot make this diagnostic highly selective against the detection of other species. A coupled set of differential equations can be used to model the change in temperature and mass of a soot particle over time. Methods for modeling the fundamental processes in LII were reviewed in this work and comparisons were made between several different models.</p><p dir="ltr">International Sooting Flame target conditions were used to form a laminar diffusion flame in a Yale burner with a range of soot levels. Soot volume fraction measurements were conducted and compared with other experimental values to validate the accuracy of the experimental setup and techniques used. A calibration was performed using a laser extinction measurement from a previous study. Results showed an overall increase in soot volume fraction with increasing percentages of ethylene, as well as a transition in the peak location. Time-resolved LII was conducted at 10 MHz to determine the primary particle size of soot particles. Larger primary particles were observed with increasing height for flames with higher ethylene content. Changes in the soot formation and surface growth rates are suspected factors in the observed trends in the data. </p><p dir="ltr">The overall objective of this study was to validate an experimental setup for Laser-Induced Incandescence using a laminar diffusion flame. LII measurements were successfully demonstrated using the same diagnostic setup in a liquid-fueled swirl-stabilized flame at aircraft engine-relevant conditions. This study sets the groundwork for further investigation into aircraft soot generation using LII. </p>

Diesel engine emissions of oxides of nitrogen and particulate matter can be reduced simultaneously through the use of high levels of exhaust gas recirculation (EGR) to achieve low temperature combustion (LTC). Although the potential benefits of diesel LTC are clear, the main challenges to its practical implementation are the requirement of EGR levels that can exceed 60%, high fuel consumption, and high unburned hydrocarbon and carbon monoxide emissions. These limit the application of LTC to medium loads. In order to implement the LTC strategy in a passenger vehicle engine, a transition to conventional diesel operation is required to satisfy the expected high load demands on the engine. The investigation presented in this thesis was therefore aimed at improving the viability of the high-EGR LTC strategy for steady-state and transient operation. An experimental investigation was carried out on a single cylinder high-speed direct injection diesel engine. This thesis presents research on engine in-cylinder performance and engine-out gaseous and particulate emissions at operating conditions (i.e. EGR rate, intake pressure, fuel quantity, injection pressure) likely to be encountered by an engine during transient and steady-state operation. At selected operating points, further investigation in terms of in-cylinder spray and combustion visualization, flame temperature and soot concentration measurements provided deeper insight into the combustion and emissions phenomena. Increased intake pressure at single injection high-EGR LTC operation was investigated as a strategy to reduce the emissions of partial combustion by-products and to improve fuel economy. The higher intake pressure, although effective in reducing partial combustion by-products emissions and improving fuel economy, increased the EGR requirement to achieve LTC. A split fuel injection strategy with advanced injection timing on the other hand was effective in reducing the EGR requirement for LTC from 62% with single injection to 52% with split injections at 120 kPa (absolute) intake pressure. Unburned hydrocarbon emissions and fuel economy were particularly sensitive to intake oxygen mass fraction, and injection and dwell timings with the split injection strategy. In-cylinder soot formation and oxidation mechanisms with the split injection strategy were found to be significantly different from the single injection high-EGR LTC case. Transient simulation of an engine during combustion mode transition identified engine operating parameters on a cycle-by-cycle basis. Steady-state investigation of these test conditions provided significant insight into the combustion conditions and their effect on emissions and performance. The results from this thesis demonstrated the importance of optimizing both the air handling system performance and the fuel injection system during engine transients. The increased emissions and impaired performance due to slow response of the EGR and turbocharger systems during transitions to and from LTC modes can in part be mitigated through split injections optimized for the specific transient point. This provides a clear direction for engine developers to pursue in optimizing engine calibration when running with LTC-conventional diesel dual-mode strategies.

U.S. Federal regulations requiring on-board diagnostics of diesel particulate filters have created a demand for compact, inexpensive, fast, and accurate sensors for measuring the particulate matter (PM) content of diesel exhaust. An electronic sensor capable of measuring the carbonaceous fraction (soot) of PM has been developed at The University of Texas at Austin. The behavior and performance of this sensor was characterized in both an older style non-emission controlled diesel engine and a modern heavy-duty diesel certified in 2008 to meet current federal emissions standards. The ability of the sensor to detect particulates at the regulated level of 15 mg/bhp-hr downstream of a leaking particulate filter was demonstrated. Under optimal conditions, the sensor was shown to have a resolution of 0.003 mg/bhp-hr, or 0.005 mg/m3. The sensor operated by measuring the flux of charged particles, ions, and electrons to an electrode immersed in an exhaust gas flow. Two distinct modes of operation were demonstrated. In the first, the sensor detected particles carrying residual charge from the combustion process. In this mode, the sensor was shown to be relatively insensitive to particle morphology and to be sensitive to exhaust gas velocity. In the second, charge carriers (particles, electrons, and ions) were created in the strong electric field produced by a second electrode at high voltage. In this mode, the sensor was found to be relatively insensitive to exhaust gas velocity, but quite sensitive to the orientation of the sensor in the exhaust flow. The size and number density of the particles was found to have a strong influence on the sensor sensitivity: as number density increased with increasing load or decreasing EGR rate, so did sensor sensitivity. Thus, as changes in engine operating condition affect particle morphology, the behavior of the sensor changes. A numerical model of the discharge mechanism in the form of an atmospheric pressure glow discharge was implemented to model the charge creation and transport. The model accurately predicted the nanoamp-level electrode currents produced in a real sensor to within a half order of magnitude with no empirical fits. The model tended to over-predict the sensitivity of sensor output to applied voltage but matched the observed sensitivity within an order of magnitude. Due to the lack of modeling flow field effects it predicted a 250% increase in sensitivity for a gap width reduced by 50% where a comparison of real sensors showed a decrease in sensitivity of 25% with a 50% reduction in gap width. / text

<p>Persistent organic pollutants (POPs) is a group of chemicals that are toxic, undergo long-range transport and accumulate in biota. Due to their persistency the distribution and recirculation in the environment often continues for a long period of time. Thereby they appear virtually everywhere within the biosphere, and poses a toxic stress to living organisms. In this thesis, attempts are made to contribute to the understanding of factors that influence the distribution of POPs with focus on processes in the marine environment. The bioavailability and the spatial distribution are central topics for the environmental risk management of POPs. In order to study these topics, various field studies were undertaken. To determine the bioavailable fraction of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated naphthalenes (PCNs), and polychlorinated biphenyls (PCBs) the aqueous dissolved phase were sampled and analysed. In the same samples, we also measured how much of these POPs were associated with suspended particles. Different models, which predicted the phase distribution of these POPs, were then evaluated. It was found that important water characteristics, which influenced the solid-water phase distribution of POPs, were particulate organic matter (POM), particulate soot (PSC), and dissolved organic matter (DOM). The bioavailable dissolved POP-phase in the water was lower when these sorbing phases were present. Furthermore, sediments were sampled and the spatial distribution of the POPs was examined. The results showed that the concentration of PCDD/Fs, and PCNs were better described using PSC- than using POM-content of the sediment. In parallel with these field studies, we synthesized knowledge of the processes affecting the distribution of POPs in a multimedia mass balance model. This model predicted concentrations of PCDD/Fs throughout our study area, the Grenlandsfjords in Norway, within factors of ten. This makes the model capable to validate the effect of suitable remedial actions in order to decrease the exposure of these POPs to biota in the Grenlandsfjords which was the aim of the project. Also, to evaluate the influence of eutrophication on the marine occurrence PCB data from the US Musselwatch and Benthic Surveillance Programs are examined in this thesis. The dry weight based concentrations of PCB in bivalves were found to correlate positively to the organic matter content of nearby sediments, and organic matter based concentrations of PCB in sediments were negatively correlated to the organic matter content of the sediment.</p>

La gazéification de la biomasse a été étudiée dans les conditions d'un réacteur à flux entraîné, à savoir à vitesse de chauffage et à température élevées. Des expériences ont été réalisées dans un four à chute entre 800°C et 1400°C, à partir de particules de bois de taille 0,35 mm et 0,80 mm, dans une atmosphère inerte (100% molaire de N2), ou contenant de la vapeur d’eau (25% molaire). Les expériences ont également été simulées grâce à un modèle 1D avec des résultats positifs, ce qui a permis de mieux comprendre les phénomènes mis en jeu. Les solides obtenus (suies et char) ont été analysés et caractérisés. Des rendements élevés en gaz et goudrons, et un faible rendement en char ont été mesurés. Par conséquent, l'évolution de la phase volatile est déterminante pour les rendements des produits finaux. Au-dessus de 1000°C, la formation de suies devient importante. Les suies sont formées à partir de C2H2 et de HAP. En présence de vapeur d’eau, le rendement en suies est nettement moins élevé, ce qui s’explique essentiellement par le vaporeformage des précurseurs de suie, mais aussi par leur gazéification. La réaction de water-gas shift joue un rôle important dans la distribution des gaz majoritaires. La gazéification du char a été mise en évidence à 1200°C et 1400°C sous atmosphère humide. L'ensemble de ces réactions conduit à un gaz riche en H2, CO et CO2. L'équilibre thermodynamique est presque atteint à 1400°C avec une concentration de 25% molaire de H2O dans l’atmosphère. La graphitisation et la désactivation du char porté à haute température ont été mises en évidence expérimentalement. Néanmoins, ces phénomènes ont une influence négligeable sur l’évolution du rendement en char lors des expériences en four à chute. Enfin, la taille des particules n’a presque aucune influence sur les résultats expérimentaux. / Biomass gasification was studied in the conditions of an entrained flow reactor, namely at high heating rate and temperature. Experiments in a drop tube reactor were performed between 800°C and 1400°C, with wood particles of 0.35 mm and 0.80 mm size, under inert and steam containing - 25 mol% of H2O - atmospheres. These experiments were also simulated with a 1D model which gave good predictions. The collected solids, soot and char, were analyzed and characterized. This study highlights the importance of gas phase reactions on the yields of the final products, mainly gaseous compounds, in these conditions. These reactions are hydrocarbons cracking, reforming and polymerization, leading to soot formation, and water-gas shift. Char graphitization and deactivation were experimentally demonstrated. However, these phenomena have a negligible influence on char evolution in the drop tube reactor. Finally, the particle size was shown to have almost no influence on experimental results.