The effect of a nozzle’s internal geometry was studied experimentally to determine the breakup of the emitted water jet when it was injected perpendicularly into a quiescent atmosphere or a subsonic air crossflow. The nozzle’s diameter, nominal surface roughness, length-to-diameter ratio and contraction angle were varied, together with the injection pressure, to find the water column’s breakup length. Photographs of the water jet at the nozzle’s exit, gave a clue as to identify the occurrence of cavitation and a hydraulic flip. On the other hand the water column’s breakup length and trajectory, in a subsonic crossflow, were measured by using a stroboscope in conjunction with a high speed CCD camera. Results agreed with previous literature that the breakup length grew with greater liquid/air momentum flux ratios for non-cavitating flows. This was true regardless of the injector nozzle. The rate of increase decreased at the inception of cavitation. On the other hand even shorter breakup lengths were observed at the inception of a hydraulic flip due to the detachment of the water jet from the internal surface of the nozzle. Increasing the nozzle’s length-to-diameter ratio eliminated the occurrence of hydraulic flip. The jet’s trajectory was correlated with the liquid/air momentum flux ratio and the nozzle’s exit diameter. The results showed that higher water jet trajectories were measured under non-cavitating conditions. Even shorter jet trajectories were measured at the inception of a hydraulic flip.

The effect of a nozzle’s internal geometry was studied experimentally to determine the breakup of the emitted water jet when it was injected perpendicularly into a quiescent atmosphere or a subsonic air crossflow. The nozzle’s diameter, nominal surface roughness, length-to-diameter ratio and contraction angle were varied, together with the injection pressure, to find the water column’s breakup length. Photographs of the water jet at the nozzle’s exit, gave a clue as to identify the occurrence of cavitation and a hydraulic flip. On the other hand the water column’s breakup length and trajectory, in a subsonic crossflow, were measured by using a stroboscope in conjunction with a high speed CCD camera. Results agreed with previous literature that the breakup length grew with greater liquid/air momentum flux ratios for non-cavitating flows. This was true regardless of the injector nozzle. The rate of increase decreased at the inception of cavitation. On the other hand even shorter breakup lengths were observed at the inception of a hydraulic flip due to the detachment of the water jet from the internal surface of the nozzle. Increasing the nozzle’s length-to-diameter ratio eliminated the occurrence of hydraulic flip. The jet’s trajectory was correlated with the liquid/air momentum flux ratio and the nozzle’s exit diameter. The results showed that higher water jet trajectories were measured under non-cavitating conditions. Even shorter jet trajectories were measured at the inception of a hydraulic flip.

An experimental investigation was conducted using particle image velocimetry to study the effect of nozzle geometry on turbulent free orifice jets. The nozzle geometries studied include the round, cross, flower, star, rectangular and elliptical nozzles (aspect ratio 2). The spread rate of the rectangular nozzle was 61% greater than the square nozzle while the elliptical nozzle was 45% greater than the round nozzle using the conventional half velocity width. The superior mixing capacity of the rectangular and elliptical nozzles is attributed to the axis-switching mechanism. Evaluation of the energy budget showed a higher level of production of turbulence and convection of the mean flow for the rectangular nozzle compared with the round nozzle. Two-point auto-correlation function revealed larger structures in the non-circular nozzles and in particular the rectangular nozzle. The Kolmogorov and Taylor microscales however, did not show any significant dependency on nozzle geometry. / October 2017

Ces travaux s'inscrivent dans le cadre du projet ESSENCYELE, projet visant à développer un nouveau modèle de véhicule hybride. L'étude thermique présentée porte sur le refroidissement de machines électriques par jets impactant. Afin d'optimiser le refroidissement des bobinages de la partie fixe de la machine (stator), des travaux ont été menés dans le but d'étudier localement les échanges de chaleur lors de l'impact d'un jet. Deux fluides ont été considérés, l'air et l'huile. Le dispositif expérimental, associé à un programme de post-traitement par méthode inverse, permet de relever la température à la surface d'un cylindre lisse lors de l'impact d'un jet. L'influence de la distance jet/surface, de la géométrie de l'injecteur ou encore des propriétés du fluide ont été testées. Des travaux numériques ont permis de donner des informations supplémentaires sur l'évolution de l'écoulement au sein des injecteurs. Pour terminer, les configurations les plus intéressantes obtenues pour l'air et pour l'huile ont été testées sur le refroidissement des bobinages de stator. / The present work is a part of an industrial project named ESSENCYELE. The main objective of this project is to develop a new hybrid vehicle. The present study is about the electrical machine cooling system by impinging jets. To improve the end winding cooling, experiments has been made to study the local heat transfers during a jet impingement. Two fluids were considered, air and oil. The experimental device, with an inverse method post-processing program, allowed to estimate the temperature at the surface of a smooth cylinder. The influence of the jet/surface distance, the nozzle geometry or the fluid properties were tested. Numerical studies have provided additional information on the fluid flow evolution inside the nozzle. Finally, the most interesting configuration obtained with air jet and oil jet were tested.

The bubble flow is used for different application in steel production and refining processes. It plays in indispensable role in the ladle refining process such as for homogenization and inclusion removal. Hence, it is important to understand the effect of the nozzle outlet geometry on the bubble formation. Three different nozzles with different outlet geometries were examined using a physical model. These geometries were: a) Circle, b) Square with round edges and c) Elliptical. All three nozzles had the same nozzle design and similar outlet cross-section areas. Therefore, the only tested parameter was the outlet geometry. The physical model is a water/air model, that consist of water tank ,the nozzles, gas gauge and a high speed camera. Each nozzle was tested under five different gas flow rates: starting from 10 L/min of air gas flow rate, until a 30 L/min of gas flow rate by incriminating with 5 L/min per experimental trial. Therefore, each nozzle was studied using a 5 experimental sets, so in total 15 experiments were made. For each set, 3000 photos were captured by the high speed camera. The photos were then analyzed using mainly the ImageJ software and the naked eye. After analyzing the photos for the experimental sets the following were found: a) The frequency of bubble formation was for the most part constant with an average of 11 bubbles per second. b) The elliptical nozzle produced for the most part the largest bubbles, while the circular produced the smallest ones. The square nozzle had similar bubble sizes comparable to the elliptical nozzle. c) The circular nozzle resulted in the bubbles with most stable surface, while the elliptical nozzle had the most unstable bubble boundary. The study had a drawback, which is the presence of a jetting flow which reduced the accuracy of the results. Thus, it is recommend that future work can solve this issue by finding at which gas flow rate pure bubbling flow stops for each nozzle geometry. / Bubbelinjektion används för olika tillämpningar inom stålproduktion och raffineringsprocesser. Det spelar en oumbärlig roll i raffineringsprocesser som homogenisering och borttagning av inneslutningar. Därför är det viktigt att förstå effekten av utloppsgeometrin hos munstycket på bubbelbildningen. Tre olika munstycken med olika utloppsgeometrier undersöktes med hjälp av en fysisk modell. Dessa geometrier är: a) Cirkel, b) Fyrkantig med rundade kanter och c) Elliptisk. Alla tre munstyckena har samma munstycksdesign och liknande utloppstvärsnittsarea. Därför är den enda parametern som testas utloppsgeometrin. Den fysiska modellen bestod av en vattentank, munstyckena där var och en undersöks separat, en gasmätare, en höghastighetskamera och modellen använde vatten/luft. Varje munstycke testades under fem olika gasflöden: startande från 10 L/min luftgasflöde, tills 30 L/min gasflöde stegvis 5 L/min per experimentuppsättning. Därför har varje munstycke 5 experimentuppsättningar, så totalt 15 experiment togs. För varje uppsättning togs 3000 bilder med höghastighetskameran. Bilderna analyserades sedan med främst ImageJ-programvara och blotta ögat. Efter att ha analyserat bilderna från experimenten så visade resultaten följande: a) Frekvensen av bubbelbildning var mestadels konstant med ett genomsnitt på 11 bubblor per sekund. b) Det elliptiska munstycket producerade mestadels de största bubblorna, medan det cirkulära producerade de minsta bubblorna. Det fyrkantiga munstycket resulterade i en liknande bubbelstorlek som det elliptiska munstycket. c) Det cirkulära munstycket resulterade i bubblorna med den mest stabila ytan, medan det elliptiska munstycket hade den mest instabila bubbelgränsen. Studien hade en nackdel, vilket är närvaron av ett jetflöde som minskade noggrannheten i resultatet. Det rekommenderas att framtida arbete kan lösa detta problem genom att hitta vid vilken gasflödeshastighet rent bubblande flöde stoppar för varje munstycke

Resumen Los sistemas de inyección directa han sido uno de los principales puntos focales de la investigación en motores, particularmente en sistemas Diésel, donde la geometría interna, movimiento de aguja y comportamiento del flujo afectan el spray externo y por tanto determinan completamente el proceso de combustión dentro del motor. Debido a regulaciones medioambientales y al potencial de los (más ineficientes) motores "Otto", grandes esfuerzos se están aportando en investigación sobre sistemas de inyección directa de gasolina. Los motores GDi tienen el potencial de incrementar sustancialmente la economía de combustible y cumplir con las regulaciones de gases contaminantes y de efecto invernadero, aunque aún existen muchos desafíos por delante. Esta tesis estudia en detalle una moderna tobera GDi que fue específicamente diseñada para el grupo de investigación conocido como Engine Combustion Network (ECN). Con metodologías punteras, este inyector ha sido usado en un amplio abanico de instalaciones experimentales para caracterizar el flujo interno y varias características clave de geometría y funcionamiento, y aplicarlo para evaluar cómo se relaciona con los efectos observados del comportamiento del chorro externo. Para la caracterización interna del flujo, el objetivo ha sido determinar la geometría de la tobera y el desplazamiento de aguja, caracterizar la tasa de inyección y el flujo de cantidad de movimiento, y evaluar el flujo cercano. Algunas metodologías nunca antes habían sido empleadas en inyectores GDi, y muchas otras lo han sido solo eventualmente. Para la geometría interna, el levantamiento de aguja y el flujo cercano, varias técnicas avanzadas con rayos-x fueron aplicadas en las instalaciones de Argonne National Laboratory. Para la tasa de inyección y flujo de cantidad de movimiento, las técnicas disponibles en el departamento han sido adaptadas desde Diésel y aplicadas en inyectores GDi multiorificio. Dado lo novedoso de las técnicas aplicadas, las particularidades de las metodologías han sido discutidas en detalle en el documento. Aún con la elevada turbulencia del flujo interno, el inyector se comporta de forma consistente inyección a inyección, incluso cuando el estudio se centra en la variabilidad orificio a orificio. Esto ha sido atribuido al comportamiento repetitivo de la aguja, evaluado en los experimentos. También fue observado que el flujo estabilizado tiene una variación de alta frecuencia que no pude ser explicado por el movimiento de la aguja, sino por el particular diseño de las toberas. El análisis de geometría interna realizado a ocho toberas nominalmente iguales resultó en la obtención de un punto vista único en la construcción de toberas y la variabilidad de dimensiones clave. Las medidas de tasa de inyección permitieron estudiar la respuesta hidráulica del inyector a varias variables como la presión de inyección, presión de descarga, temperatura de combustible y la duración de la señal de comando. Estas medidas fueron combinadas con medidas de flujo de cantidad de movimiento para estudiar el bajo valor del coeficiente de descarga, el cual fue atribuido al bajo levantamiento de aguja y coeficiente L/D de los orificios. Por otro lado, el estudio del spray externo resultó en la identificación de un importante fenómeno específico a este particular hardware, el colapso del spray. Las extensivas campañas experimentales, utilizando Schlieren e iluminación trasera difusa (DBI) permitieron identificar y describir las características macroscópicas del spray y las condiciones bajo las que el colapso ocurre. El colapso del spray se forma por una combinación de interacción de las diferentes plumas (causado por el flujo interno) y determinadas condiciones ambiente que promueven evaporación y entrada de aire. Fue determinado que a niveles de densidad y temperatura moderados se desarrolla el colapso, modificando completamente el comportamiento espera / Abstract Fuel injection systems have been one of the main focal points of engine research, particularly in Diesel engines, where the internal geometry, needle lift and flow behavior are known to affect the external spray an in turn completely determine the combustion process inside engines. Because of environmental regulation and the potential development of the more inefficient Otto engines, a lot of research efforts are currently focused into gasoline direct injection systems. GDi engines have the potential to greatly increase fuel economy and comply with pollutant and greenhouse gases emissions limits, although many challenges still remain. The current thesis studies in detail a modern type of GDi nozzle that was specifically developed for the international research group known as the Engine Combustion Network (ECN). With the objective of employing state-of-the-art techniques, this hardware has been used in a wide range of experimental facilities in order to characterize the internal flow and several geometrical and constructive aspects like needle lift; and assess how it relates to the effects seen external spray. For the internal flow characterization, the goal was to determine the nozzle geometry and needle displacement, to characterize the rate of injection and rate of momentum, and evaluate the near-nozzle flow. Some methodologies applied here have never been applied to a GDi injector before, and many have only been applied rarely. For the internal geometry, needle lift and near-nozzle flow, several advanced x-rays techniques were used at Argonne National Laboratory. For the rate of injection and rate of momentum measurements, the techniques available in CMT-Motores Térmicos have been adapted from Diesel spray research and brought to multi-hole GDi injectors. Given the novelty of the techniques used, the particular methodologies and setups are discussed in detail. Despite the high turbulence of the flow, it was seen that the injector behaves consistently injection to injection, even when studying variation in individual holes. This is attributed to the repetitive behavior of the needle that was observed in the experiments. It was also observed that the stabilized flow has a high frequency variability that could not be explained by random movement of the needle, but rather by the particular design of the nozzle. The geometrical analysis done to eight, nominally equal nozzles, allowed a unique view into the construction of the nozzle and provided insights about the variability of key dimensions. The rate of injection measurements allowed to study the hydraulic response of the injector to the main variables like rail pressure, discharge pressure, fuel temperature and command signal duration. These measurements were combined with the rate of momentum measurements to study the low value of the discharge coefficient, that ultimately was attributed to the low needle lift and low L/D ratio of the orifices. On the other hand, the study of the external spray yielded the identification of very important phenomena specific to this particular hardware, the spray collapse. The extensive experimental campaigns featuring shadowgraph (Schlieren) and Diffused Back Illumination (DBI) visualization techniques allowed identifying and describing the macroscopic characteristics of the spray and the conditions under which the collapse occurs. The spray collapse engenders from a combination of the internal flow that creates plume interaction, and ambient conditions that promote air entrainment and evaporation. At moderate density and temperature levels the collapse develops, completely modifying the expected trends in the behavior of the plumes. / Resum Els sistemes d'injecció directa han sigut un dels principals punts focals de la investigació en motors, particularment en sistemes dièsel, en què la geometria interna, el moviment de l'agulla i el comportament del flux afecten l'esprai extern i per tant determinen completament el procés de combustió dins del motor. Degut a regulacions mediambientals i al potencial dels (més ineficients) motors "Otto", grans esforços s'estan aportant en investigació sobre sistemes d'injecció directa de gasolina. Els motors GDi tenen el potencial d'incrementar substancialment l'economia del combustible i complir les regulacions de gasos contaminants i d'efecte hivernacle, encara que existeixen molts desafiaments per davant. Esta tesi estudia en detall una moderna tovera GDi que va ser especialment dissenyada per al grup d'investigació conegut com a ECN. Amb l'objectiu de desenvolupar metodologies punteres, este injector ha sigut usat en un ampli ventall d'instal·lacions experimentals per tal de caracteritzar el flux intern i diverses característiques clau de la seua geometria i funcionament, per tal d'avaluar com es relacionen amb els efectes observats del comportament de l'esprai extern. Per a la caracterització interna del flux, l'objectiu ha sigut determinar la geometria de la tovera i el desplaçament de l'agulla, caracteritzar la taxa d'injecció i el flux de quantitat de moviment, i avaluar el flux proper. Algunes metodologies no s'havien empleat abans en injectors GDi, i moltes altres ho han sigut únicament de manera eventual. Per a la geometria interna, l'alçament de l'agulla i el flux proper, s'han aplicat diverses tècniques avançades amb raigsx a les instal·lacions d'Argonne National Laboratory. Per a la taxa d'injecció i el flux de quantitat de moviment, les tècniques disponibles al departament han sigut adaptades des de Dièsel i aplicades a injectors GDi multi-orifici. Considerant la novetat de les tècniques aplicades, les particularitats de les metodologies es discuteixen en detall al document. A pesar de l'elevada turbulència del flux intern, l'injector es comporta de manera consistent injecció a injecció, inclús quan l'estudi se centra en la variabilitat orifici a orifici. Aquest fet s'ha atribuït al comportament repetitiu de l'agulla, avaluat als experiments. També es va observar que el flux estabilitzat té una variació d'altra freqüència que no pot ser explicat pel moviment de l'agulla, sinó pel particular disseny de les toveres. L'anàlisi de la geometria interna realitzat a vuit toveres nominalment iguals va permetre obtenir un punt de vista únic en la construcció de toveres i la variabilitat de dimensions clau. Les mesures de taxa d'injecció van permetre estudiar la resposta hidràulica de l'injector a diverses variables com la pressió d'injecció, la pressió de descàrrega, la temperatura del combustible i la duració de la senyal de comandament. Estes mesures van ser combinades amb mesures de flux de quantitat de moviment per tal d'estudiar el baix valor del coeficient de descàrrega, el qual va ser atribuït al baix alçament de l'agulla i al coeficient L/D dels orificis. D'altra banda, l'estudi de l'esprai extern va permetre identificar un important fenomen específic d'aquest hardware particular: el col·lapse de l'esprai. Les extensives campanyes experimentals, utilitzant Schlieren i il·luminació darrera difusa (DBI) van permetre identificar i descriure les característiques macroscòpiques de l'esprai i les condicions sota les quals el col·lapse té lloc. El col·lapse de l'esprai es forma per una combinació d'interacció de les diverses plomes (causat pel flux intern) i determinades condicions ambient que promouen evaporació i entrada d'aire. Es va determinar a quins nivells de densitat i temperatura moderats es desenvolupa el col·lapse, modificant completament el comportament esperat de l'esprai. / Vaquerizo Sánchez, D. (2018). Study on Advanced Spray-Guided Gasoline Direct Injection Systems [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/99568 / TESIS

Vegetable oil methyl esters obtained by transesterification of vegetable oils are considered to be suitable alternative fuels for diesel engines. However, higher viscosity, surface tension and boiling temperatures of biodiesels may adversely affect spray characteristics as compared to those of diesel. Thus, spray characteristics of Jatropha Methyl Ester (JME) are studied by comparing them to those of diesel in a high-pressure chamber with optical access to simulate the actual in-cylinder conditions. Also, the effect of inner-nozzle cavitation on JME and diesel sprays is studied by utilizing two nozzles, one with sharp entry-radius and the other with larger entry-radius. Finally, spray characteristics of surrogate fuels such as n-dodecane and n-hexadecane are also studied. The first part of the work concerning precise measurements of inner-nozzle geometry revealed that one of the nozzles has a hole diameter of 190-µm and entry-radius of around 70-µm, while the other has a hole diameter of 208-µm and entry-radius of around 10-µm. Injection rate-shape and coefficient of discharge for JME and diesel flow through the two nozzles were then measured. It was observed that while the coefficients of discharge (Cd) are almost identical for JME and diesel, the nozzle with entry radius of 10-µm exhibited around 20% lower Cd than that of the entry-radius of 70-µm. This observation coupled with insight from complementary CFD simulations of inner-nozzle flow showed that the lower Cd of the nozzle with entry-radius of 10-µm could be attributed to inner-nozzle cavitation. The second part of the work involved measurement of non-evaporating spray characteristics including spray-tip penetration, spray-cone angle and droplet size measurement under realistic operating conditions using techniques such as Shadowgraphy and Particle/Droplet Imaging Analysis (PDIA). The non-evaporating spray of the fuels are studied by injecting them using a common-rail fuel injection system into the high-pressure chamber maintained at room temperature. Experimental results show that JME is associated with a slightly faster spray-tip penetration and narrow spray-cone angle indicating inferior spray atomization which is confirmed by around 5% larger droplet sizes. Slower spray-tip penetration, wider spray-cone angle and around 5% smaller droplet sizes are observed for the spray from the cavitating nozzle. Thus, the inner nozzle cavitation is observed to improve the atomization of diesel and JME sprays. The differences in spray characteristics of JME and diesel reduce as the injection pressure increases. The spray-tip penetrations of both surrogates are observed to almost match that of diesel. The third part of the work involved measurements of evaporating spray liquid length, vapour penetration and spread angle for JME, diesel and surrogates at conditions of 50 bar chamber pressure and 900 K temperature. It is observed that the JME exhibits around 16% longer liquid length than that of diesel. The liquid length of n-dodecane is significantly lower than that of diesel and liquid length of n-hexadecane is around 20% higher than that of n-dodecane mimicking the trend of JME and diesel. The liquid length of n-hexadecane is very close to that of diesel at all the three test conditions. Interestingly, the vapour penetration and spread angle for all the fuels is observed to be almost identical. As the cold spray and evaporating spray characteristics of n-hexadecane match well with those of diesel, n-hexadecane can be chosen as a pure component surrogate for diesel. Finally, an analytical model for predicting the spray vapour penetration is assessed with the experimentally-observed trends of penetration and spray spread angle. The model indicated that the effect of fuel density variation is compensated by the corresponding variation in injection velocity for a given injection pressure to result in a similar vapour penetration. Overall, the present work, in addition to studying the effect of fuel physical properties and cavitation on sprays, has generated a comprehensive experimental database on non-evaporating and evaporating sprays of biodiesel, diesel, and pure component surrogates, which would aid significantly in validation of CFD simulations.

Rapid mixing and spreading of supersonic jets are two important characteristics in supersonic ejectors, noise reduction in jets and fuel mixing in supersonic combustion. It helps in changing the acoustic and thermal signature in supersonic exhaust. The supersonic nozzles in most cases result in compressible mixing layers. The subsonic nozzles form incompressible mixing layers but at high Mach numbers even they form compressible mixing layers. Compressible mixing layers have been found to have much lower mixing and spreading rates than incompressible mixing layer Birch & Eggers (1972). In order to enhance the spreading and mixing of mixing layers from supersonic nozzles various active and passive methods have been deviced. Active methods include fluid injection, fluid lobes and plasma actuation. Passive methods are mostly based on modifying the nozzle geometry such that the fluid expansion is ideal or the shock cell is broken. Many nozzles with exotic shapes have been developed to obtain mixing enhancements in supersonic jets Gutmark et al. (1995). To achieve enhanced mixing an innovative nozzle named as the Elliptic Sharp Tipped Shallow (ESTS) lobed nozzle has been developed in L.H.S.R., I.I.Sc., India Rao & Jagadeesh (2014). This nozzle has a unique geometry involving elliptical lobes and sharp tips. These lobes are generated using a simple manufacturing process from the throat to the exit. This lobed and sharp tipped structure introduces stream wise vortices and azimuthal velocity components which must help in enhanced mixing and spreading. The ESTS lobed nozzle has shown mixing enhancement with 4 lobes. The spreading rate was found to be double of the reference conical nozzle. This thesis is motivated by the need to investigate the flow physics involved in the ESTS lobed nozzle. The effect of varying the number of lobes and the design Mach number of the nozzle on the mixing and spreading characteristics will be further discussed. Visualisation studies have been performed. The schlieren and planar LASER Mie scattering techniques have been used to probe the flow. Instantaneous images were taken at axial planes with the reference conical and ESTS nozzles with three, four, five and six lobes. The nozzles are for design Mach number 2.0 and 2.5. The stagnation chamber pressure was maintained to obtain over expanded, ideally expanded and under expanded flows. LASER scattering was obtained by seeding the flow with water to observe the behaviour of the primary flow. The condensation of moisture due to the cold primary flow mixing with the ambient air was exploited to scatter laser and observe the flow structures in the mixing layer. A comparison of the images of the reference conical nozzle and the ESTS lobed nozzles shows changes in the mixing layers due to the ESTS lobed nozzles. The image of the reference conical nozzle shows a distinct potential core and mixing layers all along the length of the image. For the ESTS lobed nozzles this distinction becomes unclear shortly after the nozzle exit. Thus mixing of the primary flow and ambient air is seen to be enhanced in the case of all the ESTS lobed nozzles. The flow in the case of the ESTS lobed nozzles if found to be highly non axis symmetric. The starting process of the nozzles has been visualised using time resolved schlieren. Image processing was performed on the nozzles to quantify the spread rate. The shock structure of the nozzles has been studied and found to be modified due to the lobed geometry. The level of convolution of the mixing layer due to the lobed structure has been studied using fractal analysis. The four lobed nozzle was found to have the highest spread rate and th most convoluted shear layer. Hence this nozzle was further studied using background oriented schlieren and particle image velocimetry to quantify the flow field. These experimental results have been compared with CFD simulations using the commercial software CFX5. The computations and experiments don’t match accurately but the trends match. This allows for simulations to be used as a good first approximation. The acoustic properties of a jet are dependent on the flow structure behaviour. The ESTS lobes have been found to change the flow structure. Hence the ESTS lobed nozzle was predicted to change the acoustic signature of the flow. The acoustic measurements of the flow were carried out at National Aerospace Laboratories, Bengaluru. The screech of the overexpanded flow was seen to be eliminated and the overall sound levels were found to have been reduced in all cases. Thus the lobed nozzle was found to have acoustic benefits over the reference conical nozzle. Thus the ESTS lobed nozzle has been studied and compared with the conical nozzle using several methods. The changes due to the lobed structure have been studied quantitatively. Future studies would focus on the change in thrust due to the lobed structure. Also new geometries have been proposed inspired by the current design but with possible thrust benefits or manufacturing benefits.

[EN] In the current work, the characterization of the combustion process inside a stratified pre-chamber spark ignition (PCSI) system is performed. An extensive bibliographical review about the pre-chamber systems developed from the second half of the 20th century until modern times is presented. The review shows that the latest generation systems have the potential to accomplish the emissions limits while providing high performance and low fuel consumption. Nevertheless, many efforts of the scientific community are still needed to allow the large-scale application of the technology. Indeed, based on the outstanding challenges observed, the investigation plan is developed including both experimental and numerical parts. All experiments were performed by means of the rapid compressionexpansion machine (RCEM) in the CMT-Motores Térmicos laboratory. The original cylinder head layout was modified to allow the housing of the prechamber itself, fuel injectors, spark plug, pressure transducers in both chamber, and a thermocouple. The test methodology involved the acquisition of the pressure evolution in both main chamber and pre-chamber, the piston position (used to compute the instantaneous cylinder volume), the duration of the auxiliary injection, and the spark ignition point. These are used as input for the zero-dimensional thermodynamic model which simulates the fundamental parameters aims to characterize the PCSI system working cycle. Therefore, a deeper knowledge of the mass interchanged process, induced turbulence field, heat release rate, combustion speed, and flame regime is generated. Subsequently, to calibrate the zero-dimensional model coefficients under motoring conditions, several 3D CFD simulations were carried out by means of Converge software. Hence, the results of the simulations in terms of interchanged mass and pre-chamber turbulent kinetic energy have been used to calibrate the nozzle discharge coefficient and the turbulence sub-model coefficients for all the pre-chamber geometries. Furthermore, the 3D CFD simulations outputs are analysed to fully understand the flow field structure and the local effect induced by the different nozzles at the spark activation time. The turbulent kinetic energy in terms of intensity and orientation is investigated over several relevant pre-chamber sections. The results reveal a clear relationship between the turbulence developed within the pre-chamber and the orifices structure. Straight orifices or perpendicular jets impact, promote more intense local turbulence due to direct collision while tilted orifices guarantee more homogeneity due to the swirling motion. Additionally, increase the orifice numbers shows benefits on the fluid dynamic homogeneity. Thus, preceding the experimental campaign several fundamental aspects of the system are evaluated. The cycle-to-cycle dispersion is explored by means of the statistical assessment showing low pressure peak deviation. The auxiliary injection pressure and timing are optimized for avoiding wall wetting phenomena while ensuring proper air/fuel mixing. Finally, the spark activation point is chosen as a function of the theoretically maximum turbulent flame speed. Thereby, the experimental campaign is carried out according to tests matrix, in order to evaluate the effect of the equivalence ratio of both chambers, and how the orifices diameter, number, and distribution affect the combustion process. Moreover, chemiluminescence visualization tests, performed by means of the available optical access of the RCEM, are combined with zerodimensional and 3D CFD results to shed light on the work cycle. Conclusions suggest a slightly rich mixture inside the pre-chamber combined with the highest number of tilted orifices as the better configuration for improving combustion efficiency under lean and ultra-lean main chamber mixture conditions. Nevertheless, axial orifices should be considered for further investigations. Finally, the author proposes a series of developments considered interesting in both the experimental and numerical fields. / [ES] En el presente trabajo se realiza la caracterización del proceso de combustión dentro de un sistema de encendido por pre-cámara bajo carga estratificada. Por lo tanto, se presenta una extensa revisión bibliográfica sobre los sistemas de pre-cámara desarrollados desde la segunda mitad del siglo XX hasta los tiempos modernos. El resumen muestra que los sistemas de última generación tienen el potencial de cumplir con los límites de las emisiones, al tiempo que proporcionan un alto rendimiento y un bajo consumo de combustible. No obstante, todavía se necesitan muchos esfuerzos de la comunidad científica para permitir la difusión a gran escala de la tecnología. De hecho, sobre la base de los desafíos abiertos observados, se desarrolla el plan de investigación incluyendo tanto una parte experimental como numérica. Todos los experimentos se realizan mediante la máquina de compresión-expansión rápida (RCEM) de que dispone el laboratorio CMT-Motores Térmicos . La disposición original de la culata se modificó para permitir el alojamiento de la propia pre-cámara, los inyectores , la bujía, los sensores de presión y un termopar. La metodología de ensayo implica la adquisición de la evolución de la presión tanto en cámara principal como en pre-cámara, el volumen del cilindro, la duración de la inyección auxiliar y el punto de ignición de la bujía. Estos se utilizan como parámetros de entrada para el modelo termodinámico cero-dimensional que devuelve los parámetros fundamentales que caracterizan ciclo de trabajo del sistema PCSI. Por lo tanto, se genera un conocimiento más profundo del proceso de intercambio de masas, del campo de turbulencias inducidas, de la tasa de liberación de calor, de la velocidad de combustión y del régimen de la llama. Posteriormente, para calibrar los coeficientes del modelo cero-dimensional bajo condiciones de arrastre, se llevaron a cabo varias simulaciones CFD en 3D mediante el software Converge. Por lo tanto, los resultados de las simulaciones en términos de masa intercambiada y energía cinética turbulenta de la precámara se han utilizado para calibrar el coeficiente de descarga de la tobera y los coeficientes del sub-modelo de turbulencia para todas las geometrías de la pre-cámara. Además, se analizan los resultados de las simulaciones CFD para comprender plenamente la estructura del campo de flujo y el efecto local inducido por las diferentes geometriás en el tiempo de activación de la chispa. La energía cinética turbulenta en términos de intensidad y orientación se investiga en varias secciones relevantes de la pre-cámara. Los resultados revelan una clara relación entre la turbulencia desarrollada dentro de la pre-cámara y la estructura de los orificios. Los orificios rectos o los chorros perpendiculares, promueven una turbulencia local más intensa debido a la colisión directa mientras que los orificios inclinados del campo fluido y del dosado. Precedentemente al desarrollo de la campaña experimental se evalúan varios aspectos fundamentales del sistema. La dispersión ciclo a ciclo se explora por medio de la evaluación estadística que muestra una baja desviación de los picos de presión. La presión y el punto de inyección auxiliar se optimizan para evitar los fenómenos de mojado de las paredes, asegurando al mismo tiempo una mezcla adecuada de aire/combustible. Finalmente, el punto de activación de la chispa se elige en función de la velocidad máxima teórica de la llama turbulenta. De este modo, la campaña experimental se lleva a cabo de acuerdo con la matriz de pruebas, con el fin de evaluar el efecto del dosado equivalente de ambas cámaras, y cómo el diámetro, el número y la distribución de los orificios afectan al proceso de combustión. Además, las pruebas de visualización de quimioluminiscencia, realizadas mediante el acceso óptico disponible de la RCEM, se combinan con resultados de CFD y resultados del modelo cerodimen para arrojar luz sobre el ciclo de trabajo. Las conclusiones sugieren que una mezcla ligeramente rica dentro de la pre-cámaracombinadaconelmayornúmerodeorificiosdesfasadoseslamejor configuración para garantizar un elevada eficiencia de la combustión en condiciones de mezcla pobre y ultra-pobre de la cámara principal. No obstante, los orificios axiales deben ser considerados para investigaciones futuras. Por último, el autor propone una serie de desarrollos considerados interesantes tanto en el campo experimental como en el numérico. / [CA] En el present treball es realitza la caracterització del procés de combustió dins d'un sistema d'encesa de pre-cambra soto càrrega estratifi-cada. Per tant, es presenta una extensa revisió bibliogràfica sobre els sistemes de precambra desenvolupats des de la segona meitat del segle XX fins als temps moderns. El resum mostra que els sistemes d'última generació tenen el potencial de complir amb els límits de les emissions, al mateix temps que proporcionen un alt rendiment i un baix consum de combustible. No obstant això, encara es necessiten molts esforços de la comunitat científica per a permetre la difusió a gran escala de la tecnologia. De fet, sobre la base dels desafiaments oberts observats, es desenvolupa el pla d'investigació incloent tant una part experimental com numèrica. Tots els experiments es realitzen mitjançant la màquina de compressió-expansió ràpida (RCEM) de què disposa el laboratori CMT-Motors Tèrmics. La disposició original de la culata es va modificar per a permetre l'allotjament de la pròpia pre-cambra, els injectors , la bugia, els sensors de pressió i un termoparell. La metodologia d'assaig implica l'adquisició de l'evolució de la pressió tant en cambra principal com en pre-cambra, el volum del cilindre, la duració de la injecció auxiliar i el punt d'ignició de l'espurna. Aquests s'utilitzen com a paràmetres d'entrada per al model termodinàic zero-dimensional que retorna els paràmetres fonamen-tals que caracteritzen cicle de treball del sistema PCSI. Per tant, es genera un coneixement més profund del procés d'intercanvi de masses, del camp de turbulències induïdes, de la taxa d'alliberament de calor, de la velocitat de combustió i del règim de la flama. Posteriorment, per a calibrar els coefi-cients del model zerodimensional sota condicions d'arrossegament, es van dur a terme diverses simulacions CFD en 3D mitjançant el programari Converge. Per tant, els resultats de les simulacions en termes de massa intercanviada i energia cinètica turbulenta de la pre-cambra s'han utilitzat per a calibrar el coeficient de descàrrega de la tovera i els coeficients del sub-model de turbulència per a totes les geometries de la pre-cambra. A més, s'analitzen els resultats de les simulacions CFD per a comprendre plenament l'estructura del camp de flux i l'efecte local induït per les diferents geometries en el temps d'activació de l'espurna. L'energia cinètica turbulenta en termes d'intensitat i orientació s'investiga en diverses seccions rellevants de la pre-cambra. Els resultats revelen una clara relació entre la turbulència desenvolupada dins de la pre-cambra i l'estructura dels orificis. Els orificis rectes o els dolls perpendiculars, promouen una turbulència local més intensa a causa de la col·lisió directa mentre que els orificis inclinats garanteixen una major homogeneïtat a causa de la generació d'un macro-remolì. A més, l'augment del nombre d'orificis mostra beneficis en l'homogeneïtat fluid-dinàmica. Llavors, abans de la campanya experimental s'avaluen diversos aspectes fonamentals del sistema. La dispersió cicle a cicle s'explora per mitjà de l'avaluació estadística que mostra una baixa desviació dels pics de pressió. La pressió i el punt d'injecció auxiliar s'optimitzen per a evitar els fenòmens de mullat de les parets, assegurant al mateix temps una mescla adequada d'aire/combustible. Finalment, el punt d'activació de l'espurna es tria en funció de la velocitat màxima teòrica de la flama turbulenta. D'aquesta manera, la campanya experimental es duu a terme d'acord amb la matriu de proves, amb la finalitat d'avaluar l'efecte del dosatge equivalent de totes dues cambres, i com el diàmetre, el numero i la distribució dels orificis afecten el procés de combustió. A més, les proves de visualització de quimioluminescència, realitzades mitjançant l’accés òptic disponible de la RCEM, es combinen amb resultats de CFD i resultats del model zero-dimensional per a llançar llum sobre el cicle de treball. Les conclusions suggereixen que una mescla lleugerament rica dins de la pre-cambra combinada amb el major nombre d’orificis desfasats és la millor configuració per a garantir un elevada eficiència de la combustió en condicions de mescla pobra i ultra-pobre de la cambra principal. No obstant això, els orificis axials han de ser considerats per a investigacions futures. Finalment, l’autor proposa una sèrie de desenvolupaments considerats interessants tant en el camp experimental com en el numèric. / Pagano, V. (2020). Analysis of a stratified pre-chamber spark ignition system under lean mixture conditions [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/152486 / TESIS