1,721,028 research outputs found
Review of Numerical Methodologies for Modeling Cavitation
No full textCavitation induction is of high interest for a wide range of applications, from hydraulic machines to bioengineering applications. Numerous experimental and numerical studies have aimed to unveil the dynamics of cavitation to enhance the performance and lower the impact of erosion on machinery but also to employ its mechanics in advanced non invasive medical procedures. The current work provides a comprehensive review of the methodologies that have been developed in the framework of computational fluid dynamics in order to study cavitating flows, highlighting the link of the application with the utilized approach. The methods are presented and assessed according to the class of physical problems addressed, which, in turn, are classified into problems of single-bubble dynamics, bubble cluster dynamics, and cavitating flows at engineering scales.<br/
Machine Learning and transcritical sprays: A demonstration study of their potential in ECN Spray-A
No full textThe present work investigates the application of Machine Learning and Artificial Neural Networks for tackling the complex issue of transcritical sprays, which are relevant to modern compression-ignition engines. Such conditions imply the departure of the classical thermodynamic perspective of ideal gas or incompressible liquid, necessitating the use of costly and elaborate thermodynamic closures to describe property variation and simulation methods. Machine Learning can assist in several ways in speeding up such calculations, either as a compact, trained thermodynamic model that can be coupled to the flow solver, or as a surrogate predictive tool of spray characteristics. In this work, such applications are demonstrated and their performance is assessed against more traditional approaches. Such applications involve the prediction of macroscopic spray characteristics, for example, the spray penetration over time, or the spray distribution in space and time, and predictions of fluid properties for the thermodynamic states encountered in such applications. Macroscopic characteristics can be adequately predicted by relatively simple network structures, involving just a hidden layer of 3-4 neurons, whereas prediction of thermodynamic states requires several layers of 5-20 neurons each. The results of integrating Artificial Neural Networks in transcritical sprays are rather promising; prediction of thermodynamic properties at pressures greater than I bar has effectively zero error, yielding simulations indistinguishable from standard tabulated approaches with minimal overhead. When used as a regression method for time-histories either of spray characteristics or spray distributions, the results are within experimental uncertainty of similar experiments, not included in the training dataset. [GRAPHICS] .
VOF simulations of the contact angle dynamics during the drop spreading: Standard models and a new wetting force model
Full text linkIntroduction
In this study,a novel numerical implementation for the adhesion of liquid droplets impacting normally on solid dry surfaces is presented. The advantage of this new approach, compared to the majority of existing models, is that the dynamic contact angle forming during the surface wetting process is not inserted as a boundary condition, but is derived implicitly by the induced fluid flow characteristics (interface shape) and the adhesion physics of the gas-liquid-surface interface (triple line), starting only from the advancing and receding equilibrium contact angles. These angles are required in order to define the wetting properties of liquid phases when interacting with a solid surface.
Methodology
The physical model is implemented as a source term in the momentum equation of a Navier-Stokes CFD flow solver as an "adhesion-like" force which acts at the triple-phase contact line as a result of capillary interactions between the liquid drop and the solid substrate. The numerical simulations capture the liquid-air interface movement by considering the volume of fluid (VOF) method and utilizing an automatic local grid refinement technique in order to increase the accuracy of the predictions at the area of interest, and simultaneously minimize numerical diffusion of the interface.
Results
The proposed model is validated against previously reported experimental data of normal impingement of water droplets on dry surfaces at room temperature. A wide range of impact velocities, i.e. Weber numbers from as low as 0.2 up to 117, both for hydrophilic (θadv = 10° - 70°) and hydrophobic (θadv = 105° - 120°) surfaces, has been examined. Predictions include in addition to droplet spreading dynamics, the estimation of the dynamic contact angle; the latter is found in reasonable agreement against available experimental measurements.
Conclusion
It is thus concluded that theimplementation of this model is an effective approach for overcoming the need of a pre-defined dynamic contact angle law, frequently adopted as an approximate boundary condition for such simulations. Clearly, this model is mostly influential during the spreading phase for the cases of low We number impacts (We <80) since for high impact velocities, inertia dominates significantly over capillary forces in the initial phase of spreading
Influence of nozzle hole eccentricity on spray morphology
No full textLarge marine two-stroke diesel engines have an injector geometry, which differs substantially from the configurations used in most other diesel engine applications, as the injector orifices are distributed in a highly non-symmetric fashion. In order to experimentally assess the impact of key features of such orifice arrangements on spray morphology, orifice eccentricity relative to the injector axis in particular, a dedicated test setup has been realised, including the development and application of tailor-made data processing routines. The high-speed camera recordings of the Mie-scattering data obtained simultaneously for two perpendicular views of single sample sprays have been analysed in terms of spray tip penetration and spray angle as well as with respect to the orientation of the spray. These analyses confirm the complex three-dimensional structure of sprays at such conditions: They are in fact far from rotationally symmetric – specifically when high levels of eccentricity apply – and the actual orientation of their axis in such cases clearly deviates from the nominal one, normally assumed to be in line with the orifice axis. These deflections are in the range of 10° and they apply not only in the direction of the eccentricity but also perpendicular to it. Additional effects arise from the geometric configuration of the central bore of the injector, upstream of the orifice, and when varying the injection pressure.In the case of high eccentricity, moreover, a clear pattern can be discerned in the initial evolution of the spray deflection: Starting from a slight deflection in the direction of the eccentricity, the spray axis moves first to its nominal direction and then gradually changes orientation again towards the level of stabilisation
Influence of nozzle hole eccentricity on spray morphology
No full textLarge marine two-stroke diesel engines have an injector geometry, which differs substantially from the configurations used in most other diesel engine applications, as the injector orifices are distributed in a highly non-symmetric fashion. In order to experimentally assess the impact of key features of such orifice arrangements on spray morphology, orifice eccentricity relative to the injector axis in particular, a dedicated test setup has been realised, including the development and application of tailor-made data processing routines. The high-speed camera recordings of the Mie-scattering data obtained simultaneously for two perpendicular views of single sample sprays have been analysed in terms of spray tip penetration and spray angle as well as with respect to the orientation of the spray. These analyses confirm the complex three-dimensional structure of sprays at such conditions: They are in fact far from rotationally symmetric – specifically when high levels of eccentricity apply – and the actual orientation of their axis in such cases clearly deviates from the nominal one, normally assumed to be in line with the orifice axis. These deflections are in the range of 10° and they apply not only in the direction of the eccentricity but also perpendicular to it. Additional effects arise from the geometric configuration of the central bore of the injector, upstream of the orifice, and when varying the injection pressure.In the case of high eccentricity, moreover, a clear pattern can be discerned in the initial evolution of the spray deflection: Starting from a slight deflection in the direction of the eccentricity, the spray axis moves first to its nominal direction and then gradually changes orientation again towards the level of stabilisation
Phasenübergang des Kraftstoff Sprays bei Umgebungsbedingungen des Dieselmotors: Nichtidealität des Phasengleichgewichts
Full text linkIn mixing controlled Diesel combustion concept, mixing processes are a key phenomenon which significantly effects power, efficiency and emissions. A consequence of fuel-air-mixing is the fuel phase change, which its nature in Diesel engine fuel sprays is not clear to this date. There are different stances on the question whether fuel or a fuel-rich phase might get supercritical (one-phase mixing) or not (two-phase mixing). In this study, phase change mechanisms of sprays of different n-alkanes at high temperature and pressure conditions are investigated. Mie scattering imaging is utilized to obtain the maximum liquid penetration of Diesel surrogates (dodecane, decane, heptane and hexane) injected into nitrogen atmosphere in a constant volume chamber at fuel supercritical temperature and ambient pressures ranging from sub- to supercritical with regard to fuel critical point.
Two theoretical 1D calculations which are based on an air entrainment model derived from momentum conservation, are utilized to determine local mass ratios of ambient gas and fuel which leads to predict the liquid length based on the required enthalpy for full phase change. On the one hand, the assumption is taken that fuel evaporates without boiling or getting supercritical and on the other hand, evaporation is excluded, assuming that fuel is heated up until boiling or getting supercritical. Both models allow the calculation of values for liquid length of different fuel sprays. In following, these theoretical models are validated against experimental data (around 400 points) to identify phase change regimes for different fuels. Fitting the predicted liquid lengths from both models to the experimental data, the point of switching between those two models is the concept of “transition point”. The observed transition points for different fuels and temperatures are utilized to estimate a transition line. A transition line, at which the change from evaporation to boiling\transcritical phase change occurs, is indicator of “ideal phase equilibrium state” as well. Results indicate that along with ambient conditions, fuel properties remarkably influence the phase change mechanism in a spray. Especially fugacity plays an important role regarding the droplet evaporation rate.
An equation is calibrated to the transition lines, as function of fuel fugacity and ambient temperature. The equation can be utilized to estimate the transition line for different fuels at different conditions with satisfactory accuracy.Im Modell der mischungsgesteuerten Verbrennung von Diesel sind Mischvorgänge ein Schlüsselphänomen, welches weitreichende Auswirkungen auf Leistung, Effizienz und Emissionen hat. Eine Folge der Vermischung von Kraftstoff und Luft ist der Kraftstoff-Phasenwechsel, dessen Eigenschaften in Dieselmotor-Kraftstoffsprays bis heute nicht eindeutig geklärt werden konnte. Es bestehen unterschiedliche Standpunkte zu der Frage, ob Kraftstoff oder eine kraftstoffreiche Phase unter Umständen überkritisch werden kann (einphasige Mischung) oder nicht (zweiphasige Mischung). Im Rahmen der vorliegenden Studie sollen Phasenwechselmechanismen von unterschiedlichen Sprühnebeln aus verschiedenen n-Alkanen bei extremen Bedingungen wie hohen Temperaturen oder hohem Druck untersucht werden. Es wird eine Mie-Streuungsbildgebung genutzt, um die maximale Menge von eindringender Flüssigkeit von Diesel-Surrogaten (Dodecane, Decane, Heptane und Hexane) zu erhalten, die in einer Kammer mit konstantem Volumen bei einer überkritischen Umgebungstemperatur und überkritischen Umgebungsdrücken, die im Hinblick auf den kritischen Punkt des Kraftstoffs von unter- bis überkritisch reichen, in eine Stickstoffatmosphäre injiziert werden.
Zwei theoretische 1D-Berechnungen, die auf einem aus der Impulserhaltung hergeleiteten Luftporengehaltsmodell basieren, werden genutzt, um lokale Massenverhältnisse des Umgebungsgases und Brennstoffes zu bestimmen. Dies führt zu einer Vorhersage der Flüssigkeitslänge auf Basis der erforderlichen Enthalpie für einen vollständigen Phasenwechsel. Einerseits gilt die Annahme, dass der Brennstoff verdampft, ohne dabei zu sieden oder superkritisch zu werden. Auf der anderen Seite jedoch, ist eine Verdampfung ausgeschlossen, nimmt man an, dass der Brennstoff solange aufgeheizt wird, bis er siedet oder überkritisch wird. Beide Modelle machen die Berechnung von Werten für die Flüssigkeitslänge verschiedener Brennstoffzerstäubungen möglich. Diese theoretischen Modelle sollen in Folgenden auf Basis experimenteller Daten (ca. 400 Punkte) validiert werden, um Phasenwechselschemata verschiedener Brennstoffe zu identifizieren. Werden die vorhergesagten Flüssigkeitslängen beider Modelle an die experimentellen Daten angepasst, so ergibt sich im Punkt des Wechsels zwischen diesen beiden Modellen das Konzept des „Übergangspunktes“. Die so beobachteten Übergangspunkte verschiedener Brennstoffe und Temperaturen dienen dazu, eine Übergangslinie abzuschätzen - eine Übergangslinie, an der der Wechsel von Verdampfung zum Sieden bzw. der Wandel zur transkritischen Phase stattfindet, kann gleichermaßen als Indikator eines „idealen Phasengleichgewichtszustandes“ gesehen werden. Die Ergebnisse zeigen, dass zusätzlich zu den Umgebungsbedingungen die Brennstoffeigenschaften den Phasenänderungsmechanismus in einem Sprühnebel beachtlich beeinflussen. Vor allem Flüchtigkeit spielt hinsichtlich der Verdampfungsrate von Tropfen eine wichtige Rolle.
So soll eine Gleichung für die Übergangslinien kalibriert werden, die als Funktion der Brennstoffflüchtigkeit und der Umgebungstemperatur dient. Diese kann genutzt werden, um die Übergangslinie für verschiedene Brennstoffe unter verschiedenen Bedingungen mit ausreichender Genauigkeit abzuschätzen
Machine-learning enabled prediction of 3D spray under engine combustion network spray G conditions
Full text linkSpray and air–fuel mixing in gasoline direct-injection (GDI) engines play a crucial role in combustion and emission characteristics. While a variety of phenomenological spray models and computational fluid dynamics (CFD) simulations have been applied to identify air–fuel mixture distribution, most research efforts so far were concentrated on single axial-nozzle injectors and limited range of ambient conditions. Especially, the prediction of flash-boiling sprays in multi-hole injectors remains a great challenge due to the lack of understanding of the complicated two-phase flow dynamics. For the specific conditions, the question can arise concerning the capability of machine-learning algorithms to predict complex flash-boiling sprays. We developed a machine-learning algorithm, as a simple variant of linear regression, that is capable of predicting the spray 3D topology for various fuels and ambient conditions. A series of spray experiments were carried out in a constant-flow spray vessel coupled with high-speed diffused back-illumination extinction imaging to produce a data set for algorithm training. Nine different test fuels, including single component iso-octane (ic8) and multi-component EEE gasoline, that cover a wide range of fuel properties were injected using Engine Combustion Network (ECN) Spray G injector under ECN G2 (50 kPa absolute), G3 (100 kPa absolute), and G3HT (G3 with 393 K ambient temperature) conditions. Among the test fuels, ic8ib2 (ic8 80%, iso-butanol 20% v/v) and EEE gasoline were specified as target fuels for spray prediction by the machine-learning algorithm, thus they were not included in the training data. The macroscopic spray analysis based on projected liquid volume (PLV) and computed tomographic (CT) reconstruction showed that the spray prediction by the machine-learning algorithm showed excellent agreement with true values from the experimental data. The maximum differences in liquid penetration for ic8ib2 and EEE fuel were 3.6 mm (7.3% error) and 1.3 mm (2.32% error), respectively. The 3D spray predicted had a consistent trend to experimental data showing slight plume movement for ic8ib2 but complete spray collapsing for EEE gasoline fuel. The plume direction angle enabled by the CT data showed differences up to 2° compared to true values during the injection period. The quantitative validation results showed that the machine-learning algorithm is capable of predicting spray performance with nine input features (fuel properties and ambient conditions), and is actually superior to CFD performance for these same number of spray parameters
Cavitation and Bubble Dynamics: Fundamentals and Applications
No full textCavitation and Bubble Dynamics: Fundamentals and Applications examines the latest advances in the field of cavitation and multiphase flows, including associated effects such as material erosion and spray instabilities. This book tackles the challenges of cavitation hindrance in the industrial world, while also drawing on interdisciplinary research to inform academic audiences on the latest advances in the fundamentals.
Contributions to the book come from a wide range of specialists in areas including fuel systems, hydropower, marine engineering, multiphase flows and computational fluid mechanics, allowing readers to discover novel interdisciplinary experimentation techniques and research results.
This book will be an essential tool for industry professionals and researchers working on applications where cavitation hindrance affects reliability, noise, and vibrations
Étude numérique des effets thermiques lors de l'effondrement de la bulle
No full textL'estimation de la température qui se développe à l'intérieur de la cavitation qui s'effondre et/ou des bulles gazeuses est importante pour une large gamme d'applications. Lors de l'effondrement des bulles induit par les ultrasons, des températures de l'ordre de 10 ^ 4 K peuvent être atteintes, conduisant à une sonoluminescence et à des réactions chimiques. La génération de radicaux libres au cours de ce processus a des implications sur la pathogenèse, l'apoptose et les dommages causés aux biomolécules. L'augmentation de la température du liquide environnant peut également devenir importante en affectant les dommages par cavitation, les mécanismes d'élimination de matière, ainsi que l'efficacité des traitements médicaux par ultrasons focalisés de haute intensité (HIFU). Bien que les mesures de la température du contenu des bulles et de leurs environs restent difficiles, la majorité des études numériques dans la littérature considèrent des modèles simplifiés avec des hypothèses thermodynamiques idéales. Le cœur de la présente recherche vise à surmonter les limitations ci-dessus et à étudier les conditions dans lesquelles les températures et les pressions varient énormément. Ceci a été réalisé grâce à la modélisation de la thermodynamique des fluides réelle à l'aide d'équations d'état complexes (EoS). À cet égard, trois EoS à gaz réel sont utilisés pour la phase gazeuse en plus du gaz idéal. Pour la phase liquide, en plus de l’EoS Stiffened Gas (SG), quatre modèles avancés sont déployés. Tous les modèles de fermeture thermodynamiques susmentionnés sont implémentés et couplés à deux solveurs explicites des équations d'Euler basés sur la densité, à savoir : (1) le modèle à six équations et (2) le modèle Kapila pour les milieux d'écoulement gaz-liquide non visqueux et non miscibles. La méthode des volumes finis Godunov est utilisée pour la discrétisation. Divers cas d'effondrement de bulles sont étudiés, notamment les cas d'effondrement sphérique 1D et l'effondrement axisymétrique 2D d'une bulle s'effondrant près d'une paroi rigide ; les impulsions de pression ultrasonores typiques de celles utilisées dans les configurations de lithotriteur ont été supposées. Les résultats montrent que la fermeture thermodynamique joue un rôle important dans les effets thermiques. En ce qui concerne la thermodynamique des gaz, il est démontré qu'à mesure que l'effondrement devient plus violent, les effets réels des gaz deviennent plus importants. Il en résulte une différence substantielle de ≈33 % dans la température d'effondrement sphérique moyenne dans l'espace et de ≈41 % dans le cas d'effondrement non sphérique, correspondant à 2 700 K. Par conséquent, il est conclu que l'hypothèse des gaz parfaits est inadéquate pour prédire les températures. . En ce qui concerne la thermodynamique des liquides, la nécessité d'une thermodynamique liquide avancée dans les simulations d'effondrement de bulles est mise en évidence par la démonstration d'un saut parasite de température de liquide formé à proximité de l'interface de la bulle lors de l'utilisation d'équations d'état simplifiées. L'augmentation de la température du liquide de ≈25 K le long de la paroi est également illustrée dans l'effondrement de la lithotripsie. De plus, les impacts du rapport de pression et de la distance de sécurité initiale sont étudiés. La température du liquide prévue révèle un aperçu de la thermodynamique de la limite qui peut être utilisée pour modéliser les dommages thermiques sur les tissus mous.Estimation of the temperature developing inside collapsing cavitation and/or gaseous bubbles is important for a wide range of applications. During ultrasound-induced bubble collapse, temperatures of the order of 10^4 K can be reached, leading to sonoluminescence and chemical reactions. The generation of free radicals during this process has implications for pathogenesis, apoptosis, and damage to biomolecules. The temperature augmentation of the surrounding liquid can also become important by affecting cavitation damage, material removal mechanisms, as well as the effectiveness of high-intensity focused ultrasound (HIFU) medical treatments. While measurements of the temperature of bubble content and its surroundings remain challenging, the majority of numerical studies in the literature consider simplified models with ideal thermodynamics assumptions. The core of the present research aims to overcome the above limitations and investigate the conditions over which temperatures and pressures vary wildly. This has been achieved by modelling of real fluid thermodynamics using complex equations of state (EoS). In this regard, three real-gas EoSs are utilised for the gas phase besides the ideal gas. For the liquid phase, in addition to the Stiffened Gas (SG) EoS, four advanced models are deployed. All the aforementioned thermodynamic closure models are implemented and coupled with two explicit density-based solvers of the Euler equations, namely: (1) six-equation model and (2) Kapila model for inviscid and immiscible gas–liquid flow media. The Godunov finite volume method is utilised for discretization. Various bubble collapse cases are investigated including 1D spherical collapse cases and 2D axisymmetric collapse of a bubble collapsing near a rigid wall; ultrasound pressure pulse typical of those utilised in lithotripter setups have been assumed. The findings show that the thermodynamic closure plays a significant role in the thermal effects. Regarding gas thermodynamics, it is demonstrated that as the collapse becomes more violent, real gas effects become more significant. This results in a substantial difference of ≈33% in the space-averaged spherical collapse temperature and of ≈41% in non-spherical collapse case, corresponding to 2700 K. Hence, it is concluded that the ideal gas assumption is inadequate for predicting temperatures. With regards to liquid thermodynamics, the necessity of advanced liquid thermodynamics in bubble collapse simulations is highlighted by demonstrating a spurious liquid temperature jump formed in the vicinity of the bubble interface when using simplified equations of state. The liquid temperature augmentation of ≈25 K along the wall is also illustrated in the lithotripsy collapse. Moreover, the impacts of the pressure ratio and the initial stand-off distance are studied. The predicted liquid temperature reveals an insight into the thermodynamics of the boundary which can be utilised to model thermal damage on soft tissues
Experimentelle Messungen kavitierender Strömungen in Kraftstoffeinspritzsystemen
Full text linkIn the fuel injection equipment of internal combustion engines, the complexity of two-phase flows arising within the injectors represents a critical aspect in the intricate realm of engine design. This highly complex phenomenon involves the simultaneous presence of liquid fuel and vaporised fuel within the injector, adding layers of sophistication to the fuel atomization and the lifetime of the injector. Hence, understanding the two-phase flow is paramount for achieving efficient combustion, emission control, overall engine performance, and the ongoing evolution of internal combustion engines.
The investigation of vapour content within injectors is a complex task that relies on a combination of computational fluid dynamics (CFD) and experimental studies. CFD simulations provide a virtual platform to model and analyse the intricate fluid dynamics, thermodynamics, and phase changes occurring within the injector, and offers insights into the morphology, dynamics, and vaporisation of the fuel, allowing for a detailed examination of the two-phase flow phenomena. However, the validation of CFD models requires experimental studies to ensure accuracy and reliability. Experimental measurements offer a real-world validation of the simulated results, helping to bridge the gap between theoretical predictions and practical observations.
This work is dedicated to three major cutting-edge experimental techniques in elucidating the behaviour of the two-phase flow inside the injector as well as near nozzle spray characteristics; Initially, as research on renewable and alternative fuels is crucial for improving the energy and environmental efficiency of modern gasoline internal combustion engines, high-speed imaging using Diffuse Backlight Illumination (DBI) was employed to investigate the transient two-phase flow field arising in the internal geometry and the near-nozzle spray region of gasoline injectors. Experiments were conducted at realistic operating conditions comprising an injection pressure of 100 bar and ambient pressures in the range of 0.1 – 6.0 bar to cover the entire range of chamber pressures prevailing in Gasoline Direct Injection engines. Moreover, high-flux synchrotron radiation was employed in a time-resolved manner to characterise the distinct topology features and dynamics of different cavitation regimes arising in a throttle orifice with an abrupt flow-entry contraction. Radiographs obtained through both X-ray phase-contrast and absorption imaging have been captured at 67890 frames per second. X-ray phase-contrast imaging offers sharp refractive index gradients in the interface region for capturing fine morphological fluctuations of transient cavitation structures, whereas absorption imaging is explicitly correlated with the projected vapour thickness in a line-of-sight manner. Finally, the capability of cold neutron imaging to quantify cavitation, in terms of vapour content, within an orifice of an abruptly constricting geometry is demonstrated. Despite the time-averaged nature of this technique, the morphology of transient vaporous structures is clearly visualised due to the high spatial resolution achieved, revealing subtle differences between fluids of different rheological properties. As a result, the potential of incorporating neutron irradiation for the quantification of two-phase flows in metallic microfluidics devices is established.In der Kraftstoffeinspritzanlage von Verbrennungsmotoren stellt die Komplexität der Zweiphasenströmungen, die in den Einspritzdüsen entstehen, einen kritischen Aspekt in der komplexen Welt der Motorkonstruktion dar. Dieses hochkomplexe Phänomen beinhaltet das gleichzeitige Vorhandensein von flüssigem und verdampftem Kraftstoff in der Einspritzdüse, was die Kraftstoffzerstäubung und die Lebensdauer der Einspritzdüse noch komplizierter macht. Daher ist das Verständnis der Zweiphasenströmung für eine effiziente Verbrennung, die Emissionskontrolle, die Gesamtmotorleistung und die Weiterentwicklung von Verbrennungsmotoren von größter Bedeutung.
Die Untersuchung des Dampfgehalts in Einspritzdüsen ist eine komplexe Aufgabe, die sich auf eine Kombination aus numerischer Strömungsmechanik (CFD) und experimentellen Studien stützt. CFD-Simulationen bieten eine virtuelle Plattform zur Modellierung und Analyse der komplizierten Fluiddynamik, Thermodynamik und Phasenveränderungen, die im Injektor auftreten, und ermöglichen Einblicke in die Morphologie, Dynamik und Verdampfung des Kraftstoffs, was eine detaillierte Untersuchung der Zweiphasenströmungsphänomene ermöglicht. Die Validierung von CFD-Modellen erfordert jedoch experimentelle Untersuchungen, um Genauigkeit und Zuverlässigkeit zu gewährleisten. Experimentelle Messungen bieten eine reale Validierung der simulierten Ergebnisse und helfen, die Lücke zwischen theoretischen Vorhersagen und praktischen Beobachtungen zu schließen.
Da die Erforschung erneuerbarer und alternativer Kraftstoffe für die Verbesserung der Energie- und Umwelteffizienz moderner Benzinverbrennungsmotoren von entscheidender Bedeutung ist, wurden zunächst Hochgeschwindigkeitsaufnahmen mit diffuser Hintergrundbeleuchtung (DBI) durchgeführt, um das instationäre Zweiphasenströmungsfeld zu untersuchen, das in der internen Geometrie und im düsennahen Sprühbereich von Benzineinspritzdüsen entsteht. Die Experimente wurden unter realistischen Betriebsbedingungen mit einem Einspritzdruck von 100 bar und Umgebungsdrücken im Bereich von 0.1 – 6.0 bar durchgeführt, um den gesamten Bereich der in Benzin-Direkteinspritzungsmotoren herrschenden Kammerdrücke abzudecken. Darüber hinaus wurde Synchrotronstrahlung mit hohem Durchfluss zeitaufgelöst eingesetzt, um die unterschiedlichen Topologiemerkmale und die Dynamik verschiedener Kavitationsregime zu charakterisieren, die in einer Drosselöffnung mit abrupter Strömungseintrittskontraktion auftreten. Die Röntgenaufnahmen wurden sowohl mit Röntgen-Phasenkontrast- als auch mit Absorptionsbildgebung bei 67890 Bildern pro Sekunde aufgenommen. Die Röntgen-Phasenkontrast-Bildgebung bietet scharfe Brechungsindex-Gradienten im Grenzflächenbereich, um feine morphologische Fluktuationen transienter Kavitationsstrukturen zu erfassen, während die Absorptionsbildgebung explizit mit der projizierten Dampfdicke in Sichtlinie korreliert ist. Schließlich wird die Fähigkeit der Bildgebung mit kalten Neutronen zur Quantifizierung der Kavitation in Form des Dampfgehalts in einer Öffnung mit abrupt verengter Geometrie demonstriert. Trotz des zeitlich gemittelten Charakters dieser Technik wird die Morphologie der vorübergehenden Dampfstrukturen aufgrund der hohen räumlichen Auflösung deutlich sichtbar gemacht, wodurch subtile Unterschiede zwischen Flüssigkeiten mit unterschiedlichen rheologischen Eigenschaften sichtbar werden. Das Ergebnis ist, dass das Potenzial der Neutronenbestrahlung für die Quantifizierung von Zweiphasenströmungen in metallischen Mikrofluidikgeräten nachgewiesen ist
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