Numerical simulation of primary atomization in diesel spray at low injection pressure

[EN] Atomization involves complex physical processes and gas liquid interaction. Primary atomization on diesel spray is not well understood due to the difficulties to perform experimental measurements in the near nozzle field. Hence computational fluid dynamics (CFD) has been used as a key element to understand and improve diesel spray. A recent new code for incompressible multiphase flow with adaptive octree mesh refinement has been used to perform simulations of atomization at low injection pressure conditions. The multiphase flow strategy to manage different flows is the volume of fluid (VOF) method. The adaptive mesh allows to locally refine the mesh at each time step where a better resolution is needed to capture important gradients instead of using a static mesh with a fixed and high number of cells which, in turn, would lead to an unaffordable computational cost. Even with this approach, the cell number is very high to achieve a Direct Numerical Simulation (DNS) at reasonable computational cost. To reduce the computational cost, an idea has been explored, the possibility of setting a maximum number of cells of the domain. Following this idea, the code has been tested with different configurations to understand their effects on numerical stability, the change in different spray parameters and the benefits achieved in terms of execution time. The outcomes have been validated against a theoretical model.This work was sponsored by "Ministerio de Economa y Competitividad" of the Spanish Government in the frame of the Project "Comprensin de la influencia de combustibles no convencionales en el proceso de injeccin y combustin tipo diesel", Reference TRA2012-36932 and MTM2013-41765-P. Additionally, the hardware used for the project was purchased with funding from Ministerio de economia y competitividad FEDER-ICTS-2012-06. The authors would also like to thank the computer resources, technical expertise and assistance provided by the Universitat de Valencia in the use of the supercomputer "Tirant".Salvador Rubio, FJ.; Romero Bauset, JV.; Roselló Ferragud, MD.; Jaramillo-Císcar, D. (2016). Numerical simulation of primary atomization in diesel spray at low injection pressure. Journal of Computational and Applied Mathematics. 291:94-102. https://doi.org/10.10166/j.cam.2015.03.044S9410229

Simulation and Measurement of Transient Fluid Phenomena within Diesel Injection

Rail pressures of modern diesel fuel injection systems have increased significantly over recent years, greatly improving atomisation of the main fuel injection event and air utilisation of the combustion process. Continued improvement in controlling the process of introducing fuel into the cylinder has led to focussing on fluid phenomena related to transient response. High-speed microscopy has been employed to visualise the detailed fluid dynamics around the near nozzle region of an automotive diesel fuel injector, during the opening, closing and post injection events. Complementary computational fluid dynamic (CFD) simulations have been undertaken to elucidate the interaction of the liquid and gas phases during these highly transient events, including an assessment of close-coupled injections. Microscopic imaging shows the development of a plug flow in the initial stages of injection, with rapid transition into a primary breakup regime, transitioning to a finely atomised spray and subsequent vaporisation of the fuel. During closuring of the injector the spray collapses, with evidence of swirling breakup structures together with unstable ligaments of fuel breaking into large slow-moving droplets. This leads to sub-optimal combustion in the developing flame fronts established by the earlier, more fully-developed spray. The simulation results predict these observed phenomena, including injector surface wetting as a result of large slow-moving droplets and post-injection discharge of liquid fuel. This work suggests that post-injection discharges of fuel play a part in the mechanism of the initial formation, and subsequent accumulation of deposits on the exterior surface of the injector. For multiple injections, opening events are influenced by the dynamics of the previous injection closure; these phenomena have been investigated within the simulations

Time-resolved fuel injector flow characterisation based on 3D laser Doppler vibrometry

In order to enable investigations of the fuel flow inside unmodified injectors, we have developed a new experimental approach to measure time-resolved vibration spectra of diesel nozzles using a three dimensional laser vibrometer. The technique we propose is based on the triangulation of the vibrometer and fuel pressure transducer signals, and enables the quantitative characterisation of quasi-cyclic internal flows without requiring modifications to the injector, the working fluid, or limiting the fuel injection pressure. The vibrometer, which uses the Doppler effect to measure the velocity of a vibrating object, was used to scan injector nozzle tips during the injection event. The data were processed using a discrete Fourier transform to provide time-resolved spectra for valve-closed-orifice, minisac and microsac nozzle geometries, and injection pressures ranging from 60 to 160MPa, hence offering unprecedented insight into cyclic cavitation and internal mechanical dynamic processes. A peak was consistently found in the spectrograms between 6 and 7.5kHz for all nozzles and injection pressures. Further evidence of a similar spectral peak was obtained from the fuel pressure transducer and a needle lift sensor mounted into the injector body. Evidence of propagation of the nozzle oscillations to the liquid sprays was obtained by recording high-speed videos of the near-nozzle diesel jet, and computing the fast Fourier transform for a number of pixel locations at the interface of the jets. This 6-7.5kHz frequency peak is proposed to be the natural frequency for the injector's main internal fuel line. Other spectral peaks were found between 35 and 45kHz for certain nozzle geometries, suggesting that these particular frequencies may be linked to nozzle dependent cavitation phenomena.Comment: 12 pages, 10 figure

Development of a Multiphase Photon Monte Carlo Method for Spray Combustion and its Application in High-pressure Conditions

In this work the development of a multiphase photon Monte Carlo (PMC) method with a focus on resolving radiative heat transfer in combustion simulations is presented. The multiphase PMC solver can account for description of participating media in both Lagrangian and Eulerian frameworks. The solver is validated against exact solutions in several one-dimensional configurations. The developed solver is then applied to Diesel spray combustions, where liquid spray droplets are assumed to be cold, nonemitting, large, and isotropically scattering. Several formulations for radiative properties of the Diesel spray are first explored. The PMC solver has then been coupled with the multiphase spray combustion solver in OpenFOAM and the coupled solver is used for simulations of high pressure Diesel spray combustion. It was found that in typical Diesel spray combustion applications, such as in an internal combustion engine, impact of radiation on the evolution of the liquid spray was insignificant. Although the impact of radiation on the spray was minimal, nongray spectral properties and the assumption of semi-transparency for Diesel spray were found to impact the radiative transfer significantly, while impact of scattering was marginal. Spray radiation was also found not to have much effect on global combustion characteristics in high-pressure engine-relevant configurations. However, a small but noticeable effect on minor species distribution relevant to pollutant formation was observed

Numerical simulation of cavitation and atomization using a fully compressible three-phase model

The aim of this paper is to present a fully compressible three-phase (liquid, vapour and air) model and its application to the simulation of in-nozzle cavitation effects on liquid atomization. The model employs a combination of homogeneous equilibrium barotropic cavitation model with an implicit sharp interface capturing VoF approximation. The numerical predictions are validated against the experimental results obtained for injection of water into the air from a step-nozzle, which is designed to produce asymmetric cavitation along its two sides. Simulations are performed for three injection pressures, corresponding to three different cavitation regimes, referred to as cavitation inception, developing cavitation and hydraulic-flip. Model validation is achieved by qualitative comparison of the cavitation, spray pattern and spray cone angles. The flow turbulence in this study is resolved using the Large Eddy Simulation approach. The simulation results indicate that the major parameters that influence the primary atomization are cavitation, liquid turbulence and, to a smaller extent, the Rayleigh-Taylor and Kelvin-Helmholtz aerodynamic instabilities developing on the liquid/air interface. Moreover, the simulations performed indicate that periodic entrainment of air into the nozzle occurs at intermediate cavitation numbers, corresponding to developing cavitation (as opposed to incipient and fully-developed cavitation regimes); this transient effect causes a periodic shedding of the cavitation and air clouds and contributes to improved primary atomization. Finally, the cone angle of the spray is found to increase with increased injection pressure but drops drastically when hydraulic-flip occurs, in agreement with the relevant experiments

DEVELOPMENT OF A COMPUTATIONAL MODEL FOR A SIMULTANEOUS SIMULATION OF INTERNAL FLOW AND SPRAY BREAK-UP OF THE DIESEL INJECTION PROCESS

El proceso de atomización desde una vena o lámina líquida hasta multitud de gotas dispersas en un medio gaseoso ha sido un fenómeno de interés desde hace varias décadas, especialmente en el campo de los motores de combustión interna alternativos. Multitud de estudios experimentales han sido publicados al respecto, pues una buena mezcla de aire-combustible asegura una evaporación y combustión mucho más eficientes, aumentando la potencia del motor y reduciendo la cantidad de contaminantes emitidos. Con el auge de las técnicas computacionales, muchos modelos han sido desarrollados para estudiar este proceso de atomización y mezcla. Uno de los últimos modelos que han aparecido es el llamado ELSA (Eulerian-Lagrangian Spray Atomization), que utiliza un modelo Euleriano para la parte densa del chorro y cambia a un modelo Lagrangiano cuando la concentración de líquido es suficientemente pequeña, aprovechando de esta manera las ventajas de ambos. En el presente trabajo se ha desarrollado un modelo puramente Euleriano para estudiar la influencia de la geometría interna de la tobera de inyección en el proceso de atomización y mezcla. Se ha estudiado únicamente el proceso de inyección diésel. Este modelo permite resolver en un único dominio el flujo interno y el externo, evitando así las comunes simplificaciones y limitaciones de la interpolación entre ambos dominios resueltos por separado. Los resultados actuales son prometedores, el modelo predice con un error aceptable la penetración del chorro, el flujo másico y de cantidad de movimiento, los perfiles de velocidad y concentración, así como otros parámetros característicos del chorro.Martí Gómez-Aldaraví, P. (2014). DEVELOPMENT OF A COMPUTATIONAL MODEL FOR A SIMULTANEOUS SIMULATION OF INTERNAL FLOW AND SPRAY BREAK-UP OF THE DIESEL INJECTION PROCESS [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/43719TESISPremios Extraordinarios de tesis doctorale

Implementation and Development of an Eulerian Spray Model for CFD simulations of diesel Sprays

[EN] The main objective of this work is the modeling of diesel sprays under engine conditions, including the atomization, transport and evaporation processes pivotal in the diesel spray formation and its development. For this purpose, an Eulerian single fluid model, embedded in a RANS environment, is implemented in the CFD platform OpenFOAM. The modeling approach implemented here is based on the ⅀-Y model. The model is founded on the assumption of flow scales separation. In actual injection systems, it can be assumed that the flow exiting the nozzle is operating at large Reynolds and Weber numbers and thus, it is possible to assume a separation of features such as mass transport (large scales) from the atomization process occurring at smaller scales. The liquid/gas mixture is treated as a pseudo-fluid with variable density and which flows with a single velocity field. Moreover, the mean geometry of the liquid structures can be characterized by modeling the mean surface area of the liquid-gas interphase per unit of volume. Additionally, an evaporation model has been developed around the particular characteristics of the current engine technologies. This means that vaporization process is limited by fuel-air mixing rate and fuel droplets evaporate as long as there is enough air for them to heat up and vaporize. Consequently, the evaporation model is based on the Locally Homogeneous Flow (LHF) approach. Under the assumption of an adiabatic mixing, in the liquid/vapor region, the spray is supposed to have a trend towards adiabatic saturation conditions and to determine this equilibrium between phases Raoult's ideal law is considered. Finally, the spray model is coupled with an advanced combustion model based on approximated diffusion flames (ADF), which reduces the computational effort especially for complex fuels and is a natural step for modeling diesel sprays. First, the model is applied to a basic external flow case under non-vaporizing conditions, extremely convenient due to both the experimental database available and the symmetric layout which allows important simplification of the modeling effort. Good agreement between computational results and experimental data is observed, which encourages its application to a more complex configuration. Secondly, the model is applied to the "Spray A" from the Engine Combustion Network (ECN), under non-vaporizing conditions, in order to reproduce the internal structure of diesel sprays as well as to produce accurate predictions of SMD droplets sizes. Finally, vaporizing "Spray A" studies are conducted together with the baseline reacting condition of this database. The calculated spray penetration, liquid length, spray velocities, ignition delay and lift-off length are compared with experimental data and analysed in detail.[ES] El objetivo principal de este trabajo es el modelado de chorros diésel en condiciones de motor, incluyendo los fenómenos de atomización, transporte y evaporación fundamentales en la formación y desarrollo del chorro. Para este fin, se implementa un modelo de spray euleriano de tipo monofluido en un entorno RANS en la plataforma CFD OpenFOAM. El enfoque de modelado aplicado aquí sigue la idea de un modelo del tipo ⅀-Y. El modelo se fundamenta en la hipótesis de separación de escalas del flujo. En los sistemas de inyección actuales, es posible asumir que el flujo que sale de la tobera opera a altos números de Reynolds y Webber y por tanto, es posible considerar la independencia de fenómenos como el transporte de masa (grandes escalas del flujo) de los procesos de atomización que ocurren a escalas menores. La mezcla líquido/gas se trata como un pseudo-fluido con densidad variable y que fluye según un único campo de velocidad. Además, la geometría promedio de las estructuras de líquido se puede caracterizar mediante el modelado de la superficie de la interfase líquido/gas por unidad de volumen. Completando el modelo de chorro, se ha desarrollado un modelo de evaporación alrededor de las características particulares de las tecnologías actuales de los motores. Esto supone que el proceso de evaporación está controlado por mezcla aire-combustible y las gotas de combustible se evaporan siempre que exista suficiente aire para calentarlas y evaporarlas. Debido a esto, el modelo de evaporación implementado está basado en el enfoque de Flujos Localmente Homogéneos (LHF). Considerando una mezcla adiabática, en la región líquido/vapor, se supone que el chorro tiende a las condiciones adiabáticas de saturación y para determinar este equilibrio entre fases, se utiliza la ley ideal de Raoult. Finalmente, el modelo de chorro se acopla con un modelo avanzado de combustión basado en llamas de difusión aproximadas (ADF), que reduce el coste computacional especialmente para combustibles complejos y supone el paso lógico en el desarrollo del modelo para simular chorros diesel. En primer lugar, el modelo se aplica al cálculo de un caso básico de flujo externo no evaporativo, muy adecuado tanto por la extensa base de datos experimentales disponible como por la simetría geométrica que presenta, permitiendo una importante simplificación de la simulación. Los resultados obtenidos presentan un buen acuerdo con los experimentos, lo cual estimula su aplicación en configuraciones más complejas. En segundo lugar, el modelo se aplica al cálculo del "Spray A" del Engine Combustion Network (ECN), no evaporativo, para reproducir la estructura interna del chorro diesel así como predecir tamaños de gota (SMD) de forma precisa. Finalmente, se realizan estudios evaporativos del "Spray A" junto con la condición nominal reactiva de esta base de datos. La penetración de vapor, la longitud líquida, velocidad, el tiempo de retraso y la longitud de despegue de llama calculados se comparan con los datos experimentales y se analizan en detalle.[CA] L'objectiu principal d'aquest treball és el modelatge de dolls dièsel en condicions de motor, incloent els fenòmens d'atomització, transport i evaporació fonamentals en la formació i desenvolupament del doll. Amb aquesta finalitat, s'implementa un model de doll eulerià de tipus monofluid en un entorn RANS a la plataforma CFD OpenFOAM. L'enfocament de modelatge aplicat ací segueix la idea d'un model del tipus ⅀-Y. El model es fonamenta en la hipòtesi de separació d'escales del flux. En els sistemes d'injecció actuals, és possible assumir que el flux que surt de la tovera opera a alts nombres de Reynolds i Webber, i per tant és possible considerar la independència de fenòmens com el transport de massa (grans escales del flux) dels processos d'atomització que ocorren a escales menors. La mescla líquid / gas es tracta com un pseudo-fluid amb densitat variable i que flueix segons un únic camp de velocitat. A més, la geometria mitjana de les estructures de líquid es pot caracteritzar mitjançant el modelatge de la superfície de la interfase líquid / gas per unitat de volum. Completant el model, s'ha desenvolupat un model d'evaporació al voltant de les característiques particulars de les tecnologies actuals dels motors. Això suposa que el procés d'evaporació està controlat per la mescla aire-combustible i les gotes de combustible s'evaporen sempre que hi hagi suficient aire per escalfar i evaporar. A causa d'això, el model d'evaporació implementat està basat en el plantejament de fluxos Localment Homogenis (LHF). Considerant una mescla adiabàtica, a la regió líquid / vapor, se suposa que el doll tendeix a les condicions adiabàtiques de saturació i per determinar aquest equilibri entre fases, s'utilitza la llei ideal de Raoult. Finalment, el model de doll s'acobla amb un model avançat de combustió basat en flamelets de difusió aproximades (ADF), que redueix el cost computacional especialment per a combustibles complexos i suposa el pas lògic en el desenvolupament del model per simular dolls dièsel. En primer lloc, el model s'aplica al càlcul d'un cas bàsic de flux extern no evaporatiu, molt adequat tant per l'extensa base de dades experimentals disponible com per la simetria geomètrica que presenta, permetent una important simplificació de la simulació. Els resultats obtinguts presenten un bon acord amb els experiments, la qual cosa estimula la seva aplicació en configuracions més complexes. En segon lloc, el model s'aplica al càlcul del "Spray A" no evaporatiu de la xarxa Engine Combustion Network (ECN), per reproduir l'estructura interna del doll dièsel així com predir mides de gota (SMD) de forma precisa. Finalment, es realitzen estudis evaporatius del "Spray A" juntament amb la condició nominal reactiva d'aquesta base de dades. La penetració de vapor, la longitud líquida, velocitat, el temps de retard i la longitud d'enlairament de flama calculats es comparen amb les dades experimentals i s'analitzen en detall.Pandal Blanco, A. (2016). Implementation and Development of an Eulerian Spray Model for CFD simulations of diesel Sprays [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/68490TESI

Eulerian CFD Modeling of Multiphase Internal Injector Flow and External Sprays

The improvement of combustion systems which use sprays to atomize liquid fuel requires an understanding of that atomization process. Although the secondary break up mechanisms for the far-field of an atomizing spray have been thoroughly studied and well understood for some time, understanding the internal nozzle flow and primary atomization on which the far-field spray depends has proven to be more of a challenge. Flow through fuel injector nozzles can be highly complex and heavily influenced by factors such as turbulence, needle motion, nozzle imperfections, nozzle asymmetry, and phase change. All of this occurs within metallic injectors, making experimental characterization challenging. A review of computational studies in literature shows a trend towards engineering models based on the Eulerian description of the fluid, rather than the Lagrangian. With this approach, the internal and external flow can be simulated together. This allows for the influence of nozzle geometry on the spray to be captured. Developments and advancements applicable to these Eulerian solvers are discussed. This includes a new constraint on the turbulent mixing model, as well as the inclusion of a vaporization model and a non-equilibrium phase change model. Additionally, issues regarding thermodynamic and hydrodynamic consistency of compressible flows using a segregated solution approach are addressed, as are issues regarding dynamic mesh motion in a compressible solver. Finally, an efficient way of accounting for high pressure thermodynamic properties is presented. Applied case studies of diesel direct injection are then described. The single-field Eulerian approach is shown to perform very well at the high Reynolds and Weber numbers present in diesel DI conditions, capturing spray characteristics such as density distribution, penetration, and velocity profiles with a moderate level of accuracy. While transient needle motion is shown to cause interesting internal flow features, in converging, axisymmetric diesel DI nozzles, modeling of the internal flow is shown to only marginally benefit the solution. Finally, applied case studies of gasoline direct injection are presented, first with a parameter study varying counterbore depth, pressure drop, and the ambient to saturated pressure ratio. The significant influence of flash-boiling under a low ambient to saturated pressure ratio is shown. Next, a detailed analysis of internal nozzle and near-field flow in flashing and non-flashing multi-hole injection is presented. Excellent agreement to experimental rate of injection is achieved with transient needle motion and qualitative agreement to experimental imaging in the near-field is shown. Complex internal nozzle flow is analyzed and shown to result in string flash-boiling, perturbations and expansions of the spray angle, and oscillation in the ROI. Single-field multiphase Eulerian modeling is a useful tool for understanding and designing DI atomizers. The next generation of spray models will rely heavily on this approach to better understand and predict the influence of internal and near-field flow on the combustion system