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

Optimization of spray break-up CFD simulations by combining Sigma-Y Eulerian atomization model with a response surface methodology under diesel engine-like conditions (ECN Spray A)

[EN] This work evaluates the performance of the Sigma-Y Eulerian atomization model at reproducing the internal structure of a diesel spray with a special focus on Sauter Mean Diameter (SMD) predictions. Modeling results have been compared to x-ray radiography measurements [21,24,38] which provided unique data within dense spray region. The first step corresponds to accurately reproduce the large scale spray dispersion. Among different RANS turbulence models, the standard k-s with the round jet corrected CIE value (1.60), has shown the best performance, as shown in [12]. Then, the study is devoted to the application and optimization of the predicted interphase surface density (E). In this work, a combination of CFD modeling and the statistical Design of Experiments (DOE) technique known as Response Surface Method (RSM) is applied in order to improve Sauter Mean Diameter (SMD) predictions from E equation compared to experimental measurements. In the investigation, two different optimizations are conducted for the three modeling parameters involved in the equation, following a Central Composite Design (CCD), leading to 15 simulations for each one. After that, both optimum sets of values are validated to assure the accuracy of the method and it is decided the best choice. Finally, different injection and ambient conditions are simulated, with those selected values, providing a remarkable improvement in the modeling performance.Authors acknowledge that part of this work was possible thanks to the Programa de Ayudas de Investigacion y Desarrollo (PAID 2013 3198) of the Universitat Politecnica de Valencia. Also this study was partially funded by the Spanish Ministry of Economy and Competitiveness in the frame of the COMEFF (TRA2014-59483R) project.Pandal-Blanco, A.; Payri, R.; García-Oliver, JM.; Pastor Enguídanos, JM. (2017). Optimization of spray break-up CFD simulations by combining Sigma-Y Eulerian atomization model with a response surface methodology under diesel engine-like conditions (ECN Spray A). Computers & Fluids. 156:9-20. doi:10.1016/j.compfluid.2017.06.022S92015

A computational analysis of local flow for reacting Diesel sprays by means of an Eulerian CFD model

[EN] An implementation and validation of the coupled Sigma-gamma ADF model is presented in this work for reacting Diesel spray CFD simulations under a RANS turbulence modeling approach. An Approximated Diffusion Flamelet (ADF) model Michel et al. (2008) implemented in the OpenFOAM CFD open-source library by Winklinger (2014)15 fed with the spray description, i.e. mixing formation process, provided by the Sigma-gamma Eulerian atomization model Garcia-Oliver et al. (2013). In the present investigation, the Engine Combustion Network Spray A reference configuration is used for validation. Specifically, the model can provide accurate predictions of typical reacting spray metrics, such as the ignition delay and the lift-off length. Moreover, the internal structure is also fairly reproduced in terms of quasi-steady spatial distribution of formaldehyde and OH, related with low and high temperature reactions respectively. Additionally, modeling results have been compared to recent Particle image velocimetry (PIV) measurements Garcia-Oliver et al. (2017) under both inert and reacting conditions. Flow response to heat release is quantitatively predicted by the model, both in terms of local velocity increase as well as radial dilation. The model has been used to understand combustion-induced reduction in entrainment, in particular around the lift-off length location. Flow confinement does not seem to influence the global flame behaviour, even though some changes in the local flow hint can be observed when moving from an open to a closed domain. (C) 2017 Elsevier Ltd. All rights reserved.Authors acknowledge that this work was possible thanks to the Programa de Ayudas de Investigation y Desarrollo (PAID-2013 3198) of the Universitat Politecnica de Valencia. Also this study was partially funded by the Spanish Ministry of Economy and Competitiveness in the frame of the COMEFF(TRA2014-59483-R) project. Authors thank Gilles Bruneaux from IFPEN for the interesting suggestions and discussions.Pandal-Blanco, A.; García-Oliver, JM.; Novella Rosa, R.; Pastor Enguídanos, JM. (2018). A computational analysis of local flow for reacting Diesel sprays by means of an Eulerian CFD model. International Journal of Multiphase Flow. 99:257-272. https://doi.org/10.1016/j.ijmultiphaseflow.2017.10.010S2572729

Implementación y evaluación de un modelo euleriano de atomización y dispersión para el modelado CFD de chorros Diesel

[ES] Durante el periodo de docencia del Máster el investigador se ha dedicado al estudio del modelado CFD de chorros Diesel bajo las condiciones de funcionamiento que se dan en los motores Diesel de inyección directa y al desarrollo e implemenatción de un modelo de atomización avanzado en una plataforma CFD.[EN] During its Master teaching period, the researcher has been working on studies about CFD modeling of Diesel sprays under working conditions of direct injection Diesel engines and on the design and implementation of an advanced atomization model in a CFD platform.Pandal Blanco, A. (2013). Implementación y evaluación de un modelo euleriano de atomización y dispersión para el modelado CFD de chorros Diesel. http://hdl.handle.net/10251/3756

Introduction to the Fluid Dynamics of Biological Flows. Innovation Project using the CFD simulation of the lung air flow

Congreso Universitario de Innovación Educativa En las Enseñanzas Técnicas, CUIEET (26º. 2018. Gijón

A comparison of diesel sprays CFD modeling approaches: DDM versus Sigma-Y Eulerian atomization model

[EN] A comparison between the Σ-Y atomization model and a classical DDM approach has been carried out for diesel spray CFD simulations. The Σ-Y model, originally proposed by Vallet and Borghi (1999), is based on an Eulerian representation of the spray atomization and dispersion by means of a single-fluid variable density turbulent flow. The locally homogeneous flow approach has been applied in order to develop a spray vaporization model based on state relationships. A finite-volume solver for model equations has been created using the OpenFOAM CFD open-source C++ library (GarcíaOliver et al., 2013). In the case of the Lagrangian-DDM approach, the original dieselFoam solver of OpenFOAM is used. Model predictions have been compared to experimental measurements of free diesel sprays under vaporizing conditions from the database of the Engine Combustion Network (ECN). Accurate predictions of liquid and vapor spray penetration, as well as mixture fraction can be achieved for the nominal condition with both models, although DDM simulations tend to be less accurate. Additionally, near nozzle flow structure of the Spray A condition of ECN is also studied with both models. The conclusion is a more accurate prediction of the near-field internal structure of the spray in the case of the Eulerian model, due to both a higher mesh resolution and a more adequate modelling approach. Consequently, results shown in this work put in evidence the benefits of using an Eulerian model to predict qualitatively and accurately the diesel spray behaviour under different ambient conditions and injection pressures.Part of this work was possible thanks to the Programa de Ayudas de Investigacion y Desarrollo (PAID-2013) of the Universitat Polite`cnica de València. Also,we thank the collaboration of I. Garc´ıa Ayala and R. Redon Moreno, students of final degree project (PFC) at the Universitat Politècnica de ValènciaDesantes Fernández, JM.; García Oliver, JM.; Pastor Enguídanos, JM.; Pandal-Blanco, A. (2016). A comparison of diesel sprays CFD modeling approaches: DDM versus Sigma-Y Eulerian atomization model. Atomization and Sprays. 26(7):713-737. doi:10.1615/AtomizSpr.2015013285S71373726

Diesel spray CFD simulations based on the sigma-Y eulerian atomization model

[EN] This work presents an implementation and evaluation of the Sigma-Y atomization model for Diesel spray CFD simulations. The Sigma-Y model is based on an Eulerian representation of the spray atomization and dispersion by means of a single-fluid variable density turbulent flow within a RANS framework. The locally homogeneous flow approach has been applied in order to develop a spray vaporization model based on state relationships. A finite-volume solver for model equations has been created using the OpenFOAM CFD open-source C++ library. Model predictions have been compared to experimental data from free Diesel sprays under nonvaporizing and vaporizing conditions. High-speed imaging, PDPA, and Rayleigh-scattering measurements have been used in order to assess the CFD model. Accurate predictions of liquid and vapor spray penetration, as well as axial velocity and mixture fraction profiles, can be simultaneously achieved for a wide range of injection pressure and ambient conditions, despite only having qualitatively correct predictions of droplet size. The success of these predictions supports the mixing-limited vaporization hypothesis. Model accuracy is better for high ambient density and injection pressure conditions. It is proposed that under low ambient density and injection pressure conditions, interfacial dynamics become more important and the single velocity field assumption is less appropriate.This work was partially funded by the Spanish Ministry of Education and Science in the frame of the ENE2010-18542 project. The authors acknowledge support from the Army Research Office under grant no. W911NF-08-1-0171.García Oliver, JM.; Pastor Enguídanos, JM.; Pandal Blanco, A.; Trask, N.; Baldwin, E.; Schmidt, D. (2013). Diesel spray CFD simulations based on the sigma-Y eulerian atomization model. Atomization and Sprays. 23(1):71-95. https://doi.org/10.1615/AtomizSpr.2013007198S719523

Computational simulation of aqueous humour dynamics in the presence of a posterior-chamber versus iris-fixed phakic intraocular lens.

PURPOSE:To compare aqueous humour (AH) dynamics in the presence of a precrystalline (Implantable Collamer Lens®; ICL) or iris-fixed (Artiflex®) phakic intraocular lens (PIOL). METHODS:By computational fluid dynamics simulation, AH flow was modelled through a peripheral iridotomy (PI) or central lens hole (both 360 μm) in the presence of an Artiflex or ICL lens, respectively. The impacts of AH flow were then determined in terms of wall shear stress (WSS) produced on the endothelium or crystalline lens. Effects were also modelled for different scenarios of pupil diameter (PD 3.5 or 5.5 mm), ICL vault (100, 350, 800 μm) and number of Artiflex iridotomies (1 or 2) and location (12 or 6 o'clock). RESULTS:For a PD of 3.5 mm, AH volumes flowing from the posterior to the anterior chamber were 37.6% of total flow through the lens hole (ICL) and 84.2% through PI (Artiflex). For an enlarged PD (5.5 mm), corresponding values were 10.3% and 81.9% respectively, so PI constitutes a very efficient way of evacuating AH. Central endothelial WSS in Pa was lower for the large vault ICL and the Artiflex (1-03 and 1.1-03 respectively) compared to the PIOL-free eye (1.6-03). Crystalline lens WSS was highest for the lowest vault ICL (1-04). CONCLUSIONS:AH flow varied according to the presence of a precrystalline or iris-fixed intraocular lens. Endothelial WSS was lower for an implanted ICL with large vault and Artiflex than in the PIOL-free eye, while highest crystalline WSS was recorded for the lowest vault ICL