222 research outputs found

    Numerical study of the characteristics of CNG, LPG and Hydrogen turbulent premixed flames

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    Numerical simulations have proven itself as a significant and powerful tool for accurate prediction of turbulent premixed flames in practical engineering devices. The work presented in this thesis concerns the development of simulation techniques for premixed turbulent combustion of three different fuels, namely, CNG, LPG and Hydrogen air mixtures. The numerical results are validated against published experimental data from the newly built Sydney combustion chamber. In this work a newly developed Large Eddy Simulation (LES) CFD model is applied to the new Sydney combustion chamber of size 50 x 50 x 250 mm (0.625 litre volume). Turbulence is generated in the chamber by introducing series of baffle plates and a solid square obstacle at various axial locations. These baffles can be added or removed from the chamber to adapt various experimental configurations for studies. This is essential to understand the flame behaviour and the structure. The LES numerical simulations are conducted using the Smagorinsky eddy viscosity model with standard dynamic procedures for sub-grid scale turbulence. Combustion is modelled by using a newly developed dynamic flame surface density (DFSD) model based on the flamelet assumption. Various numerical tests are carried out to establish the confidence in the LES based combustion modelling technique. A detailed analysis has been carried out to determine the regimes of combustion at different stages of flame propagation inside the chamber. The predictions using the DFSD combustion model are evaluated and validated against experimental measurements for various flow configurations. In addition, the in-house code capability is extended by implementing the Lewis number effects. The LES predictions are identified to be in a very good agreement with the experimental measurements for cases with high turbulence levels. However, some disagreement were observed with the quasi-laminar case. In addition a data analysis for experimental data, regarding the overpressure, flame position and the flame speed is carried out for the high and low turbulence cases. Moreover, an image processing procedure is used to extract the flame rate of stretch from both the experimental and numerical flame images that are used as a further method to validate the numerical results. For the grids under investigation, it is concluded that the employed grid is independent of the filter width and grid resolution. The applicability of the DFSD model using grid-independent results for turbulent premixed propagating flames was examined by validating the generated pressure and other flame characteristics, such as flame position and speed against experimental data. This study concludes that the predictions using DFSD model provide reasonably good results. It is found that LES predictions were slightly improved in predicting overpressure, flame position and speed by incorporating the Lewis number effect in the model. Also, the investigation demonstrates the effects of placing multiple obstacles at various locations in the path of the turbulent propagating premixed flames. It is concluded that the pressure generated in any individual configuration is directly proportional to the number of baffles plates. The flame position and speed are clearly dependent on the number of obstacles used and their blockage ratio. The flame stretch extracted from both the experimental and numerical images shows that hydrogen has the highest stretch values over CNG and LPG. Finally, the regime of combustion identified for the three fuels in the present combustion chamber is found to lie within the thin reaction zone. This finding supports the use of the laminar flamelet modelling concept that has been in use for the modelling of turbulent premixed flames in practical applications

    Center for Modeling of Turbulence and Transition (CMOTT). Research briefs: 1990

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    Brief progress reports of the Center for Modeling of Turbulence and Transition (CMOTT) research staff from May 1990 to May 1991 are given. The objectives of the CMOTT are to develop, validate, and implement the models for turbulence and boundary layer transition in the practical engineering flows. The flows of interest are three dimensional, incompressible, and compressible flows with chemistry. The schemes being studied include the two-equation and algebraic Reynolds stress models, the full Reynolds stress (or second moment closure) models, the probability density function models, the Renormalization Group Theory (RNG) and Interaction Approximation (DIA), the Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS)

    On the application of Large-Eddy simulations in engine-related problems

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    In internal combustion engines the combustion process and the pollutants formation are strongly influenced by the fuel-air mixing process. The modeling of the mixing and the underlying turbulent flow field is classically tackled using the Reynolds Averaged Navier Stokes (RANS) modeling method. With the increase of computational power and the development of sophisticated numerical methods the Large Eddy Simulation (LES) method becomes within reach. In LES the turbulent flow is locally filtered in space, rather than fully averaged, as in RANS. This thesis reports on a study where the LES technique is applied to model flow and combustion problems related to engines. Globally, three subjects have been described: the turbulent flow in an engine-like geometry, the turbulent mixing of a gas jet systemand the application of flamelet-basedmethods to LES of two turbulent diffusion flames. Because of our goal to study engine-related flow problems, two relatively practical flow solvers have been selected for the simulations. This choice was motivated by their ability to cope with complex geometries as encountered in realistic, engine-like geometries. A series of simulations of the complex turbulent, swirling and tumbling flow in an engine cylinder, that is induced by the inlet manifold, has been performed with two different LES codes. Additionally one Unsteady RANS simulation has been performed. The flow field statistics from the Large-Eddy simulations deviated substantially between one case and the next. Only global flow features could be captured appropriately. This is due to the impact of the under-resolved shear layer and the dissipative numerical scheme. Their effects have been examined on a square duct flow simulation. An additional sensitivity that was observed concerned the definition of the inflow conditions. Any uncertainty in the mass flow rates at the two runners, that are connected to the cylinder head, greatly influences the remaining flow patterns. To circumvent this problem, a larger part of the upstream flow geometry was included into the computational domain. Nevertheless, the Large-Eddy simulations do give an indication of the unsteady, turbulent processes that take place in an engine, whereas in the URANS simulations all mean flow structures are very weak and the turbulence intensities are predicted relatively low in the complete domain. The turbulent mixing process in gaseous jets has been studied for three different fuel-to-air density ratios. This mimicked the injection of (heavy) fuel into a pressurized chamber. It is shown that the three jets follow well the similarity theory that 152 Abstract was developed for turbulent gas jets. A virtual Schlieren postprocessingmethod has been developed in order to analyze the results similarly as can be done experimentally. By defining the penetration depth based on this method, problems as typically in Schlieren experiments, related to the definition of the cutoff signal intensity have been studied. Additionally it was shown that gaseous jet models can be used to simulate liquid fuel jets, especially at larger penetration depths. This is because the penetration rate from liquid sprays is governed by the entrainment rate, which is similar as for gaseous jets. However, it remains questionable if gas jet models can in all cases replace the model for fuel sprays. The cone angle for gas jets can deviate strongly from those observed in spray experiments. Only when corrected for this effect, the penetration behavior was similar. Two turbulent diffusion flames have been investigated with a focus on the modeling of finite rate chemistry effects. Concerning the first flame, the well known Sandia flame D, two methods are compared to each other for the modeling of the main combustion products and heat release. These methods are described by the classical flamelet method where the non-premixed chemistry is parameterized using a mixture fraction and the scalar dissipation rate, and a relatively new method, where a progress variable is used in non-premixed combustion problems. In the progress variable method two different databases have been compared: one based on non-premixed flamelets and one based premixed flamelets. It is found that the mixture fraction field in the Large-Eddy simulation of Sandia flame D is best predicted by both the classical flamelet method and the progress variable method that is based on premixed chemistry. In these cases the flame solution was mostly located close to its equilibrium value. However, when correcting for the prediction of the mixture fraction in the spatial coordinates, it is shown that the progress variable method based on non-premixed chemistry is better, compared to experiments. Especially at locations where a flame solution near chemical equilibrium is not adequate this model is more appropriate. Additionally a sooting turbulent benzene diffusion flame has been investigated. Therefore a steady laminar flamelet library has been applied which is based on a very detailed reaction mechanism for premixed benzene flames. In the Large-Eddy simulations the total PAH/soot mass and mole fractions have been computed explicitly, while the source terms for these variables are based on a classical flamelet parametrization. The regions of PAH/soot formation have been identified, showing distributed parcels where PAH/soot formation takes place. The results show a growth of PAH/soot volume fraction up to levels of about 4 ppm. The average particle size increases steadily in this flame, up to about 30 nm

    Annual Research Briefs, 1990

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    The 1990 annual progress reports of the Research Fellows and students of the Center for Turbulent Research (CTR) are included. It is intended primarily as a contractor report to NASA, Ames Research Center. In addition, numerous CTR Manuscript Reports were published last year. The purpose of the CTR Manuscript Series is to expedite the dissemination of research results by the CTR staff. The CTR is devoted to the fundamental study of turbulent flow; its objectives are to produce advances in physical understanding of turbulence, in turbulence modeling and simulation, and in turbulence control

    Investigation of the effects of turbulence modeling on the prediction of compression-ignition combustion unsteadiness

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    This is the author¿s version of a work that was accepted for publication in International Journal of Engine Research. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published as https://doi.org/10.1177/1468087421990478[EN] Adverse effects of global warming due to the greenhouse gas emissions is changing the actual paradigm for the use energy resources. In the absence of a mid-term solution for reducing these emissions in transportation, internal combustion (IC) engines are going to coexist in the social spheres in the foreseeable future. Therefore, the study of other IC engine-related problems remains relevant to ensuring the health of the society. In this investigation, a numerical methodology for comprehensive understanding of Noise, Vibration and Harshness in internal combustion engines is proposed. Due to its inherent complexity and lack of awareness, the main objective is to evaluate the impact of the turbulence modeling framework on the in-cylinder acoustic field recreation. Modal decomposition methods have been applied to isolate the coherent flow structures and to analyze how they change with the turbulence approach. Results demonstrate that the choice of the turbulence model is a critical aspect for noise modeling. Unsteady Reynolds-Averaged Navier-Stokes schemes predict a raw estimation of the internal acoustic field with the added value of being computationally less expensive. However, the use of more complex turbulence approaches such us large eddy simulation offers an accurate prediction of the acoustic structures and their cyclic dispersion.The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The equipment used in this work has been partially supported by FEDER project funds "Dotacion de infraestructuras cientifico tecnicas para el Centro Integral de Mejora Energetica y Medioambiental de Sistemas de Transporte (CiMeT)'' [grant number FEDER-ICTS-2012-06], framed in the operational program of unique scientific and technical infrastructure of the Spanish Government. The submitted manuscript was created partly by UChicago Argonne, LLC, Operator of Argonne National Laboratory. Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. This research was partly funded by U.S. DOE Office of Vehicle Technologies, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC0206CH11357.Broatch, A.; Novella Rosa, R.; Garcia Tiscar, J.; Gómez-Soriano, J.; Pal, P. (2022). Investigation of the effects of turbulence modeling on the prediction of compression-ignition combustion unsteadiness. International Journal of Engine Research. 23(4):541-559. https://doi.org/10.1177/146808742199047854155923

    Reynolds-stress turbulence model in the KIVA code for engine simulation

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    A Reynolds-Stress Turbulence Model has been incorporated with success into the KIVA code, a computational fluid dynamics hydrocode for three-dimensional simulation of fluid flow in engines. The newly implemented Reynolds-stress turbulence model greatly improves the robustness of KIVA, which in its original version has only eddy-viscosity turbulence models. Validation of the Reynolds-stress turbulence model is accomplished by conducting pipe-flow and channel-flow simulations, and comparing the computed results with experimental and direct numerical simulation data. Flows in engines of various geometry and operating conditions are calculated using the model, to study the complex flow fields as well as confirm the model’s validity. Results show that the Reynolds-stress turbulence model is able to resolve flow details such as swirl and recirculation bubbles. The model is proven to be an appropriate choice for engine simulations, with consistency and robustness, while requiring relatively low computational effort

    Finite-Rate Chemistry Effects in Turbulent Premixed Combustion

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    In recent times significant public attention has been drawn to the topic of combustion. This has been due to the fact that combustion is the underlying mechanism of several key challenges to modern society: climate change, energy security (finite reserves of fossil fuels) and air pollution. The further development of combustion science is undoubtedly necessary to find improved solutions to manage these combustion science related challenges in the near and long term future. Combustion is essentially an exothermic process, this exothermicity or heat release essentially occurs at small scales, by small scales it meant these scales are small relative to the fluid length scales, for example heat release layer thicknesses in flames are typically much less than the fluid integral length scales. As heat release occurs at small scales this means that in turbulent combustion the small scales of the turbulence (which can be of the order of the heat release layer thickness) can possibly interact and influence the heat release and thus chemistry of the flame reaction zone. Premixed combustion is a combustion mode where the fuel and oxidiser are completely premixed prior to the flame reaction zone, this mode of combustion has been shown to be a promising method to maximise combustion efficiency and minimise pollutant formation. The continued and further application of premixed combustion to practical applications is limited by the current understanding of turbulent premixed combustion, these limitations in understanding are linked to the specific flame phenomena that can significantly influence premixed combustion in a combustion device, examples of such phenomena are: flame flashback, flame extinction and fuel consumption rate – all phenomena that are influenced by the interaction of the small scales of turbulence and chemistry. It is the study and investigation of the interaction of turbulence and chemistry at the small scales (termed finite-rate chemistry) in turbulent premixed flames that is the aim of this thesis which is titled “Finite-rate chemistry effects in turbulent premixed combustion”. Two very closely related experimental burner geometries have been developed in this thesis: the Piloted Premixed Jet Burner (PPJB) and the Premixed Jet Burner (PJB). Both feature an axisymmetric geometry and exhibit a parabolic like flow field. The PPJB and PJB feature a small 4mm diameter central jet from which a high velocity lean-premixed methane-air mixture issues. Surrounding the central jet in the PPJB is a 23.5mm diameter pilot of stoichiometric methane-air products, the major difference between the PPJB and the PJB is that the PJB does not feature a stoichiometric pilot. The pilot in the PPJB provides a rich source of combustion intermediates and enthalpy which promotes initial ignition of the central jet mixture. Surrounding both the central jet and pilot is a large diameter hot coflow of combustion products. It is possible to set the temperature of the hot coflow to the adiabatic flame temperature of the central jet mixture to simulate straining and mixing against and with combustion products without introducing complexities such as quenching and dilution from cold air. By parametrically increasing the central jet velocity in the PPJB it is possible to show that there is a transition from a thin conical flame brush to a flame that exhibits extinction and re-ignition effects. The flames that exhibit extinction and re-ignition effects have a luminous region near the jet exit termed the initial ignition region. This is followed by a region of reduced luminosity further downstream termed the extinction region. Further downstream the flame luminosity increases this region is termed the re-ignition region. For the flames that exhibit extinction and re-ignition it is proposed that intense turbulent mixing and high scalar dissipation rates drives the initial extinction process after the influence of the pilot has ceased (x/D>10). Re-ignition is proposed to occur downstream where turbulent mixing and scalar dissipation rates have decreased allowing robust combustion to continue. As the PJB does not feature a pilot, the flame stabilisation structure is quite different to the PPJB. The flame structure in the PJB is essentially a lifted purely premixed flame, which is an experimental configuration that is also quite unique. A suite of laser diagnostic measurements has been parametrically applied to flames in the PPJB and PJB. Laser Doppler Velocimetry (LDV) has been utilised to measure the mean and fluctuating radial and axial components of velocity at a point, with relevant time and length scale information being extracted from these measurements. One of the most interesting results from the LDV measurements is that in the PPJB the pilot delays the generation of high turbulence intensities, for flames that exhibit extinction the rapid increase of turbulence intensity after the pilot corresponds to the start of the extinction region. Using the LDV derived turbulence characteristics and laminar flame properties and plotting these flames on a traditional turbulent regime diagram indicates that all of the flames examined should fall in the so call distributed reaction regime. Planar imaging experiments have been conducted for flames using the PPJB and PJB to investigate the spatial structure of the temperature and selected minor species fields. Results from two different simultaneous 2D Rayleigh and OH PLIF experiments and a simultaneous 2D Rayleigh, OH PLIF and CH2O PLIF experiment are reported. For all of the flames examined in the PPJB and PJB a general trend of decreasing conditional mean temperature gradient with increasing turbulence intensity is observed. This indicates that a trend of so called flame front thickening with increased turbulence levels occurs for the flames examined. It is proposed that the mechanism for this flame front thickening is due to eddies penetrating and embedding in the instantaneous flame front. In the extinction region it is found that the OH concentration is significantly reduced compared to the initial ignition region. In the re-ignition region it is found that the OH level increases again indicating that an increase in the local reaction rate is occurring. In laminar premixed flames CH2O occurs in a thin layer in the reaction zone, it is found for all of the flames examined that the CH2O layer is significantly thicker than the laminar flame. For the high velocity flames beyond x/D=15, CH2O no longer exist in a distinct layer but rather in a near uniform field for the intermediate temperature regions. Examination of the product of CH2O and OH reveals that the heat release in the initial ignition region is high and rapidly decreases in the extinction region, an increase in the heat release further downstream is observed corresponding to the re-ignition region. This finding corresponds well with the initial hypothesis of an extinction region followed by a re-ignition region that was based on the mean chemiluminescence images. Detailed simultaneous measurement of major and minor species has been conducted using the line Raman-Rayleigh-LIF technique with CO LIF and crossed plane-OH PLIF at Sandia National Laboratories. By measuring all major species it is also possible to define a mixture fraction for all three streams of the PPJB. Using these three mixture fractions it was found that the influence of the pilot in the PPJB decays very rapidly for all but the lowest velocity flames. It was also found that for the high velocity flames exhibiting extinction, a significant proportion of the coflow fluid is entrained into the central jet combustion process at both the extinction region and re-ignition regions. The product of CO and OH conditional on temperature is shown to be proportion to the net production rate of CO2 for certain temperature ranges. By examining the product of CO and OH the hypothesis of an initial ignition region followed by an extinction region then a re-ignition region for certain PPJB flames has been further validated complementing the [CH2O][OH] imaging results. Numerical modelling results using the transported composition probability density function (TPDF) method coupled to a conventional Reynolds averaged Naiver Stokes (RANS) solver are shown in this thesis to successfully predict the occurrence of finite-rate chemistry effects for the PM1 PPJB flame series. To calculate the scalar variance and the degree of finite-rate chemistry effects correctly, it is found that a value of the mixing constant ( ) of approximately 8.0 is required. This value of is much larger than the standard excepted range of 1.5-2.3 for that has been established for non-premixed combustion. By examining the results of the RANS turbulence model in a non-reacting variable density jet, it is shown that the primary limitation of the predictive capability of the TPDF-RANS method is the RANS turbulence model when applied to variable density flows

    Modelling of explosion deflagrating flames using Large Eddy Simulation

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    Encouraged by the recent demand for eco-friendly combustion systems, advancements in the predictive capability of turbulent premixed combustion are considered to be essential. The explosion and deflagrating flame are modelled with the numerical method by applying the Large Eddy Simulation (LES) technique. It has evolved itself as a powerful tool for the prediction of turbulent premixed flames. In the LES, Sub-Grid Scale (SGS) modelling plays a pivotal role in accounting for various SGS effects. The chemical reaction rate in LES turbulent premixed flames is a SGS phenomenon and must be accounted for accurately. The Dynamical Flame Surface Density (DFSD) model which is based on the classical laminar flamelet theory is a prominent and well accepted choice in predicting turbulent premixed flames in RANS modelling. The work presented in this thesis is mainly focused upon the implementation of a dynamic flame surface density (DFSD) model for the calculation of transient, turbulent premixed propagating flames using the LES technique. The concept of the dynamism is achieved by the application of a test filter in combination with Germano identity, which provides unresolved SGS flame surface density information. The DFSD model is coupled with the fractal theory in order to evaluate the instantaneous fractal dimension of the propagating turbulent flame front. LES simulations are carried out to simulate stoichiometric propane/air flame propagating past solid obstacles in order to validate the model developed in this work with the experiments conducted by the combustion group at The University of Sydney. Various numerical tests were carried out to establish the confidence of LES. A detailed analysis has been carried out to determine the regimes of combustion at different stages of flame propagation inside the chamber. LES predictions using the DFSD model are evaluated and validated against experimental measurements for various flow configurations. The LES predictions were identified to be in strong agreement with experimental measurements. The impact of the number and position of the baffles with respect to ignition origin has also been studied. LES results were found to be in very good agreement with experimental measurements in all these cases

    Large eddy simulations para modelado de combustión de hidrógeno. Aplicaciones a unidades balísticas de reducción de arrastre de base y análisis de secuencias de accidentes nucleares

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    [SPA] Esta tesis doctoral se presenta bajo la modalidad de compendio de publicaciones. En este trabajo se han desarrollado modelos de simulación mediante herramientas de mecánica de fluidos computacional (CFD) utilizando modelado de turbulencia Large Eddy Simulation (LES) para abordar el análisis de problemas en los que, tradicionalmente, se han utilizado de forma extendida simulaciones con modelado de turbulencia Reynolds Averaged Navier Stokes Equations (RANS), en las que los resultados alcanzados presentan ,en muchas ocasiones, diferencias significativas comparados con datos experimentales. En la actualidad, simulaciones CFD con modelado de turbulencia LES se están convirtiendo en una atractiva alternativa a simulaciones RANS, siendo abordable en términos de coste computacional y tiempo de simulación para muchas aplicaciones industriales, debido principalmente a la evolución y avances en materia de recursos y potencia computacional. En ese contexto, el objetivo principal de este trabajo consiste en desarrollar y validar modelos y estrategias de simulación CFD para ser aplicados y extraer conclusiones relevantes en problemas donde tradicionalmente simulaciones con modelos RANS han sido ampliamente aplicadas, pero con limitaciones en su validación experimental. Estos problemas son el análisis de balística exterior incluyendo unidades de reducción de resistencia de base mediante tecnología Base Bleed, así como el estudio de problemas de combustión en secuencias de accidente nuclear. Ambas aplicaciones tienen en común que involucran procesos de combustión hidrógeno-aire en condiciones de flujo turbulento. Para cada una de estas aplicaciones, diferentes metodologías y estrategias numéricas han sido desarrolladas y validadas. Adicionalmente, junto al desarrollo de estos modelos, se proponen metodologías para optimizar el coste computacional con limitado impacto en la precisión de los resultados alcanzados. La tecnología conocida como Base Bleed ha sido, y es, ampliamente utilizada con el objetivo de reducir la resistencia aerodinámica de cuerpos esbeltos mediante la destilación de gases (procedentes de una combustión) en su zona posterior. Los modelos desarrollados en este trabajo permiten estimar el coeficiente de resistencia aerodinámica (CD) cuando el cuerpo, con unidad de Base Bleed (activa o no), posee rotación axial (spin) y se considera vuelo cuasi - estacionario en régimen transónico y supersónico (Mach 0.99-1.5). Se han comparado los resultados de varios modelos bidimensionales y tridimensionales con datos experimentales obtenidos mediante técnicas de trayectografía. Los resultados alcanzados evidencian que los modelos de turbulencia RANS y Detached-Eddy Simulation (DES) obtienen buenas predicciones de CD en ausencia de unidades Base Bleed. Sin embargo, el efecto de reducción de resistencia provocado por estas no aparece reflejado en las predicciones de CD calculados con estos modelos de turbulencia. En cambio, con modelos de turbulencia LES, se obtienen predicciones más realistas. En relación al estudio de procesos de combustión en secuencias de accidente nuclear, estos precisan de simulaciones de combustión premezclada turbulenta en espacios confinados, simulaciones que presentan comúnmente la limitación del elevado coste computacional requerido, así como el reducido número de datos experimentales disponibles para la validación. De forma general, ciertos modelos de combustión turbulenta basados en RANS han obtenido resultados satisfactorios para predecir parámetros globales de la combustión, pero presentan limitaciones para modelar correctamente algunos fenómenos transitorios, especialmente interacciones dinámicas de los frentes de llama en un medio turbulento y su influencia en la combustión. En este contexto, los modelos de combustión basados en LES se presentan como una alternativa eficiente en términos de coste computacional para analizar secuencias de accidente involucrando la combustión del hidrógeno. En este trabajo, dos modelos diferentes han sido desarrollados y propuestos para analizar la evolución de la velocidad de combustión de deflagraciones y la interacción de estas en medios turbulentos. Estas estrategias han sido, un modelo de variable de progreso (Flamelet Progress Variable, LES-FPV) y otro con modelado de tasa de reacción química de gases multicomponente (Finite-Rate chemistry model) denominado Thickened Flame Model (LES-TFM) en el que se pretende modelar la interacción entre el mecanismo de cinética química con la turbulencia. Se ha llevado a cabo la validación de estos modelos para predecir fenómenos tales como la velocidad de combustión, aceleración turbulenta y evolución de la presión. Adicionalmente, se han propuesto técnicas para reducir el coste computacional y para hacer abordable su aplicación en problemas industriales, de mayor escala que los ensayos de laboratorio para validación. Estas técnicas incluyen: Dynamic Adaptive Chemistry (DAC), in-situ Adaptive Tabulation (ISAT) y mallados dinámicos adaptativos. Esta última técnica tiene el objetivo de aumentar la resolución espacial localmente en el frente de llama, manteniendo un coste computacional y tiempos de simulación abordables. Finalmente, se ha aplicado los modelos previamente validados para analizar dos secuencias de pérdidas de vacío en ITER (Loss Of Vacuum Accident, LOVA). Con ellos se han obtenido conclusiones relevantes sobre dichos accidentes. Adicionalmente, otra aproximación basada en la hipótesis de “Reactor Perfectamente Agitado” (Perfectly Stirred Reactor, PSR) ha sido propuesta y validada para predicción de variables globales en secuencias de combustión de hidrógeno-aire premezclado. Esta aproximación tiene la ventaja de una menor complejidad desde el punto de vista de modelado, a expensas de requerir un mayor coste computacional, además de presentar una aplicabilidad limitada en determinados regímenes de combustión. Se ha llevado a cabo una validación y evaluación de estos modelos comparando con datos experimentales y con otros estudios numéricos de aceleración de llama en un canal con obstáculos. Los resultados permiten identificar las principales deficiencias a tener en cuenta al utilizar esta aproximación y evaluar las incertidumbres relacionadas con el uso de diferentes modelos de turbulencia sub-grid scale. Por último, se ha desarrollado un modelo, para simular problemas de combustión bifásicos de flujos reactivos en presencia de partículas de grafito a partir de los modelos LES-TFM. La modelización numérica de la combustión turbulenta de mezclas de H2-aire con partículas sólidas de grafito es un reto clave en muchos problemas industriales, incluyendo el ámbito de la seguridad nuclear. El modelo se basa en una aproximación Euler-Euler acoplada con diferentes cinéticas químicas detalladas para simular la combustión de mezclas de gases y partículas. El modelo se ha empleado para predecir la evolución transitoria de las secuencias de combustión turbulenta de mezclas de H2, aire y partículas de grafito en condiciones de baja concentración de este último, obteniendo resultados que se ajustan a los experimentales obtenidos en una bomba esférica. El modelo permite predecir ciertas tendencias experimentales, como la composición de productos de la combustión, mostrando que una baja concentración inicial de partículas de grafito (~96 g/m3) influye en la dinámica de la combustión del H2 para mezclas de 20% en volumen de H2 en aire. En estas condiciones, se aumentaron los niveles de presión alcanzados en las paredes de la esfera y se redujo el tiempo de combustión respecto al caso sin presencia de partículas. Los resultados muestran la viabilidad de utilizar este tipo de modelado para caracterizar parámetros globales como la evolución temporal de la presión en las paredes. [ENG] This doctoral dissertation has been presented in the form of thesis by publication. In this work, Computational Fluid Dynamics (CFD) simulations using Large Eddy Simulation (LES) turbulence modeling are proposed for analyzing problems where traditionally Reynolds Averaged Navier Stokes Equations (RANS) have been extensively used, but with results that did not find good agreement when compared with experimental data. Nowadays, as a consequence of the increase in computational efficiency and power during last years, LES models has become an affordable alternative for being applied on a lot of fluid-dynamics problems even from an industrial perspective. This work is focused on two problems: external ballistics for slender bodies with drag reduction (Base Bleed) units, and nuclear accident sequences. Both problems have in common that involve hydrogen-air combustion processes under turbulent flow conditions. For each application, different approaches have been developed and tested, and methodologies for improving computational cost with low (or not) penalty on the results accuracy have been analyzed and proposed. Base Bleed technology is a common strategy used for body drag reduction. This work studied analyzes CFD models to estimate the drag coefficient of slender bodies with spin and Base Bleed technology under transonic and supersonic (Mach number 0.99–1.5) quasi-steady conditions. 2-dimensional and 3-dimensional numerical models based on RANS, Detached Eddy Simulation (DES) and LES models were presented and benchmarked against ad-hoc experimental flight measurements performed with both active and inactive Base Bleed units. Results show that RANS and DES models predict well the drag coefficient in the absence of Base Bleed units. However, they have a very limited accuracy in drag prediction when facing a problem involving a high temperature jet mixing layer with a transonic wake as in the case of active Base Bleed. Notwithstanding, a reasonable agreement is found between numerical predictions of drag reduction and experimental data for the case of LES. On the other hand, the modelling of premixed combustion in three-dimensional confined scenarios is also studied in this work. Accurate modelling of combustion sequences is difficult due to computational costs and the limited ad-hoc experiments available to validate the models. RANS based combustion models have shown to be successful in predicting gross features of combustion, nevertheless, they have serious deficiencies to predict transient phenomena, such as combustion instabilities, cycle-to-cycle variations, self-ignition, and pollutant emission. LES seems to be a cost-effective method to reach this goal when analyzing H2 combustion dynamics in accident sequences. In this work, two different LES models have been proposed and assessed for predicting flame combustion acceleration and interaction in the presence of turbulence: a Flamelet Progress Variable (LES-FPV) and a Thickened Flame Model (LES-TFM). With the aim of reducing computational costs, Dynamic Adaptive Chemistry (DAC) and in-situ adaptive tabulation (ISAT) methods have been exploited when facing detailed kinetic mechanism for hydrogen combustion. Moreover, an adaptive meshing technique was used with the aim of tracking the flame front to ensure an adequate local spatial resolution, where the model requires such level discretization. Experimental validation was performed to assess the ability of the different studied approaches to predict the flame burning speed, flame acceleration, and pressure evolution for lean H2-Air volume percent mixtures from 16 to 28% propagating within a turbulent field. Results revealed that both approaches led to accurate predictions in terms of flame burning speed. When considering DAC and ISAT methods with detailed chemistry, LES-TFM model was found to be a cost-efficient solution, which relies less on experimental inputs than the LES-FPV alternative. Once this model has been validated, it is used to analyze two loss of vacuum accident (LOVA) sequences within the International Thermonuclear Experimental Reactor (ITER) Vacuum Vessel. Results permitted to get key insights into these accidents. Additionally, LES turbulence with perfectly stirred reactor (PSR) assumption and detailed chemistry have been also assessed to predict global variables of unsteady, premixed, hydrogen combustion sequences. This approach requires less modeling efforts but increases the need of computational resources and it shows application limitations. The assessment is faced by benchmarking the model with hydrogen-air experimental tests and with numerical data of flame acceleration in an obstructed channel obtained with other models. Results permit to identify major shortcomings that should be addressed with this approach and to assess the uncertainties linked to the use of different sub-models. Finally, LES-TFM approach have been proposed for modeling two-phase combustion problems to describe reacting flows in presence of graphite particles. The model proposed is benchmarked against experimental combustion data obtained in a spherical bomb. The numerical modelling of turbulent combustion of H2-air mixtures with solid graphite particles is a challenging and key issue in many industrial problems including nuclear safety. The model relies in an Eulerian–Eulerian approach coupled with different detailed chemical kinetics to simulate the combustion of mixtures of gases and particles. The model is applied to predict the transient evolution of turbulent combustion sequences of mixtures of hydrogen, air, and a low concentration of graphite particles. Results show a good agreement between experimental and numerical data. Moreover, the model is able to predict some key experimental tendencies and reveals that the presence of a low concentration of graphite particles (~96 g/m3) influences the hydrogen combustion dynamics for mixtures of 20% (in volume) of hydrogen in air. Under these conditions, pressure levels reached at the walls of the sphere are increased and the combustion time is shortened. The results also show the viability of using this kind of models for obtaining global combustion parameters such as the temporal evolution of the wall pressure.Esta tesis doctoral se presenta bajo la modalidad de compendio de publicaciones. Está formada por un total de cuatro artículos: Article 1.- F. Nicolás-Pérez, F.J.S. Velasco, J.R. García-Cascales, R.A. Otón-Martínez, A. López-Belchí, D. Moratilla, F. Rey, A. Laso, On the accuracy of RANS, DES and LES turbulence models for predicting drag reduction with Base Bleed technology, Aerospace Science and Technology, Volume 67, 2017, Pages 126-140, ISSN 1270-9638, https://doi.org/10.1016/j.ast.2017.03.031. Article 2.- F. Nicolás-Pérez, F.J.S. Velasco, José R. García-Cascales, Ramón A. Otón-Martínez, Ahmed Bentaib, Nabiha Chaumeix, Evaluation of different models for turbulent combustion of hydrogen-air mixtures. Large Eddy Simulation of a LOVA sequence with hydrogen deflagration in ITER Vacuum Vessel, Fusion Engineering and Design, Volume 161, 2020, 111901, ISSN 0920-3796, https://doi.org/10.1016/j.fusengdes.2020.111901. Article 3.- F. Nicolás-Pérez, F.J.S. Velasco, Ramón A. Otón-Martínez, José R. García-Cascales, Ahmed Bentaib and Nabiha Chaumeix, Capabilities and limitations of Large Eddy Simulation with perfectly stirred reactor assumption for engineering applications of unsteady, hydrogen combustion sequences, Engineering Applications of Computational Fluid Mechanics (TCFM) 2021 https://doi.org/10.1080/19942060.2021.1974092. Article 4.- F. Nicolás-Pérez, F.J.S. Velasco, Ramón A. Otón-Martínez, José R. García-Cascales, Ahmed Bentaib and Nabiha Chaumeix, Mathematical Modelling of Turbulent Combustion of Two-Phase Mixtures of Gas and Solid Particles with a Eulerian–Eulerian Approach: The Case of Hydrogen Combustion in the Presence of Graphite Particles, Mathematics 2021, 9(17), 2017; https://doi.org/10.3390/math9172017.Escuela Internacional de Doctorado de la Universidad Politécnica de CartagenaPrograma de Doctorado en Energías Renovables y Eficiencia Energétic
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