Suitability of different RANS models in the description of turbulent forced convection flows: application to air curtains

The main motivation of this thesis is the analysis of turbulent flows. Turbulence plays an important role in engineering applications due to the fact that most flows in industrial equipment and surroundings are in turbulent regime. The thesis has a double purpose and is divided in two main parts. The first one is focussed on the basic and fundamental analysis of turbulence models. In the second part the know-how acquired in the first part is applied to the study of air curtains.Regarding to the first part, the principal difficulty of computing and modelling turbulent flows resides in the dominance of non-linear effects and the continuous and wide spectrum of time and length scales. Therefore, the use of turbulence modelling employing statistical techniques for high Reynolds numbers or complex geometries is still necessary. In general, this modelization is based on time averaging of the Navier-Stokes equations (this approach is known as Reynolds-Averaged Navier-Stokes Simulations, RANS). As consequence of the average new unknowns, so-called Reynolds stresses, arise. Different approaches to evaluate them are: i) Differentially Reynolds Stress Models (DRSM), ii) Explicit Algebraic Reynolds Stress Models (EARSM), and iii) Eddy Viscosity Models (EVM).Although EVM models assuming a linear relation between the turbulent stresses and the mean rate of strain tensor are extensively used, they present various limitations. In the last few years, with the even-increasing computational capacity, new proposals to overcome many of these deficiencies have started to find their way. Thus, algebraic or non-linear relations are used to determinate the Reynolds stress tensor without introducing any additional differential equation.Therefore, the first part of this thesis is devoted to the study of several EARSM and EVM models involving linear and higher order terms in the constitutive relation to evaluate turbulent stresses. Accuracy and numerical performance of these models is tested in different flow configurations such as plane channel, backward facing step, and both plane and round impinging jets. Special attention is paid to the verification of the code and numerical solutions, and the validation of the mathematical models used. In the impinging plane configuration, improvements of models using higher order terms in the constitutive relation are limited. Whereas, in the rest of studied cases these non-linear models show a reasonably good behaviour.Moreover, taken into account models convergence, robustness and predictive realism observed in the analysis of these benchmark flows, some of them are selected for the study of air curtains and their interaction with the environment where they are placed. Air curtains are generally one or a set of vertical or horizontal plane jets used as ambient separator of adjacent areas presenting different conditions. The jet acts as a screen against energy losses/gains, moisture or mass exchanges between the areas.As was indicated before, the main purpose of the second part of this thesis is to characterize in detail actual air curtains using both experimental and different numerical approaches. Semi-empirical models to design air curtains are presented. Then, an experimental set-up used to study air curtain discharge and jet downstream is explained. Experimental measurements of velocity and temperature are shown. As a result of the experiments carried out, an improved air curtain with a new design of the discharge nozzle is obtained. Furthermore, air curtain experiments are numerically reproduced and predictions validated against the experimental data acquired. Good agreement between numerical and experimental results is observed.Finally, systematic parametric studies of air curtains in heating and refrigeration applications are done. Global energetic balances are specially considered together with global parameters selected in order to evaluate air curtain performance. It is found that discharge velocity, discharge angle and turbulence intensity of the jet are the most sensitive parameters. Inadequate values for these variables can produce undesirable effects and contribute to increase energy gains/losses

Solución numérica de las ecuaciones de Navier-Stokes incompresibles por el método de los volúmenes finitos

ResumenEl objetivo principal de este artículo es la resolución de las ecuaciones de conservación de cantidad de movimiento y de masa de Navier-Stokes, para un fluido incomprensible. Por esto, se presenta el planteamiento numérico con una discretización por medio de volúmenes finitos (MVF) y se hace uso del método de los pasos fraccionados, para la resolución del acoplamiento entre la velocidad y la presión. Con el propósito de validar el modelo matemático y verificar el código se resolvió un problema tipo “benchmark”, el “Driven Cavity” en dos dimensiones. Se estudiaron dos números de Reynolds en régimen laminar: 100 y 1000. Los resultados obtenidos con la herramienta computacional desarrollada fueron similares a los esperados. Se usó un refinamiento del tipo h para la verificación.Palabras clave: ecuaciones de Navier Stokes, volúmenes finitos, método de paso fraccional, driven cavity.Numerical solution of the incompressible Navier-Stokes equations with finite volume methodAbstractThe main goal of this paper is the numerical solution of the Navier-Stokes equations for an incompressible flow. A numerical approach with a finite volume discretization technique and using the method of fractional stepsare presented to solver the coupling between velocity and pressure. In order to validate the mathematical model and the code a the Driven Cavity problem in two dimensions for Reynolds numbers between 100 and 1000 was solved. The results given by the code are very similar to the expected. In order to verify the numerical results a h-refinement study is carried out.Keywords: Navier Stokes equations, finite volumes, fractional step method, driven cavity

Solución numérica de las ecuaciones de Navier-Stokes incompresibles por el método de los volúmenes finitos

ResumenEl objetivo principal de este artículo es la resolución de las ecuaciones de conservación de cantidad de movimiento y de masa de Navier-Stokes, para un fluido incomprensible. Por esto, se presenta el planteamiento numérico con una discretización por medio de volúmenes finitos (MVF) y se hace uso del método de los pasos fraccionados, para la resolución del acoplamiento entre la velocidad y la presión. Con el propósito de validar el modelo matemático y verificar el código se resolvió un problema tipo “benchmark”, el “Driven Cavity” en dos dimensiones. Se estudiaron dos números de Reynolds en régimen laminar: 100 y 1000. Los resultados obtenidos con la herramienta computacional desarrollada fueron similares a los esperados. Se usó un refinamiento del tipo h para la verificación.Palabras clave: ecuaciones de Navier Stokes, volúmenes finitos, método de paso fraccional, driven cavity.Numerical solution of the incompressible Navier-Stokes equations with finite volume methodAbstractThe main goal of this paper is the numerical solution of the Navier-Stokes equations for an incompressible flow. A numerical approach with a finite volume discretization technique and using the method of fractional stepsare presented to solver the coupling between velocity and pressure. In order to validate the mathematical model and the code a the Driven Cavity problem in two dimensions for Reynolds numbers between 100 and 1000 was solved. The results given by the code are very similar to the expected. In order to verify the numerical results a h-refinement study is carried out.Keywords: Navier Stokes equations, finite volumes, fractional step method, driven cavity

Análisis numérico del comportamiento térmico y fluidodinámico de los gases de combustión en un horno tradicional para la producción de panela

Introduction: Panela is a product derived from sugar cane that is prepared using a traditional burner designed especially for this purpose. According to studies found in the literature, it was identified that the thermal efficiency of panela burners is 30% on average. Objective: The objective of this investigation is to contribute to the search for new alternatives for the improvement of the low efficiency present on these systems, mainly affecting the flue gases duct. Methodology: The development of this study is as follows: first, a research of the radiation and optical thickness effect in a simplified furnace is carried out. Afterward, a series of simulations with modifications in the design of the flue gas duct for a real size furnace are analyzed. Results: The results showed that the radiation effect must be considered and, even though the optical thickness is low, it has a relevant impact in the heat transfer process due to the high temperatures in the furnace. A chaotic movement of the gases implied more heat transferred to the heaters and high values of Nusselt with the addition of new elements in the duct were obtained. Conclusions: Arrangement 1, provides the best results with a Nusselt and thermal efficiency increase. No significant differences between the DOM and the P-1 radiation were found.Introducción: La panela es un producto derivado de la caña de azúcar. En su elaboración se utiliza una hornilla tradicional, diseñada especialmente para este propósito. Según estudios encontrados en la literatura, se ha identificado que la eficiencia térmica de las hornillas paneleras se estima en un 30% promedio. Objetivo: Esta investigación tiene como objetivo contribuir en la búsqueda de nuevas soluciones para el mejoramiento del nivel de eficiencia, modificando principalmente el ducto de humos. Metodología: El desarrollo de este estudio es el siguiente: primero, se realiza una investigación del efecto de la radiación y del espesor óptico en un horno simplificado. Posteriormente, se realiza una serie de simulaciones con modificaciones en el diseño del ducto de humos para un horno de tamaño real. Resultados: Los resultados mostraron que se debe considerar el efecto de la radiación. Aunque el espesor óptico sea bajo, tiene un impacto relevante en el proceso de transferencia de calor debido a las altas temperaturas en el horno. Un movimiento caótico de los gases implicó más calor transferido a las pailas, y se obtuvieron altos valores de Nusselt con la adición de nuevos elementos en el conducto. Conclusiones: El arreglo 1, proporciona los mejores resultados con un aumento de la eficiencia térmica y de Nusselt. No se encontraron diferencias significativas entre los modelos de radiación DOM y P-1

Recuperación energética y análisis térmico del proceso de cocción en la industria cerámica

The scientific research is not focused on downdraught beehive kilns, which are the most used in the Colombian brick (ceramic) industry. In this work, an analytical mathematical model is developed in order to simulating the exhaust gas behavior. To do so, gas combustion, and heat and mass transfer to the products and walls are modelled within the kiln. The obtained model is validated against experimental data. Finally, with the experimental data acquired in the discharge duct and chimney, a heat recovery system is proposed to make use of the residual energy.Las investigaciones encontradas en la literatura no son específicas para hornos colmena, los cuales son los más usados en la industria ladrillera (cerámica) en Colombia. Es por esto que en este trabajo se desarrolla un modelo matemático analítico que simula el comportamiento de los gases de combustión en ellos. En el horno se modela la combustión, la transferencia de calor y masa a los productos y las pérdidas por las paredes. El modelo matemático usado es validado con resultados experimentales. Además, con los datos experimentales medidos en el ducto de descarga y chimenea, se selecciona un sistema recuperador para aprovechar la energía residual

Modelado CFD de la combustión en calderas de biomasa – Revisión del estado del arte

Combustion is the main method of converting biomass to energy, either by direct heating systems or by boilers. By means of CFD models, it is possible to optimize the behavior of those systems and improve significantly its performance, without incurring the economic and environmental cost of experimental studies. However, modelling of biomass combustion is a complex process that requires a large number of sub-models and computational resources for a detailed description, therefore, different approaches have been developed which depend on the system and simulation objective. In this work, a review of the state of art of modelling of solid biomass combustion in the last years is presented, including classification, description and analysis of several of the main models about the subject.La combustión es el principal método de transformación de biomasa en energía, ya sea en sistemas de calefacción directa o en calderas. Por medio de los modelos CFD se puede optimizar el funcionamiento de estos sistemas y lograr mejoras significativas en su desempeño, sin incurrir en los costos económicos y ambientales que los estudios experimentales acarrean. No obstante, el modelado de la combustión de biomasa sólida es un proceso complejo que requiere de gran cantidad de sub-modelos y recursos computacionales para una descripción detallada, por lo que se han desarrollado diversos enfoques que dependen del sistema a modelar y del objetivo de la simulación. En el presente trabajo se realiza una revisión del estado del arte sobre el modelado de la combustión de biomasa sólida en los últimos años, incluyendo la clasificación, descripción y análisis de varios de los principales modelos desarrollados sobre el tema