Simulation of heat and mass transfer phenomena in the critical elements of H2O-LiBr absorption cooling machines. Experimental validation and application to design

Degut a la tendència a l'increment del preu de la energia, i el seu ús cada cop més estès per aire condicionat en els paisos desenvolupats, els sistemes de refrigeració basats en energia solar tenen cada cop més atractiu. El objectiu final d'aquesta tesi és el desenvolupament d'eines de simulació numèrica pel disseny de màquines de refrigeració per absorció que tinguin la possibilitat de funcionar amb energia solar. Malgrat existeixen en el mercat màquines d'absorció d'aquestes característiques des de fa anys, hi ha una deficiència en el desenvolupament de sistemes de petita capacitat. Els sistemes de petita capacitat impliquen problemes addicionals en el seu disseny (sistemes refrigerats per aire, compacitat ...) que només es poden abordar fent ús d'eines de disseny adequades, tant pel sistema com pels seus components. Tanmateix, hi ha també certa deficiència en la literatura especialitzada en el desenvolupament de models matemàtics adequats per la descripció dels processos de transferència de calor i de massa en les màquines de refrigeració per absorció: àrea mullada en les superfícies d'intercanvi de calor i de massa, paper dels additius, etc. Per aquestes raons aquest treball ha estat enfocat en aquests objectius:- Estudi de processos bàsics de transferència de calor i de massa juntament amb els fenomens fluid-dinàmics implicats en absorbidors de màquines d'absorció. Aquest estudi ha estat fet mitjançant simulacions detallades resolent les equacions de Navier-Stokes sota ertes hipòtesis.- Desenvolupament d'eines de simulació numèrica pel disseny i predicció de sistemes de refrigeració per absorció, aprofitant la informació donada per models més detallats.- Desenvolupament d'eines de simulació numèrica pel disseny dels elements crítics d'intercanvi de calor i de massa de sistemes de refrigeració per absorció (absorbidor, generador, evaporador, condensador) mantenint el càlcul en un raonable temps de CPU. Aquest model recolza el mencionat en el punt anterior.- Desenvolupament de un prototipus de màquina d'absorció, refrigerada per aire, fent servir H2O-LiBr com a fluid de treball, amb les eines numèriques desenvolupades. - Contrastació experimental dels models desenvolupats.- Estudi del funcionament de la màquina d'absorció anteriorment mencionada. - Avaluació dels resultats per millorar els criteris de disseny i optimització del mateix de cara a prototipus de segona generació.Després del desenvolupament d'aquestes eines de simulació numèrica que s'han fet servir per problemes específics sortits en el procés d'estudi d'una màquina en concret, un marc de treball ha estat creat per l'estudi d'altres sistemes de refrigeració per absorció.Due to the increasing trend of the price of the energy, mainly obtained from fossil combustibles, and its also increasing use for air-conditioning in developed countries, solar cooling has been becoming more attractive from the point of view of economics and environment conservation. The final aim of this thesis is the development of numerical simulation tools for the design of absorption machines with the possibility of being driven by solar energy. Although there are available in the market absorption chillers of such characteristics for years, there is a lack in development of small capacity systems. Small capacity systems imply additional problems of design (air-cooled systems, compactness ...) that only can be afford with adequate design tools for system and components. Moreover, there is also a lack in the specialised literature in the development of adequate mathematical models for the description of the heat and mass transfer processes in absorption machines: wetted area of the heat and mass transfer surfaces, role of additives, complex geometries etc.For these reasons this work has been focused on the following detailed objectives: - Study of basic heat and mass transfer processes together with the fluid-dynamic phenomena implied in absorbers of absorption chillers. This study has beencarried out by means of detailed simulations solving the Navier-Stokes equations under certain hypotheses. - Development of numerical simulation tools for design and prediction of absorption systems, taking advantage of information given by more detailed models. - Development of numerical simulation tools for design of the heat and mass exchange components of absorption systems keeping the calculation in a reasonable CPU time. This model provides of the necessary information for the model mentioned in the previous point.- Development of a prototype of an air cooled absorption machine based on the numerical results obtained from the models.- Validation of the models developed by means of comparison of numerical results and experimental data obtained from the prototypes developed.- Study of the performance of the above mentioned absorption system. - Evaluation of the results in order to improve the design criteria for a second generation of prototypes.After the development of these numerical simulation tools and their applicationin specific problems, a framework has been created for the study of other type of absorption systems.Postprint (published version

Numerical and experimental investigation of a vertical LiBr falling film absorber considering wave regimes and in presence of mist flow

The absorber represents the most critical component in absorption systems and one of the key issues. In this component complex heat and mass transfer phenomena during the absorption process takes place simultaneously. For this reason the development of mathematical models validated against experimental data always constitutes useful tools for the design and improvement of falling film absorbers. A testing device has been designed and built to reproduce absorption phenomena in vertical LiBr-H2O falling film absorbers with the primary objective to obtain experimental data. On the other hand, a mathematical model of falling film absorption of H2O vapour in LiBr aqueous solutions has been implemented. Wave regime is considered by including and solving the Free Surface Deflection Equation. The numerical results are validated using the experimental data. During the development of this work, the authors have paid careful attention to the verification of experimental data. Such verification consists of performing energy and mass balances in the fluid film side. Important discrepancies were found in our experimental data. Therefore, an extensive study was carried out in order to find the source of such errors. The conclusion is that there is a drag of LiBr solution in the water vapour which increases with the Re number. This mist flow cannot be measured experimentally, but can be evaluated in an indirect way. The mathematical models have been adapted in order to consider the influence of mist flow. On the other hand, in the literature there are not many experimental works related to falling film absorbers which expose enough information to verify the reliability of their experimental data.Peer ReviewedPostprint (author's final draft

A level-set model for thermocapillary motion of deformable fluid particles

A new level-set model is proposed for simulating immiscible thermocapillary flows with variable fluid-property ratios at dynamically deformable interfaces. The Navier–Stokes equations coupled with the energy conservation equation are solved by means of a finite-volume/level-set approach, adapted to a multiple marker methodology in order to avoid the numerical coalescence of the fluid particles. The temperature field is coupled to the surface tension through an equation of state. Some numerical examples including thermocapillary driven convection in two superimposed fluid layers, and thermocapillary motion of single and multiple fluid particles are computed using the present method. These results are compared against analytical solutions and numerical results from the literature as validations of the proposed model.Peer ReviewedPostprint (author's final draft

On estimating the interface normal and curvature in PLIC-VOF approach for 3D arbitrary meshes

Volume of fluid (VOF) method with its Piecewise Linear Interface Calculation (PLIC) reconstruction algorithm is one of the most popular approaches in numerical simulation of interfacial flows with a wide range of applications in different areas. In an effort to evaluate the similarity of the PLIC-generated planes in comparison with the exact interface, a point-cloud, based on the polygon centers of PLIC planes is extracted, which later is used to form a triangular grid that represents the estimated interface. The main objective of this article is to evaluate the interface geometrical properties based on the extracted triangular grid of the interface. The methods presented in this article, characterized by a higher spatially convergence ratio, are compared with the commonly used methods. The proposed methods are tested for two 3-dimensional general test cases, where an evident improvement is seen in calculation accuracy and spatial convergence of the errors of interface normal vector and curvature.This work has been financially supported by MCIN/AEI/10.13039/ 501100011033 Spain, project PID2020-115837RBI00. E. Schillaci acknowledges the financial support of the Programa Torres Quevedo (PTQ2018-010060).Peer ReviewedPostprint (author's final draft

Minimization procedure of experimental tests for calibration purposes, within HVAC&R energy efficiency framework

Simulation models and predictive tools need to be fast, accurate and robust at the same time. The models that have to provide numerical solutions under transient conditions for a long period of time need to be simple with the aim of minimizing the time respond, without losing the accuracy. Thus, previous experimental data and a calibration methodology are necessary to assure this objective, both are strictly necessary to reproduce the behaviour with accuracy expected. Consequently, even accurate information (e.g. look-up tables) for HVAC&R components (e.g. heat exchangers, fan/compressor, auxiliary elements, etc.) is known and all coupling system is developed, the minimization of experimental tests for calibration purposed based scattered data interpolation is now an important aspect, which looks for reducing the quantity of experiments necessary to assure the accuracy expected from an optimization point of view. The present work shows an optimization procedure based on test number minimization according detailed error comparison against existing previous data. Illustrative results for a specific component are presented highlighting test number reduction without losing accuracy.This project has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 755517.Peer ReviewedPostprint (published version

Three-dimensional direct numerical simulation (dns) Of taylor bubbles rising in non-Newtonian environments

Three-dimensional numerical simulation of Taylor gas bubbles as primary unites of slug flow patterns rising in non-Newtonian environments is performed in the context of Direct Numerical Simulation (DNS) of the governing equations, where the whole physics of fluid motions will be taken into account. State-of-the-art numerical tools are proposed to tackle the numerical challenges in the DNS study of this problem. E.g. a coupled level-set volume-of-fluid (CLSVOF) interface capturing method is used to solve the topological changes of the interface. Physical formulations are integrated with moving-mesh (MM) technique to decrease the computational cost of 3D simulations and adaptivemesh-refinement (AMR) technique to increase the local accuracy around the interface. The governing equations are solved using High-Performance Computing (HPC) parallel approaches. To the best of the authors’ knowledge, this is the first work dealing with three-dimensional direct numerical simulation of Taylor bubbles rising in non-Newtonian environments.Postprint (published version

Oral tests. Strategies for teachers and students to get rid of anxiety

Esta comunicación intenta valorar el papel que el examen oral puede desempeñar dentro de la interacción enseñanza/aprendizaje. A modo de definición-presentación enumeramos contrastivamente con otras pruebas las condiciones especiales de estos exámenes que afectan a profesores y alumnos, y que los hacen cualitativamente diferentes. Nuestro interés por una mejor investigación hace que consideremos el aspecto humano como elemento fxssitivo. De este modo las estrategias, bien sean psicológicas o de otro tipo, juegan un papel fundamental en la liberación de la tensión. No obstante, una investigación eficiente debería tener en cuenta todos los factores relevantes, que hemos tratado de analizar. En primer lugar ofrecemos todos los objetivos posibles junto con las necesidades y expectativas. Además presentamos los distintos tipos de técnicas de pruebas orales, a modo de estrategias, clasificados gráficamente de acuerdo con la interacción de mensaje y participantes, así como con el grado de dificultad y espontaneidad, con el fin de facilitar la secuenciación y temporización. La segunda parte se centra en las posibilidades de explotar la valoración y calificación de acuerdo con determinada investigación. La evaluación del test debe ser un asjjecto ineludible en cualquier estudio de este tipo. Si todos estos aspectos resultan válidos, la investigación se convierte elemento vivo y constante en nuestro quehacer.This paper is based on the value that oral tests may have within the teaching and learning interaction. We begin with a definition-introduction of the main constraints that affect teachers and students which make this type of test qualitatively different from others. Research could be better planned if the human side is looked at as a positive aspect, for which strategies, psychological or others, play a fundamental role in getting rid of anxiety. Nevertheless, in order to find a more efficient searching way, all aspects that take part in oral tests should be taken into account and re-examined. In trying to do so, we have firstly compiled and questioned all possible aims alongside with needs and expectations. Oral test techniques (used as strategies) are shown in a chart according to the interaction of message and participants, as well as the degrees of difficulty and spontaneity so that lime and sequencing are controlled in order to suit the learner's needs. The second part explains how assessment and marking could be exploited to favour some particular research. Test evaluation is essential in this kind of research. Once these aspects have been questioned and proved to be valid, we may find ourselves on a constant research

Oral tests. Strategies for teachers and students to get rid of anxiety

Esta comunicación intenta valorar el papel que el examen oral puede desempeñar dentro de la interacción enseñanza/aprendizaje. A modo de definición-presentación enumeramos contrastivamente con otras pruebas las condiciones especiales de estos exámenes que afectan a profesores y alumnos, y que los hacen cualitativamente diferentes. Nuestro interés por una mejor investigación hace que consideremos el aspecto humano como elemento fxssitivo. De este modo las estrategias, bien sean psicológicas o de otro tipo, juegan un papel fundamental en la liberación de la tensión. No obstante, una investigación eficiente debería tener en cuenta todos los factores relevantes, que hemos tratado de analizar. En primer lugar ofrecemos todos los objetivos posibles junto con las necesidades y expectativas. Además presentamos los distintos tipos de técnicas de pruebas orales, a modo de estrategias, clasificados gráficamente de acuerdo con la interacción de mensaje y participantes, así como con el grado de dificultad y espontaneidad, con el fin de facilitar la secuenciación y temporización. La segunda parte se centra en las posibilidades de explotar la valoración y calificación de acuerdo con determinada investigación. La evaluación del test debe ser un asjjecto ineludible en cualquier estudio de este tipo. Si todos estos aspectos resultan válidos, la investigación se convierte elemento vivo y constante en nuestro quehacer.This paper is based on the value that oral tests may have within the teaching and learning interaction. We begin with a definition-introduction of the main constraints that affect teachers and students which make this type of test qualitatively different from others. Research could be better planned if the human side is looked at as a positive aspect, for which strategies, psychological or others, play a fundamental role in getting rid of anxiety. Nevertheless, in order to find a more efficient searching way, all aspects that take part in oral tests should be taken into account and re-examined. In trying to do so, we have firstly compiled and questioned all possible aims alongside with needs and expectations. Oral test techniques (used as strategies) are shown in a chart according to the interaction of message and participants, as well as the degrees of difficulty and spontaneity so that lime and sequencing are controlled in order to suit the learner's needs. The second part explains how assessment and marking could be exploited to favour some particular research. Test evaluation is essential in this kind of research. Once these aspects have been questioned and proved to be valid, we may find ourselves on a constant research

A generalized computational model for the simulation of adsorption packed bed reactors – Parametric study of five reactor geometries for cooling applications

Environmental concerns regarding global warming and ozone depletion urge towards sustainable solutions for satisfying the increasing cooling demand. Adsorption cooling technology could form part of the solution since it can be driven by solar energy and industrial or vehicular waste heat, as well as it employs non ozone-destructive refrigerants. However, its low performance hinders its extensive development and commercialization. The design of the adsorption reactor is crucial for its performance improvement, since its inherent cyclic operation imposes a compromise between the Specific Cooling Power and the Coefficient of Performance. A generalized three-dimensional computational model based on unstructured meshes is presented, capable to simulate all potential geometries. Dynamic conjugate simulations of the packed bed and the heat exchanger allow to study the latter’s influence on the reactor performance. A parametric study of five reactor geometries was conducted, demonstrating quantitatively the strong impact of the solid volume fraction, fin length and fin thickness on the performance. Within the studied range, the Specific Cooling Power is maximized for the highest solid volume fraction and for the lowest fin thickness and fin length. The effect of the adsorbed mass spatial distribution on the desorption phase is discussed. A sensitivity analysis exhibits the importance of the heat transfer coefficient between the two domains. Copper and aluminium are compared as heat exchanger materials, revealing that the former performs more effectively, although the difference is appreciable only for longer fin lengths. The presented numerical model can be employed for improving the design of adsorption packed bed reactors.Peer ReviewedPostprint (author's final draft

Caracterización eléctrica de películas delgadas de SIO2 mediante esprectoscopia de electrones auger

Depto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu