Non-Uniform Heating Effects on the Convective Airflow in an Open Cavity with Ventilation Ports under Non-Boussinesq Conditions

The influence of non-uniform heating conditions on the buoyancy-driven flow established in an open square cavity with ventilation ports is studied. Cavity configuration is shaped to electronic device with passive cooling. Numerical results for relevant parameters are presented as a function of the Rayleigh number, considering both fixed temperature and heat flux heating conditions. The results obtained retaining the temperature-dependent thermophysical properties are compared with those calculated under the Boussinesq approximation. The impact that the non-uniformly heated wall produces on the thermal and dynamic behavior of the airflow is analyzed. The choice of a given heating distribution slope can produce a thermal behavior improvement, even increasing heat transfer above 20%. Some practical engineering correlations are presented. In fact, a correlation for the critical heating parameter for isoflux heating condition, concerned to the burnout phenomenon, is obtained from numerical results. The effects of the non-uniform heating condition (heat flux case) are not too relevant on this particular phenomenon.I would like to acknowledge to Prof. J. Hernández (UNED, Spain), for his great teaching

Determining correlations for solar chimneys in buildings with wind interference: A numerical approach

The impact of wind on the performance of a solar thermal chimney is analyzed by numerical investigation. The chosen geometry for the dwelling-solar chimney system can be considered as sufficiently generic and representative, and similar to the buildings assisted with passive systems that are currently being designed. Both the wind driving and the buoyancy forces are included in the study. The developed numerical approach takes into account the atmospheric boundary layer for specifying the acting wind, and includes a low-Reynolds treatment of turbulence at walls for obtaining a detailed description of the airflow. The procedure is validated enough, and permit us to address a systematic analysis of the influence of temperature difference (10–100 K, corresponding to Rayleigh numbers from 3.616 × 10^9 to 3.616 × 10^10 and wind speed ranging 0–10 m/s) on the behavior of chimney in ventilation mode. Two opposite directions of the wind are studied, finding clear different impacts on the solar chimney performance. For the more favorable wind direction, a complete set of correlations for the induced mass-flow rate and the dimensionless heat transfer coefficient is proposed, aimed at practical engineering applications

Comparative assessment of liquid sodium and gallium as emergency coolants in nuclear energy applications

The influence of the thermophysical variable properties on the buoyancy-driven flows of liquid sodium and gallium established in a cylindrical cavity is numerically investigated, for the entire range of Rayleigh number corresponding to practical applications. Axisymmetric turbulent simulations (initially steady) are obtained, considering appropriate boundary and reference conditions for simulating the emergency cooling of a nuclear reactor. The results obtained for different heating intensities are analyzed and compared. In the case of sodium, the expected decay in the heat transfer coefficients is less relevant than that previously obtained for airflows. In turn, strong differences in the gallium thermal behavior are encountered. In fact, a trend opposite to that of sodium is detected for wide ranges of Rayleigh number and heating parameter. Additional transient simulations are carried out to complete a comparative assessment of the performance of both liquid sodium and gallium as cooling agents

Enhancing the performance of a rooftop solar chimney through flow disturbers

Enhancing the performance of passive solar chimneys constitutes a key point for successful applications in bioclimatic architecture. Present work assesses applications of several kinds of flow disturbers in a rooftop solar chimney, under isoflux heating and windless conditions, and including surface radiative effects. Systematic numerical calculations are conducted aiming a comprehensive analysis, by means of a low-Reynolds turbulence model, being the range of Rayleigh number considered 2.170× 10^12 −2.170×10^13. Effect of different geometrical parameters is analyzed, although main attention is posed on the influence of disturbers elements on the thermohydraulic behavior of the established airflow, for obtaining best performance conditions. Some obstacles cause a clear decrease in the efficiency of the system, but given disturbers appropriately located produce valuable enhancements in the thermal or dynamic efficiency. Insertion of intermediate plates proves to be the best option, achieving maximum increases of even approximately 50% in the ventilation capacity

Morphological comparative assessment of a rooftop solar chimney through numerical modeling

An assessment on the performance of three different configurations at the outlet sections of a rooftop solar chimney is conducted in this work. The starting configuration corresponds to a representative dwelling-solar chimney system. On the basis of the knowledge reached in the technical bibliography, a numerical modeling is developed, considering both the wind-driving and the buoyancy-driving forces. The turbulent nature of the airflow is simulated through an enough validated two-transport equations model, including a low-Reynolds treatment at walls. Special care is taken in the numerical simulation of the wind, implementing a logarithmic speed profile to properly introduce the atmospheric boundary layer. At walls, a heat-flux heating condition ( uniform) is imposed in order to realistically simulate the heating from irradiation. Focus is posed on a comparative and systematic study of the behavior of airflow through the three considered morphologies, free outlet, covered outlet and side outlet. The more relevant differences found in the performance of each constructive shape are highlighted, both for near-calm as well as for wind-dominant conditions. A wide range of heating conditions are considered, 0.5 ≤ ≤ 1000 W/m^2 (corresponding to Rayleigh numbers from 1.085 × 10^10 to 2.170 × 10^13), as well as reference wind velocities from windless condition up to 10 m/s. Several correlations for the air ventilation rate and the dimensionless heat transfer coefficient are proposed, oriented to engineering practical applications. Wind reveals as dominant from reference velocities in the range 2-3 m/s. The conditions under which a certain better performance wall-to-wall spacing between walls forming the solar tower can be encountered, are analyzed and discussed. Illustratively, in some cases, the maximum value of the average Nusselt number reaches twice the value corresponding to aspect ratio equal to unity

Effects of Thermophysical Variable Properties on Liquid- Sodium Convective Flows in a Square Enclosure

The influence of the variable properties on the buoyancy-driven flows of liquid sodium established in a square enclosure including two inner heated plates is numerically investigated. Two-dimensional turbulent simulations are obtained, considering uniform wall temperature heating conditions. The low-Reynolds k–omega turbulence model is employed. The average Nusselt number and the dimensionless mass-flow rate evaluated between the inner plates are obtained for a wide range of the Rayleigh number, varying from 10^3 to 10^12. Several practical correlations are presented. The results obtained for different heating intensities are analyzed and compared. The expected decay in the heat transfer coefficients is less relevant than that previously obtained for airflows. The thermal structure of the flow into the enclosure is also shown

Numerical studies on laminar, transitional and turbulent convective airflows in channels with generalised geometry, including applications to thermal-ventilation passive systems

The flows induced by natural convection appear in many engineering problems. Configurations formed by heated plates where these processes occur, were the topic of intense study in past years. However, new systems with a different constructive disposition and applications in several fields (bioclimatic architecture, electronic cooling, nuclear energy cooling systems) are at the moment of great interest. Since typical applications of buoyancy-driven airflows in smooth vertical channels are usually in small-scale devices (i.e. in electronic cooling equipment), most investigations were carried out for laminar flows. As a consequence of the great scale of certain passive ventilation systems, as solar chimneys, Trombe walls, or roof collectors, the flow established becomes transitional or even fully turbulent. The regarded geometries frequently involve structures based on converging and sloped channels formed by heated plates, where buoyancy-driven flows take place. Therefore, the study of problems such as the transition to turbulent regime or flows with walled-channel geometries including sloped and converging walls are key to assertively find out the real heating transmission existing in these new systems in a more realistic manner. Nowadays, the need to achieve human comfort by passive heating and ventilation techniques is greater as is the requirement for energy saving. Passive solar systems are the basic elements of bioclimatic design and they do not involve the use of mechanical or electrical devices. The Trombe Wall is the primary example of the technique called indirect gain, whose typical configuration is usually formed by a thick, darkened, masonry wall and a glazed wall. In ventilation applications, other passive systems, called thermosyphons, heat syphons or solar chimneys, can yield natural motions of air due to the induced temperature differences by solar heating. Although several reported works provided useful results for the analysis and design of passive solar devices in buildings, these works cannot determine the necessary details of convection for numerical simulations. Furthermore, as an important lack of design correlations is detected, it is necessary to carry out a systematic study that supplied the heat transfer coefficient and the mass-flow rate as a function of relevant parameters, for several configurations

Non-linear dynamic analysis of the response of moored floating structures

© 2016 Elsevier Ltd. The complexity of the dynamic response of offshore marine structures requires advanced simulations tools for the accurate assessment of the seakeeping behaviour of these devices. The aim of this work is to present a new time-domain model for solving the dynamics of moored floating marine devices, specifically offshore wind turbines, subjected to non-linear environmental loads. The paper first introduces the formulation of the second-order wave radiation-diffraction solver, designed for calculating the wave-floater interaction. Then, the solver of the mooring dynamics, based on a non-linear Finite Element Method (FEM) approach, is presented. Next, the procedure developed for coupling the floater dynamics model with the mooring model is described. Some validation examples of the developed models, and comparisons among different mooring approaches, are presented. Finally, a study of the OC3 floating wind turbine concept is performed to analyze the influence of the mooring model in the dynamics of the platform and the tension in the mooring lines. The work comes to the conclusion that the coupling of a dynamic mooring model along with a second-order wave radiation-diffraction solver can offer realistic predictions of the floating wind turbine performance.Postprint (published version

Máquinas hidráulicas: teoría y problemas

Máquinas Hidráulicas (Teoría y Problemas), es un texto orientado a la docencia de las Máquinas Hidráulicas en titulaciones relacionadas con las Ingenierías Industrial y Energética. El enfoque del libro es fundamentalmente académico para un nivel de posgrado, por lo que presenta una colección completa de problemas propuestos, y otros resueltos, que pueden ayudar a conseguir un conocimiento firmemente asentado en sus principios físicos sobre las Máquinas Hidráulicas y sus instalaciones. Se ha prestado atención a los aspectos energéticos y a las Centrales Hidroeléctricas, buscando una respuesta adecuada a las nuevas necesidades competenciales de los Másteres vinculados con las titulaciones mencionadas. El estudio de las Máquinas Hidráulicas se realiza siguiendo el siguiente esquema. En primer lugar, se aplican las ecuaciones de conservación de la Mecánica de Fluidos en forma integral a las Máquinas Hidráulicas y se definen los distintos rendimientos; se aplican a continuación las herramientas del análisis dimensional y de la semejanza física; se exponen las teorías unidimensional y bidimensional con un enfoque sobre todo ideal, tanto a bombas como a turbinas, y para máquinas axiales y radiales, y posteriormente se exponen distintos aspectos del comportamiento real y del diseño de bombas y turbinas. Se analizan con mayor detalle las Turbomáquinas, puesto que son las Máquinas Hidráulicas más utilizadas, y son además las más interesantes desde el punto de vista fluidomecánico. Finalmente, se introduce el comportamiento de las Centrales Hidroeléctricas y de otras máquinas hidráulicas, volumétricas y de compresibilidad despreciable. El contenido se ha dividido en 4 grandes bloques (con 16 capítulos en total): I. Generalidades sobre las máquinas hidráulicas. Energía hidráulica. II. Teoría general de turbomáquinas hidráulicas. III. Flujo real, pérdidas y cavitación en turbomáquinas. IV. Cálculo de turbomáquinas y de sus instalaciones. Otras máquinas hidráulicas

La verbalización como método facilitador en la enseñanza de la ingeniería en el contexto del proceso de Bolonia

[ESP] La Enseñanza de la Ingeniería actual requiere que el alumno adquiera competencias de simulación numérica, para lo que precisa otras competencias previas que frecuentemente no posee. La rigidez crediticia no permite actuaciones recuperadoras para estas carencias competenciales, y se recurre en los desarrollos docentes al uso masivo de programas comerciales de simulación, plagando los planes de estudio de cajas negras. El abuso de estas cajas negras perturba el proceso de enseñanza-aprendizaje a corto y largo plazo. En este trabajo, se propone una forma de verbalización conceptual, con la triple finalidad de evitar el uso indiscriminado de cajas negras, facilitar el proceso de enseñanza-aprendizaje de la simulación numérica y mejorar la adquisición de competencias específicas de Ingeniería. El alumno conoce la estructura eidética de los programas, pero no aprende su lenguaje específico; maneja el lenguaje común, verbaliza. El alumno diseña la simulación y la presenta verbalizada, como una demanda textualizada exacta y rigurosa del proceso de simulación. Si la demanda verbalizada es correcta, el Tutor Facilitador la traduce al lenguaje de programación. Esta verbalización, constructivista, permite esquivar los hábitos utilitaristas de las cajas negras y motiva el uso posterior, conceptualmente coherente, de herramientas complejas de simulación numérica. [ENG] Today, Engineering Education requires that students acquire skills concerned with numerical simulation, for which other previous skills (that often they don‘t have) are mandatory. The credit rigidity doesn‘t allow the recovery of these skill shortcomings; so, it is normal the use of commercial simulation packages in educational developments, which leads to the black boxes. The black boxes abuse disrupts the teaching-learning process in the short and long term. In this work, we propose a conceptual form of verbalization, with the threefold purpose of preventing the indiscriminate use of black boxes, facilitate the teaching-learning process of the numerical simulation subject and improve the acquisition of specific skills in Engineering. The student knows the eidetic structure of simulation codes, but he doesn‘t learn its specific language; by contrast, uses the common language, i.e., verbalizes. The student designs the simulation and shows it in a verbalized manner, as an accurate and rigorous demand of the whole simulation process. If the verbalized demand is correct, the Tutor Facilitator translates the outlined problem to a programming language. This constructivist verbalization permits us to avoid the massive use of black boxes and then produces the conceptually consistent use of complex tools for numerical simulation