Experimental estimation of the residual fatigue life of in-service wind turbine bolts

This study presents an experimental methodology aimed at estimating the residual fatigue life of in-service wind turbine bolts. The main objective is to assess the residual life of the bolts to plan their replacement and to avoid unexpected breakages of wind turbine blade connections. To develop the methodology, M16 bolts of quality 10.9 with controlled predamage were used, simulating in-service operating conditions. The fatigue tests were carried out taking care to place the nut at the point on the bolt that produces the highest damage at the same point where the predamage was performed. In addition, the influence of a possible angular positioning error on the residual fatigue life has been investigated. The residual fatigue life is estimated from the difference in fatigue life of new bolt tests and the fatigue life of predamaged bolt tests, simulating service conditions. Special care has been taken to guarantee that the most damaged zone of the bolt in service is also in the position that produces the highest damage during tests. An experimental procedure for determining the fatigue life of a new bolt from tests conducted on a bolt under the same operating conditions was developed. The developed methodology has been applied to M20 bolts belonging to real turbines in service

Experimental and Computational Model for a Neonatal Incubator with Thermoelectric Conditioning System

This work describes the design, construction and testing of a thermo-electric conditioning device installed in a neonatal incubator with the aim of improving the precision in the regulation of the interior air temperature, reducing noise and interior vibration, and improving the life of the neonate. A simplified one-dimensional thermal model has been developed, made up of resistances and thermal capacities that simulate the thermal behaviour of all the elements of the system from end to end. All the equations of the model are obtained in a nodal way, allowing the mathematical relationship between the input and output to be known. This model makes it possible to improve temperature control, avoiding the deviations that occur in the traditional model controlled by sensors at both ends. The computational model allows to predict the variation of temperatures in transient and permanent regime. This model allows the design and sizing of the thermoelectric system for different outdoor environmental conditions and the selection of the number of Peltier modules needed to satisfy the heating demand of other incubators with different geometry and capacity. The results of the computational model show good agreement with the experimental tests, despite being a simplified 1D nodal model. The results obtained show a coefficient of operation (COP) of 1.38, achieving higher performance than the current traditional electrical resistance system (COP = 1). In addition, a CFD study has been carried out to check the air patterns, to see the temperature uniformity and to estimate the number of air changes per hour (HVAC) inside the incubator.This work was funded by University of Cadiz

Convergence Analysis of the Straightforward Expansion Perturbation Method for Weakly Nonlinear Vibrations

There are typically several perturbation methods for approaching the solution of weakly nonlinear vibrations (where the nonlinear terms are "small" compared to the linear ones): the Method of Strained Parameters, the Naive Singular Perturbation Method, the Method of Multiple Scales, the Method of Harmonic Balance and the Method of Averaging. The Straightforward Expansion Perturbation Method (SEPM) applied to weakly nonlinear vibrations does not usually yield to correct solutions. In this manuscript, we provide mathematical proof of the inaccuracy of the SEPM in general cases. Nevertheless, we also provide a sufficient condition for the SEPM to be successfully applied to weakly nonlinear vibrations. This mathematical formalism is written in the syntax of the first-order formal language of Set Theory under the methodology framework provided by the Category Theory

Influence of the Fractal Geometry on the Mechanical Resistance of Cantilever Beams Designed through Topology Optimization

In this work, the complex geometry of beams obtained from topology optimization is characterized through the fractal dimension (F-D). The fractal dimension is employed as an efficiency measure of the mass distribution in the beams, that is, the capacity of the optimized solutions to be efficiently distributed in the design space. Furthermore, the possible relationships between the fractal dimension and beams' mechanical properties are explored. First, a set of theoretical beams are studied based on their well-known fractal dimension. A 3D fractal called Menger sponge is reproduced on a Michell's beam (cantilever with a single force applied at the end). The programming codes that generate those beams are created in Matlab software, as are the algorithms for estimating the fractal dimension (box-counting method). Subsequently, identical beams are modelled in the software Inspire in order to apply the topology optimization and determine the mechanical parameters from the static analysis. Results indicate that the fractal dimension is affected by the design geometry and proposed optimized solutions. In addition, several relationships among fractal dimension and some mechanical resistance parameters could be established. The obtained relations depended on the objectives that were initially defined in the topology optimization

Calibration Methodology for CFD Models of Rooms and Buildings with Mechanical Ventilation from Experimental Results

This chapter describes a methodology for the development and calibration of computational fluid dynamics (CFD) models of three-dimensional enclosures for buildings with combined forced and natural convection from experimental result. The models were validated with physical test measurements of room air temperature. The developed CFD models included a model of an internal wall-mounted air conditioning (HVAC) split unit. The methodology proposed here aims at selecting the correct grid size and the appropriate boundary conditions from experimental data. The experimental campaign took place in an empty office room within an educational building. A set of experiments was performed with varying boundary conditions of two main variables, the fan speed of the HVAC unit and the surface wall temperature of the opposite wall to the HVAC unit. The developed CFD models used the standard k-ε turbulence model and the SIMPLE algorithm. The variable of interest was the room air temperature and its distribution within the internal environment. The application of the methodology has shown satisfactory results, finding a maximum error of 9% between the CFD model and the experimental result. This methodology can be used by other researchers to calibrate CFD models in existing rooms and then carry out detailed studies of temperature distribution, comfort and energy demand analysis

Characterizing the Air Temperature Drop in Mediterranean Courtyards from Monitoring Campaigns

As microclimate modifiers, courtyards may be a good passive strategy for enhancing thermal comfort and reducing the energy demands of buildings. Thus, it is necessary to be able to quantify their tempering effect in dominant summer climates. This is frequently done using calculation methods based on CFD, but these have the drawback of their high computational cost and complexity, so their use is limited to advanced users with a high level of knowledge. Thus, an alternative is required based on a simplified method that can explain and predict the air temperature drop in courtyards. This would be extremely useful for professionals looking for the optimal design of this kind of space through energy assessment programs integrating these methods. This study proposes a simplified method of characterization that aims to identify the functional dependencies of the decrease in air temperatures in courtyards, and so to predict the air temperature inside them from that outside, if available. From the results of several experimental campaigns, three variables have been identified that characterize the decrease in the air temperature in courtyards, all of which depend on the confinement factor of the courtyard. Finally, the proposed predictive method was validated by means of an additional monitoring campaign. The results show a good fit of the calculated values to the measured ones, R2 being equal to 0.98Junta de Andalucía TEP-7985 Proyectar Arquitecturas de Transición desde una Investigación Objetiva (PATIO

Building and surroundings: thermal coupling

Energy building performance can be different according to outdoor conditions or urban environment, at the same time that this last assess, buildings are also affected by the building envelope, as obvious consequence of the thermal and Aeraulic coupling existing between the indoor and outdoor conditions in buildings. Thus, in this coupling is fundamental to typify the transmission phenomenon through the building envelope. Doing this, it is possible to estimate transmission heating losses and gains and also the superficial temperatures of the envelope. In order to assess the transient behaviour of the building envelope it is necessary to develop a predictive model, precise enough, to be integrated in a simulating tool. Detailed and multidimensional models, based in numerical methods, like Finite Element Method (FEM), has a high precision, but its complexity imply resources consumption and computational time, too high to be integrated in these kind of tools. On the contrary, simplified methods are good enough because they are simple and fast, with an acceptable precision in almost all the situations. The present work is focused: (a) Firstly, to develop a simplified RC-network model. The aim of the model is to characterize and to implement with precision the behaviour of a wall in a simulating software tool based on urban environment, (b) secondly, to express in form of equivalences, the different indoor and outdoor excitations that can exist in the building envelope, and (c) finally, to calibrate the simplified model through its characteristic parameters. For a homogeneous wall and two types of excitations, it has been obtained the characteristic parameters of the model that represent the better adjustment to the real wall. In a first step, it has been obtained the results of the proposal model and a reference model based on FEM, in terms of wall external surface heat flow. Results of both models have been compared, and the resultant characteristic parameters of the model have been obtained through an optimisation method. Results for the wall and for the excitations under analysis show: (1) Characteristic longitude ec, or capacitive node position, it is determined according to a certain value of Fo equal to 2 for both excitations, this value remains constant in time, (2) useful wall thickness, on the contrary, vary as time function, according to a logarithmic law for both excitations, although this function is different depending on the considered excitation, (3) using a constant excitation, coefficients from the previous logarithmic function depends on the range of the excitation, while these are practically independent of the lineal excitation gradient

A New Forced Convection Heat Transfer Correlation for 2D Enclosures

This work presents a new parametric correlation for 2D enclosures with forced convection obtained from CFD simulation. The convective heat transfer coefficient of walls for enclosures depends on the geometry of the enclosure and the inlet and outlet openings, the velocity and the air to wall temperature difference. However, current correlations not dependent on the above parameters, especially the position of the inlet and outlet, or the temperature difference between the walls. In this work a new correlation of the average Nusselt number for each wall of the enclosure has been developed as a function of geometrical, hydrodynamic and thermal variables. These correlations have been obtained running a set of CFD simulations of a 3 m high sample enclosure with an inlet and outlet located at opposite walls. The varying parameters were: a) the aspect-ratio of the enclosure (L/H = 0.5 to 2), b) the size of the inlet and outlet (0.05 m to 2 m), c) the inlet and outlet relative height (0 m to 3 m high), and d) the Reynolds number (Rein = 103 to 105). Furthermore, a parametric analysis has been performed changing the temperature boundary conditions at the internal wall and founds a novel correlation function that relates different temperatures at each wall. A specifically developed numerical model based on the SIMPLER algorithm is used for the solution of the Navier–Stokes equations. The realisable turbulence k-ε model, and an enhanced wall-function treatment have been used. The heat transfer rate results obtained are expressed through dimensionless correlation-equations. All developed correlations have been compared with CFD simulations test cases obtaining a R2 = 0.98. This new correlation function could be used in building energy models to enhance accuracy of HVAC demands calculation and estimate the thermal load

Influence of the Fractal Geometry on the Mechanical Resistance of Cantilever Beams Designed through Topology Optimization

In this work, the complex geometry of beams obtained from topology optimization is characterized through the fractal dimension (FD). The fractal dimension is employed as an efficiency measure of the mass distribution in the beams, that is, the capacity of the optimized solutions to be efficiently distributed in the design space. Furthermore, the possible relationships between the fractal dimension and beams’ mechanical properties are explored. First, a set of theoretical beams are studied based on their well-known fractal dimension. A 3D fractal called Menger sponge is reproduced on a Michell’s beam (cantilever with a single force applied at the end). The programming codes that generate those beams are created in Matlab software, as are the algorithms for estimating the fractal dimension (box-counting method). Subsequently, identical beams are modelled in the software Inspire in order to apply the topology optimization and determine the mechanical parameters from the static analysis. Results indicate that the fractal dimension is affected by the design geometry and proposed optimized solutions. In addition, several relationships among fractal dimension and some mechanical resistance parameters could be established. The obtained relations depended on the objectives that were initially defined in the topology optimization

Comparativa de ciclos de refrigeración con compresión por eyector en términos del coeficiente de rendimiento. Caracterización del eyector mediante el ratio de entrada y la eficiencia de compresión

La mayor parte de la energía que se consume en el funcionamiento de equipos de refrigeración proviene de combustibles fósiles cuyas reservas se están agotando. El propósito de este artículo es el de mostrar los potenciales beneficios del uso de sistemas de refrigeración con compresión por eyector para mejorar la eficiencia energética de éstos. Los eyectores también presentan otra serie de ventajas respecto a los compresores convencionales como son su bajo coste de fabricación y mantenimiento. Se ha llevado a cabo una revisión de las posibles configuraciones de ciclos de refrigeración donde se puede aplicar el eyector y se han comparado con un ciclo convencional para las mismas condiciones de trabajo y potencia térmica, así como para distintos refrigerantes de interés (R134a, R1234yf, R600a). Los resultados mostraron mejoras en el coeficiente de rendimiento de hasta un 45%. El eyector ha sido caracterizado mediante correlaciones su ratio de flujo de entrada y una nueva definición de la eficiencia de compresión. Estas correlaciones han sido obtenidas gracias a un método de análisis cuasi-unidimensional. Los sistemas de refrigeración han mostrado ser una potencial alternativa a los sistemas convencionales de refrigeración.Este estudio ha sido parcialmente financiado por el proyecto FEDER INTERCONECTA “Sistemas de climatización eficientes de capacidad variable para autobuses eléctricos”