Estudios del flujo laminar en conductos sometidos a cambio térmicoexperimentación en captadores de energía solar térmica

Laburpena

This doctoral thesis is structured into five closely related chapters, each based in part on the conclusions and results of the previous one. The general objective has been to simulate the temperature profiles inside the circulating fluid through tubes of a flat solar thermal collector. The first chapter raises the theoretical and experimental bases on which this research is based. The second chapter expands the theoretical basis applying the Discriminated Dimensional Analysis to the study of pipelines under the conditions of the Graetz Problem, and acts as a link between the general foundations and the concrete investigation of solar collectors. After a numerical analysis, a new design variable for the heat transfer fluid in ducts, denominated characteristic length, is proposed. From it, the profiles of the universal temperature and heat curves of the circulating fluid have been obtained. In the third chapter, the three-dimensional Graetz Problem has been extended to break the symmetry of the problem bringing it closer to the actual thermal phenomenon through variations in the velocity profile and in the boundary conditions. In the fourth chapter, simultaneously to the previous ones, the process of taking experimental measures, useful for the scientific community, as well as the data analysis based on the standard EN-12975: 2006 is described. The data collected in the points indicated by the standard are compared with the other data obtained in this experiment, taken in more than 80 points. The experimental data are used to compare and validate the results of the numerical simulation models elaborated in this research. The fifth chapter presents the basis of a 3D simulation model which contemplates all the considerations and conclusions of all the previous chapters. Proposals are made to refine the model by adding boundary conditions for future research. In the general conclusions of the thesis it is summarized that the numerical simulation models proposed in the second and third chapter solve different problems. These models approximate the results more to the experimental ones than those found in the scientific literature in the four studied transversal sections of measurement. The numerical simulation model proposed in chapter five approximates the results to those measured experimentally in chapter four even better than previous models because a constant heat contour condition has been imposed. Concerning the experimental study, two alternative methods have been provided to that indicated in the standard in order to obtain the average temperature of the absorber of a solar collector, one of them is more approximate and the other one is more operative than the one indicated in the standard. It is provided a comparison of thermal efficiencies of two different systems of plate-tube bonding. The assembly consisting of copper tubes welded by points to an aluminum absorber is more efficient than the one formed by copper tubes mechanically embraced and welded with tin to a copper absorber. The static and dynamic analyses of the elements of the collector provide a description of the thermal inertia of each of these elements, finding that the effects of the back cover, glazed surface and insulation should be considered on the overall inertia of the collector. It is confirmed that indoor air has no influence on the forecast. An important agreement has been obtained between the predictions made with the new quasi-dynamic model and the actual results observed. Finally, the annexes complete the results obtained in some chapters, substantiate part of the methodology used throughout the research, and add suggestions for the use of this methodology and results applied to teaching in technical education.