Numerical simulation and experimental validation of vapor compression refrigerating systemsspecial emphasis on natural refrigerants

Resumo

The aim of this work is to study the thermal and fluid-dynamic behavior of vapor compression refrigerating systems and their constitutive elements (heat exchangers, expansion devices, compressors and connecting tubes) focused on the use of natural refrigerants (carbon dioxide, isobutane and ammonia). The specific topics analyzed throughout this thesis have arisen from the growing interest in environmentally friendly refrigerants that has led the CTTC group (Centro Tecnológico de Transferencia de Calor) to undertake significant research efforts and to take part in several projects with national and European institutions. The information reported herein represents a summary of the work carried out by the author during the last years together with the contributions provided by other members of the CTTC group. This thesis has led to the creation of some publications in International Conferences and indexed journals. The main achievement of this work has been the development of a flexible numerical tool based on several subroutines. The whole numerical infrastructure is the result of coupling the specific resolution procedures for each vapor compression refrigerating system component together with the whole system resolution algorithm. The simulations have been oriented to study the system thermodynamical characteristics as well as some relevant aspects of its particular elements. In addition to the numerical results a significant experimental work has also been carried out in the CTTC facilities due to the need for experimental validation. The author has been fully involved in data acquisition procedures and has also collaborated in the setting up of the experimental units. In general, all the studies conducted in this work have been presented following the same structure: i) numerical model and resolution procedure explanation; ii) model validation against experimental data; and iii) simulation results. The specific topics tackled within this thesis include the implementation of a two-phase numerical model to simulate the thermal and fluid-dynamic behavior of single- and two-phase flows inside ducts, the study of heat transfer coefficient empirical correlations for both cooling of carbon dioxide at transcritical conditions and evaporation of ammonia at overfeed conditions, the implementation of a numerical model to simulate capillary tubes in order to study their behavior at typical operational conditions found in household refrigerators working with isobutane, the development of a two-phase flow distribution model to simulate heat exchangers made up by manifold systems, and the study of vapor compression refrigeration cycles with special emphasis on carbon dioxide transcritical situations. The set of the numerical models implemented has demonstrated to be a flexible tool as several different aspects of refrigeration vapor compression systems have been successfully simulated and study. It has also demonstrated to be an accurate tool as the numerical results achieved have shown good agreement against experimental data