O termo limitação ao fluxo expiratório (LFE) refere-se a uma condição na qual o fluxo expiratório máximo obtido durante um ciclo da respiração espontânea é menor que o previsto e permanece constante apesar do aumento do gradiente pressórico. A LFE pode estar presente durante o envelhecimento pulmonar fisiológico, e em afecções pulmonares obstrutivas, como na doença pulmonar obstrutiva crônica (DPOC). A aferição direta para determinar a LFE requer a mensuração do volume pela relação entre o fluxo e a pressão transpulmonar, porém é um método invasivo. Os testes usualmente empregados estão baseados na comparação da curva fluxo-volume expiratório máximo e corrente, porém estas técnicas requerem alto grau de colaboração do indivíduo e podem gerar alteração do tônus muscular brônquico. Estudos sugerem que a Técnica de Pressão Expiratória Negativa (NEP) pode ser aplicada para detecção da LFE, sendo um método simples, não invasivo e que não requer esforço dos voluntários. Contudo, índices quantitativos para a avaliação deste fenômeno ainda não foram definidos, assim como também não foram estudadas a LFE no processo de envelhecimento e nos diversos estágios de obstrução das vias aéreas na DPOC. Neste contexto, os objetivos deste estudo foram: (1) avaliar o comportamento da LFE durante o processo de envelhecimento e (2) estudar a LFE presente nos portadores de DPOC. Trata-se de um estudo transversal controlado com avaliação de casos prevalentes, tendo como unidade de avaliação o indivíduo. Os exames realizados incluíram medidas de espirometria e NEP. Foram selecionados indivíduos saudáveis para o grupo envelhecimento separados em três grupos: grupo jovem (GJ), n=17; grupo meia idade (GMI), n=17 e grupo idoso (GI), n=17. No grupo DPOC foram selecionados indivíduos tabagistas e com doença obstrutiva, sendo classificados de acordo com o nível de obstrução sugerido pela espirometria. Essa classificação resultou em cinco categorias: indivíduos normais ao exame espirométrico (NE, n= 18); com distúrbio ventilatório obstrutivo leve (DVOL, n=15); distúrbio ventilatório obstrutivo moderado (DVOM, n= 18); distúrbio ventilatório obstrutivo acentuado (DVOA, n= 18) e distúrbio ventilatório obstrutivo muito acentuado (DVOMA, n= 18). Todos os indivíduos realizaram os exames de NEP e posteriormente foram submetidos à espirometria. No estudo sobre envelhecimento o parâmetro LFE% foi o que melhor caracterizou a LFE, apresentando uma correlação moderada com a idade. Os parâmetros ∆EF0-50% e ∆EF25-75% apresentaram uma correlação razoável com o progredir da idade, possivelmente devido a LFE no idoso apresentar componentes relacionadas às vias aéreas superiores. No grupo DPOC a NEP caracterizou adequadamente a LFE, sendo o melhor parâmetro a LFE%. Alterações significativas também foram encontradas com os parâmetros ∆EF0-50% e ∆EF25-75%. Avaliando-se a influência da idade neste grupo, pode-se observar que a idade é um fator de contribui para a LFE, no entanto, o efeito preponderante foi a gravidade da doença. Pode-se concluir que: (1) a NEP é útil no estudo da LFE em idosos saudáveis; (2) nestes indivíduos a LFE ocorre principalmente nas vias aéreas extratorácicas; (3) que a NEP é útil na avaliação de pacientes com DPOC, e: (4) que os efeitos da DPOC se sobrepõe ao efeito do envelhecimento. / The term expiratory flow limitatition (EFL) refers to a condition in which the peak flow obtained during a spontaneous breathing cycle is less than expected and remains constant despite the increased pressure gradient. The EFL may be present during the physiological aging lung and obstructive lung diseases such as chronic obstructive pulmonary disease (COPD). Direct assessment of EFL requires determination of iso-volume relationships between flow and trans-pulmonary pressure, however, this is an invasive method. The conventional method to detect EFL is based on comparison of tidal with maximal flow-volume curves, but this test requires a high degree of pacients cooperation and the maneuvers of forced expiration and inspiration can change the bronchial tone. Previous studies suggest that the Negative Expiratory Pressure technique (NEP) can be applied to detect EFL, and that it is a simple and noninvasive technique, which requires no effort of volunteers. However, quantitative indices for evaluating this phenomenon has not been defined, as well as the EFL have not been studied in aging and in various stages of COPD airway obstruction. In this context, the aims of this study were (1) evaluate the EFL during the aging process and (2) study the EFL present in COPD. This is a cross-controlled study evaluation of prevalent cases, with the evaluation unit the individual. The examinations included measurement of NEP and spirometry. We selected healthy individuals divided in three groups: young group (GJ, n = 17); middle aged group (GMI, n = 17) and elderly group (GI, n = 17). In the COPD group we selected smoking subjects and people with obstructive disordered, classified in five categories according to the level of obstruction suggested by spirometry: normal spirometric exam (n = 18); mild obstruction (n = 15); moderate obstruction (n=18), severe obstruction (n = 18) and very severe obstruction (n=18). In the aging group, LFE% parameter was the more adequate to characterize the EFL present during the aging process, showing a moderate correlation. The parameters ∆EF0-50% and ∆EF25-75% showed a reasonable correlation with the progression of age, possibly because the EFL in the elderly have components of the upper airways. In the COPD group, the more adequate parameter to characterize the EFL was the LFE%. Significant changes were also found with the parameters ∆EF0-50% and ∆EF25-75%. Evaluating the influence of age in this group, it can be seen that age contributes for the LFE. However, the effect of the disease was higher than that associated with ageing. It can be concluded that: (1) NEP is a useful tecnique in the study of LFE in healthy elderly people, (2) In these individuals the LFE occurs mainly in the extrathoracic airways, (3) NEP tecnique is useful in evaluating patients with COPD, and: (4) the effects of COPD diseases overlaps the effect of aging.

Stratified two-phase flows were investigated at different test facilities with horizontal test sections in order to provide an experimental database for the development and validation of computational fluid dynamics (CFD) codes. These channels were designed with rectangular cross-sections to enable optimal observation conditions for the application of optical measurement techniques. Consequently, the local flow structure was visualised with a high-speed video camera, delivering data with high-resolution in space and time as needed for CFD code validation. Generic investigations were performed at atmospheric pressure and room temperature in two air/water channels made of acrylic glass. Divers preliminary experiments were conducted with various measuring systems in a test section mounted between two separators. The second test facility, the Horizontal Air/Water Channel (HAWAC), is dedicated to co-current flow investigations. The hydraulic jump as the quasi-stationary discontinuous transition between super- and subcritical flow was studied in this closed channel. Moreover, the instable wave growth leading to slug flow was investigated from the test section inlet. For quantitative analysis of the optical measurements, an algorithm was developed to recognise the stratified interface in the camera frames, allowing statistical treatments for comparison with CFD calculation results. The third test apparatus was installed in the pressure chamber of the TOPFLOW test facility in order to be operated at reactor typical conditions under pressure equilibrium with the vessel atmosphere. The test section representing a flat model of the hot leg of the German Konvoi pressurised water reactor (PWR) scaled at 1:3 is equipped with large glass side walls in the region of the elbow and of the steam generator inlet chamber to allow visual observations. The experiments were conducted with air and water at room temperature and maximum pressures of 3 bar as well as with steam and water at boundary conditions of up to 50 bar and 264°C. Four types of experiments were performed, including generic test cases as well as transient validation cases of typical nuclear reactor safety issues. As an example, the co-current flow experiments simulate the two-phase natural circulation in the primary circuit of a PWR. The probability distribution of the water level measured in the reactor pressure vessel simulator was used to characterise the flow in the hot leg. Moreover, the flooding behaviour in this conduit was investigated with dedicated counter-current flow limitation experiments. A comparison of the flooding characteristics with similar experimental data and correlations available in the literature shows that the channel height is the characteristic length to be used in the Wallis parameter for channels with rectangular cross-sections. Furthermore, for the analysis of steam/water experiments, condensation effects had to be taken into account. Finally, the experimental results confirm that the Wallis similarity is appropriate to scale flooding in the hot leg of a PWR over a large range of pressure and temperature conditions. Not least, different examples of comparison between experiment and simulation demonstrate the possibilities offered by the data to support the development and validation of CFD codes. Besides the comparison of qualitative aspects, it is shown exemplarily how to treat the CFD results in order to enable quantitative comparisons with the experiments.

In the event of hypothetical accident scenarios in PWR, emergency strategies have to be mapped out, in order to guarantee the reliable removal of decay heat from the reactor core, also in case of component breakdown. One essential passive heat removal mechanism is the reflux condensation cooling mode. This mode can appear for instance during a small break loss-of-coolant-accident (LOCA) or because of loss of residual heat removal (RHR) system during mid loop operation at plant outage after the reactor shutdown. In the scenario of a loss-of-coolant-accident (LOCA), which is caused by the leakage at any location in the primary circuit, it is considered that the reactor will be depressurized and vaporization will take place, thereby creating steam in the PWR primary side. Should this lead to ¿reflux condensation¿, which may be a favorable event progression, the generated steam will flow to the steam generator through the hot leg. This steam will condense in the steam generator and the condensate will flow back through the hot leg to the reactor, resulting in counter-current steam/water flow. In some scenarios, the success of core cooling depends on the behaviour of this counter-current flow. Over several decades, a number of experimental and theoretical studies of counter-current gas¿liquid two-phase flow have been carried out to understand the fundamental aspect of the flooding mechanism and to prove practical knowledge for the safety design of nuclear reactors. Starting from the pioneering paper of Wallis (1961), extensive CCFL data have been accumulated from experimental studies dealing with a diverse array of conditions A one-dimensional two field model was developed in order to predict the counter-current steam and liquid flow that results under certain conditions in the cold leg of a PWR when a SBLOCA (small break loss of coolant accident) in the hot leg is produced. The counter-current model that has been developed can predict the pressure, temperature, velocity profiles for both phases, also by taking into account the HPI injection system in the cold leg under a counter-current flow scenario in the cold leg. This computer code predicts this scenario by solving the mass, momentum and energy conservation equations for the liquid and for the steam separately, and linking them by using the interfacial and at the steam wall condensation and heat transfer, and the interfacial friction as the closure relations. The convective terms which appear in the discretization of the mass and energy conservation equations, were evaluated using the ULTIMATE-SOU (second order upwinding) method. For the momentum equation convective terms the ULTIMATE-QUICKEST method was used. The steam-water counter-current developed code has been validated using some experimental data extracted from some previously published articles about the direct condensation phenomenon for stratified two-phase flow and experimental data from the LAOKOON experimental facility at the Technical University of Munich. / Romero Hamers, A. (2014). STUDY OF THE THERMAL STRATIFICATION IN PWR REACTORS AND THE PTS (PRESSURIZED THERMAL SHOCK) PHENOMENON [Tesis doctoral]. Editorial Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/36536 / Alfresco