Liquid water formation and flooding in PEM fuel cell gas distribution channels can significantly degrade fuel cell performance by causing substantial pressure drop in the channels and by inhibiting the transport of reactants to the reaction sites at the catalyst layer. A better understanding of the mechanisms of discrete water droplet transport by air flow in such small channels may be developed through the application of quantitative flow visualization techniques. This improved knowledge could contribute to improved gas channel design and higher fuel cell efficiencies. An experimental investigation was undertaken to gain better understanding of the relationships between air velocity in the channel, secondary rotational flows inside a droplet, droplet deformation, and threshold shear, drag, and pressure forces required for droplet removal. Micro-digital-particle-image-velocimetry (micro-DPIV) techniques were used to provide quantitative visualizations of the flow inside the liquid phase for the case of air flow around a droplet adhered to the wall of a 1 mm x 3 mm rectangular gas channel model. The sidewall against which the droplet was adhered was composed of PTFE treated carbon paper to simulate the porous GDL surface of a fuel cell gas channel. Visualization of droplet shape, internal flow patterns and Velocity measurements at the central cross-sectional plane of symmetry in the droplet were obtained for different air flow rates. A variety of rotational secondary flow patterns within the droplet were observed. The nature of these flows depended primarily on the air flow rate. The peak velocities of these secondary flow fields were observed to be around two orders of magnitude below the calculated channel-averaged driving air velocities. The resulting flow fields show in particular that the velocity at the air-droplet interface is finite. The experimental data collected from this study may be used for validation of numerical simulations of such droplet flows. Further study of such flow scenarios using the techniques developed in this experiment, including the general optical distortion correction algorithm developed as part of this work, may provide insight into an improved force balance model for a droplet exposed to an air flow in a gas channel.

Liquid water formation and flooding in PEM fuel cell gas distribution channels can significantly degrade fuel cell performance by causing substantial pressure drop in the channels and by inhibiting the transport of reactants to the reaction sites at the catalyst layer. A better understanding of the mechanisms of discrete water droplet transport by air flow in such small channels may be developed through the application of quantitative flow visualization techniques. This improved knowledge could contribute to improved gas channel design and higher fuel cell efficiencies. An experimental investigation was undertaken to gain better understanding of the relationships between air velocity in the channel, secondary rotational flows inside a droplet, droplet deformation, and threshold shear, drag, and pressure forces required for droplet removal. Micro-digital-particle-image-velocimetry (micro-DPIV) techniques were used to provide quantitative visualizations of the flow inside the liquid phase for the case of air flow around a droplet adhered to the wall of a 1 mm x 3 mm rectangular gas channel model. The sidewall against which the droplet was adhered was composed of PTFE treated carbon paper to simulate the porous GDL surface of a fuel cell gas channel. Visualization of droplet shape, internal flow patterns and Velocity measurements at the central cross-sectional plane of symmetry in the droplet were obtained for different air flow rates. A variety of rotational secondary flow patterns within the droplet were observed. The nature of these flows depended primarily on the air flow rate. The peak velocities of these secondary flow fields were observed to be around two orders of magnitude below the calculated channel-averaged driving air velocities. The resulting flow fields show in particular that the velocity at the air-droplet interface is finite. The experimental data collected from this study may be used for validation of numerical simulations of such droplet flows. Further study of such flow scenarios using the techniques developed in this experiment, including the general optical distortion correction algorithm developed as part of this work, may provide insight into an improved force balance model for a droplet exposed to an air flow in a gas channel.

[pt] O presente trabalho consiste no projeto, construção e qualificação de um aparato para o estudo experimental de bolhas de separação laminar sobre uma placa plana. A previsão do comportamento das bolhas de separação é importante para o projeto de aeronaves, turbinas e geradores eólicos, pois o desprendimento dessas bolhas tem grande impacto na performance de aerofólios. A dinâmica das bolhas não é bem compreendida, além de ser difícil de ser capturada por simulações numéricas que utilizam modelagem da turbulência. Por isso, ensaios experimentais são muito utilizados para a calibração dos modelos utilizados em simulações de engenharia. Neste trabalho, foram criados equipamentos para simular experimentalmente bolhas de separação laminar sobre uma placa plana. Os dispositivos foram projetados para o canal de água do Laboratório de Engenharia de Fluidos da PUC-Rio. O gradiente de pressão necessário para induzir a separação da camada limite e consequente formação da bolha foi ajustado com uma parede falsa, de modo a criar um canal convergente-divergente com a placa plana. Foi projetado um mecanismo de sucção da camada limite na parede falsa para evitar a separação do escoamento nessa superfície. A localização e as vazões de cada ponto de sução foram determinadas com o auxílio de simulações numéricas. Também foi projetado e testado um sistema de sopro e sucção para a excitação de ondas de instabilidade do tipo Tollmien-Schlichting na camada limite da placa plana. O funcionamento de cada dispositivo foi avaliado através da medição da velocidade do escoamento. Para isso, foram empregadas técnicas de medição por velocimetría laser doppler e velocimetría por imagem de partículas. Os resultados obtidos validam o projeto e qualificam o aparato para o estudo de bolhas de separação laminar. / [en] The present work involves the design, construction and performance test of an apparatus for the investigation of laminar separation bubbles in a flat plate boundary layer. Laminar separation bubbles are relevant for many engineering applications and the dynamic of such bubbles has a strong impact on the performance of aircrafts and turbines. The separated boundary layer reattaches to the surface due to the laminar-turbulent transition in the bubble region. This dynamic process is highly challenging for flow simulation tools used for engineering purposes. Thus, there is a demand for experimental studies that can be used for calibration of models present in those simulation tools. To this end, an apparatus was designed and built for the water channel of the Laboratory of Fluid Engineering at PUC-Rio. The boundary layer separation on the flat plate was induced by imposing a constant adverse pressure gradient to the flow. To this end a false wall was built, in order to form a converging-diverging channel with the flat plate. Flow separation on the false wall was avoided using a suction mechanism that was designed to reduce locally the boundary layer thickness. Location of suction and suction flow rates were determined with aid of numerical simulations. In addition, it was designed and built a disturbance source to generate Tollmien-Schlichting waves in the boundary layer of the flat plate. This device was used to trigger the boundary layer transition in a controlled manner. All equipment were tested and their designs were validated against experimental measurements. Laser Doppler anemometry and Particle Image Velocimetry techniques were adopted for assessment of each equipment. Results validate the design and show that separation bubbles can be investigated in detail using this apparatus.

To quench a thermal runaway reaction in a chemical rector, an efficient approach is the introduction of a small quantity of a liquid inhibiting agent, named a “killer”, into the mixing vessel. In this thesis, an experimental approach has been coupled tightly with numerical modelling using Computational Fluid Dynamics (CFD). The first part of this thesis is devoted to a study of the hydrodynamics of partially-baffled mixing vessels, including the free-surface deformation caused by the central vortex. The use of an inhomogeneous, multiphase approach allowed simulation of the free-surface deformation. The capability of this novel method was demonstrated by very good agreement between the numerical predictions and experimental data. In the second part, liquid jet injection at the free-surface was coupled with the vessel hydrodynamics. Numerical results, obtained using an Eulerian-Lagrangian approach, have again shown good agreement with experimental data. These results allowed the jet trajectory to be modelled and its penetration into the agitated vessel was quantified. New mixing criteria were introduced that are specific to this application. Finally, the numerical methods validated at the pilot scale were applied at the industrial scale and allowed the proposal of practical improvements to the safety of the synthesis reactors studied

No description available.

Cette étude est consacrée à l'élaboration d'une méthodologie de détermination des vitessesfondamentales des flammes laminaires, en utilisant un diagnostic de Vélocimétrie par Imagerie deParticules (PIV). Ce dernier est appliqué aux écoulements réactifs avec point de stagnation, permettant lastabilisation de flammes planes, stationnaires et en conditions quasi adiabatiques. Les effets d’étirementssubits par la flamme sont également quantifiables et parfaitement maîtrisés. L’approche ici développée atout d’abord été appliquée aux mélanges méthane/air pour validation. Une comparaison exhaustive desrésultats obtenus avec les données de la littérature est effectuée. Les codes de combustion 1D (PREMIX,OPPDIF) et 2D (Fluent©) ont été utilisés afin de confirmer la fiabilité et la précision de l’approche proposée.Une attention particulière a été accordée à la caractérisation du mouvement des particules ensemencéesdans les écoulements réactifs divergents, avec notamment la prise en considération de la force dethermophorèse. La méthode développée a ensuite été appliquée à la détermination des vitesses deflammes laminaires de divers mélanges de syngas (H2+CO). Une étude comparative sur ces mélanges aété conduite en utilisant des approches expérimentales multiples comprenant : les flammes à contrecourant,les flammes à propagation sphérique ainsi que les flammes stabilisées coniques. Les résultatsobtenus pour chaque approche ont été confrontés et la sensibilité à l’étirement des flammes de syngas aété caractérisée pour une large gamme de richesses (E.R.=0.4 to 5.0) et de compositions de mélanges(5/95 to 50/50 % H2/CO). / In the context of CO2 emission reduction, the present study is devoted to the development of alaminar flame speed measurement methodology, using the Digital Particle Image Velocimetry (DPIV)diagnostic. The latter is applied to stagnation flow flames, seen to have considerable assets for suchstudies. Indeed, flames stabilized in these diverging flows are planar, steady and in near-adiabaticconditions, while subtraction of strain effects on flame is intrinsically allowed. The methodology developedherein has been applied to the well-characterized methane/air mixtures for validation. An extensivecomparison with the literature datasets has been provided. Both 1D (PREMIX, OPPDIF) as well as 2D(Fluent©) numerical tools have been used to confirm the reliability and accuracy of the developed approach.A particular attention has been given to the characterization of the seeding particle motion within thediverging flow, with consideration of the often-neglected thermophoretic force. Fundamental flame velocitiesof various syngas (H2+CO) mixtures have been investigated using multiple experimental approachesincluding the aforementioned counterflow methodology as well as spherical and conical flameconfigurations. Performed measurements from the different approaches have been confronted and flamesensitivities to stretch have been characterized for a wide range of equivalence ratios (E.R.=0.4 to 5.0) andmixture compositions (5/95 to 50/50 % H2/CO).

La propagation d'une onde acoustique en guide est associée, pour de forts niveaux, à un certain nombre de phénomènes de l'acoustique non linéaire. Parmi ces phénomènes, les écoulements redressés (ou vent acoustique), l'effet d'une discontinuité et la transition à la turbulence, à l'étude dans ce mémoire, sont associés à la génération d'écoulements induits. L'étude expérimentale de ces phénomènes repose sur l'adaptation des méthodes de vélocimétrie Laser : Vélocimétrie Laser par effet Doppler (VLD) et Vélocimétrie par Images de Particules (PIV) à la mesure des différents écoulements. Ainsi, des mesures PIV en sortie de convergent, viennent compléter des mesures VLD réalisées antérieurement. Dans l'espoir de mieux appréhender les spécificités de la transition à la turbulence en guide d'onde acoustique, l'évolution de la couche limite de Stokes est étudiée pour des amplitudes de vitesse acoustique croissantes. Une étude expérimentale des écoulements redressés dans un guide d'onde à section carrée est proposée et les spécificités liées à cette géométrie sont recherchées. En outre, l'évolution des tourbillons du vent acoustique en guide d'onde cylindrique est analysée lorsque le vent devient rapide et certains facteurs pouvant être à l'origine de cette évolution sont modifiés. La répartition harmonique dans le guide est ainsi modifiée, puis l'influence des conditions thermiques est abordée en couplant les mesures de vitesses à des mesures de température moyenne dans le guide et en paroi. Une comparaison avec des résultats issus de simulations numériques permet de conforter l'évolution des écoulements redressés observée. / High amplitude acoustic propagation in a guide is associated with several non linear phenomena including acoustic streaming, discontinuity effects and transition to turbulence. Those phenomena are studied in this work and are all associated with acoustically induced flows. The present experimental study therefore is based on velocimetry laser techniques: Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV), wich are fitted to the measurement of the different flow velocity components. Firstly, PIV measurements at the exit of a convergent enable to complement previous LDV measurements. Then, in order to a better understanding of the specificity of transition to turbulence in acoustics, the evolution of the Stokes boundary layer is studied for increasing acoustic velocity amplitudes. Then an experimental study of acoustic streaming in a square channel is reported, and the influence of the geometry is examined. Moreover, the evolution of acoustic streaming vortices in a cylindrical waveguide is analyzed for fast streaming and some parameters that could control such evolution are modified. The harmonicdistribution inside the guide is changed and then the influence of thermal conditions is studied by coupling velocity measurements and mean temperature measurements inside the waveguide and along the wall. Some comparisons between measured streaming velocities and numerical simulation results are presented.

[pt] No presente trabalho foi realizado um estudo experimental sobre escoamento bifásico em regime intermitente através de tubos horizontais e de pequena inclinação, com o objetivo de determinação dos campos de velocidade instantâneos nas regiões a montante do nariz, a jusante da cauda das bolhas de gás, assim como na região do filme de líquido sob as bolhas. Foi implementada uma combinação de três técnicas ópticas não invasivas. Velocimetria por Imagem de Partículas bi-dimensional (Particle Image Velocimetry – PIV) foi utilizada para determinar os campos de velocidade instantâneos nas regiões de interesse, enquanto luz de fundo pulsada e sincronizada proveniente de uma matriz de LED’s vermelhos iluminava os contornos das bolhas aumentando o contraste das interfaces líquido-gás (Pulsed Shadow Technique – PST). Uma técnica baseada na fluorescência induzida por laser foi utilizada (Laser Induced Fluorescence - LIF) para separar a luz verde intensa proveniente do laser associado à técnica PIV. Os testes foram conduzidos em seção transparente tendo água e ar como fluidos de trabalho. Os resultados obtidos revelaram informações detalhadas sobre o escoamento de líquido no escoamento intermitente. Foram produzidos também resultados estatísticos de algumas variáveis globais do escoamento como, a velocidade e os comprimentos das bolhas e pistões de líquido, além da frequência de passagem desta estruturas do escoamento. / [en] instantaneous liquid velocity fields at the film, nose and tail regions of slugs in horizontal and inclined two phase flow. To this end, a combination of three nonintrusive optical techniques was employed. Two-dimensional particle image velocimetry (PIV) was used to measure the instantaneous liquid velocity field at a meridional plane of the horizontal pipe test section, while a synchronized pulsed back lighting, provided by a matrix of red LED s, illuminated the bubble contours thereby enhancing contrast of the interfaces (Pulsed Shadow Technique - PST). A laser-induced fluorescence technique (LIF) was employed to separate the strong reflections coming from the gas-liquid interfaces produced by the green PIV laser, The tests were conducted on a specially built transparent pipe test section, using air and water as the working fluids. The velocity fields were obtained for flow regimes where the slugs were lightly aerated. The velocity field results in the nose, tail and film regions revealed valuable detailed information that helped to better understand the physics of the flow, besides contributing to the formation of a data bank for supporting the development of two-phase, horizontal slug flow simulations. The work conducted also provided statistical information on the main global variables that characterize the flow, such as, speed and length of gas bubbles and liquid slugs, and the frequency of passage of these structures.

Variable Valve Timing (VVT) presents a powerful tool in the relentless pursuit of efficiency improvements in the internal combustion engine. As the valves have such ultimate control over the gas exchange processes, extensive research effort in this area has shown how valve event timing can be manipulated to reduce engine pumping losses, fuel consumption and engine out emissions. Pumping losses may be significantly reduced by use of throttleless strategies, making use of intake valve duration for load control, while alternative cycles such as the Miller cycle allow modification of the effective compression ratio. More recently, the value of single valve operation in part load conditions is exploited, bringing with it the concept of asymmetric valve lifts. Work in this area found the side effect of asymmetric valve operation is a significant change in the behaviour of the in-cylinder flow structures, velocities and turbulence intensity. Work presented in this thesis exploits asymmetric valve strategies to modify the in-cylinder flow conditions. The Proper Orthogonal Decomposition (POD) is a method employed in the fluids dynamics field to facilitate the separation of coherent motion structures from the turbulence. In the presented work, the application of POD to in-cylinder flow analysis is further developed by the introduction of a novel method for identifying the POD modes representative of coherent motion and those representative of the turbulence. A POD mode correlation based technique is introduced and developed, with the resulting fields showing evidence of coherence and turbulence respectively. Experimental tests are carried out using a full length optically accessible, single cylinder research engine equipped with a fully variable valve train (FVVT) to allow full control of both valve timing and lift. In-cylinder flow is measured through the use of Particle Image Velocimetry (PIV) at several crank angle timings during the intake stroke whilst the engine is operated under a range of asymmetric valve strategies. The exhaust valves and one intake valve have their respective schedules fixed, while the second intake valve schedule is adjusted to 80\%, 60\%, 40\%, 20\%, 0\% lift. The resulting PIV fields are separated into coherent motion and turbulence using the developed technique, allowing for analysis of each constituent independently. The coherent element gives insight to large scale flows, often of the order of magnitude of the cylinder. These structures not only give a clear indication of the overall motion and allow assessment of flow characteristics such as swirl and tumble ratio, but the variation in the spatial location of these structures provides additional insight to the cyclic to cycle variation (CCV) of the flow, which would not otherwise be possible due to the inclusion of the turbulent data. Similarly, with the cyclic variation removed from the turbulent velocity field, a true account of the fluctuating velocity, u' and derived values such as the Turbulent Kinetic Energy (TKE) may be gained. Results show how manipulation of a one intake valve timing can influence both the large scale motions and the turbulence intensity. By the reduction of lift, the swirl ratio is increased almost linearly as the typical counter-rotating vortex pair becomes asymmetric, before a single vortex structure is observed in the lowest lift cases. A switching mechanism between the two is identified and found to be responsible for increased levels of CCV. With the reduction in lift, TKE is observed not only to increase, but change the spatial distribution of turbulence. Of course, the reduction in valve lift comes with the penalty of a reduced valve curtain area. However, it was identified both in literature and throughout this study that the reduction in lift did not negatively influence the engine breathing as the same trapped mass was achieved under all cases with no adjustment of manifold pressure. While literature shows both bulk motion and turbulence are key in liquid fuel break-up during the intake stroke, the mixing effects under port-injected natural gas were investigated experimentally using Laser Induced Fluorescence (LIF). The valve strategy was found to have no significant effect on the mixture distribution at the time of spark.

Natural turbulent convection studies encompass a wide range of flows occurring in nature, for example, atmospheric and oceanic flows, con-vection in the Earth’s mantle, convection in the stars and also in many engineering applications. Rayleigh-Benard convection (RBC), i.e. con-vection in a horizontal fluid layer confined between two plates with a temperature differential maintained across them, has been a proto-type problem in the studies of turbulent natural convection. Many small scale and global features of the flow in the turbulent regime of RBC are known, yet the flow dynamics is not fully understood, es-pecially at high Rayleigh numbers (Ra). Present work comprises of experimental investigations of a different type of flow, high Rayleigh number turbulent convection in a long vertical tube (abbreviated as tube convection or TC). The tube of aspect ratio (length to diameter) of about 10, open at both the ends interconnects two large tanks. The flow driven by an unstable density difference created between the two tanks, has some unique features, different from RBC. The net flow at any tube cross-section is zero and the time averages of the velocities, the Reynolds shear stress and the mean shear are also zero. Turbu-lent energy production is therefore solely due to buoyancy. The flow is axially homogeneous and axisymmetric. In the homogeneous region, the mean density gradient is linear. Rayleigh number in TC is conve-niently defined based on the mean (linear) density gradient (denoted by Rag). Two sets of experiments are carried out. In one set of experiments, the density difference is created using brine and fresh water and in another set, it is created using heat. The ranges of Rag achieved are 3 × 108 < Rag < 8 × 109 in the experiments using salt (Schmidt number, Sc ≈ 600) and 5 × 104 < Rag < 5 × 106 in the experiments using heat (Prandtl number, P r ≈ 6). From the measured salt and heat fluxes in both the sets of experiments, the non dimensional flux 1 1 scaling above a certain value of Rag is obtained as N ug ∼ Rag2 P r 2 and from the velocity measurements in the experiments using salt, the 1 Reynolds number scaling is obtained as Re ∼ Rag2 P r− 12 . Both these are as per the predicted scalings by the mixing length model proposed by Arakeri et al. (2000) for high Rag convection in the vertical tube. The flux scaling N u ∼ (RaP r)2 , also known as the ‘ultimate regime’ of convection, expected at very high Ra but not yet observed in the experiments in classical RBC, is easily achieved in TC at relatively lower values of Ra. The fluxes and Reynolds numbers in TC are orders of magnitude higher as compared to those obtained in RBC for similar values of Ra and P r. In the lower range of Rag values for P r ≈ 6, a transition to a new flux scaling, N u ∼ (RaP r)0.29 is found. Similar transitions are also found to be present in the results of Tovar (2002) for Sc ≈ 600 and in the DNS results of Schmidt et al. (2012) for P r = 1, at different values of Rag. Collecting all these data, it is shown that the transition occurs at a fixed Grashof number of 1.6 × 105, independent of P r. Velocity measurements are carried out using particle image velocime-try (PIV) in the salt experiments. Kinetic energy spectra computed from the velocity fields are presented for the locations from the tube axis to the wall, for the lowest and the highest values of Rag achieved in the experiments. The spatial energy spectrum of lateral velocity at the tube axis follows Kolmogorov-Obukhov (KO) scaling (−5/3 scaling exponent) while the spatial spectrum of longitudinal velocity shows a scaling slightly higher than −5/3 but lower than −11/5 (the Bolgiano-Obukhov (BO) scaling). The scalar spectra is computed from the concentration fields obtained from planar laser induced fluorescence (PLIF) in the experiments using salt, and also from the temperature measurements from the experiments using heat. Both the concentra-tion and temperature fluctuations spectra show some evidence of dual scaling - BO scaling (−7/5 scaling exponent) in the inertial subrange followed by Obukhov-Corrsin (OC) scaling (−5/3 scaling exponent) over a narrow range of scales. Light propagation through the buoyancy driven turbulent flow in TC has also been experimentally investigated. Light propagation through convective turbulence is encountered in many situations. In some cases e.g. in observational astronomy it is undesirable, while in some other cases it is useful, e.g. in remote sensing of meteorological parameters. In the present study, light intensity and angle of arrival fluctuations in a parallel beam of light are measured. Laser shadowgraphy is used in the intensity measurements while the angle of arrival is obtained by measuring deflections of narrow laser beams, created by passing collimated laser light through a mask having equispaced grid of holes. Background oriented schlieren (BOS) measurements have also been carried out to obtain the displacements, which are proportional to the angle of arrivals. The equations for frequency spectrum of intensity and angle of arrival from the literature, developed for isotropic, ho-mogeneous turbulent media, are modified for the flow in the present case and the asymptotic scalings for high and low frequency ranges are obtained. The scalings in the frequency spectra computed from the measurements of intensity and angle of arrival fluctuations are com-pared with the obtained asymptotic scalings. The results from the present work are also compared with results from studies in the atmo-sphere and lab experiments.