Um dos atributos fundamentais associados aos escoamentos multifásicos é a existência de estruturas características segundo as quais as diferentes fases do líquido escoam. O surgimento de uma dessas estruturas, conhecidas como configurações ou regimes de escoamento, é determinado pelas vazões e propriedades físicas dos componentes, além de parâmetros geométricos como diâmetro e inclinação do conduto. O desenvolvimento de metodologias de caracterização de regimes, bem como a caracterização e o diagnóstico da transição destes regimes de escoamento são de fundamental importância. Este trabalho utiliza a análise tempo-frequência da transformada de Gabor para caracterizar os regimes de escoamento horizontais gás-líquido intermitentes. Mais especificamente, o principal objetivo está em investigar a existência de sub-regimes dentro do regime intermitente, para tanto recorremos à covariância tempo-frequência da transformada de Gabor, que é capaz de detectar transições através da não-estacionaridade associada com as correspondentes transições. Testes experimentais foram conduzidos no circuito TALC em CEA-Grenoble e uma extensiva base de dados foi obtida, cobrindo diversos tipos de escoamento intermitente. Uma sonda de condutividade elétrica, consistindo de dois anéis de eletrodos montados junto à tubulação, produziu sinais dos quais a covariância tempo-frequência foi calculada através da correspondente transformada de Gabor. / One of the main features associated to multiphase flows is the existence of characteristic dynamic structures according to which the different phases of a mixture of immiscible fluids can flow. The manifestation of one of these structures, known a flow pattern or regime, is determined by the flow rates as well as by physical and geometrical properties of the fluids and piping. The development of flow pattern characterization and diagnostic methods, and the associated transitions in between, is of crucial importance for an efficient engineering of such phenomena. Time-frequency analysis based on the Gabor transform is used in this work to characterize horizontal air-water intermittent flow regimes. More specifically, our main objective is to reveal the existence of sub-regimes inside the intermittent regimes region with the help of the corresponding time-frequency covariance based on the Gabor transform, which is capable of detecting transitions by assessing the unstationarity associated with the corresponding transitions. Experimental tests were conducted at the TALC facility at CEA-Grenoble and an extensive database was obtained, covering several types of intermittent flow. A conductivity probe, consisting in two ring electrodes flush mounted to the pipe, delivered signals from which the time-frequency covariance were calculated from the corresponding Gabor transform.

Slot-rectangular spouted beds, with rectangular cross-section and slotted gas inlets, have been proposed as a solution to overcoming scale-up difficulties with conventional axisymmetric spouted beds. They can be utilized in gas/particle processes such as drying of coarse particles and coating of tablets. However, application of this spouted bed was limited because of instability and insufficient hydrodynamic studies. The present work is therefore aimed at the study of hydrodynamics, stability and scale-up of slot-rectangular spouted beds. The hydrodynamic study was carried out in four slot-rectangular columns of various width-to-thickness ratios combined with various slot configurations, particles of different properties and a range of operating conditions. Hydrodynamics of slot-rectangular spouted beds showed major similarity with conventional spouted beds. However, equations and mechanistic models adopted from conventional axisymmetric spouted beds generally failed to provide good predictions for the three-dimensional slot-rectangular geometry. New empirical correlations were derived for the minimum spouting velocity and maximum pressure drop for different slot configurations. Slot-rectangular spouted beds also showed more flow regimes than conventional spouted beds. Nine flow regimes, as well as unstable conditions, were identified based on frequency and statistical analysis of pressure fluctuations. Slot geometrical configuration was found to be the main factor affecting the stability of slot-rectangular spouted beds. A comprehensive hydrodynamic study on the effect of slot configuration was therefore carried out. Slots of smaller length-to-width ratio, smaller length and greater depth were found to provide greater stability. Stable criteria for the slot configuration were found consistent with the conventional axisymmetric spouted beds with extra limitation on slot length-to-width ratio and slot depth. Local distributions of pressure, particle velocity and voidage, as well as spout shape and particle circulating flux, were compared for different slot configurations. Higher slot length-to-width ratios lead to slightly higher particle circulation rates. A previously proposed scale-up method involving multiple chambers was tested in the present work using multiple slots. Instability caused by the merging of multiple spouts and asymmetric flow was successfully prevented by suspending vertical partitions between the fountains. Some criteria and guidelines were also proposed for scale-up using multiple chambers. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Crashback operation of a propeller is a common emergency slowing maneuver for ships and submarines. The reversing of the propeller while the vessel is moving forward results in large loads on the propeller blades and highly detached flow, which presents both practical concerns and fundamental fluid physics inquiries. This thesis contains a comprehensive numerical analysis of two propellers in crashback operation. Available numerical and experimental data for David Taylor Model Basin (DTMB) 4381 propeller are used for validation of the computational fluid dynamics solver used, REX. A second propeller, Maritime Research Institute Netherlands (MARIN) 7371R is used to classify the common crashback flow behavior into regimes. Four regimes were identified, each existing for a range of operating conditions. The most prominent and deciding feature of the flow regimes is the presence of a ring vortex, resulting from the opposing action of the free-stream flow and the propeller induced flow. The position, shape and strength changes between regimes, dominating the dynamics of the flow by altering the induced flow into the propeller disk. Flow conditions resulting from regime transitions are described. Changes in the ring vortex structure lead to two stable flow conditions of interest. One condition produces a reduction of thrust despite the increase in flow speed into the propeller and negligible side-forces. The other condition creates large side-forces capable of rotating a vessel, resulting from an asymmetry forming in the ring vortex. Additionally, massive flow separation occurs at high free-stream speeds that cause extreme blade loading. An extensive description of each flow regime is provided, with further investigation and discussion of the flow regimes that present more practical concerns and novel characteristics of the crashback flow.

Hydrogen Sulfide (H2S) is a harmful gas produced during petroleum extraction that leads to corrosion of drilling tools and pipelines. However, a H2S-scavenging liquid compound, when added to pipelines, interacts with liquids that absorbed H2S to create a non-corrosive bi-product. The interaction is associated with the mixing of gases and liquids. This thesis is a study on the effect of superficial gas and liquid velocities on the scavenger's efficiency. This study employs two experimental setups designed to simulate the mixing of gases and liquids within pipelines. A high pressure closed loop was designed and fabricated to determine the influence of gas, liquid velocities and liquid volume on the scavenger's efficiency. All experiments were conducted in this high pressure loop with a thousand feet of coiled tubing to simulate the horizontal section of the pipeline that runs along the ocean floor from the reservoir. This provided practical understanding to petroleum companies to make a better forecast of how the scavenger used in eliminating the H2S, is affected in the process of transporting the liquids and gases from the reservoir to the surface. For an adequate analysis, experiments on four liquid and four gas velocities ranging from 0.2m/s to 0.5m/s and 0.4m/s to 1.1m/s respectively were conducted. Results in this study indicated that increases in superficial gas velocity at low superficial liquid velocity decreases the scavenger efficiency while the opposite is seen at high superficial liquid velocity. In addition, the H2S mass absorption was not a function of liquid volume as would be seen in static reservoirs but more of a function of superficial liquid and gas velocities. With the scavenger interacting with the liquid absorbed H2S, it was expected that the efficiency would increase with the increase in volume but in this study this was not the case. The second experiment is a flow visualization loop which was designed to understand the flow regimes at high pressures. This was done by constructing four 25ft section hoses together with four foot long breaks for visualization. This provided a more fundamental study of the fluid's behavior inside the pipelines allowing for the creation of appropriate flow regime maps in air-water flow. A hundred experiments for two different pressures were conducted at the 25ft location. At high pressures, the flow regime map appeared to shift the transition zones.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Since the industrial processes efficiency depends on the granular flow regime established under given operating conditions on the rotary drum, the ability to predict the particle motion inside this equipment, including the particle properties effect, is of primary importance. So, in this work an experimental and numerical study was carried out in order to investigate: the transition phenomenon between different flow regimes, the mixture and segregation phenomenon and the particle dynamic behavior inside an unbaffled rotary drum, under different operating conditions, using particles of different physical properties. A modification of the Blumberg and Schlünder model equation for rolling-cascading transition was proposed by the introduction of the particle shape effect, represented here by the sphericity. It was observed, for the first time, the hysteresis phenomenon in the transition between cataractingcentrifuging regimes which was shown to be dependent on the physical properties of the particles such as sphericity, density and particle-wall friction coefficient. A new expression relating the critical rotation speed for centrifuging as a function of the filling degree, which takes into consideration the particle properties and the hysteresis effects, was proposed. Regarding the segregation phenomenon, radial segregation due to particle diameter and density differences was observed in all systems studied after a few drum rotations. Size induced axial segregation (banding) was observed, as expected. However density differences alone did not induce axial segregation. As regards the numerical investigation, two different approaches were used for the granular flow prediction inside a rotary drum: Eulerian and Lagrangian. For the Eulerian approach, the results indicated that the kinetic model, which has been successfully applied in many dilute granular flow simulations, may also be applied in the dense granular flow treatment present in rotary drums. It was also observed that, the drag force can be neglected in the case of a rotating drum operated in the rolling regime where there is no fluid entering or leaving the system. Taking the computational efforts into account, this force can be set to zero in the entire calculation domain. For the Lagrangian model calibration, a sensitivity analysis of the numerical dynamic angle of repose due to variations in the friccional coefficient (μf) and the damping ratio (β), both varying from 0.149 to 0.701, was assessed using a Central Composite Design. The smallest deviation from the experimental data when using rice grains was obtained in the simulation whose parameters values were μf = 0.425 and β = 0.149 with an error of about 2.9%. As regards the glass beads, the smallest deviation between experiment and simulation was found using a friction coefficient μf = 0.701 and damping ratio β = 0.425 with an error of about 3.4%. For the case of the rotary drum using rice grains, which are characterized by irregular shapes, the calibrated model was affected by neither the filling degree nor the drum rotation speed. On the other hand, in the case of rounded particles (glass beads), the Lagrangian model parameters should be calibrated to specific conditions of rotation speed and filling degree and cannot be generalized. / A eficiência dos processos industriais desenvolvidos em um tambor rotatório depende da forma como as partículas ou grãos movimentam-se em seu interior. O entendimento dos principais mecanismos que regem estes movimentos e a capacidade de prevê-los através de técnicas de modelagem computacional tornam-se de importância primordial para questões de operação, projeto e otimização. Neste sentido, este trabalho teve como foco a aplicação de metodologias experimentais e numéricas com o intuito de estudar: 1) o fenômeno de transição entre os diferentes regimes de escoamento; 2) mistura e segregação; 3) dinâmica de partículas em um tambor rotatório sob diferentes condições operacionais utilizando partículas com diferentes propriedades físicas. Foi proposta uma nova expressão para a velocidade crítica de rotação para centrifugação função, além do grau de preenchimento do tambor, das propriedades físicas das partículas. Uma modificação no modelo desenvolvido por Blumberg e Schlünder (1996) para a transição rolamento-cascateamento através da inclusão do efeito da forma das partículas foi também introduzida. Observou-se, pela primeira vez na literatura, o fenômeno de histerese quando da transição entre os regimes catarateamento-centrifugação o qual se mostrou dependente das propriedades físicas dos materiais particulados tais como: esfericidade, massa específica e coeficiente de atrito partícula-parede. Foi possível a realização de uma investigação, tanto quantitativa quanto qualitativa, acerca dos efeitos das diferenças de diâmetro, massa específica e da condição inicial do material no interior do tambor rotatório sobre o fenômeno de segregação radial e axial (mistura binária). O fenômeno da segregação axial foi observado apenas quando da diferença de diâmetros entre as partículas. A diferença tão somente da massa específica, independente das condições de velocidade de rotação e grau de preenchimento, não causou segregação axial. No que diz respeito à abordagem numérica, duas diferentes aproximações para a previsão do escoamento granular no interior do tambor foram utilizadas: Euleriana e Lagrangeana. Comparações entre os perfis radiais de velocidade de sólidos experimentais e simulados através do modelo Euleriano mostraram que o modelo cinético-colisional, o qual tem sido aplicado com sucesso em muitas simulações de escoamento granulares diluídos pode, também, ser aplicado no tratamento de um escoamento granular denso presente em tambores rotatórios. Neste caso, observou-se também que a força de arrasto ou resistiva fluido-sólido pode ser negligenciada no caso de um tambor rotatório operando no regime de rolamento, onde não há entrada ou saída de fluido do sistema. No processo de calibração do modelo Lagrangeano através de um Planejamento Composto Central, os valores determinados dos parâmetros coeficiente de atrito (μp) e razão de amortecimento (ζ) para o arroz foram de, respectivamente, 0,425 e 0,149 (erro relativo de 2,9%), enquanto que para as esferas de vidro foram de 0,701 e 0,425 (erro relativo de 3,4%), respectivamente. Os parâmetros da modelagem Lagrangeana determinados para as partículas arredondadas (esferas de vidro) mostraram-se dependentes das condições operacionais. Já para as partículas irregulares (arroz), a calibração dos parâmetros através de um tambor em escala reduzida, mostrou-se adequada para o emprego na previsão do escoamento em tambores rotatórios em uma escala superior, podendo futuramente ser empregada em uma escala industrial. / Doutor em Engenharia Química

Reservoir releases for generating power need to be reconciled with efforts to maintain healthy ecosystems in regulated rivers having irregular channel topography. Fluctuating, complex flow patterns near river obstructions such as boulders and large woody debris provide unique habitat for many aquatic organisms. Numerical modeling of the flow structures surrounding these obstructions is challenging, yet it represents an important tool for aquatic habitat assessment. Moreover, efforts for modeling the morphologically and biologically important transient flows, as well as quantifying their impacts on physical fish habitat during the unsteady-flow period remain rare. In this dissertation, the ability of two- (2-D) and three-dimensional (3-D) hydraulic models to reproduce the localized complex flow features at steady base and peak flows is examined first. The performance of the two hydraulic models is evaluated by comparing the numerical results with measurements of flow around a laboratory hemisphere and boulders located at a reach of the Smith River in Virginia. Close agreement between measured values and the velocity profiles predicted by the two models is obtained outside the wakes behind these obstructions. However, results suggest that in the vicinity of theses obstructions the 3-D model is better suited for reproducing the circulation flow behavior favored by many aquatic species over a broad range of flows. Further, time-dependent flow features affecting channel morphology and aquatic physical habitat are investigated using the numerical models for the same reach in the Smith River. Temporal variation measurements of water surface elevation and velocity profile obtained in the field during a reservoir release are in good agreement with the numerical results. A hypothetical "staggering" flow release scenario simulated by the 3-D model leads to reduced erosional area and longer refugia availability for juvenile brown trout during hydropeaking. Finally, an unsteadiness parameter β is proposed for determining whether an unsteady flow regime can be either modeled using a truly dynamic flow approach or a quasi-steady flow method. / Ph. D.

Um dos principais problemas relacionados ao transporte e manipulação de fluidos multifásicos refere-se a existência de regimes de escoamento e sua forte influência sobre importantes parâmetros de operação. Um bom exemplo disto ocorre em reatores químicos gás-líquido no qual máximos coeficientes de reação podem ser alcançados mantendo-se um escoamento a bolhas disperso, maximizando a área interfacial total. Assim, a habilidade de identificar automaticamente regimes de escoamento é de importância crucial, especialmente para operação adequada de sistemas multifásicos. Este trabalho se constituirá no desenvolvimento e implementação de uma rede neural auto-organizativa especializada ao problema de identificação de regimes de escoamento bifásico ar-água em tubo horizontal. Os regimes reconhecidos em tubo horizontal são seis: estratificado liso, estratificado ondulado, estratificado rugoso, intermitente, bolhas e anular. Para tanto, pretende-se tomar como ponto de partida as medidas capacitivas, variação de pressão da tubulação e medida de pressão \"flutuante\" como padrões associativos a serem armazenados na rede neural. Posteriormente, a fase de treinamento da rede neural consistirá em identificar os coeficientes sinápticos apropriados, a partir de um conjunto representativo de ensaios. Nesse trabalho optou-se por uma arquitetura com 10 neurônios na camada de entrada, portanto uma quantidade maior do que o número de regimes que se deseja identificar. O objetivo é ver se a rede neural consegue encontrar de forma autônoma os seis regimes já conhecidos, mesmo tendo 10 neurônios na grade. Isso demonstra a habilidade da rede neural auto-organizativa em identificar regimes de escoamento mesmo em situações onde não há conhecimento prévio dos mesmos. Serão feitas simulações para verificar o desempenho da rede neural a partir de dados experimentais coletados no oleoduto piloto do Núcleo de Engenharia Térmica e Fluidos da Escola de Engenharia de São Carlos. / One of the main problems related to the transport and manipulation of multiphase flow is the existence of flow regimes and its strong influence on important parameters of operation. As an example of it occurs in gas-liquid chemical reactor in which maximum coefficients of reaction can be reached by keeping a maximal flow bubbly dispersed into a total interfacial area. Thus, the ability to identify flow regimes automatically is very important, specially in order to obtain a properly operation of multiphase systems. This work consists in the development and implementation of a self-organizing neural networks specially for the problem of identification of regimes of two-phase flow air-water in horizontal pipe. The regimes recognized in horizontal pipe are six: estratified smooth, estratified wavy, estratified rugged, intermittent, bubbly and annular. The capacitive measures, variation of pressure of the piping and measure of flutuante pressure were considered as a starting point to be stored as associative standards in the neural networks. After, the phase of training of the neural networks consisted of an appropriate identification for the sinaptic coefficients from a representative set of tests. An architectural input-layer of 10 neurons was opted. Therefore a bigger amount of regimes than the ones usually required. The objective is to see if the neural network is able to find independently the six regimes already known, even having 10 neurons in the grid. This demonstrates the ability of the self-organizing maps neural network in identifying flow regimes in situations where one does not have previous knowledge of them. Simulations will be made to verify the performance of the neural network from experimental data collected in the pilot pipe-line of the Nucleus of Thermal Engineering and Fluids of the School of Engineering of São Carlos.

Um dos principais problemas relacionados ao transporte e manipulação de fluidos multifásicos refere-se a existência de regimes de escoamento e sua forte influência sobre importantes parâmetros de operação. Um bom exemplo disto ocorre em reatores químicos gás-líquido no qual máximos coeficientes de reação podem ser alcançados mantendo-se um escoamento a bolhas disperso, maximizando a área interfacial total. Assim, a habilidade de identificar automaticamente regimes de escoamento é de importância crucial, especialmente para operação adequada de sistemas multifásicos. Este trabalho se constituirá no desenvolvimento e implementação de uma rede neural auto-organizativa especializada ao problema de identificação de regimes de escoamento bifásico ar-água em tubo horizontal. Os regimes reconhecidos em tubo horizontal são seis: estratificado liso, estratificado ondulado, estratificado rugoso, intermitente, bolhas e anular. Para tanto, pretende-se tomar como ponto de partida as medidas capacitivas, variação de pressão da tubulação e medida de pressão \"flutuante\" como padrões associativos a serem armazenados na rede neural. Posteriormente, a fase de treinamento da rede neural consistirá em identificar os coeficientes sinápticos apropriados, a partir de um conjunto representativo de ensaios. Nesse trabalho optou-se por uma arquitetura com 10 neurônios na camada de entrada, portanto uma quantidade maior do que o número de regimes que se deseja identificar. O objetivo é ver se a rede neural consegue encontrar de forma autônoma os seis regimes já conhecidos, mesmo tendo 10 neurônios na grade. Isso demonstra a habilidade da rede neural auto-organizativa em identificar regimes de escoamento mesmo em situações onde não há conhecimento prévio dos mesmos. Serão feitas simulações para verificar o desempenho da rede neural a partir de dados experimentais coletados no oleoduto piloto do Núcleo de Engenharia Térmica e Fluidos da Escola de Engenharia de São Carlos. / One of the main problems related to the transport and manipulation of multiphase flow is the existence of flow regimes and its strong influence on important parameters of operation. As an example of it occurs in gas-liquid chemical reactor in which maximum coefficients of reaction can be reached by keeping a maximal flow bubbly dispersed into a total interfacial area. Thus, the ability to identify flow regimes automatically is very important, specially in order to obtain a properly operation of multiphase systems. This work consists in the development and implementation of a self-organizing neural networks specially for the problem of identification of regimes of two-phase flow air-water in horizontal pipe. The regimes recognized in horizontal pipe are six: estratified smooth, estratified wavy, estratified rugged, intermittent, bubbly and annular. The capacitive measures, variation of pressure of the piping and measure of flutuante pressure were considered as a starting point to be stored as associative standards in the neural networks. After, the phase of training of the neural networks consisted of an appropriate identification for the sinaptic coefficients from a representative set of tests. An architectural input-layer of 10 neurons was opted. Therefore a bigger amount of regimes than the ones usually required. The objective is to see if the neural network is able to find independently the six regimes already known, even having 10 neurons in the grid. This demonstrates the ability of the self-organizing maps neural network in identifying flow regimes in situations where one does not have previous knowledge of them. Simulations will be made to verify the performance of the neural network from experimental data collected in the pilot pipe-line of the Nucleus of Thermal Engineering and Fluids of the School of Engineering of São Carlos.

Les débitmètres sont des technologies connues et fortement répandues mais pas exemptes d’erreurs, dont les causes peuvent être assez variées. Ces capteurs installés dans les systèmes d’assainissement ne sont de plus pas rigoureusement étalonnables in situ. Une méthode de vérification in situ des débitmètres, utilisable pour des pseudos étalonnages, est présentée. En effet, l’injection ponctuelle d’un traceur dans un écoulement permet de calculer le débit et son incertitude d’une manière indépendante des sondes en place. Cette méthodologie, en sept étapes, offre des résultats comparables à ceux fournis par des méthodes considérées comme des références (traçages aux sels et débitmètre électromagnétique). Des premiers essais en réseaux unitaires ont été réalisés pour la vérification de débitmètres (le long d’un collecteur du Grand Lyon) et pour le calage de courbes de tarages (sur le Syndicat Intercommunal d’Assainissement Grand Projet). Pour les mesures qualitatives des effluents, de nombreuses études antérieures ont montrées de bonnes corrélations entre les concentrations en polluants et les signaux fournis par des turbidimètres et des spectromètres UV/visible. Un pilote expérimental et des campagnes d’échantillonnages sur des effluents prétraités de la station d’épuration de Fontaines sur Saône ont été mis en place pour répondre aux objectifs suivants : i) concevoir et tester un site de mesure de nouvelle génération, ii) chercher des modèles de régressions entres différents capteurs (turbidimètres mono et bi-longueur(s) d’onde(s), conductimètre, pHmètre, spectromètre UV/visible, capteurs à micro-ondes) et les concentrations en polluants pour les échantillons de temps sec, de temps de pluie et la totalité, iii) caractériser la performance de ces modèles et iv) tester la robustesse des méthodes proposées sous des conditions atypiques mais susceptibles d’être rencontrées en réseau d’assainissement. Les résultats confirment les bonnes corrélations entre certains paramètres (turbidité, conductivité et spectre UV/visible) et les concentrations en polluants. Aucun capteur n’est le plus performant pour l’ensemble des polluants. La majorité des capteurs délivrent des estimations comparables aux incertitudes près mais ces estimations sont peu voire pas redondantes aux analyses effectuées sur les échantillons. La conception du banc (et les variations des matrices des eaux usées) et/ou les conditions expérimentales lors des tests de robustesse sont peut être en cause. / Flowmeters are well known technologies and highly prevalent but not free of errors, the causes can be quite varied. These sensors installed in sewer systems can’t be in situ calibrate. A method of in situ verification of flow meters, used for pseudo-calibrations, is presented. Indeed, the injection of a tracer in a flow allows to calculate the flow and its uncertainty in a manner independent of probes in place. This methodology, in seven steps, provides results comparable to those provided by methods considered standards (tracer salts and electromagnetic flowmeter). The first applications in combined sewers were conducted for verification of flowmeters (along a main pipe of Greater Lyon) and for calibrations curves (the Syndicat Intercommunal Grand Sanitation Project). For qualitative measures of influents, many previous studies have shown good correlations between pollutant concentrations and the signals provided by turbidimeters and UV/visible spectrometers. A pilot and experimental samples taken during campaigns on the pretreated influent of the waste water treatment plant of Fontaines-sur-Saône, a bench was built to meet the following objectives: i) design and test a site measuring of a new generation, ii) seek regression models among different sensors (single and dual length(s)-wavelength(s) turbidimeters, conductivity meter, pH meter, UV/visible spectrometer, microwave sensors) and pollutant concentrations for samples of dry weather, wet weather and all, iii) characterize the performance of these models and iv) test the robustness of the proposed methods under atypical conditions but succeptibles to be encountered in sewers. The results confirm the good correlations between few parameters (turbidity, conductivity and UV/Vis fingerprint) and pollutant concentrations. No sensor is the most efficient for all pollutants. The majority of sensors deliver similar estimates to uncertainties but these estimates are little or no redundant with laboratory analysis. The design of the bench (and changes in wastewater matrices) and / or experimental conditions during the tests of robustness may be involved.

Turbulent mixed convection is a complicated flow where the buoyancy and shear forces compete with each other in affecting the flow dynamics. This thesis deals with the near wall dynamics in a turbulent mixed convection flow over an isothermal horizontal heated plate. We distinguish between two types of mixed convection ; low-speed mixed convection (LSM) and high-speed mixed convection (HSM). In LSM the entire boundary layer, including the near-wall region, is dominated by buoyancy; in HSM the near-wall region, is dominated by shear and the outer region by buoyancy. We show that the value of the parameter (* = ^ determines whether the flow is LSM or HSM. Here yr is the friction length scale and L is the Monin-Obukhov length scale. In the present thesis we proposed a model for the near-wall dynamics in LSM. We assume the coherent structure near-wall for low-speed mixed convection to be streamwise aligned periodic array of laminar plumes and give a 2d model for the near wall dynamics, Here the equation to solve for the streamwise velocity is linear with the vertical and spanwise velocities given by the free convection model of Theerthan and Arakeri [1]. We determine the profiles of streamwise velocity, Reynolds shear stress and RMS of the fluctuations of the three components of velocity. From the model we obtain the scaling for wall shear stress rw as rw oc (UooAT*), where Uoo is the free-stream velocity and AT is the temperature difference between the free-stream and the horizontal surface.A similar scaling for rw was obtained in the experiments of Ingersoll [5] and by Narasimha et al [11] in the atmospheric boundary layer under low wind speed conditions. We also derive a formula for boundary layer thickness 5(x) which predicts the boundary layer growth for the combination free-stream velocity Uoo and AT in the low-speed mixed convection regime.