Chute aerators are widely used in spillways to avoid cavitation damage. When the water flow passes the aerator, two jets form – upper and lower jet. The purpose of this thesis has been to study the effects from the aerator by conducting experiments in a model with a flow depth large enough to ensure that the upper and lower jet remain separated. This means that the effects from the self-aeration at the upper surface has no effect on the process in the lower jet, thus making it possible to quantify the effects from the aerator. This thesis has also provided information of the bubble formation in the lower jet to aid in the ongoing research at Sichuan University. The following questions were set up for this thesis: • What is cavitation and how is it harmful? • What is the working principle of an aerator? • How is air concentration and bubble frequency distributed in the flow? • How well do the experimental results coincide with theoretical calculations? • How are air bubbles formed and transported within the flow? The effects from the aerator have been quantified by measuring the air concentration and bubble frequency throughout the cavity zone. The model was modified and the velocity was varied between the experiments to study how different parameters effected the aeration. The results indicate that much air is being entrapped in the lower surface, but only a small amount of the entrapped air is being entrained into the flow and that the bubble frequency increases with both distance from the aerator and with an increased flow velocity. No difference in behaviour was noticed between the different modifications of the model. The bubble formation was studied by recording the flow with a high-speed camera. These recordings were used to obtain data about important parameters for the ongoing research at Sichuan University.

The importance of hydraulic structures has become an essential mitigating mean for floodsthat occur more often due to climate change. Thus, the importance and safety of flooddischarge tunnels has promoted further studies and experiments on the topic to mitigatedamages, such as cavitation that arise because of high speed flows.After an experimental study on a physical model was carried out on the flood discharge tunnelin Zipingpu Water Conservancy project, a CFD model was designed and simulated in thecommercial software ANSYS Fluent. The simulations aimed to evaluate and examine the riskfor cavitation in the tunnel, examine the design problems of the structure and analyse theinstalled aerators for the mitigation of cavitation. Moreover, using CFD models as acomplementary form to physical models was analyzed.A three dimensional geometry of the discharge tunnel was built in ANSYS Spaceclaim and themesh conducted with ANSYS mesh generator. The known boundary condition such as thedesign flow conditions, velocity inlet, pressure inlets and pressure outlet were set. For themodel a multiphase VOF scheme with RANS approach, k-ϵ turbulence model and a standardwall function was set.The results from the initial simulations showed that the discharge tunnel was under cavitationrisk, since the recorded cavitation index in the tunnel was below 1.8. After having revised thelayout of the aerators in order to mitigate cavitation risk, the results from the simulations withadded aerators were sufficient to mitigate the risk as the cavitation index was still below 1.8.The results for the cavitation index remained unchanged even in the simulated models with adifferent solver setup that were used in the comparison with the experimental data in order tovalidate them.As a conclusion, it was recommended that the tunnel design has to be revised and improvedby adding more aerators and air vents to mitigate the cavitation risk. Furthermore, more studieson the discharge tunnel or similar tunnels with similar conditions should be carried out in orderto validate the results of this study and determine if numerical models are preferable to physicalmodels

A inserção de ar em escoamentos de altas velocidades ao longo de estruturas hidráulicas é uma técnica bastante eficiente para prevenir a cavitação. A sua importância é majorada quando se considera os custos econômicos e as questões de segurança que estão associadas à estabilidade de uma barragem. No presente trabalho são apresentados equacionamentos para quantificar a entrada de ar em vertedores através de aeradores de fundo. Essas equações foram obtidas utilizando princípios físicos de conservação de massa, energia e quantidade de movimento nos escoamentos de ar e água que ocorrem no aerador, permitindo organizar informações advindas de cada fase. Ressalta-se que buscou-se tornar o equacionamento independente da subpressão do jato, uma vez que esta subpressão é um parâmetro de difícil determinação a priori pelo projetista. Entretanto, toda formulação é direcionada justamente para melhor representar este parâmetro utilizando os princípios físicos clássicos e as variáveis decorrentes da sua utilização. Buscou-se a validação de modelos teóricos obtidos por meio de formulações baseadas nas leis de conservação de massa, quantidade de movimento e energia para aeradores de fundo. Neste contexto, comparam-se os resultados dos equacionamentos propostos e os dados experimentais encontrados na literatura, tendo se verificado boas correlações. Este tipo de quantificação essencialmente teórica de incorporação do ar em aeradores de fundo ainda é raro e o presente trabalho visa contribuir na validação de modelos com estas características. Adicionalmente, efetua-se a comparação com as equações empíricas e semi-empíricas encontradas na literatura. A experiência adquirida na área mostra que esta é a forma mais adequada de abordar o problema. / The introduction of air in flows around bottom aerators in spillways of dams is an efficient technique to prevent cavitation. Its importance is increased when one considers the costs involved and the safety issues that are associated with the stability of a large dam. Equations are presented in this study to quantify the air inlet through bed aerators in flows along spillways. The equations were obtained using the physical principles of conservation of mass, energy and momentum in both the flows of air and water in the aerator, allowing to organize the information obtained from each phase. It was possible to show the parameters that are relevant for quantifying the induced air flow in bed aerators. In addition, a comparison was conducted between the equations resulting from this analysis and empirical and semi-empirical expressions found in the literature. It is noteworthy to mention that one of the objectives of this study was to obtain a final equation independent of the relative pressure under the jet, since this low pressure is a parameter difficult to determine a priori by the designer. However, the entire formulation was directed precisely to better represent this parameter using the principles of classical physics and the variables arising from their use. The experience acquired in this area shows that this is the most appropriate way to address this problem.

Water is a fundamental need for existence of mankind. Only 0.01 % of total global water is readily available for human consumption as fresh water. The rapid increase in human population and consequent rise in urbanization and industrialization is producing a stress on this meager water resource. Water at the same time is a renewable resource, ie with suitable treatment it can be made re-useable. Aeration is one of the important processes employed in activated sludge process of the biological treatment units of wastewater. In this process the level of dissolved oxygen in the effluent is raised to the required amounts to decompose organic matters present in the effluent and thereby to reduce the BOD (biochemical oxygen demand) of the effluent by a physical means called “aeration process”. The aeration process consumes as much as 60-80% of total power requirements of wastewater treatment plants. Therefore, the efficiency in design of aeration process is required so that treatment and its power consumption can be economized. With the objective of optimizing the aeration process the present work in this thesis endeavors to develop an aeration which is efficient as well as economical. The various geometric parameters that affect the aeration process in mechanical surface aerators have been optimized. In the present work circular surface aeration tanks have been used. There are two types of circular tanks: Baffled and unbaffled. Separate optimal geometric parameters have been obtained for baffled and unbaffled circular tanks. With optimal geometric similitude scale up studies were done. Reynolds number and Froude number criteria has been found unsuitable for scaling oxygen transfer rates. Theoretical power per unit volume parameter is the most suitable scaling parameter for oxygen transfer rates in both baffled and unbaffled circular tanks. Baffled circular tanks are found to give better performance in terms of oxygen transfer rates as compared to unbaffled tanks. In contrast unbaffled tanks give better performance in terms of power consumption as compared to baffled circular tanks. General correlations have been developed for oxygen transfer rates for both baffled and unbaffled circular surface aerations tanks which incorporate all the geometric and dynamic parameters. These correlations help in the design of new treatment facilities as well as evaluating and up gradation of existing facilities. Power consumption studies have also been conducted on circular surface aeration tanks. Geometric parameters affect the power consumption significantly. Using the optimal geometric similarity conditions obtained for oxygen transfer rates the scale up studies for power consumption has also been done. Reynolds and Froude criteria are found to be giving scale effects for non dimensional power consumption parameter, power number. Theoretical power per unit volume parameter is found to be the scaling parameters for power number and a suitable correlation equation has been developed for baffled circular surface aeration tanks. General correlations have been developed for power number in baffled and unbaffled circular tanks. A novel type of self aspirating tube sparger system has been developed. It is like a bubble aerator with a rotor. The various geometric parameters that affect oxygen transfer rates have been optimized in baffled circular surface aeration tank. The optimal geometrically similar tanks have been used for scale up studies. Theoretical power per unit volume parameter is found to be the scaling parameter for oxygen transfer rates in circular surface aeration tanks with self aspirating sparger systems. Circular baffled tanks with a special sparger system gives very much higher oxygen transfer rates (as much as 5.7 times) as compared to circular tanks. The oxygen transfer rates data from literature also show lower values as compared to the system developed in this thesis. Geometrically similar unbaffled tanks have also been used with self aspirating sparger system. For same power consumption oxygen transfer rate in circular surface aeration tanks with self aspirating sparger system is higher as compared to circular tanks without self aspirating system. Mixing mechanisms in surface aeration tanks depend upon two different extreme length scales of time, namely macromixing and micromixing. Small scale mixing close to the molecular level is referred to as micromixing; whereas macromixing refers to the mixing on a large scale. The effect of geometrical parameters on macromixing time has been studied. The scaling parameters for macromixing and micromixing have been developed and simulation equations governing these time scales are also presented.

Dissolved oxygen refers to the mass of oxygen that is contained in water. The concentration of dissolved oxygen is an important indicator of the environments water quality. The presence of oxygen in water is desirable therefore it is a positive sign; whereas the absence of oxygen is a sign of severe pollution. An adequate supply of dissolved oxygen is important for waste water treatment processes. Many naturally occurring biological and chemical processes use oxygen, thereby diminishing the dissolved oxygen concentration in the water. The physical process of oxygen transfer or oxygen absorption from the atmosphere acts to replenish the used oxygen. This process has been termed aeration. Aeration is the primary requirement of the biological treatment of water and wastewater treatment. As reported in the literature, the aeration process consumes as much as 60-80% of total power requirements in wastewater treatment plants. Therefore, it is necessary that the design and operation of aeration process should be economized in terms of their energy efficiency. The performance of surface aeration systems is rated in terms of their oxygen transfer rate; hence the choice of a particular surface aeration system depends on its performance and efficiency of oxygen transfer rates. Oxygen transfer rate and the corresponding power requirement to rotate the rotor are very vital parameters for the design and scale-up of surface aerators. Basically two types of operation are in use for surface aeration systems – batch operation and continuous operation. Batch operation involves a single vessel which is filled, aerated then completely emptied. Continuous operation method of operating a biological treatment plant is characterized by a steady input stream (in terms of chemical and biological composition and flow rate, when flow and concentration equalization is practiced), steady process conditions during the treatment steps and by a fairly consistent flow of treated material with only little variation in its composition. The work presented in this thesis consists of two parts. The first one deals with the experimental investigations on the three types of batch surface aeration tanks. A comprehensive design analysis has been worked out and presented on these types of surface aeration systems. In the second category, experimental investigations have been carried out extensively on continuous flow surface aeration systems of different sizes. Analysis has led to the formulation of optimal geometric dimension and the simulation criteria for the design purposes. As far as the first category of investigations is concerned, a substantial work has been reported on batch surface aerators on various issues, during the past several years. Still, a general methodology to scale up or scale down the process phenomena is lacking. In the present work, experiments were done on different shaped batch surface aeration system for generalizing or devising the scale up and scale down criteria for oxygen transfer coefficient and power consumption. Present work through experimental observations established that unbaffled circular tanks are more energy efficient than baffled when used as surface aerator. Power consumption in surface aeration systems is characterized by a fundamental non-dimensional parameter named power number. This number relates drag force to the inertial force in fluid flow system. Power number scaling up of unbaffled surface aerators of square, circular and rectangular shaped tanks is one of the most important contributions of the present work. Design charts have been developed for all the three shape of tanks for the installation as the batch surface aeration systems. Based on the experimental analyses in the present work, it was found that circular shape is the most efficient than any other shape and it is also established that generally a number of smaller sized tanks were more economic and efficient than using a single big tank while aerating the same volume of water. Based on the energy economy analysis, present work suggests the optimal speed range of batch systems of different shaped surface aeration tanks. Different sized rectangular aeration tanks with different aspect ratios (that is length to width ratio) were tested along with a series of square and circular tanks for comparing their relative performances. Present work by doing experiments answered this fact and found that square tank (aspect ratio =1) was more efficient than any other aspect ratio rectangular tanks. Vortexes are inherently present in any type of unbaffled tanks. Present work analyzed the vortex behavior of unbaffled surface aeration systems to determine the critical impeller speed in unbaffled batch surface aeration systems at which oxygen transfer rates are more. The second part of the present work establishes the optimal geometrical parameters of a continuous flow surface aeration systems. These types of operations were found to be least reported in the available literature and there appears to be of no report in the literature on optimal geometrical parameters. Extensive experimental work is reported in the present thesis on the establishment of the optimal geometrical parameters of continuous flow surface aeration systems. From there, simulation criteria are established by maintaining optimal geometrical similarity in different sized continuous flow surface aeration tanks; so that the scale up or scale down criteria can be applied to predict oxygen transfer rates and power number.

A inserção de ar em escoamentos de altas velocidades ao longo de estruturas hidráulicas é uma técnica bastante eficiente para prevenir a cavitação. A sua importância é majorada quando se considera os custos econômicos e as questões de segurança que estão associadas à estabilidade de uma barragem. No presente trabalho são apresentados equacionamentos para quantificar a entrada de ar em vertedores através de aeradores de fundo. Essas equações foram obtidas utilizando princípios físicos de conservação de massa, energia e quantidade de movimento nos escoamentos de ar e água que ocorrem no aerador, permitindo organizar informações advindas de cada fase. Ressalta-se que buscou-se tornar o equacionamento independente da subpressão do jato, uma vez que esta subpressão é um parâmetro de difícil determinação a priori pelo projetista. Entretanto, toda formulação é direcionada justamente para melhor representar este parâmetro utilizando os princípios físicos clássicos e as variáveis decorrentes da sua utilização. Buscou-se a validação de modelos teóricos obtidos por meio de formulações baseadas nas leis de conservação de massa, quantidade de movimento e energia para aeradores de fundo. Neste contexto, comparam-se os resultados dos equacionamentos propostos e os dados experimentais encontrados na literatura, tendo se verificado boas correlações. Este tipo de quantificação essencialmente teórica de incorporação do ar em aeradores de fundo ainda é raro e o presente trabalho visa contribuir na validação de modelos com estas características. Adicionalmente, efetua-se a comparação com as equações empíricas e semi-empíricas encontradas na literatura. A experiência adquirida na área mostra que esta é a forma mais adequada de abordar o problema. / The introduction of air in flows around bottom aerators in spillways of dams is an efficient technique to prevent cavitation. Its importance is increased when one considers the costs involved and the safety issues that are associated with the stability of a large dam. Equations are presented in this study to quantify the air inlet through bed aerators in flows along spillways. The equations were obtained using the physical principles of conservation of mass, energy and momentum in both the flows of air and water in the aerator, allowing to organize the information obtained from each phase. It was possible to show the parameters that are relevant for quantifying the induced air flow in bed aerators. In addition, a comparison was conducted between the equations resulting from this analysis and empirical and semi-empirical expressions found in the literature. It is noteworthy to mention that one of the objectives of this study was to obtain a final equation independent of the relative pressure under the jet, since this low pressure is a parameter difficult to determine a priori by the designer. However, the entire formulation was directed precisely to better represent this parameter using the principles of classical physics and the variables arising from their use. The experience acquired in this area shows that this is the most appropriate way to address this problem.