The hydrodynamics and the breakup of single oscillating drops which are accelerating from rest through a continuous phase were studied. Specially purified chlorobenzene, 1,2-dichloroethane, and ethylbromide were used as the drop phase liquids and double distilled water was used as the continuous phase liquid. The motion of the drops was recorded on cine-film using shadow and schlieren optical systems and these films were analysed frame by frame. Data is presented for the variation with time of the velocity of fall, the frequency of oscillation and the eccentricity of nonbreaking drops and the changes in the structure of the wake behind these drops is described. A transition of the wake behind the accelerating drops from class I to the wake class of the terminal region was observed. The first formation of a class III attached wake was followed by the onset of the terminal oscillations of the drop. A mechanism is proposed for the sustaining of these oscillations. The mode of breakup of freely falling drops was investigated. Secondary drops were formed by a necking process of the liquid columna which were formed both at the rear and at the front of the primary drop. The occurrance and the sizes of the secondary drops are related to the size and the oscillations of the primary drop. Theoretical predictions are made of the onset of necking and of the rate of necking using respectively surface free energy considerations and a momentum balance on the liquid in the column.

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This research comprises of device designing and processing of biomaterial suspensions based on co-flowing medium and air. The preparation of stable microbubble suspensions is fundamental to a wide range of technological applications across the scientific, engineering, medical and industrial sectors; from the production of basic foodstuffs to the self-assembly of smart materials. A key requirement in such preparation is to control the size distribution (i.e. mean and standard deviation) of the resulting microbubbles, with ideally a monodispersion being achieved. Such quality control is important because when the microbubbles are used in, for example, in ultrasound imaging, the response of such a microbubble to ultrasound excitation is strongly influenced by the radius and the thickness of the microbubble and also the mechanical properties of any coating material. This response, in tum, determines the amplitude and frequency of the ultrasound signal scattered by the microbubbles and the threshold for microbubble destruction. In first part of the work, a device is designed and optimized to microbubble silicone oils of different viscosities. All the important physical parameters like liquid and air flow rate, capillary diameter, gap between two capillaries, viscosity and surface tension are investigated to understand the mechanisms of bubble formation and to idealise the processing technique. The condition of bubble formation; a ratio of air to liquid pressure is found to be related to viscosity of liquid which is important in materials processing. Bubble dynamics are also modelled to predict the air jet diameter which can control the diameter of bubble being produced. It was shown that air jet diameter has a strong relationship to viscosity of the liquid. Secondly, using a similar device and technique, lipid suspension and nano gold particulate colloids are microbubbled to show that process is suitable for producing near monodisperse microbubbles with more than 20 minutes life time. These microbubbles have immediate application as ultrasound contrast agent and drug delivery vehicle. Moreover, it is also found that nano-particulate reinforced bubbles can enhance the non-linearity of backscatter at low amplitude pressure. To further reduce the diameter of bubbles, a hybrid flow focussing and electrospraying device is developed to show that monodisperse size of bubbles less than 10 μm can be produced. This process has been syslematically studied to conclude that the product of applied voltage and flow ratio governs the diameter of bubbles generated.

This research project is concerned with the Eulerian-Eulerian mathematical modelling of fluidized suspensions. We first derive new averaged equations of motion for particulate systems made up of a finite number of monodisperse particle classes; this clarifies the mathematical origin and physical meaning of the terms featuring in the equations and allows to attain a well-posed multiphase model. We then tackle the closure problem of the fluid-particle interaction force in monodisperse fluidized suspensions, laying emphasis on the buoyancy, drag and elastic forces. We analyze critically several constitutive relations used to express these forces, we identify their shortcomings and we advance new, and more accurate, closure equations. To validate them we study, analytically and computationally, the expansion and collapse of homogeneous fluidized beds and their transition to the bubbling regime, comparing the result with experimental data. We then address the mathematical modelling of polydisperse fluidized suspensions, which are characterized by a continuous distribution of the particle properties, such as size or velocity. Here we adopt a more powerful modelling approach based on the generalized population balance equation (GPBE). Whereas the classical transport equations of continuum mechanics are three-dimensional, the GPBE is usually higher-dimensional and incompatible with customary computational schemes. To solve it, we use the method of moments (MOM), which resorts to a limited number of GPBE moments to derive three-dimensional transport equations that can be handled by normal CFD codes. The limited set of equations, which replaces the single multidimensional GPBE, keeps the problem tractable when applied to complicated multiphase flows; the main obstacle to the method is that the moment transport equations are mathematically unclosed. To overcome the problem, we present two very efficient methods, the direct quadrature method of moments (DQMOM) and the quadrature method of moments (QMOM). Both approximate the volume density function (VDF) featuring in the GPBE by using a quadrature formula. The methods are very flexible: the number of nodes in the quadrature corresponds to the number of disperse phases simulated. The more the nodes, the better the quadrature approximation; more nodes, however, entail also more complexity and more computational effort. For monovariate systems, i.e., systems with only one internal coordinate in the generalized sense, the methods are entirely equivalent from a theoretical standpoint; computationally, however, they differ substantially. To conclude the work, we use DQMOM to simulate the dynamics of two polydisperse powders initially arranged as two superposed, perfectly-segregated packed systems. As fluidization occurs, the simulation tracks the evolution in time and physical space of the quadrature nodes and weights and predicts the mixing attained by the system. To validate the method, we compare computational predictions with experimental results.

Electronic instrumentation was developed for the measurement of the oxygen partial pressure, P1, in a sample gas using fully-sealed zirconia pump-gauge oxygen sensors operated in an AC mode. These sensors, operated typically at 700°C, consisted of two discs of zirconia with porous platinum electrodes on each face separated by a gold seal and enclosing a small internal volume. One disc was operated as a pump enabling oxygen to be electrochemically transferred into and out of the enclosed volume; the other disc operated as a gauge, the Nernst EMF across the electrodes providing a measure of the ratio of the internal to the external oxygen partial pressure. By careful design of the circuitry it was possible to measure the oxygen partial pressure, P, without the need for a separate reference gas supply. Subsequently, a novel "tracking" mode of operation was proposed and implemented in which leakage effects generally associated with sealed pump-gauge devices were minimised: the sensor was operated in a feedback control-loop in order to adjust automatically the mean internal reference oxygen partial pressure, P0, so as to maintain the ratio (Px/P0) close to unity. The signal-to-noise ratio was markedly improved by using gauge EMFs with high amplitudes which inevitably display a distorted sinusoid due to the logarithmic term in the Nernst equation. Surprisingly, mathematical analysis predicted that the linearity of the output of the instrument using phase-sensitive detection should not be affected by the deviation from a sinusoid and this was confirmed experimentally: signal processing was practically implemented using simple analogue electronics. As anticipated there was a strong influence of sensor temperature on the output of the instrument: consequently, methods for temperature compensation were proposed and shown to be feasible with minimum hardware. The theory of Operation of leaky pump-gauge was also developed which indicated that a physical leak in the sensor should cause a phase shift and amplitude change in the sensor output. Experimental results were, in general, in agreement with the theory demonstrating the influences of the geometry and dimensions of the leak and of the operating frequency. Importantly, the theory predicted that, when operated in the AC mode, devices with major leakage may still be used for oxygen partial pressure measurement: again this was confirmed by experiment and the additional benefit of a concomitant substantial simplification of the electronic circuitry also realised. Interestingly an unexpected but small influence of oxygen concentration on the phase shift was observed: this requires additional study.