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|Title:||Acoustic cavitation in emerging applications : membrane cleaning and flow processing.||Authors:||Prabowo, Firdaus.||Keywords:||DRNTU::Science::Physics::Acoustics
DRNTU::Engineering::Mechanical engineering::Fluid mechanics
|Issue Date:||2012||Source:||Prabowo, F. (2012). Acoustic cavitation in emerging applications : membrane cleaning and flow processing. Doctoral thesis, Nanyang Technological University, Singapore.||Abstract:||Using theoretical and experimental methods, we investigate potential applications of acoustic cavitation bubbles for cleaning and flow processing. An important physical parameter for a successful cleaning is the wall shear stress generated on the contaminated surface. In this thesis, a theoretical investigation of wall shear stress generated from weakly oscillating bubbles near a rigid boundary was conducted. We approximate the effect of the rigid boundary using image theory. The important damping parameters, i.e. acoustic, viscous, and thermal damping, are included for radially oscillating bubbles. The model is a modified Keller-Miksis equation for two interacting bubbles. For small-amplitude oscillations, we obtain an analytical solution for the wall shear stress. Parameter study finds that the wall shear stress has maxima with varying frequency, equilibrium bubble radius, lateral distance, and viscosity. At resonance, we observe maximum wall shear stress of up to 1 kPa despite the limitation of the small-amplitude oscillations. The wall shear stress generated from this oscillating bubbles may already be sufficient to clean water filtration membranes used in low pressure system. Yet, to remove very small particulates which are found in the semiconductor industry, it is necessary to drive bubbles above their linear regime. Here we also study experimentally oscillating gas bubble attached to a rigid surface. By varying the amplitude of the sinusoidal driving, we observed the onset of surface oscillations and the growth of surface oscillation modes. To quantify the shape of surface oscillations, we employ a Fourier transform on the contour. With increasing the driving amplitude three distinctive regimes are observed: radial oscillation with a monotonous increase of the first and second mode, sudden jump and excitation of higher surface modes, and chaotic surface oscillations. We suggest that the chaotic oscillations are the result of circumferential travelling surface modes. Interestingly we find low velocity jetting. Repeated jetting build up a liquid cushion on the surface. Hence, we conclude the jets may not contribute to surface erosion or cleaning. The cleaning of water filtration membrane is investigated experimentally. We propose a novel tandem frequency (combination of low and high frequency) cleaning method. Here, the high frequency is used to grow bubbles on the membrane surface and the low frequency to drive these bubbles into large amplitude oscillations which are the cause of the cleaning. We show experimentally using a high speed camera and compare the results with a single frequency method. We measure the trans-membrane pressure (TMP) of a fouled membrane before and after cleaning and find a TMP approximately equal to that of a clean membrane demonstrating the effectiveness of the tandem frequency method. This also suggests that the membrane is not damaged. Finally, we investigate the effect of flow on acoustic cavitation bubble dynamics. We investigate the multi-bubble sonoluminescence (MBSL) in an acoustic cell with flowing liquid using photomultiplier and ICCD camera. We use the light emission as an indicator for the strength of bubble dynamics. It indicates how rapid the bubble dynamics during collapse. We find that low flow rates do not affect sonoluminescence, however at higher flow rates a strong decrease in sonoluminescence signal is found. Using a high speed camera, we investigate bubble dynamics and find that at low flow rate the typical bubble configuration is similar to Miller arrays, i.e. big bubbles surrounded by small satellite bubbles which are spiralling. However, at high flow rate we find that the big bubbles are dragged away by the flow, hence disturbing the stable configuration of the Miller array. We conclude that the disappearance of the stable Miller array is the cause of the reduction in light emission.||URI:||http://hdl.handle.net/10356/51103||metadata.item.grantfulltext:||open||metadata.item.fulltext:||With Fulltext|
|Appears in Collections:||SPMS Theses|
checked on Dec 20, 2019
checked on Dec 20, 2019
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