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Title: Millimetre-Scale Ultrasonic Concentrator for Microparticles in Fluid
Keywords: ultrasonic,separation,acoustic,particle,millimetre-scale,concentrator
Issue Date: 27-Dec-2011
Source: THEIN MIN HTIKE (2011-12-27). Millimetre-Scale Ultrasonic Concentrator for Microparticles in Fluid. ScholarBank@NUS Repository.
Abstract: Previous studies on particle concentration by the ultrasonic standing wave technology show that microparticles can easily and effectively be concentrated in the micrometer-scale concentrators. However, microparticles are difficult to concentrate in millimeter-scale concentrators because of wide disparity between the small particle size and large channel width. One of the millimeter-scale devices that can concentrate microparticles at a relatively large volume flow rate is the h-shaped acoustic concentrator. Previous studies on the h-shaped concentrator have presented design guidelines, performance characterization and some design improvements. However, more studies are still needed to fully understand the insights of device¿s operation and the behaviour of microparticles inside the concentrator. This study conducts systematic investigation into the operation of the h-shaped concentrator by measuring separation heights and particle concentrations at different voltage and flow rates. Specifically, this study (1) performs the characterization of concentration effectiveness of the h-shaped concentrator and (2) investigates the existence of particle trapping due to the lateral radiation forces. One-dimensional layered piezoelectric model was first used to obtain the design criterion for the layer thicknesses. Next, two h-shaped concentrators were constructed, one with nominally chosen layer thicknesses and one with properly designed layer thicknesses. Separation height measurements were performed to characterize the concentration effectiveness of the devices. The results validated that correct choice of layer thicknesses would help to improve the maximum achievable flow rate compared to nominally designed channel. Secondly, concentration effectiveness of the h-shaped concentrator was characterized by particle concentration experiments at inlet and two outlets using turbidity measurements. The results showed that particle trapping exists in the chamber and can be very significant at high voltage and high inlet particle concentration. These results shed the light into the insights of the device¿s operation since particle trapping can be beneficial or detrimental depending on the required mode of operation. The results suggested that the device can be used as a particle trapping device at high inlet particle concentration and high driving voltage. However, the device is suitable as a continuous flow-through concentrator only at low inlet particle concentration and moderate voltage levels. Finally, to further characterize the sound field inside the channel, energy density measurements were conducted. Acoustic and flow field analysis was performed by using the finite element models. Proper matching between the experimental and calculated energy densities was done and primary axial as well as lateral radiation forces inside the channel were estimated. Lateral radiation forces are found to be in comparable order of magnitude with viscous drag forces. Next, acoustic forces together with viscous drag forces were applied to the particle to predict particle trajectories. The general trends in separation heights from the particle trajectory model agree well with the experimental results. Moreover, numerical results show that particle trajectory came to a stop at high voltage and this could explain the particle trapping observed experimentally.
Appears in Collections:Ph.D Theses (Open)

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