No description available.

Tese de mestrado integrado. Engenharia Química. Faculdade de Engenharia. Universidade do Porto. 2010

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This research project was part of the microfluidic program in the CRC for Microtechnology, Australia, during 2000 to 2003. The aim of this research was to investigate the feasibility of applying geometric variations in a microchannel to create effects other than pure molecular diffusion to enhance microfluidic mixing. Geometric variations included the shape of a microchannel, as well as the various obstacle structures inside the microchannel. Generally, before performing chemical or biological analysis, samples and reagents need to be mixed together thoroughly. This is particularly important in miniaturized Total Analysis Systems (�TAS), where mixing is critical for the detection stage. In scaling down dimensions of micro-devices, diffusion becomes an efficient method for achieving homogenous solutions when the characteristic length of the channels becomes sufficiently small. In the case of pressure driven flow, it is necessary to use wider microchannels to ensure fluids can be pumped through the channels and the volume of fluid can provide sufficient signal intensity for detection. However, a relatively wide microchannel makes mixing by virtue of pure molecular diffusion a very slow process in a confined volume of a microfluidic device. Therefore, mixing is a challenge and improved methods need to be found for microfluidic applications. In this research, passive mixing using geometric variations in microchannels was studied due to its advantages over active mixing in terms of simplicity and ease of fabrication. Because of the nature of laminar flow in a microchannel, the geometric variations were designed to improve lateral convection to increase cross-stream diffusion. Previous research using this approach was limited, and a detailed research program using computational fluid dynamic (CFD) solvers, various shapes, sizes and layouts of geometric structures was undertaken for the first time. Experimental measurements, published experimental data and analytical predictions were used to validate the simulations for selected samples. Mixing efficiency was evaluated by using mass fraction distributions. It was found that the overall performance of a micromixer should include the pressure drop in a microdevice, therefore, a mixing index criterion was formulated in this research to combine the effect of mixing efficiency and pressure drop. The mixing index was used to determine optimum parameters for enhanced mixing, as well as establish design guidelines for such devices. Three types of geometric variations were researched. First, partitioning in channels was used to divide fluids into mixing zones with different concentrations. Various designs were investigated, and while these provided many potential solutions to achieving good mixing, they were difficult to fabricate. Secondly, structures were used to create lateral convection, or secondary flows. Most of the work in this category used obstacles to disrupt the flow. It was found that symmetric layouts of obstacles in a channel had little effect on mixing, whereas, asymmetric arrangements created lateral convection to enhance crossstream diffusion and increase mixing. Finally, structures that could create complex 3D advections were investigated. At high Reynolds numbers (Re = 50), 3D ramping or obstacles generated strong lateral convection. Microchannels with 3D slanted grooves were also investigated. Mixers with grooved surfaces generated helicity at low Reynolds numbers (Re � 5) and provided a promising way to reduce the diffusion path in microchannels by stretching and folding of fluid streams. Deeper grooves resulted in better mixing efficiency. The 3D helical advection created by the patterned grooves in a microchannel was studied by using particle tracing algorithms developed in this research to generate streaklines and Poincare maps, which were used to evaluate the mixing performance. The results illustrated that all the types of mixers could provide solutions to microfluidic mixing when dimensional parameters were optimized.

Graduation date: 1998

The Mixing Height (MH), Convective Condensation Level (CCL), and Convective Available Potential Energy (CAPE) are computed with different methods and we examined whether these parameters can help to discriminate between weak and strong convection. The observational data set contains soundings released from Stony Plain in Alberta and The Pas in Manitoba for the summers of 2006 and 2007. The major findings were: 1) The Mixing Height values computed with the Heffter method were reliable provided the critical inversion criterion was adjusted for Prairie conditions. 2) The Mixing Height values computed with the Moist Mixed layer method were in good agreement with Mixing Heights computed with the Heffter method. 3) The Mixing Height values computed with the Holzworth parcel method were less useful in that often the potential temperature did not decrease with height above the ground. 4) Observed convective cloud base heights tended to be lower than the CCL computed using the surface parcel method, the 50 mb mixed parcel method, and the moist mixed parcel method. 5) The MH, the sounding-based CCL, and the CAPE did not differentiate between weak and strong convection. 6) We derived a new parameter: the difference between the convective cloud base and the Moist Air Mixing Height. This parameter did discriminate between the likely occurrence of strong and weak convection.

Thesis (M.S.M.E.)--University of Delaware, 2006. / Principal faculty advisor: Ajay K. Prasad, Dept. of Mechanical Engineering. Includes bibliographical references.

Tests were conducted to characterize mixing in a square and a rectangular duct with respect to suitability for single point sampling of contaminants. Several configurations, such as a straight duct with unidirectional flow at the entrance section and straight ducts preceded by mixing elements (a 90° mitred bend, double 90° bends in S- and U-type configurations) were tested. For a straight duct of square cross section, the COV of tracer gas concentration at 19 duct diameters downstream of the gas release location is 143% (Center release). COVs of velocity and tracer gas concentration downstream of each mixing element in square duct setups were verified throughout this study. In the case of a rectangular duct with a 3:1 (width to height) aspect ratio, COVs of velocity and tracer gas concentration only downstream of a 90° mitred bend were verified. Tests were conducted to develop improved inlets for a Battelle bioaerosol sampling system. New inlets have been developed called the All Weather Inlets (AWI), which are designed to prevent entry of precipitation while maintaining aerosol penetration. The AWI has two inlets - one that samples at a flow rate of 780 L/min and the other one that is operated at a flow rate of 90 L/min. The initial version of the AWI-780 L/min unit featured an internal cone, which was removed because the penetration of the AWI-780 without the bottom chamber was higher than that of the Battelle inlet – 81% with the cone while 86% without the cone for around 9.5 µm AD at 2 km/h. The best bug-screen configuration was verified and a cutpoint management process was performed. The inlets were tested with different wind speeds from 2 to 24 km/h to verify the wind sensitivity of those inlets.

Current understanding of small-scale physical processes, such as mixing, in tropical water bodies is lacking and observations are scarce at best. This study sheds more light on these processes through a combined observational-modeling approach. For this purpose, observations were made in Valle de Bravo's freshwater reservoir, about 100 km west of Mexico City and at an elevation of 1830 m above sea surface. Turbulence kinetic energy dissipation (TKED) rates were estimated by fitting a theoretical Batchelor spectrum to the temperature gradient spectrum. From similarity scaling of dissipation rates, it was found that in the surface layer, winds were the main driving force in generating turbulence during the day, while convective forces were responsible during the night. Bottom boundary layer (BBL) mixing was mainly driven by internal wave (first vertical and first horizontal mode) breaking at the bottom. Lognormality of turbulence dissipation rates is also discussed for surface, intermediate and bottom boundary layers. For our modeling efforts, a state-of-the-art one-dimensional turbulence model was used and forced with the observed surface meteorology to obtain simulated temperature and dissipation rate profiles. The model results were found to be in good agreement with the observations, though minor differences in dissipation rates were found in the vicinity of the thermocline and the BBL.

Thesis (S.M.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), February 2001. / Includes bibliographical references (p. 116-118).