An experimental study was performed to investigate the mechanics of fluid flow in a 30<SUP>o</SUP> annular diffuser, and to study the periodicity of flow oscillations in this region. Surface oil-flow patterns and smoke-flow visualisation experiments were performed with a centrebody concentric and eccentric in the diffuser. Above a threshold offset, the measurements revealed two contra-rotating periodic vortices in antiphase with each other, symmetrically disposed about the plane of minimum clearance in the annulus, and originating in the 30<SUP>o</SUP> diffuser. Steady pressure measurements indicated that the steady fluid forces acting on the centre body are decentralising, and any small perturbation will result in the centrebody being pushed towards the wall of the diffuser. Unsteady pressure and force measurements showed that for a mean eccentric centrebody position, there was a predominantly tangential unsteady vortex force present with the centre body both fixed and vibrating. This vortex force scaled linearly with flow velocity, indicating a Strouhal-type mechanism. The magnitude of the vortex force was independent of both amplitude and frequency of vibration of the centrebody, indicating a forced vibration effect. The exception was when the centre body frequency approached the vortex shedding frequency, in which case lock-on occurred. For the geometry considered, lock-on does <i>not</i> significantly increase the unsteady forces acting on the centrebody. During lock-on it was found that the vortices could not only be influenced by centrebody motion, but could be completely suppressed by closely controlling the amplitude and frequency of the centrebody. The effect that shaking the centrebody has on the different flow regimes in which the annular diffuser operates is explained. The vortices could also be eliminted by a) attaching a small helical fence to the surface of the centrebody, and b) by inserting a perforated liner within, and downstream of, the diffuser section. A small perturbation theoretical analysis of the unsteady flow in the diffuser has been developed. The flow was computed numerically, and the predicted self-induced forces examined. The analysis predicted mainly negative damping for the configurations examined. The predicted magnitude of the unsteady forces agreed with experimental results. Finally, the flow was also predicted analytically, and a good level of agreement with the numerical study was found.

No description available.

<p>Dynamic Air Diffuser</p>

Dynamic Air Diffuser

The performance of a vertical axis wind turbine with and without a diffuser was studied using direct force measurement technique applied to a scaled model of the rotor in a water tunnel. The experiment was conducted at different tip-speed ratios. The maximum power coefficient for the turbine was found to be equal to 0.35 for the rotor with diffuser and to 0.26 for the rotor without diffuser. Therefore, the maximum power coefficient was increased by 35% when the diffuser was used in the configuration. In the second part of this work, the flow patterns downstream of the turbine were studied by the particle image velocimetry (PIV) technique. Six different tip-speed ratios were considered for each configuration (with and without a diffuser). The vorticity and the streamline plots provide insight into the flow physics in each configuration. In addition, the swept area of a full-scale rotor was calculated for both a diffuser-augmented and a bare turbine for a range of power outputs. / Graduate

No description available.

Diffusers are devices that are designed to spread or scatter light. Many different types of diffusers are readily available in the commercial and scientific industries, but most are designed with a specific application in mind. Thus, when attempting to use a diffuser in an unconventional way, it is important to understand how it will behave in the system it will be used in. One example is a diffuser in a coherently illuminated, wide field of view system such as a laser spot tracker. For some diffusers, coherent illumination and/or off-axis illumination can have a major impact on their diffusing properties, which may or may not be acceptable to system performance. This project evaluates the pros and cons of several types of diffusers in order to identify the most effective solution based on the parameters of a system, using a laser spot tracker as an example. First, three diffusers are down selected from the list based on their refractive and diffractive properties. Next, the candidates are evaluated analytically and/or experimentally to characterize their behavior when illuminated by a collimated laser over a wide range of incident angles. Results suggest that a computer generated diffractive diffuser is the most flexible design option for the described laser spot tracker, but some drawbacks such as diffraction efficiency and maximum diffusion angle do need to be considered. This selection method can be extended to serve as a guide for making the proper diffuser choice in many different applications.

A subsonic wind tunnel facility was built and tested as part of a base line test investigating flow within a diffuser-collector. Facility controls allowed the quarter scale model to match both Reynolds number and Mach number. Mass averaged conditions at the diffuser inlet during testing were determined as 1.939 ? 106 for Reynolds number based upon diffuser inlet hydraulic diameter, and 0.418 for Mach number. A flow conditioning section prior to test section contained several interchangeable sections. Flow conditioning components were used to create flow characteristic of that leaving the last stage of a land-based gas turbine. The diffuser-collector subsystem was evaluated through the use of wall static pressure measurements, a variety of probe traverse measurements, and Stereo-PIV. Flow within the collector and diffuser were determined to be heavily dependent upon the collector geometry. PIV measurements showed the development of two large counter rotating vortices within the collector. Each symmetric vortex grew and shifted according to the collector geometry while creating complex regions of flow. Pressure recovery within the diffuser was in range of 0.47 to 0.78, and would drop to 0.52 at the collector exit. The drop in pressure recovery was presumed to be a combination of inefficient diffusion in the collector and losses due to the vortices. The baseline test was found to be successful in terms of facility design, and determining the critical flow phenomena. Further testing and experimentation are necessary to evaluate specific details of the collector geometry's effect upon the pressure recovery and flow development. / Master of Science

Eight decreasing adverse pressure gradient flows, and the similar regions of an initially increasing adverse pressure gradient flow, are examined in terms of the two experimentally observed half-power regions. The existing semi-empirical and analytical mean velocity profiles are examined and their range of applicability is determined in terms of the ratio of outer to inner half-power slopes. Three variations of the k-e model of turbulence are evaluated in terms of how well they predict the turbulence field in an eight degree conical diffuser. The model of Nagano and Tagawa (1990) is seen to be superior to the others. It is possible for Nagano and Tagawa’s model to yield reasonable prediction of k and E because they implemented the Hanjalic and Launder (1980) modification for the irrotational strains. However, the k-e models prediction of the Reynolds stresses is poor. / Thesis / Master of Engineering (ME)

Stratification is a natural occurrence in deep lakes and reservoirs. This phenomenon results in two distinct layers, the warmer, less dense epilimnion on top and the colder, denser, hypolimnion on the bottom. The epilimnion remains saturated with dissolved oxygen (DO) from mass transfer with the atmosphere, while the hypolimnion continues to undergo oxygen-depleting processes. During seasons of high oxygen demand the hypolimnion often becomes anoxic and results in the release of compounds, such as Iron, Manganese, Hydrogen Sulfide, and Phosphorous from the sediment. Iron, Manganese, and Hydrogen Sulfide can require addition Chlorine for water treatment plants, thus increasing cost and the potential production of DBP's, while the release of phosphorous results in algal blooms the following year. Spring Hollow Reservoir, located in Roanoke County, Virginia is a deep reservoir that undergoes stratification during the summer months. During 1997 Roanoke County purchased a bubble-plume diffuser from Tennessee Valley Authority (TVA) to oxygenate the hypolimnion to maintain long-term water quality. Spring Hollow currently operates the diffuser, with compressed air, during late summer months when DO levels in the hypolimnion reach approximately 4 mg/L. Observations during oxygenation have identified changing DO addition rates during diffuser operation and changing DO depletion rates following termination of oxygenation. Future research should focus on developing a quantitative understanding of the changing rates as they are related to diffuser induced oxygen demand. / Master of Science