| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 88 === Abstract
Large mixing effect induced by flow turbulence usually is the primary mechanism occurred in the traditional mixers designed for two or more streams of different fluids, e. g., the jet, bluff body, and swirler. The production of turbulence, however, has to be three dimensional according to the classical turbulence theory. In the applications where the turbulence is hardly generated, for instance, in a micro combustion chamber with a very narrow, almost-two-dimensional channel, most of the mixing is proceeded by molecular diffusion. To achieve a rapid mixing in a small space becomes a challenge to investigators. Therefore, there is a special need to find a mixer which is able to attain an apt mixing effect during a short period in a short distance. The mixing performances of applying three traditional mixers, jet, bluff body, and swirler, to a planar flow are evaluated in this study.
Air and nitrogen streams are supplied separately to an almost two-dimensional channel, which is made of transparent acrylics. Laser-light-sheet-enhanced smoke-trace flow visualization method and oxygen concentration measurements are conducted over the whole field to evaluate the mixing performance of different means. The plane- jet presents a small spatial gradient of oxygen concentration down to a far distance from the jet exit. The shear layer is not developed quickly in the narrow space, partly because of the reduced entrainment in the small space. The bluff-body mixer although initiates appreciable mixing in a short distance, the over-all performance is not as good as the jet mixer. The recirculation bubble in the wake does not provide a good stirring pool for the laminar mixing. Encouragingly is that the swirl mixer expedites the mixing rapidly and attains an excellent performance in a short distance. The physical mechanism may be resorted to the rotating motion of flows generated in the swirl mixer, which causes a prolonged residence time between the participating fluids and thus increases the molecular mixing time during the mixing process. The swirl mixer is hence a potential candidate for rapid mixing in a planar flow.
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