Graduate Thesis Or Dissertation
 

Experimental investigation of natural convection of mercury in an open, uniformly heated, vertical channel

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  • An experimental investigation was conducted to determine characteristics of natural convection of mercury in a vertical channel, which was open at both ends. Two cases were investigated separately: 1) both walls uniformly and symmetrically heated, and 2) one wall heated with the other insulated. Heat transfer correlations in laminar flow were obtained for several aspect ratios (channel height/width ratio), using streamwise distance from the leading edge as the characteristic dimension. These were generalized into one correlation which contains aspect ratio, parametrically, for each set of boundary conditions. The influence of flow in from the open sides of the channel was investigated by installing insulating plates along the vertical edges. It was observed that the average velocity of a low Prandtl number fluid in buoyant flow increases as the channel width is reduced. This results in a reduction of average wall temperature until the channel is sufficiently narrow that viscous forces begin to play an important role in the integral momentum balance across the channel. The channel was thus observed to be more thermally efficient than a single heated plate for aspect ratios less than 20. Velocity profiles were obtained in the region of laminar flow for each set of boundary conditions and several channel aspect ratios. Integration of the profiles gave mass flux predictions for various conditions. A hot film anemometer was used, and has proven to be very accurate for velocity measurement in the 5-100 in/min range. The documentation of laminar instability in buoyant channel flow was also an integral facet of this experiment. The criterion used was Grashof modulus based on the streamwise coordinate where the first "observable" velocity disturbance waves were determined. Channel width decrease initially had a destabilizing influence on the flow. The transition criterion displayed a minimum value at a particular aspect ratio. It was also observed that, in narrow channel cases at the highest heat flux used, flow underwent transition then relaminarized as the boundary layers met and fully-developed conditions were established. The thesis also discusses the apparatus and unique aspects of the experimental program. Data reduction methods, property determination and some anomalies of hot film anemometry in a liquid metal are also discussed.
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