| Summary: | 碩士 === 國立清華大學 === 動力機械工程學系 === 104 === PIV and Infrared Thermometry measurements are respectively conducted to study the effects of entrance geometry and Reynolds number (Re) on the detailed flow fields and local heat transfer distributions in a stationary two-pass smooth parallelogram channel with 180-deg sharp turn. Two entrance geometries, including a fully developed inlet condition (FDI) as well as an asymmetrically and suddenly contracted inlet condition (ASI), are investigated. The smooth parallelogram channel has equal adjacent sides of 45.5 mm in length and two pairs of adjacent angles are 45-deg and 135-deg. Local (Nu0) and regionally averaged ((Nu) ̅_0) Nusselt numbers over entire top and bottom walls along the first and second passages and through the bend region with the associated pressure drop are examined at Re ranging from 5,000 to 20,000. Moreover, cross-sectional secondary- flow patterns as well as the near-wall streamwise mean velocity components and turbulent kinetic energy are analyzed to correlate the relationship between flow characteristics and heat transfer distributions at Re=10,000. The most distinct finding of present study is the asymmetric thermal and fluid flow features on the top and bottom wall side, in contrast to symmetric ones in the corresponding square and rectangular channels. The Nu0 distributions over top and bottom walls of ASI channel respectively extends downstream to the mid-turn and the middle of second pass. Compared with FDI channel, the (Nu) ̅_0 for ASI are elevated to the levels about 65.3-70.1%, 14.2-13.7%, and 23.9-14.0% in the first passage, turn region, and second passage, respectively, under constant flow rate condition. Thermal performance factors of the ASI channel are about 43.5% and 53.7% higher than the FDI channel at Re=5,000 and 20,000, respectively. Moreover, the correlations of Nu0 and fanning friction factor (f0) with Re as the controlling parameter are obtained and validated by those of the comparable square channels available from the literature.
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