Influence of entrance geometry on flow field and heat transfer performance in stationary two-pass smooth parallelogram channels

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 temperature 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 under Re ranging from 5000 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 the present study is that 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 affected top and bottom wall Nu0 distributions of ASI respectively extends downstream to the mid-turn and the middle of second pass. Compared with FDI, ASI elevates the Nu‾0 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 are about 43.5% and 53.7% higher than the FDI at Re = 5000 and 20,000, respectively. Moreover, the correlations of Nu0 and fanning friction factor (f0) with Re are obtained and further compared with those of the corresponding square channels available from the literature.