Smooth Channel Results

Figure 1.5 shows the results of Nusselt number ratios from leading and trailing surfaces. For the non-rotating (Re =5000) heat transfer ratio decreases monotonically for both leading and trailing surfaces from 2.2 near the thermal entrance of the first passage to about 1.4 near the downstream

X/D = 5 and than recovers to 2 before the flow enters the sharp 180° turn. The non-rotating heat transfer ratio reaches the highest value 3.0 right after the sharp 180° turn around (X/D = 10) due to the induced secondary flows of the 180° turn. The Nusselt number ratio approaches the fully developed turbulent flow value 2 near the outlet region. The non-rotating results agree with the previous study of local heat transfer ratios in a typical multi-pass coolant passage with smooth surface (see Azad et al.19). However, in the rotation case, rotation induced Coriolis forces produce cross- stream flow patterns, which push the main stream flow from the core towards the trailing surface in first pass. The situation is opposite in second pass. Coriolis forces push the main stream flow towards the leading surface. Therefore, the trailing surface heat transfer is higher than the leading surface in the first pass and similarly, the leading surface heat transfer is higher than trailing surface in the second pass. It is also noticed that the rotation effect on Nusselt number ratios for second channel leading and trailing surfaces is not as strong as first pass. This is because Coriolis and buoyancy forces move in the same direction in first pass while in second pass they move against each other (see Han et al.9). At channel orientation P =90°, rotation secondary flows vortices produced by Coriolis forces are impinging normally on trailing surface first pass and leading surface second pass. In contrast, at channel orientation P=135°, the secondary flow vortices migrate diagonally away from the corner region toward the center of the passage.

■" ■ " ■ Leading, Ro=0 --n--Trailing, Ro=0

■" ■ " ■ Leading, Ro=0 --n--Trailing, Ro=0

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