The objective of this study was to investigate, through experiments, the combined effects of a sharp 180 deg turn and rib patterns on the pressure drop performance and distributions of the local heat transfer coefficient in an entire two-pass rib-roughened channel with a 180 deg turn. The rib pitch-to-equivalent diameter ratio P/de was 1.0, the rib-height-to-equivalent diameter ratio e/de was 0.09, and the rib angle relative to the main flow direction was varied from 30 ∼ 90 deg with an interval of 15 deg. Experiments were conducted for Reynolds numbers in the range 4000 ∼ 30,000. It was disclosed that, due to the interactions between the bend-induced secondary flow and the rib-induced secondary flow, the combination of rib patterns in the channel before and after the turn causes considerable differences in the pressure drop and heat transfer performance of the entire channel.

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Experimentation was conducted to examine the heat transfer and pressure drop characteristics in a rib-roughened rectangular passage with aspect ratio 2:1 for four rib configurations: 90 deg, 75 deg, 60 deg and 45 deg oblique ribs. The ribs were attached to two opposing long side walls instead of short side walls. In this study the oblique ribs were intended to function as secondary flow inducers as well as turbulators to improve the heat transfer of the bottom wall (one of the short side walls). The results revealed that, in order to enhance the heat transfer of the bottom wall, the oblique ribs should be arranged so that the secondary flow along the ribs hits the top wall instead of the bottom wall. Flow visualization test was performed to understand the heat transfer mechanisms. It was confirmed that the heat transfer enhancement at the bottom wall was attributed to the rib-induced secondary flow where the flow along the ribs hit the top wall, turned back and carried cold air from the passage core region towards the bottom wall. The highest average heat transfer was achieved for the 60 deg rib pattern due to the strong rotational momentum of the secondary flow and higher heat transfer enhancement on the rib-roughened walls in comparison to the other three rib patterns.

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