Visualization of effect of turbulators on local heat transfer distribution in a 180 degree turn with liquid crystals

Abstract

Liquid crystal experiments have been conducted to study the turbulent heat transfer for fully developed flow of air in a two-pass square channel with two opposite walls roughened with parallel and cross arrays of full ribs and with V-shaped ribs. The rib-roughened channel models the serpentine cooling passages in modern gas turbine blades. Results are obtained for a rib height-to-channel hydraulic diameter ratio of 0.0625, a rib pitch-to-height ratio of 10, angles-of-attack of 45', 90', and, 135', and Reynolds numbers of about 10,000 and 38,000. The results are presented as constant Nusselt number lines on the surface of one of the principal walls of the test section. The smooth channel heat transfer around the 180' turn is influenced by flow impingement, flow recirculation, secondary flow induced by centrifugal force at the turn, and flow separation at the tip of the divider or inner wall. The heat transfer in the after-turn region is generally higher compared with that in the before-tum region. There is a small recirculation zone at the first outer corner in the turn. The 90' ribs increase not only the heat transfer in the ribbed walls and the smooth walls in the two straight passes of the test channel, but also the heat transfer in the sharp turn. Between consecutive parallel 450 ribs, there are regions of high heat transfer near the upstream ends of the ribs. The high heat transfer regions may be near the outer wall or near the inner wall in the two-pass channel, depending on the orientation of the ribs. In the after-tum region, the main flow is forced toward the outer wall. It is advantageous to have the angled ribs in the after-tum region oriented in such a way that secondary flow caused by the angled ribs is directed toward the inner wall. The crossed 450 ribs cause the heat transfer distributions in the sharp turn to be quite different from those in the corresponding parallel rib cases. They have generally lower thermal performance than the parallel 45' ribs. Between consecutive V-450 ribs, there is a large region of high heat transfer. The V-450 ribs cause higher heat transfer in the sharp turn than both the parallel 45' ribs and the 900 ribs. The V-135' ribs do not perform as well as the V-450 ribs and cause generally lower heat transfer in the sharp turn than the V-45' ribs. In the lower Reynolds number cases, there are larger variations of the heat transfer distributions in the sharp turn.