The three dimensional flow field in multi-pass channels with and without ribs was measured by magnetic resonance velocimetry (MRV), while heat transfer performance on the endwall of channels in the same geometry was investigated using transient thermochromic liquid crystal (TLC) technique. The whole field three-dimensional velocity distribution resolved by MRV was used to reveal the flow mechanism driving heat transfer enhancement in deferent regions of multi-pass channels with and without ribs. Results show that the flow characteristics in the right-angle bend as well as the second pass are dramatically different for the smooth and ribbed channel, resulting in totally different features of heat transfer distribution on the end wall in those two channels. For the smooth channel, strong Dean vortices form around the bend region near the outer wall where heat transfer is enhanced substantially. For the ribbed channel, flow separates and reattaches downstream of ribs and heat transfer close to the reattachment regions is strengthened noticeably. Comparison between velocity and heat transfer results suggest that the principle mechanisms driving heat transfer enhancement are both endwall directed velocity, even though they are induced by different flow structures for smooth and ribbed channel.

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