Thermo-fluidic heat transfer analysis in U-shaped mini-channel sinks

The thermal and fluid dynamic performance of a U-shaped mini water-cooled heat sink featuring a semicircular bend is investigated experimentally at three Reynolds numbers (Re ≈ 320, 735 and 1540) and a fixed heat source temperature of 50°C. Thermal characterization is performed through K-type thermocouples placed in the aluminium basement and Infrared Thermography (IRT), while velocity and vorticity fields are obtained by means of high-speed micro Particle Image Velocimetry (µPIV). A thermal transfer function is derived to relate the external Plexiglass surface temperature, measured by the infrared camera, to the temperature at the internal channel roof. Infrared images are pre-processed through a combined temporal and spatial filtering approach based on Proper Orthogonal Decomposition and an edge-preserving elliptic Gaussian kernel. Lumped system analysis is applied to both thermocouple and infrared data to extract thermal transient rate fields, which serve as a proxy for the local convective heat transfer coefficient. Results indicate that the thermal transient rate increases with Reynolds number, reflecting a faster cooling effect at higher flow rates. Its spatial distribution is, however, non-uniform along the channel. At the lowest Reynolds number, the region of enhanced heat transfer is located in the second pass, and progressively shifts upstream, toward the first pass as the Reynolds number increases. PIV results reveal a consistent flow behaviour: as the flow approaches the semicircular bend, it detaches from the convex inner wall and is displaced toward the concave outer wall. This displacement intensifies with increasing Reynolds number. Vorticity fields show a bifurcation of two positive vorticity regions, indicative of the onset and propagation of Dean vortices. The Dean numbers computed for the three flow conditions range from approximately 152 to 725, above the critical threshold for the formation of primary counter-rotating vortex pairs. The observed heat transfer enhancement in the bend region is attributed to the additional fluid mixing induced by these coherent structures, which transport colder fluid from the channel core toward the heated wall. The spatial correspondence between the high thermal transient rate region and the area of vorticity bifurcation supports this interpretation.