The Measurement of Local Wall Heat Transfer in Stationary U-Ducts of Strong Curvature, With Smooth and Rib Roughened Walls

The paper presents some of our recent experimental investigations of convective heat transfer in flow through stationary passages relevant to gas-turbine blade-cooling applications. The main objective of this effort is to produce local heat transfer data for CFD validation. Local Nusselt number measurements in flows through round-ended U-bends of square cross-section, with and without artificial wall roughness are presented. Our earlier LDA measurements of flows through these passages are first briefly reviewed and then the liquid-crystal technique for the measurement of local wall heat transfer inside passages of complex geometries is then presented. Tightly curved U-bends generate strong secondary motion and cause flow separation at the bend exit, which substantially raise turbulence levels. Wall heat transfer is significantly increased, especially immediately downstream of the U-bend, where it is over two times higher than in a straight duct. The local heat-transfer coefficient around the perimeter of the passage is also found to vary considerably because of the curvature-induced secondary motion. The introduction of surface ribs, results in a further increase in turbulence levels, a reduction in the size of the curvature induced separation bubble and a complex flow development after the bend exit with additional separation regions along the outer wall. Heat-transfer levels in the straight sections are more than doubled by the introduction of ribs. The effects of the bend on the overall levels of Nusselt number are not as strong as in the smooth U-bend, but are still significant. The effects of the bend on the perimetral variation of local heat-transfer coefficients within the ribbed downstream section are also substantial.