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 convec tive heat transfer in flow through stationary passages relevant to gas turb ine blade-cooling applications. The main objective of this effort is to pro duce local heat transfer data for CFD validation. Local Nusselt number meas urements in flows through round-ended U-bends of square cross section, with and without artificial Mall roughness, are presented Our earlier LDA measu rements of flows through these passages are first briefly reviewed and then the liquid-crystal technique for the measurement of local wall heat transf er inside passages of complex geometries is presented. Tightly curved U-ben ds generate strong secondary motion and cause flow separation at the bend e xit, which substantially raise turbulence levels. Wall heat transfer is sig nificantly increased, especially immediately downstream of the U-bend, wher e it is over two rimes higher than in a straight duct. The local heat trans fer coefficient around the perimeter of the passage is also found to vary c onsiderably because of the curvature-induced secondary motion. The introduc tion of surface ribs results in a further increase in turbulence levels, a reduction in the size of the curvature-induced separation bubble, and a com plex flow development after the bend exit with additional separation region s along the outer wall. Heat transfer levels in the straight sections are m ore than doubled by the introduction of ribs. The effects of the bend on th e overall levels of Nusselt number are not as strong as in the smooth U-ben d, but are still significant. The effects of the bend on the perimetral var iation of local heat transfer coefficients within the ribbed downstream sec tion are also substantial. [S0889-504X(00)00802-3].