Experimental investigation of single- and two-phase convective heat transfer in slender rectangular 180° return bends.

• Single- and two-phase heat transfer of slender rectangular 180° return bends. • Local heat transfer ratios between the concave and convex surface. • Conjugated heat transfer is considered via a Biot number approach. The convective heat transfer characteristics of single-phase water flows and two-phase air-water flows through a slender 180° return bend with a rectangular cross-section are investigated. A moderately curved section is considered with a curvature ratio around 58, a hydraulic diameter of 3.9 mm and an aspect ratio of 1.067. The focus of this work is to investigate if there is a recognizable difference in heat transfer, respectively Nusselt numbers, between the outer (convex) and inner (concave) curved surface of the duct. An approach that considers the conjugated heat transfer is applied to adequately investigate this difference. Additionally the impact of the secondary gas-phase on the heat transfer characteristics is discussed. [ABSTRACT FROM AUTHOR]