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Fully developed turbulent flow through three concentric annuli was investigated experimentally for a Reynolds-number rangeRe= 2 × 104−2 × 105. Measurements were made of the pressure drop, the positions of zero shear stress and maximum velocity, and the velocity distribution in annuli of radius ratios α = 0.02, 0.04 and 0.1, respectively. The results for the key problem in the flow through annuli, the position of zero shear stress, showed that this position is not coincident with the position of maximum velocity. Furthermore, the investigation showed the strong influence of spacers on the velocity and shear-stress distributions. The numerous theoretical and experimental results in the literature which are based on the coincidence of the positions of zero shear stress and maximum velocity are not in agreement with reality.

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Abstract-On the basis of friction factor relationships of a simple model a method is developed which allows the prediction of friction factors for turbulent flow in non-circular channels if only the geometry factor of the pressure drop relationship for laminar flow is known. The proposed method of calculation is tested with numerous experimental results from the literature with respect to non-circular channels such as triangular shaped ducts, eccentric annuli and rod bundles in hexagonal and square arrays in circular tubes, hexagonal and square channels. It turns out that the proposed method provides an excellent description of all the experimental data at hand.channel wall and rods; annular zone parameter; wall distance; dimensionless wall distance y+ =

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Axial and azimuthal turbulence intensities in the rod-gap region are shown, for developed turbulent flow through parallel rod arrays, to increase strongly with decreasing rod spacing. Two array geometries are reported: one was constructed from a rectangular cross-section duct containing four rods and spaced at five pitch-to-diameter or wall-to-diameter ratios; the second was a test section containing six rods set in a regular square-pitch array to represent the interior flow region of a large array.Measurements were made of the mean axial velocity, wall-shear-stress variation, axial-pressure distribution and Reynolds stresses. Techniques for resolving secondary-flow velocities to within ±1% of the local axial velocity failed to detect significant non-zero mean secondary-flow components. Analysis of the turbulent flow structure showed an energetic azimuthal turbulent-velocity component in the open rod gap for both geometries. The axial turbulent velocity has a coupled large-scale semiperiodic structure, with an antiphase relationship across the rod-gap centre or subchannel boundary. This structure is considered to be generated by an incompressible-flow parallel-channel instability, and, for closely spaced rod arrays, is the dominant process for intersubchannel mass, heat or momentum exchange.