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Heat transfer from an array of parallel longitudinal fins to a turbulent air stream passing through the interfin spaces has been investigated both analytically/numerically and experimentally. The fins were integrally attached to a heated base plate, while the fin tips were shrouded to avoid leakage. In the analytical/numerical work, a conjugate problem was solved which encompassed turbulent flow and heat transfer in the air stream and heat conduction in the fins and in the base plate. The turbulence model and computational scheme were verified by comparison with experiment. It was found that the local heat transfer coefficients varied along the fins and along the surface of the base plate, with the lowest values in the corners formed by the fin/base plate intersections and the fin/shroud intersections. The numerically determined fin efficiencies did not differ appreciably from those calculated from the conventional pure-conduction fin model. Average Nusselt numbers, evaluated from the experimental data in conjunction with the numerically determined fin efficiencies (for derating the fin surface area), agreed well with those for fully developed heat transfer in a uniformly heated circular tube.

Experiments were performed to determine the response of the heat transfer from a longitudinal fin array to the presence of clearance between the fin tips and an adjacent shroud. During the course of the experiments, the clearance was varied parametrically, starting with the no-clearance case; parametric variations of the fin height and of the rate of fluid flow through the array were also carried out. Air was the working fluid, and the flow was turbulent. The fully developed heat transfer coefficients corresponding to the presence and to the absence of clearance were compared under the condition of equal air flowrate, and substantial clearance-related reductions were found to exist. For clearances equal to 10, 20, and 30 percent of the fin height, the heat transfer coefficients were 85, 74, and 64 percent of those for the no-clearance case. The ratio of the with-clearance and no-clearance heat transfer coefficients was a function only of the clearance-to-fin-height ratio, independent of the air flowrate, the fin height, and the fin efficiency model used to evaluate the heat transfer coefficients. The presence of clearance slowed the rate of thermal development in the entrance region.

The heat transfer characteristics of spokelike rods that extend outward from a rotating shaft have been determined experimentally. The experiments encompassed a number of geometrical parameters, including the length and diameter of the rods, the number of rods deployed around the circumference of the shaft, and the distance between the rods and the free end of the shaft. Also varied during the experiments was the rotational Reynolds number. Per-rod Nusselt numbers were evaluated from the experimental data. By incorporating the rod dimensions in the definition of the characteristic velocity that appears in the Reynolds number, a universal Nusselt–Reynolds–Prandtl correlation was obtained which is independent of the dimension ratios of the problem. It was also found that the Nusselt number results are independent of the number of rods in the circumferential array and are very weakly dependent on the distance from the rods to the free end of the shaft. The final correlation is conveyed by equations (18) and (19), with the Reynolds number defined by equations (7) and (10).

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