A systematic experimental study was carried out to determine how the heat transfer characteristics of a turbulent tube flow are affected by the length and diameter of a cylindrical plenum chamber which delivers fluid to the tube. The net pressure loss due to the presence of the plenum was also measured. The experimental arrangement was such that the fluid experiences a consecutive expansion and contraction in the plenum before entering the electrically heated test section. Air was the working fluid, and the Reynolds number was varied over the range from 5,000 to 60,000. It was found that at axial stations in the upstream portion of the tube, there are substantially higher heat transfer coefficients in the presence of longer plenums. Thus, a longer plenum functions as an enhancement device. On the other hand, the plenum diameter appears to have only a minor influence in the range investigated (i.e., plenum diameters equal to three and six times the tube diameter). The fully developed Nusselt numbers are independent of the plenum length and diameter. With longer plenums in place, the thermal entrance length showed increased sensitivity to Reynolds number in the fully turbulent regime. The pressure loss coefficient, which compares the plenum-related pressure loss with the velocity head in the tube, increases more or less linearly with the plenum length. With regard to experimental technique, it was demonstrated that guard heating/cooling of the electrical bus adjacent to the tube inlet is necessary for accurate heat transfer results at low Reynolds numbers but, although desirable, is less necessary at higher Reynolds numbers.
Effect of Plenum Length and Diameter on Turbulent Heat Transfer in a Downstream Tube and on Plenum-Related Pressure Losses
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Lau, S. C., Sparrow, E. M., and Ramsey, J. W. (August 1, 1981). "Effect of Plenum Length and Diameter on Turbulent Heat Transfer in a Downstream Tube and on Plenum-Related Pressure Losses." ASME. J. Heat Transfer. August 1981; 103(3): 415–422. https://doi.org/10.1115/1.3244479
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