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Laminar flow heat transfer downstream from U-bends

Abdelmessih, Amanie Nassif

The use of horizontal U-tubes is common in several types of heat exchange equipment. However, the present understanding of laminar flow heat transfer downstream from the U-bend is limited. Consequently, the laminar flow heat transfer downstream from the unheated, vertical bends in horizontal U-tubes with electrically heated straight tube sections was investigated. Four U-tubes with curvature ratios of 4.84, 7.66, 12.35 and 25.36 were studied. Distilled water and almost pure ethylene glycol solutions (water content 1 to 5 %) were the test fluids. For each test section, local axial and peripheral wall temperatures were measured and the local peripheral heat transfer coefficients at the various locations were calculated. The experiments covered the local bulk Reynolds number range of 120 to 2500. The local bulk Prandtl number varied between 4 and 110, while the Grashof number ranged from 2,500 to 1,130,000. The uniform wall heat flux ranged from 900 to 4230 Btu/hr.sq.ft (3.12 to 13.33 kW/sq.m.).

This investigation permitted a better understanding of the interaction of the primary, secondary and tertiary flow patterns; i.e. the combination of forced and natural convection with the centrifugal effects. Also, the following correlation was developed:

Nu = {4.364 + 0.3271Gr^0.25 Pr^0.25 + 1.955x10^-6 Re^1.6 De^0.8 e^-0.0725(X/di)} (μD/μW)^0.14

This correlation predicts the heat transfer coefficient downstream from unheated U-bends as well as the heat transfer coefficient in straight tubes.