Optimization of Heat Transfer Enhancement in Single-Phase Tubeside Flows with Twisted-Tape Inserts

摘要

The heat transfer enhancement in circular-tube exchangers with twisted-tape inserts is evaluated on the basis of two energy-conservation criteria: increased heat load for fixed pumping power and size, and reduced surface area (or size) for fixed heat duty and pressure drop. The consequent relative compactness of exchangers is also discussed. Single-phase flows of viscous liquids (5 ≤ Pr ≤ 1000) in both laminar and turbulent regimes, with inserts of different twist ratios (2 ≤ y ≤ ∞) are considered. The fundamental character of twisted-tape induced helical swirl flows is described, and in laminar flows the heat transfer is seen to increase with flow Reynolds number Re, tube length-to-diameter ratio (L/d), and severity of tape-twist y; in turbulent flows, the relatively less improvement is simply dependent upon y. The inserts are seen to be especially useful in laminar-regime applications, and optimal enhancement is obtained with higher Re flows in larger (L/d) exchangers. Depending upon Pr, Re, and (L/d), up to six times higher heat transfer rates can be sustained for a fixed tubular geometry and pumping power, and only 40% of equivalent empty-tubes surface area required for fixed heat load and pressure drop. This lends to the design of exchangers with significantly higher thermal effectiveness and size compactness. Twisted tapes, however, have practically insignificant enhancement benefits in the turbulent flow regime.