Heat transfer enhancement of intube flows in process heat exchangers by means of twisted-tape inserts.

摘要

Energy conservation and abatement of environmental degradation warrant the use of high performance heat transfer equipment. The application of enhancement technologies can result in substantial energy, materials, and cost savings. This has motivated the current research on heat transfer enhancement in viscous liquid flows by means of twisted-tape inserts.;An exhaustive survey of twisted tape literature is presented, highlighting the performance benefits for process heat exchangers. It also illustrates the lack of generalized applicability of existing thermal-hydraulic design correlations; accurate predictions of f and Nu for effective inclusion of twisted tapes in practical heat exchangers.;In an attempt to devise generalized correlations, a limiting case of twisted-tape usage is analyzed. Numerical solutions for constant property, laminar flow heat transfer in circular-segment ducts with uniform wall temperature are presented; the circular segment geometry models a straight tape insert of finite thickness. Two variations of the thermal boundary condition, that describe the two extremes of the tape fin effects, are considered. Results for hydrodynamically developed but thermally developing, and fully developed flows are obtained.;Furthermore, the development of the experimental test facility is delineated. It consists of two identical double-pipe heat exchangers, set up in a forced-circulation, closed-loop configuration with shellside flows of hot/cold water. Data for f and Nu are presented for intube flows with twisted tapes having y = 3.0, 4.5, and 6.0; ethylene glycol and water were used as test fluids with snug fitting tapes. The data are strongly influenced by the tape geometry, heat transfer conditions, and fluid flow parameters. Free convection also comes into play at low flow rates with large twist-ratio tapes and wall-to-bulk fluid temperature differences.;Flow regime maps are presented that describe the dominant enhancement phenomena in different Reynolds number regions. Based on a balance of forces, the parameter Sw(= Re