Twisted tapes

B23. Bundy, R. D., Gambill, W. R., and Wansborough, R. W., Heat transfer, burn-out and pressure drop for water in swirl flow through tubes with internal twisted tapes, Natl. Heat Transfer Conf, 4th, A.I.Ch.E., August 1960. 

Twisted tapes. These consist of a thin strip of twisted metal with the same width as the tube inside diameter that is slid into the tube. The flow is caused to spiral along the tube length. 

Static mixers. Static mixers are manufactured from thin metal strips with the same width as the tube internal diameter. In contrast with twisted tapes, the helix is not continuous. The flow is caused to twist through 180° and then twist in the reverse direction through 180°, and so on. The continuous splitting, reorientation and recombination of the fluid cause the enhancement as it flows through the insert. 

On the basis of heat transfer enhancement and pressure drop considerations, Twisted Tape H/d/8.5 would be chosen. However, a detailed examination of capital cost might cause this to be revised. 

EURAEC (1966) 0.0025 (0.064) Saturated twisted tapes Straight tube Decrease by 20%... 

Enhancement of CHF subcooled water flow boiling was sought to improve the thermal hydraulic design of thermonuclear fusion reactor components. Experimental study was carried out by Celata et al. (1994b), who used two SS-304 test sections of inside diameters 0.6 and 0.8 cm (0.24 and 0.31 in.). Compared with smooth channels, an increase of the CHF up to 50% was reported. Weisman et al. (1994) suggested a phenomenological model for CHF in tubes containing twisted tapes. 

Weisman, J., J. Y. Yang, and S. Usman, 1994, A Phenomenological Model for Boiling Heat Transfer and the CHF in Tubes Containing Twisted Tapes, Int. J. Heat Mass Transfer 37(l) 69-80. (4) Weisman, J., and S. H. Ying, 1983, Theoretically Based CHF Prediction at Low Qualities and Intermediate Flows, Trans. Am. Nuclear Soc. 45 832. (5)... 

FIGURE 5.5 Polarized PL from a gel-processed, uniaxially drawn film of EHO-OPPE (cf. Figure 5.4) in UHMW-PE. Twisted tapes (drawn to a draw ratio A = 80) are shown under excitation with UV light (365 nm) and the pictures were taken through a linear polarizer with its polarization axis oriented horizontally (a) and vertically (b). (After Weder, C., Sarwa, C., Bastiaansen, C., and Smith, P., Adv. 

Static mixers. Static mixers or motionless mixers are pressure-driven continuous mixing devices through which the melt is pumped, rotated, and divided, leading to effective mixing without the need for movable parts and mixing heads. One of the most commonly used static mixers is the twisted tape static mixer schematically shown in Fig. 3.25. 

The convection heat transfer coefficient can also be increased by inducing pulsating flow by pulse generators, by inducing swirl by inserting a twisted tape into the tube, or by inducing secondary flows by coiling the tube. 

Many commercially viable methods of inducing swirl flow inside of tubes are available in the form of either swirl flow tube inserts (twisted tapes, helical cores, spiral wire inserts) or special tube configurations... 

Mercerization has been recognised as a method for removing immature (dead) fibres to obtain level dyeing effect on cotton fabrics. The dead fibres are underdeveloped and appear as flat or slightly twisted tapes. They are non-crystalline, convolutions are sometimes absent, cell walls are extremely thin and the lumen is collapsed and hence do not contain dye to same extent as matured fibres. 

In a fully ripened cotton fibre there are between 200 and 300 convolutions per inch. They are more numerous in the better qualities than in coarser varieties. Sometimes, however, there are cells with comparatively little twist they are the unripe or only partially matured ones. On occasions twist is entirely absent, the cell walls are extremely thin and the lumen is collapsed so that the fibres look more like flat pieces of slightly twisted tape. These are dead fibres in which growth came to an end before they were mature. All cotton contains a few dead and unripe fibres, but where conditions for growth are favourable the number is small. Bad seasons, overcrowding, neglect of cultivation, and worn-out plants are amongst the causes of excessive undeveloped fibres. Their microscopic appearance is shown in Fig. 3.6. The detection of unripe or dead fibres is facilitated by treatment with... 

The enhancement of heat transfer inside a circular duct is often achieved by inserting a thin, metal tape in such a way that the tape is twisted about its longitudinal axis, as indicated in Fig. 5.47. Swirl flow is created in this manner. The width of the tape is usually the same as the internal diameter of the duct. The tape twist ratio XL is defined as H/d. When XL approaches infinity, the circular duct with the twisted tape becomes two semicircular straight ducts separated by the tape. 

FIGURE 5.47 A circular tube with a twisted tape inserted [1],... 

R. M. Manglik, and A. E. Bergles, Heat Transfer and Pressure Drop Correlations for Twisted-Tape Inserts in Isothermal Tubes Part I—Laminar Flows, J. Heat Transfer, (115) 881-889,1993. 

S. W. Hong, and A. E. Bergles, Augmentation of Laminar Flow Heat Transfer in Tubes by Means of Twisted-Tape Inserts, tech. rep. HTL-5, ISU-EMI-Ames 75011, Eng. Res. Inst., Iowa State University, Ames, 1974. 

Swirl-flow devices include a number of geometric arrangements or tube inserts for forced flow that create rotating and/or secondary flow inlet vortex generators, twisted-tape inserts, and axial-core inserts with a screw-type winding. 

Sununu [173] and Genetti and Priebe [174] used Kenics static mixers for heating of viscous oils (Figs. 11.26 and 11.28, curves e and f). These mixers consist of 360° segments of twisted tapes every second element is inverted and the segments are tack-welded together. The... 

Data for uniform-wall-temperature heating are plotted in Fig. 11.32, and the isothermal friction factors are plotted in Fig. 11.33. Twisted tapes and propellers were used by Koch [4] to heat air (curves a-d). Propellers produce higher heat transfer coefficients than twisted tapes however, this enhancement is at the expense of a rather large increase in friction factor, as seen in Fig. 11.33. Up to Re = 200, the friction factor for the twisted tape is the same as that for the empty half-tube (y = °o). The twisted-tape data of Marner and Bergles [114] with ethylene glycol exhibit an enhancement of about 300 percent above the smooth-tube values. Swirl at the pipe inlet does not produce any effective enhancement [192],... 

Turbulent-flow heat transfer in uniformly heated tubes with twisted-tape inserts has been correlated by [195]... 

FIGURE 11.33 Isothermal friction factor data for twisted-tape inserts. 

FIGURE 11.34 Performance of twisted-tape inserts with air (FG-2a). 

Twisted tapes are quite popular because of their simplicity and their adaptability to existing heat exchange equipment. They are ideal for hot-spot applications, since a short tape can cure the thermal problem with little effect on the overall pressure drop. Boiling curves for subcooled boiling with twisted tapes are similar to those for empty tubes [207] however, CHF can be increased by up to 100 percent [200], as shown in Fig. 11.36. Because of a dra-... 

FIGURE 11.36 Influence of twisted-tape inserts on subcooled boiling CHF of water (data from Ret 200). 

---