Spirally fluted tubes

The extension of these PECs to two-phase heat transfer is complicated by the dependence of the local heat transfer coefficient on the local temperature difference and/or quality. Heat transfer and pressure drop have been considered in the evaluation of internally finned tubes for refrigerant evaporators [14] and for internally finned tubes, helically ribbed tubes, and spirally fluted tubes for refrigerant condensers [15]. Pumping power has been incorporated into the evaluation of inserts used to elevate subcooled boiling critical heat flux (CHF) [16, 17]. A discussion of the application of enhancement to two-phase systems is given by Webb [373],... 

Cox et al. [101] used several kinds of enhanced tubes to improve the performance of horizontal-tube multiple-effect plants for saline water conversion. Overall heat transfer coefficients (forced convection condensation inside and spray-film evaporation outside) were reported for tubes internally enhanced with circumferential V grooves (35 percent maximum increase in U) and protuberances produced by spiral indenting from the outside (4 percent increase). No increases were obtained with a knurled surface. Prince [102] obtained a 200 percent increase in U with internal circumferential ribs however, the outside (spray-film evaporation) was also enhanced. Luu and Bergles [15] reported data for enhanced condensation of R-113 in tubes with helical repeated-rib internal roughness. Average coefficients were increased 80 percent above smooth-tube values. Coefficients with deep spirally fluted tubes (envelope diameter basis) were increased by 50 percent. 

Several studies considered commercial deep spirally fluted tubes for horizontal singletube and tube-bundle condensers [152-154]. Commercially available configurations bear some resemblance to the Gregorig profile shown in Fig. 11.21. The derived condensing coefficients (envelope basis) range from essentially no improvement to over 300 percent above the plain-tube values. 

A major study of condensing on the outside of vertical enhanced tubes has been carried out at Oak Ridge National Laboratory in connection with geothermal Rankine cycle condensers. About 12 tubes were tested with ammonia, isobutane, and various fluorocarbons. The report by Domingo [163] on R-ll concluded that the best surface was the axially fluted tube, followed, in order, by the deep spirally fluted tube, spiral tubes, and roped tubes. The composite (vapor and tube wall) heat transfer coefficient was as much as 5.5 times the smooth-tube value. This high performance was further improved to a factor of 7.2 by using skirts to periodically drain off the condensate. 

S. M. MacBain and A. E. Bergles, Heat Transfer and Pressure Drop Characteristics of Forced Convection Evaporation in Deep Spirally Fluted Tubing, in Convective Flow Boiling, J. C. Chen ed., pp. 143-148, Taylor Francis, Washington, DC, 1996. 

The enhancement of vertical condensers remains an area of high interest due to potential large-scale power and process industry applications, for example, desalination, reboilers, and OTEC power plants. Tubes with exterior longitudinal fins or flutes, spiral flutes, and flutes on both the interior and exterior (doubly fluted) have been developed and tested. The common objectives are to use the Gregorig effect to create thin-film condensation at the tips of the flutes and to drain effectively. 

FIGURE 11.25 Heat transfer enhancement in vertical tube evaporator (tubes 48, 70, 94 axial flutes others spiral flutes) [164]. 

With falling film evaporation inside vertical tubes, Thomas and Young [135] found that heat transfer coefficients could be increased by more than a factor of 10 with loosely attached internal fins. While distorted tubes (e.g., doubly fluted and spirally corrugated) have been... 

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