Helically ribbed 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],... 

Satisfactory performance is obtained with tubes having helical ribs on the inside surface, which generate a swirling flow. The resulting centrifugal action forces the water droplets toward the inner tube surface and prevents the formation of a steam film. The internally rifled tube maintains nucleate boiling at much higher steam temperature and pressure and with much lower mass velocities than those needed in smooth tubes. In modern practice, the most important criterion in drum boilers is the prevention of conditions that lead to DNB. 

The use of ordinary surface roughness elements tends to reduce the critical heat flux. However, by introducing helical and fin-form deformations on the tube surface, it is possible to induce swirl and turbulence in the flow, which promotes transfer of the liquid phase to the surface and thus gives enhancement. An example of work of this kind is illustrated in Fig. 15.135, which shows results obtained by Nishikawa et al. [329] using rifled and ribbed tubes. 

The fuel element is a steel tube of 12 mm outer diameter filled with fuel it has 4 helical ribs on the outer surface to ensure self-spacing in a bundle. The volumetric fractions of materials in the physical cell corresponding to a single fuel element are as follows ... 

The pin-type elements consist of a number of cast, cylindrical, fuel pins stacked end pn end within a thin-waU, circular, metal tube with a thin sodiumbond between the fuel and the tube wall (Fig. 43). The tvibes are spaced in a hexagonal pattern within each subassembly by a slingle, helical rib welded on the outside of each tube. The only difference between the twQ designs of the pin-type is that the Modified Reference Design has smaller diametral dimensions. In the pin-type element the coolant flows on the outside of the tubes. 

The fuel pin and fluid are sealed into the tube by means of end plugs silver soldered into place. The tube is rnade of AISI Type 347 stainless steel of 0.188 in. OD and 0.008 in. wall thickness. The helical rib outside the fuel tube is made of low-carbon steel for these tests, although stainless steel or other material will ultimately be used. The rib (cross section 0.065 in. by 0.025 in.), is made of flattened wire. The stock is edge wound on a 1-in. diameter arbor, set, and finally pulled out lengthwise to form a helix of the proper diameter and a 4-inch right hand pitch. The helix is slipped over the fuel tube and soldered into place. One hundred and sixty-nine fuel tubes are required for each subassembly. 

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. 

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