Patent Number: 039309410
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows part of a cladding tube 1 of a fuel element 2. A plurality of fin rows 3 are provided on the outer tube surface at right angles to the axial direction 4 of the fuel element 2. Each fin row 3 (of which only two are designated) with the reference numeral is constituted of single fins 5. The individual fins 5 are circular segments of a square, rectangular or trapezoidal cross section or have a rhombic shape, respectively. The individual fins 5 of single fin rows 3 are offset relative to each other. This means that any gap between two fins of a given fin row is flanked on both sides by two fins belonging to the two fin rows that are immediately adjacent the given fin row. In this manner meandering passages are obtained. The fins of every other row are always located on the same generatrix of the fuel element 2. The distance x (e.g., x = 1.6 mm) between the fin rows 3 is twice the fin height h (h= 0.8 mm), so that the unobstructed clearance between two fins 5 that are in alignment parallel to the axis of the tube 1 is four times the height h. The width b of the fins 5 in this embodiment is 3 mm. The thickness d of the fins 5 is measured parallel to the axis of the cladding tube 1. FIG. 2 is a sectional view of the fuel element 2 showing the individual fins 5 of one of fins. The clearance e between the fins 5 is 2.9 mm in this example, thus slightly smaller than the width b of the fins, which is 3.0 mm in this embodiment. FIG. 3 is a diagram illustrating the roughness parameter R (h.sup..sup.+) as a function of the so-called dimensionless roughness height h.sup..sup.+ = h/d.sub.h .sup.. Re .sup.. .sqroot. f/2. Two curves 6 and 7 are plotted, curve 7 showing a measured result with offset roughnesses and curve 6 indicating a measured result with the values of p/h = 9.9 and h/b = 1.68. Evidently, much smaller values of R (h.sup..sup.+) are achieved with offset roughnesses, as is shown in curve 7. FIG. 4 shows a diagram illustrating the ratios St.sub.R /St.sub.o as a function of f.sub.R /f.sub.o of the offset fins according to curve 8 as compared with circumferential fins with different cross sections. These are curves 9, 10, 11, 12 and with the respective values of p/h = 9.9; 10.0; 47.2; 8.0 and 4.1 and the values for h/b = 1.68; 1.0; 1.7; 2.45 and 1.55. These measurements were performed in a rod bundle with the values of p.sub.R /d = 1.4; Re = 10.sup.5 ; f.sub.o = 4.55 .times. 10.sup..sup.-3 and St.sub.o = 2.8 .times. .sup..sup.-.sup.3. It is evident that the shape of roughness according to the present invention greatly improves the heat transfer coefficient. FIG. 5 is a diagram illustrating the ratio (St.sub.R /St.sub.o).sup.3 /f.sub.R /f.sub.o as a function of f.sub.R /f.sub.o. Again, a rod bundle with the same data as those shown in FIG. 4 has been used. Curve 14 again shows measured results with the offset roughness elements according to the present invention, while curves 15, 16, 17, 18 and 19 indicate the parameters p/h and h/b as curves 9 to 13 according to FIG. 4. This makes it particularly clear that the shape of roughness according to the present invention furnishes optimum results. The rod bundle investigated with the fuel element 1 was fabricated by the spark erosion technique. However, it can also be made as an opposed thread by cutting in such a way that a shape of roughness can be generated in which the webs have not a rectangular but a rhombic shape.