Heat Transfer and Hydraulic Resistance in Rough Tubes Including with Twisted Tape Inserts

Abstract

The spiral and cross-section wire insertions, knurls of a various configuration, microfinning, spherical, cylindrical, cone-shaped both other ledges and depressions, stamped surfaces etc. refer to the heat transfer intensifiers allowing considerably to augment a heat transfer at moderate or comparable growth of a pressure drop. The effect of a heat transfer intensification on rough surfaces is attained due to the additional vortex generation leading to raise of a turbulent diffusion in a conversion zone, to a turbulent kernel and due to lowering of stability and width of a viscous boundary layer with molecular thermal conduction at a surface. W. Nunner (1956) has determinated that in rough tubes at growth of ledge height of a roughness the heat transfer factors it is more to 3 times than value in smooth tubes. Intensifying agency of a roughness has been displayed in many subsequent papers (Dipprey & Sabersky, 1963; Isachenko et al., 1965; Kolar, 1965; Sheriff et al., 1964; Sheriff & Gumley, 1966; et al.). Among artificially roughened surfaces there are surfaces with a continuous roughness (for example, in the form of a thread) and with a discrete roughness (the roughness ledges pitch considerably exceeds their absolute sizes). The discrete roughness is more often preferable for heat transfer enhancement. However the continuous roughness of the outer and inner surfaces of tubes also can be effective for raise of heat transfer, especially at boiling and condensation (Berenson, 1962; Buznik et al., 1969; Danilova & Belsky, 1965; Ivanov et al., 1988; Nishikawa et al., 1982; et al.). The basic flow regularity in tubes with a continuous uniform granulous roughness has been determined in the first half of 20th century (Nikuradze, 1933; Schlichting, 1979; et al.). However the subsequent researches for tubes with a various uniform continuous sand roughness (organized by the single-start and multiple-start cross threads with triangular, rectangular and rounded profiles, and also in the form of ring bores and spherical ledges with passage and chess arrangement) have displayed considerable divergences with an existing explanation of the action mechanism of a roughness and with theoretical models of a boundary layer on a rough surface (Ibragimov et al., 1978; Isachenko et al., 1965; et al.). Thus intensity of a heat transfer and pressure drop in tubes with various aspects of roughnesses is rather individually and also is defined by not only a relative height of