Hydraulic Resistance of Flows in Tubes with Inserted Straight and Twisted Tapes
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HYDRAULIC RESISTANCE OF FLOWS IN TUBES WITH INSERTED STRAIGHT AND TWISTED TAPES Yu. Ya. Pechenegov
UDC 532.529
The results of an experimental study of hydraulic resistance of a laminar flow with air stream macrovortices in a long tube with inserted straight and twisted tapes are presented. It was found that there is a minimum in the relationship of the hydraulic resistance coefficient with the rotation pitch of the tape inserted into the tube. An empirical equation, recommended for use in a wide tape rotation pitch variation range, has been derived for calculating hydraulic resistance. Keywords: coefficient of hydraulic resistance, swirled flow in tube, straight tape, twisted tape, hydrodynamic stabilization, minimum resistance, generalized equation.
To intensify heat transfer processes in tubular apparatuses, straight and twisted tapes are inserted in the tubes [1–3]. When straight tapes are placed in the diametral plane of the tubes, semicircular channels are formed. In such channels with angular zones, an additional secondary flow in the transverse direction joins the main flow of the heat carrier streams in the longitudinal direction, which exerts an activating effect on the heat and pulse transfer processes. Insertion of twisted tapes ensures swirling of the flow of the medium, which leads to vortex formation, radical restructuring of the flow vis-à-vis its straight flow and, as a result, to further intensification of pulse and heat transfer processes. Data on hydraulic resistance of flows of homogeneous media in tubes with tape inserts are scarce in the literature. Calculating equations obtained by various authors to determine the characteristics of hydraulic resistance are often not congruent. In this respect, primarily short channels with tube length to tube diameter ratio l /d ≤ 70, tapes with a relative twist pitch s/d ≤ 20 ( s – absolute pitch corresponding to full 360° rotation of the tape and d – inner diameter of the tube), and streams with developed turbulent flow conditions were investigated. In the region of laminar flow with macrovortices where the Reynolds number (Re) is greater than its critical value Re cr , which is in accord with transition to turbulent conditions of intensification of heat transfer with swirling of the flow in the tubes by the twisted tape, is economically expedient [3]. In this case, the economic efficiency of intensification of heat transfer increases with increase of s/d value. In the region of developed turbulent flow (where Re > Re cr ), flow swirling by use of twisted tape becomes economically inexpedient. Evidently, further investigations of hydraulic resistance during flow of various media in tubes having tape inserts should be aimed at economically preferable ranges of variation of the parameters that affect hydraulic resistance. Engels Technological Institute (Branch), Yu. A. Gagarin Saratov State Technical University (Éngel’skii Tekhnologicheskii Institut (Filial) Saratovskogo Gosudarstvennogo Tehnicheskogo Universiteta im. Yu. A. Gagarina), Russia; e-mail: y.pec
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