Hydraulic Resistance of Pipes with Uniform Continuous Roughness in the Form of a Metric Thread of Varying Profile and an
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 5, September, 2020
HYDRAULIC RESISTANCE OF PIPES WITH UNIFORM CONTINUOUS ROUGHNESS IN THE FORM OF A METRIC THREAD OF VARYING PROFILE AND AN INSERTED TWISTED TAPE A. V. Zlobina and S. É. Tarasevichb
UDC 532.529.5:536.24
Results of experimental investigations into the hydraulic resistance of pipes with a metric thread of triangular, rectangular, and rounded profiles with an asperity height of 0.25 to 1.25 mm, which is cut inside, in air flow and with an inserted twisted tape have been given. The degree of swirling varied in the range from 2.5 to 7. A large data array has been obtained on the hydraulic resistance of pipes with a metric thread of varying profile and an inserted twisted tape. The influence of the Reynolds number Re, the degree of swirling, and the relative roughness height on the hydraulic resistance has been analyzed in the laminar regime of swirling flow with macrovortices and in the turbulent regime with macrovortices. Generalized dependences have been obtained for calculating the coefficient of hydraulic resistance of pipes with a triangular profile as a function of the degree of swirling and the relative roughness height at Re = 3000–80,000. Keywords: hydraulic resistance, metric thread, roughness, flow swirl, twisted tape. Introduction. Continuous roughness of the exterior and interior surfaces of pipes is efficient for enhancing boiling heat transfer in steam generators of steam thermal plants, particularly in the process of evaporation of cryogenic liquids [1–4]. The possibility of intensifying substantially the processes of heat transfer in condensation and boiling of refrigerants on horizontal pipes with a rough surface has been shown in [5]. One efficient method to intensify heat transfer is to organize a flow swirl. Investigations into the thermohydraulic efficiency of joint use of intensifiers in the form of artificial roughness and a flow swirl are of scientific interest, since it is expected that this combined form of intensification will be efficient in practical application [3, 6–8]. At the present time, numerous publications are available on investigations into the hydrodynamics in straight rough pipes with continuous artificial roughness for singe-phase flows [7, 8], but there are practically no experimental data for the case of joint use of rough walls formed by the cut metric thread [10] and the flow swirl [7]. Experimental Results. Experimental investigations into the hydraulic resistance of an air flow were conducted on the bench and on the same pipes with a metric thread whose description was given in [10] for the following geometric and dimensionless parameters of the pipes, the roughness, and the twisted tape: pipe length L = 0.66 m, volume mean pipe diameter d = 0.013–0.0136 m, relative roughness height Δ = Δ/d = 0.012–0.055, and tape thickness δ = 1 mm; the degree of swirling s/d ranged from 2.5 to 7. Metric-thread profiles of the investigated pipes are shown in Table 1. The Reynolds number was determined from
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