Fluid Flow and Heat Transfer Features at a Cross-Flow of Dimpled Tubes in a Confined Space

Abstract

This paper provides the primary results of an experimental study into the fluid flow and heat transfer features at a cross-flow of a dimpled tube in a rectangular-shaped duct between two adjacent dimpled tubes. The cylindrical dimples were engraved on each tube surface both in the staggered and in-line mode; altogether nine dimpled tubes were tested in the range of the Reynolds number Re from 8,000 to 115,000. The first group (four samples) represents tubes structured with symmetrical dimples drilled in the radial direction, while the second group (five samples) is tubes with asymmetrical dimples. In the latter case each dimple was made in such a way that its axis is parallel to the tube diameter with a certain clearance between axes. For comparisons a row of smooth tubes of the same configuration was tested under identical fluid boundary conditions. Three factors primarily influencing heat transfer are under consideration in this paper: a) increase in a heat exchange surface due to a tube dimpling, b) variations in the flow pattern, c) interaction between boundary layer and main flow. Behind a smooth tube in confined space the reverse flow zone grows initially to Re = 37,000 however decreases at larger Reynolds numbers. Unlike this, behind a dimpled tube in confined space the reverse flow zone reduces at low Reynolds numbers to reach minimum magnitude at Re = 10,000–28,000, and increases afterwards to become approximately constant at Reynolds numbers over 45,000. It has been found, the reverse flow length depends on the Reynolds number, dimple parameters and configuration. The frequency spectrum of the dimpled tube is different from that occurring for a smooth tube. A few frequency ‘picks’ with corresponding the Strouhal numbers were registered including those typical to a single dimple on a flat plate. The heat transfer enhancement rates of around 45%–55% compared with a smooth tube in confined space were obtained depending on dimple parameters and flow regimes. Increase in the heat transfer enhancement rate for tubes with shallow dimples exceeds growth of heat exchange surface due to a dimpling. Increases in a pressure drop at the tube bundle caused by dimpling do not exceed 14%.

Experimental study of heat transfer and hydraulic resistance at cross flow of tube bundle with indentations

“…In [3], hydraulic characteristics of the flow of a circular tube with cylindrical symmetrical and asymmetrical indentations were investigated. Studies Reynolds numbers (up to 10 4 ), the presence of indentations on the surface reduces the length of the flow separation zone behind the tube in comparison with the case of a smooth tube, and this effect is intensified with increasing depth of the dimples.…”

“…In [1][2][3][4][5], devoted to the cross flow of a single tube, the main emphasis is on the study of hydrodynamic regimes, the point of flow separation, the frequency of vortex shedding, the dependence of the drag resistance on the Reynolds number. In the works devoted to tube bundles, hydraulic losses, heat transfer and thermal and hydraulic efficiency were investigated.…”

“…Available publications [1][2][3][4][5][6][7][8] consider the cross flow of both a single tube and tube bundles with surface indentations in a wide range of Reynolds numbers. In [1][2][3][4][5], devoted to the cross flow of a single tube, the main emphasis is on the study of hydrodynamic regimes, the point of flow separation, the frequency of vortex shedding, the dependence of the drag resistance on the Reynolds number.…”

Features of Flowing Round and Heat Transfer of One-Row of Cylinders With Spiral Grooves in Interaction With a Cross - Flow of Air

“…Основним методом збільшення передачі теплоти є оребрення різноманітної форми [2]; однак воно характеризується збільшеною вагою теплообмінного обладнання и підвищенням втрат тиску. Як показали дослідження, виконані в роботах [3,4], застосування заглиблень на теплообмінній поверхні труби істотно покращують теплогідравлічні і масогабаритні харак-теристики обладнання. Ці дослідження підтвердили раніше зроблений висновок про те, що заглиблення різної форми на теплообмінній поверхні дозволяють інтенсифікувати теплообмін при зменшених втратах тиску [4].…”

“…Як показали дослідження, виконані в роботах [3,4], застосування заглиблень на теплообмінній поверхні труби істотно покращують теплогідравлічні і масогабаритні харак-теристики обладнання. Ці дослідження підтвердили раніше зроблений висновок про те, що заглиблення різної форми на теплообмінній поверхні дозволяють інтенсифікувати теплообмін при зменшених втратах тиску [4]. Одним з варіантів заглиблення на поверхні є спіральна канавка прямокутного перерізу на зовнішній поверхні труби, яка генерує вихрові структури.…”