The effect of adjacent tubes on the diabatic friction factors in the transitional flow regime

Abstract

Heat exchangers are used throughout the world in important processes such as the generation of electrical energy. Modern heat exchangers are often forced to operate in the transitional flow regime, where flow can be unpredictable. Most of the research that has been done on the transitional flow regime has focussed on the influence of heat transfer and the inlet effects. However, all these studies made use of only a single tube, while most heat exchangers would typically have a bundle of tubes such as in shell-and-tube type heat exchangers. The purpose of this study was to investigate the effect of adjacent tubes on the transitional flow regime during diabatic conditions. An experimental set-up was purposefully built for this investigation and two test sections were investigated. A single-tube test section was built for validation purposes, since similar work has been done. A triple-tube test section was built with three tubes spaced at a pitch distance of 1.4 outer diameters. The mass flow rate, as well as the pressure drops over the fully-developed section was measured for each tube. From the pressure drop data the friction factors were calculated. Furthermore, a heat flux of 3 kW/m2 was applied to each tube and the inlet, outlet and wall temperatures were measured, to ensure that specifically the diabatic friction factors were determined. Water was used as the working fluid and tests were run over a Reynolds number range of 1 000 - 6 500. An uncertainty analysis showed the maximum uncertainty of the friction factors to be 8.3%. The laminar, transitional and turbulent flow regimes could be identified from the friction factor data. The results from the single-tube test section correlated well to the literature with transition starting at a Reynolds number of 2 380 and ending at 3 050. The results from the triple-tube test section showed the start of transition to be initiated by the presence of adjacent tubes, with the Centre-tube entering transition at a Reynolds number of 1 970. The outer tubes experience a delayed start in transition at Reynolds numbers of 3 000 and 2 800 for the Left-tube and Right-tube respectively. The end of transition occurred at approximately the same Reynolds number (3 100) for all three tubes of the triple-tube test section. Since the Centre-tube entered transition earlier than the outer tubes, maldistribution was evident, with the water taking the path of least resistance. The flow rate in the Centre-tube showed an average difference of 2.8% in the Reynolds number range of 1 970 to 3 150. Maldistribution proved to be negligible when all three tubes were in the laminar or turbulent flow regimes.