The inlet effects of multiple tubes on the adiabatic pressure drop of smooth horizontal tubes in the transitional flow regime

Abstract

There exists many investigations in the field of pressure drop inside smooth tubes; however, there is a research gap where this flow is in the transitional flow regime. All work in the transitional flow regime thus far has been concerned with a single tube and in limited cases the effect of different types of inlets were investigated. Many heat exchangers such as shell-and-tube heat exchangers consist of a number of closely packed tubes leading from a common header and in some cases may operate in or close to the transitional flow regime. However, it is not known what effect adjacent tubes will have on pressure drop and the flow distribution in the transitional flow regime. The purpose of this study is therefore to investigate the effect that the presence of adjacent tubes will have on the pressure drop in the transitional flow regime. The study is limited to smooth and circular, horizontal tubes in the fully developed, transitional flow regime and adiabatic pressure drops. The transition effects were investigated experimentally by developing and building an experimental set-up on which, firstly, the pressure drops could be measured of one tube to be used as a reference and then secondly, the pressure drop of three tubes in parallel, equally spaced, 1.4 diameters apart. The internal tube diameters of all tubes were 3.97 mm and the tube lengths were 6 m. The pressure drops were measured over a length of 1.97 m, at the end of the tubes where the flow was fully developed. The pressure drops were measured with pressure transducers while the inlet and outlet temperatures of the water were measured with PT100 probes. All the tubes were connected to a calming section to ensure a square-edge inlet. Experiments were conducted with water at Reynolds numbers from 700 to 5 100 to ensure that the pressure drops and thus friction factors could be determined for fully developed flow throughout the laminar, transitional and turbulent flow regimes. The uncertainty of the friction factors were all less than 1%. It was found that the centre tube experienced an earlier onset of transition than that previously seen in single tube tests, at a Reynolds number of 1 840. In the outer tubes, transitional flow was delayed well beyond that previously seen in literature. The flow in all three tubes underwent transition into fully developed turbulent flow by the maximum Reynolds number of 3 340. The effect of having multiple, adjacent inlets caused a maldistribution in the mass flow rate, with a 5.8% difference in the flow rates of the outer tubes in the transitional flow regime. New correlations were developed to predict the friction factor for transitional flow in each of three adjacent tubes at an inlet pitch distance of 1.4 times the inner tube diameter. Overall, it can be concluded that multiple tube entrances have an effect on the transitional flow in all of the tubes and should be further investigated for other pitch distance and tube arrangements.