The influence of a multiple tube-inlet condition on heat transfer in the transitional flow regime

Abstract

In the industrial design of heat exchangers, engineers have long followed the general rule of avoiding transitional flow, and have rather designed a system operating in the turbulent flow regime. Whilst the turbulent regime is better for heat transfer, the higher friction inside the tube results in a much higher pressure drop which inevitably results in the system requiring a more powerful pump than if the system were to operate in the laminar regime. Designing a heat exchanger that operates in the turbulent flow regime is often the safer option, since little published design data is available for flow in the transitional flow regime, giving rise to numerous unwanted uncertainties during the design phase. Recent research into the transitional flow regime has resulted in promising experimental data that shows the regime is not as unstable as previously suspected. The regime allows for higher heat transfer than flows in the laminar regime, yet lower pressure drops than flows in the turbulent regime. Numerous investigations have previously been performed on a single uniformly heated tube operating in the transitional flow regime, however, there exists no data on the influence of a multiple tube inlet condition, as typically found in shell and tube heat exchangers, on the heat transfer characteristics. The purpose of this study was thus to determine the influence of varying tube pitch ratios on the fully developed heat transfer characteristics of three smooth circular horizontal tubes. An experimental set up was designed and built to accommodate a single tube heat exchanger used for validation purposes, and a multiple tube heat exchanger comprising of three identical and equally spaced tubes. Using a DC power supply, the tubes were uniformly heated at 2, 3 and 4 kW/m2 along the length of the test section. The heat transfer characteristics were determined experimentally for outer diameter tube pitch ratios of 1.25 and 1.5 of three 4 mm inner diameter tubes, each 6 m in length for a range of Reynolds numbers of 1 000 to 7 000. Water was used as the test fluid. Using PT100 probes and thermocouples at the inlet, outlet and outer surface of the test section, it was found that the presence of multiple tubes at the inlet of the heat exchanger for a pitch ratio of 1.25 promoted the onset of transition for the centre tube, and sharpened the transition gradient of the outer tubes. This effect noticeably increased with increasing heat flux and was absent at the higher pitch ratio of 1.5.