Heat Transfer and Pressure Drop of Developing Flow in Smooth Tubes in the Transitional Flow Regime

Abstract

Heat exchangers have a wide range of applications and engineers need accurate correlations to optimise the design of these heat exchangers. During the design process, the best compromise between high heat transfer coefficients and relatively low pressure drops is usually in the transitional flow regime. Limited research has been done on tube flow in the transitional flow regime. These studies considered either fully developed flow, or average measurements of developing flow across a tube length. No research has been done with the focus on developing flow in smooth tubes in the transitional flow regime. Therefore, the purpose of this study was to experimentally investigate the heat transfer and pressure drop characteristics of developing flow in the transitional flow regime. An experimental set-up was designed, built and validated against literature. Heat transfer and pressure drop measurements were taken at Reynolds numbers between 500 and 10 000 at three different heat fluxes (6.5, 8.0 and 9.5 kW/m2). A total of 398 mass flow rate measurements, 19 158 temperature measurements and 370 pressure drop measurements were taken. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. The test section was a smooth circular tube and had an inner diameter and length of 11.52 mm and 2.03 m, respectively. An uncertainty analysis showed that the uncertainties of the Nusselt numbers and Colburn j-factors varied between 4% and 5% while the friction factor uncertainties varied between 1% and 17%. Five different flow regimes (laminar, developing laminar, transitional, low-Reynolds-number-end and turbulent) were identified in the first part of the tube during the experiments and nomenclature was developed to more clearly identify the boundaries of the different flow regimes. The developing laminar regime was unique to developing flow and decreased along the tube length. Both the start and end of transition were delayed along the tube length and the width of the transition region decreased slightly. This is in contrast with the results obtained in literature where the effect of the non-dimensional distance from the inlet on fully developed flow in the transition region was investigated. Transition was also slightly delayed with increasing heat flux, but secondary flow effects had no significant influence on the width of the transition region. The relationship between heat transfer and pressure drop was investigated and correlations were developed to predict the Nusselt number as a function of friction factor, Reynolds number and Prandtl number in the laminar, transitional, low-Reynolds-number-end and turbulent flow regimes. Overall, it can be concluded that the heat transfer characteristics of developing and fully developed flow differ significantly and more work needs to be done to fully understand the fundamentals before the heat transfer and pressure drop characteristics are fully understood.