Heat transfer and pressure drop in the transitional flow regime for a smooth circular tube with twisted tape inserts and a square-edged inlet

Abstract

HIGHLIGHTS• Experimental investigation of transitional flow regime in tubes with twisted tapes.• Results based on heat transfer coefficients and pressure drops.• Transitional flow regime was influenced by variation in twist ratios.• Variation in heat fluxes influenced the transitional flow regime.• Transition is earlier as the twist ratios and heat fluxes reduced. ABSTRACTThe purpose of this study was to experimentally investigate the heat transfer and pressure drop characteristics in the transitional flow regime of twisted tape inserts in a circular tube.Experiments were conducted in a circular tube with an internal diameter of 19.0 mm and a length of 5.27 m, and twisted tape inserts with twist ratios of 3, 4 and 5. A square-edged geometry was used at the tube inlet and it was experimentally operated with water flowing through it while the tube was heated with a constant heat flux. The experiments were conducted at three different heat fluxes of 2, 3 and 4 kW/m 2 . The experimental set-up was operated between Reynolds numbers of 400 and 11 400, and the Prandtl numbers varied between 2.9 and 6.7. Two methods were used to identify the transition points of the different heat fluxes and twist ratios. The first method used the standard deviation of the temperature measurements, and the second method used three linear curve fits on a log-log scale. The curve fits made it possible for correlations to 1 be developed for the non-dimensionalised heat transfer coefficients and friction factors, which took twist ratio, heat flux and Reynolds number into consideration. For the same heat flux, it was found that the Colburn j-factors increased as the twist ratios decreased, and transition started earlier. When the twist ratio was kept constant and the heat flux was varied, higher heat fluxes delayed the transition from laminar to transitional flow. The friction factors were found to increase as the twist ratio decreased. When both the twist ratio and the Reynolds number were kept constant, an increase in heat flux was found to decrease the friction factor.

Relationship between pressure drop and heat transfer of developing and fully developed flow in smooth horizontal circular tubes in the laminar, transitional, quasi-turbulent and turbulent flow regimes

“…This corresponds to the Chilton-Colburn analogy [3], which was developed for laminar and turbulent flow over flat plates, that shows that there is a direct relationship between friction factor (pressure drop) and Colburn j-factor (heat transfer). Similar findings were also made by Everts [50] and Meyer and Abolarin [26]. However, Tam et al [7] found that the transition region, ΔRe, was wider for heat transfer than for pressure drop.…”

“…The results Nu/Pr 0.42 are plotted as a function of (Re -1 000) in Fig. 14(a), and a power curve-fit was done to obtain the following correlation: (26) The method of Churchill and Usagi [52] was used to obtain a single correlation that is valid for all four flow regimes: respectively, in terms of the modified Grashof number. Table 2 summarises the valid ranges and performances of the average friction factor and average Nusselt number correlations for developing and fully developed flow, which were developed in this study.…”

“…Although their data contained developing flow, the focus was on the effect of inlet geometries and enhanced tubes and not on the local heat transfer and pressure drop characteristics along the tube length. Meyer and co-workers also investigated transitional flow in nanofluids [23], micro-channels [24], annular flow [25] and using twisted tape inserts [26].…”

Single-phase mixed convection of developing and fully developed flow in smooth horizontal circular tubes in the laminar and transitional flow regimes

“…It has been found from analysing the experimental data of this study and literature [41,44], that all mixed convection data in the laminar and transitional flow regimes could be represented as shown schematically in Fig. 5.…”

“…The transitional flow regime itself has also been extensively investigated by Ghajar and co-workers [27][28][29][30][31][32][33][34][35][36] as well as Meyer and co-workers [37][38][39][40][41][42][43][44], however, the focus of these studies were on the effects on inlet geometries, enhanced tubes, nanofluids, micro-channels, annuli and twisted tape inserts. While Ghajar and co-workers considered mainly fully developed flow, Meyer and co-workers used the average measurement across a tube length.…”