Abstract:
Design constraints and energy requirements have often led to heat exchangers operating outside of their design parameters. These parameters often involve the exchanger operating in the transition region of flow. Adiabatic as well as diabatic experiments were conducted inside smooth tubes with diameters of 15.88 mm (5/8 in.) and 19.02 mm (3/4 in.). Four inlet profiles were investigated; hydrodynamically fully developed, square-edged, re-entrant, and bellmouth. The test fluid was water that was cooled, with Reynolds numbers ranging between 1000 and 20,000, Prandtl numbers between 4 and 6, and Grashof numbers in the order of 105. Adiabatic results showed that transition from laminar to turbulent flow was strongly dependent on the inlet profile, with transition being delayed to Reynolds numbers as high as 12,000, confirming results of previous studies. Diabatic heat transfer and friction factor results showed that transition was independent of the inlet, with transition occurring at a Reynolds number of approximately 2100. This was due to the secondary flow suppressing the disturbance of the inlets. Laminar heat transfer and friction factors were also substantially higher than when compared with their theoretical counterparts. This could also be attributed to secondary flows, confirming previously published results. A direct relationship between friction factor and heat transfer exists and is shown to predict 88% of the friction factor data to within 15%, with a mean absolute error of 8.7% when using well-known laminar and turbulent heat transfer correlations.
