Abstract:
The operating conditions of many heat exchangers are in, or close to, the transitional flow regime. However, in this regime,
not a lot of design information is available and some design books even recommend to not design heat exchangers to operate
in the transitional flow regime. Furthermore, it is known that the type of inlet of heat exchangers influences the transition
characteristics. It was therefore the purpose of this study to measure heat transfer and pressure drop characteristics in smooth
horizontal tubes using different types of inlets. The types of inlets were hydrodynamically fully developed, square-edged,
re-entrant, and bellmouth. Experiments were conducted on a 14.48-mm inner diameter horizontal tube in which the water
was cooled. Reynolds numbers ranged between 1000 and 20,000 and Grashof numbers were on the order of 105. It was found
that for adiabatic flow the square-edged inlet delayed transition to Reynolds numbers of around 2600, while the bellmouth
inlet delayed it to about 7000. However, for diabatic flow, the transition was independent of the type of inlet. Laminar friction
factors were much higher than their theoretically predicted values due to the secondary flows increasing the amount of
mixing in the tube. Heat transfer measurements showed that transition with water was totally independent of the type of inlet
used.
