Heat transfer and pressure drop characteristics of circular smooth tubes in the transitional flow regime (original) (raw)

Normally, literature advises designers not to develop heat exchanger equipment that operates in the transitional flow regime. However, design constraints or changes in process conditions often force heat exchanger equipment to operate in or close to the transitional flow regime. Although much work has been done in fluid mechanics in the transitional flow regime with special emphasis on better understanding the phenomenon of turbulence and the development of turbulence models, very little work has been done on the influence of transition on heat transfer. The purpose of this keynote paper is to present measured data for the heat transfer and pressure drop characteristics in the transitional flow regime for water flowing inside horizontal tubes. Adiabatic as well as diabatic experiments were conducted on water inside two horizontal smooth tubes with different diameters. The tubes were configured in a tube-in-tube arrangement, with the warm water stream in the inner tube and the chilled water stream in the annulus. Reynolds numbers in the transitional flow regime ranged between 1 000 and 20 000, Prandtl numbers varied between 4 and 6, and Grashof numbers were in the order of 10 5 . The tube tested had an outside diameter of 15.88 mm, inner diameter of 14.482 mm and a length of 5 m. Four inlet profiles were investigated, namely hydrodynamically fully developed, square-edged, re-entrant and bellmouth. Adiabatic results of friction factor confirmed that transition from laminar to turbulent flow was strongly dependent on the inlet profile, with transition being delayed to Reynolds numbers as high as 7 000. However, diabatic friction factor and Nusselt number results confirmed that transition was independent of the inlet, with transition occurring at a Reynolds number of approximately 2 100. The transition and Nusselt number independence was attributed to the buoyancyinduced secondary flows in the tube, which suppress any disturbances found in the inlet.