Abstract
An experimental study has been carried out to investigate flow characteristics of the liquid film flowing down on a vertical tube wound spirally with strip fins. We have developed this type of finned tube as an element of an absorber for absorption heat pumps. The liquid was supplied through a crescent feeding section to the outside top of the spiral finned tube. By introducing the parameter k=f/(16/Re), the friction factor through the feeding section was correlated within 20 percent error with an expression derived from Asino's analysis for a flow through the gap between an outer tube and an eccentric inner cylinder. The spirally wound strip fins could generate a dynamically oscillating falling liquid film with short wave length. In the range of film Reynolds number from 500 to 2500, the frequency of the fluctuating liquid thickness was 0.3 to 3 Hz or one-tenth of that on a bare tube, and its wave length was 25 to 30 mm, which is shorter than the wave length of 55 to 120 mm of the falling liquid film on a bare tube. Hence, the number of the waves per unit vertical length is dense, being 1.8 to 4.8 times that on a bare tube. The average falling liquid film velocity and wave velocity have been expressed as functions of film Reynolds number. With increasing film Reynolds number, the ratio of the latter to the former approaches 1.3 for single spirally stacked fins and 2.0 for double spirally stacked fins. The dimensionless film thickness representing the holdup on these finned tubes has been correlated by Eqs. (19)–(21) as functions of film Reynolds number.
