This paper describes the initial thermal-hydraulic and flow visualization studies of inserts to enhance turbulent heat transfer in tubes, with particular application to fire-tube boilers. An electrically heated flow facility was used to deliver hot air to a water-cooled steel tube of 2 .675 in (0.0679 m) i.d. and 71.75 in (1.823 m) length. Tube wall temperatures, fluid bulk temperatures, and flow rates were measured to derive sectional average heat transfer coefficients for four segments of the tube. Reference data for the empty tube are in excellent agreement with the usual correlations. Seven different geometrical variations of one type of bent-strip insert were tested. Heat transfer coefficient increases of 175% to 285% were recorded at a Reynolds number of 10,000; the respective friction factor increases were 400% to 1800%. A preliminary correlation of these data is given.

Two methods of flow visualization were used to study the flow patterns created by this type of insert. Both the dye injection (with water) and the oil/lamp-black (with air) techniques provided insight into the mechanisms of enhancement.

Units: Dual