TWO-PHASE FLOW AND HEAT TRANSFER IN VERTICAL U-SHAPED TUBES. (II). ELIMINATING BOILING HEAT TRANSFER DETERIORATION BY MEANS OF INSERTED TURBULATORS.

Vertical U-shaped tubes have been used in some waste heat boilers of large synthetic ammonia plants. In operation, accident of tube failure at the bend due to overheating occurs frequently. The present tests showed that under the action of both centrifugal and gravitational forces, sratification of steam-water flow occurs, leading to heat transfer deterioration on the inner side of the bend. In order to solve the problem, three types of inserted tubulator - a single twisted tape, a double twisted tape and a spiral coil - were tested in a high-pressure electrically heated water-loop experimental installation. Boiling heat transfer and pressure drop characteristics were investigated. The Chisholm equation was used to correlate the data of the two-phase pressure drop, and a correlation for determining the two-phase frictional multiplier was proposed. In order to understand the operating mechanism of the inserted turbulator, flow pattern observations of the single twisted tape were carried out in a transparent tube for air-water flow and an overall flow-pattern map was established in terms of superficial liquid and gas velocities.