Enhancement of heat transfer to a steam-water mixture in the postcritical region with the aid of spherical cavities

The paper describes the experimental stand and the procedure for investigation of heat transfer from a surface with spherical cavities under conditions of boiling of water in the postcritical region. The experimental section is provided by a 14/10 mm round channel 590 mm long, whose replaceable inner pipe is smooth or covered with spherical cavities 3.8 mm in diameter and 1.0 mm deep. The arrangement of cavities is in-line, with a longitudinal step of 6 mm and a transverse step of 6.3 mm. Experiments are performed at a mass velocity ranging from 300 to 500 kg/(m(2) s) and a pressure of 17.7 and 21.7 MPa in a wide range of relative enthalpy. The results of comparison of the values of heat-transfer coefficient a for a surface with cavities with the values of alpha for a smooth surface (obtained by us and other researchers) reveal a considerable enhancement of heat transfer thanks to cavities: an increase in alpha by a factor of 1.5-4 and a decrease in the pipe wall temperature by 100-200 K.