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 $\alpha$ 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.