Abstract
Experiments of natural convection for a horizontal cylindrical annulus of the inverse relative gap width Di/L=6.7 were carried out with water in the range of Rayleigh number from 8.23×10^2 to 2.16×10^5, in which the unsteady flow pattern is characterized by the 3-D spiral flow. In order to clarify the heat transfer characteristics, flow patterns were visualized by adding the tracer particles of nylon to a flowing fluid. A stable crescent-shaped flow pattern is observed at low values of the Rayleigh number, but the flow does not develop completely into the upper portion of the annulus. As the Rayleigh number is increased, counter-rotating cells are appeared in the longitudinal direction and the flow pattern becomes a 3-D spiral flow. Then the flow reverts to the stable crescent-shaped pattern developed fully in the upper portion of the annulus at about 3×10^3 of the Rayleigh number. Heat transfer characteristics can be explained well from the flow patterns revealed by the flow visualization.
