Free Convection in Open Thermosyphon Tubes of Non-Circular Section

Abstract

This paper describes theoretical and experimental studies of laminar and turbulent free convection in the vertical open thermosyphon. Attention is concentrated on the effect of changes in the shape of the cross-section on the heat transfer. Two-dimensional theory developed in the paper gives a reliable indication of the termination of laminar boundary-layer flow in tubes of rectangular cross-section. Detailed measurements clearly illustrate the adverse effects of confined corner regions on heat transfer, particularly in laminar impeded flow. Even with a viscous fluid, confined rectangular sections seem capable of inducing a complete transition to fully mixed turbulent flow at unexpectedly low Rayleigh numbers. The circular section gives overall heat transfer generally superior to that of any non-circular section in laminar boundary-layer flow.

In turbulent flow, at low Rayleigh numbers, unconfined circular and triangular sections provide best heat transfer; at high Rayleigh numbers, for fully developed, fully mixed turbulent flow, their performance is greatly exceeded by that of unconfined rectangular and aerofoil sections. The Nusselt number then increases uniformly with Rayleigh number according to a two-fifths power law usually associated only with turbulent boundary-layer flow. Nusselt numbers of sections of different shape cannot satisfactorily be correlated in terms of hydraulic radius, particularly at high Rayleigh numbers. The investigation suggests that hollow gas turbine blades would promote better thermosyphon cooling than blades having circular holes.