In the present study, an experimental investigation
of heat transfer and fluid flow characteristics of
buoyancy-driven flow in horizontal and inclined annuli
bounded by concentric tubes has been carried out. The
annulus inner surface is maintained at high temperature by
applying heat flux to the inner tube while the annulus outer
surface is maintained at low temperature by circulating
cooling water at high mass flow rate around the outer tube.
The experiments were carried out at a wide range of
Rayleigh number (5 · 104Ra5 · 105) for different
annulus gap widths (L/Do = 0.23, 0.3, and 0.37) and different
inclination of the annulus (a = 0, 30 and 60). The
results showed that: (1) increasing the annulus gap width
strongly increases the heat transfer rate, (2) the heat
transfer rate slightly decreases with increasing the inclination
of the annulus from the horizontal, and (3)
increasing Ra increases the heat transfer rate for any L/Do
and at any inclination. Correlations of the heat transfer
enhancement due to buoyancy driven flow in an annulus
has been developed in terms of Ra, L/Do and a. The prediction
of the correlation has been compared with the
present and previous data and fair agreement was found.