While the Rayleigh number is given as zero, it is corresponding
to the case of heat conduction. The calculated mean Nusselt
numbers for both the hot and cold tubes are approximate to 1.02,
which is close to unity of the pure heat conduction state in an
infinite large space. Fig. 4 shows the temperature and velocity
profiles at various Rayleigh numbers while the hot and cold tubes
are side by side. It shows that there is only one vortex clockwise
surrounding the tube pair at very low Rayleigh number (Ra ¼ 1).
However, there appear three vortexes at Ra ¼ 10, the bigger one
covers the tube pair and the fluid flow circulates clockwise through
the top right corner and the left bottom corner, and the two smaller
ones locate respectively at the right bottom corner and left top
corner circulating counter clockwise. The bigger vortex gets unstable
with an increase of Rayleigh number, and is continuously
squeezed by the two smaller ones so that it gets narrower and
narrower, and splits into two independent smaller ones while Ra is
increased over 1000.
The local Nusselt numbers of hot and cold tube for the case of
side by side arrangement are shown in Fig. 5. It shows that the
maximum local Nusselt numbers for the hot and cold tubes are
located in places closest to the other, i.e. qH-max ¼ 2p, and qCmax
¼ p.
Fig. 6 shows the temperature and velocity profiles in a circular
enclosure at various Rayleigh numbers while the hot and cold tubes