Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.
Helically coiled tubes are commonly used as heat exchangers in
hot water storage tanks. An experimental investigation was
carried out to analyze the thermal performance of a helical coil
submerged in a water tank. Special attention was paid to the
natural convection process on the outer surface of the helically
coiled tube. Two experimental configurations were considered.
In the first one, the helical coil was placed at the bottom of the
tank and hot water at constant inlet temperature was forced
through it. The hot water mass flow was varied. Then, the
heating process of the water in the tank was analyzed by
measuring the transient water temperature profiles and the
actual heating powers exchanged by the coil were obtained.
In the second configuration, the helical coil was placed at the
top of the tank and cold water at a constant inlet temperature
was forced inside the tube, while the water in the tank was
heated by means of an electrical heater fixed at the bottom of
the tank. This configuration, once reached the steady state,
provided a practically constant water temperature around the
coil at the top of the tank. This temperature was varied between
45 ºC and 90 ºC at intervals of 5 ºC. The cooling water mass
flow was also varied within a velocity range between 0.25 and
3 m/s. For each set of experimental runs, the water temperature
distribution inside the tank, as well as, the cold water mass flow
in the coil and its inlet and outlet temperatures were measured.
Then, the natural convection heat transfer coefficients from the
outer surface of the tube were determined.
In this paper, the experimental setups are described, the
reduction data procedures are indicated and the outstanding
experimental results are shown and discussed. General
correlations (Nu=C·Ran), considering several characteristic
lengths (tube diameter, tube length and coil height), for the
natural convection process on the outer surface of the coiled
tube were obtained. Moreover, general correlations found in the
open literature are compared with that proposed in this work
and with the experimental data obtained.
