Thermal analysis of a helically coiled tube in a domestic hot water storage tank

Abstract

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.