Abstract
In this study, we investigated ways of improving the performance of heat exchangers, which are the heat pumps for use in hot-water supply systems on the hot-water supply side. Therefore, we verified experimentally the pressure drop and the heat transfer characteristics within the coiled flow channel. Five internally helical-grooved copper tubes with an outside diameter of 12.7 mm and coil diameters of 100 mm, 120 mm, 140 mm, 160 mm and 180 mm and a smooth copper tube with an outside diameter of 12.7 mm and coil diameters of 160 mm were used as the heat-transfer coiled tubes in the experiments. Experiments were conducted under conditions of constant isothermal heating and fluid flow inside coil tubes with an inlet temperature of 20°C, and the flow rates of the fluid flow inside the coil tubes were adjusted and varied to change the Reynolds number within the range of 900 to 25,000. From the experimental results, the following conclusions were obtained: (1) The friction factor of the coiled internally helical-gro oved tubes displayed a smaller value the larger the curvature radius ratio, and displayed larger values than those of a coiled smooth tube within a low flow rate range. (2) The Nusselt number of the coiled internally helical-grooved tubes was about 50% higher than that of a coiled smooth tube with a Reynolds number of about 2500. (3) The critical flow rate necessary for the effective diffusion of the thermal boundary layer depends upon the curvature radius ratio. That the enhanced heat transfer is performed effectively indicates an optimum curvature radius ratio. (4) In the Reynolds number of 2000+ to the turbulent flow region, we proposed the friction factor of coiled internally helical-grooved tubes be correlated within ±10%, and the heat transfer coefficient of those tubes was correlated within ±30% based on the experimental values.
