An experimental investigation of natural convection heat transfer from a helically coiled heat exchanger

The natural convection heat transfer over the outer surface of a helically coiled tube heat exchanger located inside a water storage tank is investigated experimentally in the present study. The fluid inside the coil is R-134a with phase changing. Seventeen helical coils are applied to study the effect of coil diameter, pitch, turns, and mass flow rate on the outside Nusselt number (Nuof), overall coefficient of performance of the refrigeration cycle (COPov), and destroyed exergy (EXdes). A new empirical correlation is presented for Nuof with respect to the Rayleigh number (Ra) and coil parameters as the characteristic length. Several characteristic lengths are used to numerically predict the temperature of water based on the presented correlation. The best result, in comparison with the experimental data, was obtained for coil length as the characteristic length. The Rayleigh number based on coil length is in the range of 5.01×1010−7×1011. The best coil based on minimum EXdes and maximum COPov is introduced. Results indicate that Nuof, COPov and EXdes increase with enhancing coil diameter while the other parameters are constant.