Numerical study on thermal performance of multirow helically coiled tube heat exchanger for surface water heat pump system

Owing to their good heat transfer advantage, multirow helically coiled tubes (MHCTs) have been widely used in industrial engineering, including surface water heat pump systems. To investigate the thermal performance of MHCTs at low surface-water velocities, a detailed numerical simulation is conducted based on a previous experimental system. The SST k-omega turbulence model and temperature dependent properties are used. The special feature in this study is that the heat convection inside and outside the tube is concerned instead of using constant heat flux or wall temperature. The results show that the variation characteristics of the Nusselt number and the outer wall temperature of the MHCT are consistent with the experimental results. Owing to the velocity attenuation of surface water flowing through the MHCT, the convective intensity in the negative area is lower than that in the positive area by more than 50%. When the natural convection is enhanced and the forced convection is weakened on the surface-water side, the outside Nusselt number decreases with the rows. Therefore, if the temperature of the surface water is not ideal or the velocity is small, the row number should not be extremely high. Moreover, the average inside Nusselt numbers corresponding to the average coil diameter are similar to the results of the MHCT. The effect of the flow direction of the medium on the thermal performance is insignificant and hence negligible.