Investigation on heat transfer and pressure drop characteristics of multi-row helically coiled tube heat exchanger for surface water-source heat pump

A new multi-row helically coiled tube (MHCT) heat exchanger was applied in surface water-source heat pump system in recent years. To investigate the heat transfer and pressure drop characteristics of the MHCT heat exchanger, an experiment system was built to realize the heat extraction in winter and heat rejection in summer with low velocities of surface water. Four experiment modes were conducted by varying the parameters of surface water and medium. The results show that the increase of the inlet temperature or velocity of medium and the temperature of surface water can improve inside convection intensity, among which the effect of medium velocity is the most distinct. Besides, the mixed heat convection was carried out on surface water side, and the intensity was also affected by the variation of the medium parameters. As the temperature of surface water or medium and the medium velocity do a change with a constant surface water velocity, the effect of natural convection is critical to outside convective intensity. Through comparisons, it is acceptable to take arithmetic average coil diameter into the SHCT correlations of inside Nusselt number and friction factor for the engineering design of the MHCT. The correlations of purely forced convection on the MHCT underestimate the experimental outside Nusselt numbers and cannot consider the effects of medium side.