Heat transfer analysis of multi-row helically coiled tube heat exchangers for surface water-source heat pump

An experiment system simulating the uniform flow of surface water with low velocities was established to investigate the heat transfer performance of multi-row helically coiled tube (MHCT) heat exchangers for surface water-source heat pump. Four experimental modes of variable temperatures and velocities of surface water and medium were performed on eight MHCT configurations with different geometric parameters including horizontal spacing, vertical spacing and tube length. The results demonstrate that when the surface water velocity changes and the other parameters are constant, the outside Nusselt number depends on the Reynolds number. Conversely, it depends on the Rayleigh number. Both the inside and outside Nusselt numbers increase with vertical spacing and horizontal spacing. The effect of vertical spacing is more obvious on the outside Nusselt number, but can be neglected on the inside Nusselt number. By comparison, the correlations of single-row helically coiled tubes cannot accurately calculate the inside Nusselt number of the MHCT, and the influence of surface-water velocity on the outside Nusselt number is not negligible. The correlations for the prediction of the inside and outside Nusselt numbers are developed. Additionally, the effect of tube length on the Nusselt number is small, and can be ignored for engineering applications.