Experimental study on the thermal performance of multi-row helically coiled tube heat exchanger for surface water-source heat pump

Highlights: • Velocity attenuation results in greater convection intensity in the positive area. • The increase of surface water temperature can increase inside Nusselt number. • The effect of surface water parameters on outside mixed convection was analyzed. • Large thermal-conduction resistance weaken the positive effect of flow velocity. • The comparison with the previous work on Nusselt number was conducted. -- Abstract: To study the thermal performance of multi-row helically coiled tube (MHCT) heat exchanger for surface water-source heat pump, various experimental cases of heat extraction in winter and heat rejection in summer were investigated. The results showed that a velocity attenuation phenomenon occurred around the MHCT heat exchanger, leading to a greater convection intensity in the positive area. The heat exchange rate and inside Nusselt number were affected obviously by variable surface water temperature, while were insensitive to the change of flow velocity. With the surface water temperature increasing at a constant velocity, the inside Nusselt number was climbed in winter and summer, whereas outside convection intensity depended on the temperature difference between the surface water and outer wall of MHCT. As the surface water velocity increased, the outside Nusselt number was increased by 25.6% in winter and 20.1% in summer. Moreover, the thermal-conduction resistance of tube wall accounted for 65%, which can weaken the positive effect of surface water flow velocity on heat exchange rate. Through a comparison, the correlations of single-row helically coiled tube cannot predict the inside Nusselt number well, and the effect of velocity cannot be neglected in the outside Nusselt number.