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The heat transfer performance of a high-density polyethylene helical coil heat exchanger (HCHE) which is adopted by a seawater-source heat pump system (SWHP) is experimentally and numerically studied. The effects of seawater flow rate on heat transfer performance of the HCHE are investigated and the correlation between Nusselt and Reynolds number is fitted by the experimental results of cooling condition. The fitted results are compared with other similar studies of helical heat exchangers. Mathematical models in cooling condition, non-icy condition and icy condition are developed with the calculated external convective heat transfer coefficient. The simulation results are verified by the experimental data of icy condition and they show good agreement with each other. The effects of different key parameters on the heat transfer performance of the HCHE are investigated, such as inlet temperature, velocity of intermediate medium, pipe length and diameter, temperature of seawater and icing outside the pipes. The results indicates that the consideration of seawater flow rate in the study of subsea heat exchangers is much closer to the reality and the development of the mathematical models is very helpful in the designing and optimizing of the subsea heat exchangers used for the SWHP system. [ABSTRACT FROM AUTHOR]

• A novel heat exchanger with unequally-pitch arrangement is proposed. • The thermal, hydraulic and thermal-hydraulic performance are analyzed. • The novel heat exchanger is superior to that with equally-pitch arrangement. A novel staggered tube-bundle seawater heat exchanger with unequally-pitch arrangement, which the transversal and longitudinal tube pitches are inhomogeneous, is firstly proposed and applied in the closed-loop seawater source heat pump system. An experimental system that can induce the regular wave to simulate the seawater flow is established to investigate the thermal resistance ratio, overall heat transfer coefficient, thermal efficiency of heat exchanger under various wave parameters. The thermal performance, hydraulic performance, and comprehensive thermal-hydraulic performance of heat exchangers with unequally-pitch arrangement and equally-pitch arrangement are compared. The results indicate that the unequally-pitch arrangement of tube-bundle heat exchanger can significantly enhance the overall heat transfer coefficient and thermal efficiency, meanwhile decrease the pressure drop, and consequently improve the comprehensive performance. The present study is conducive to improve the performance potential of staggered tube-bundle seawater heat exchanger and promote the exploitation of ocean thermal energy. [ABSTRACT FROM AUTHOR]

• An unsteady mathematical model was developed based on wave theory. • The model was validated by the tested results. • Effects of key parameters on the thermal performance were studied. Seawater heat pump system exhibits merits of reducing building energy consumption, while enhancing the heat transfer of seawater heat exchangers is the key to improve the system performance. The effect of seawater flow behavior on the thermal performance of seawater heat exchanger immersed in oscillating flow is significant but often neglected. An unsteady mathematical model was firstly developed to describe heat transfer of oscillating flow around the staggered tube-bundle heat exchanger and verified by experimental results. Effects of the structural and operational parameters, such as the tube length, heat transfer fluid velocity, wave parameters and sinking depth on the thermal performance were analyzed. It was shown that the thermal efficiency of seawater heat exchanger ascended with the increase of wave amplitude and decrease of the heat exchanger sinking depth. Additionally, the tube length and heat transfer fluid velocity could be reasonably designed to achieve the optimum value between cost and performance. The heat transfer mechanism in seawater side was predominated by forced convection to natural convection with the increasing sinking depth under various wave parameters. The proposed model is favorable to delicately characterize the heat transfer of seawater heat exchanger with the consideration of seawater flow. The results in this paper is helpful to guide the optimization on designing the staggered tube-bundle heat exchanger in oscillating flow and to promote the application of ocean thermal energy. [ABSTRACT FROM AUTHOR]