Computational fluid dynamics analysis of PCM-based ocean thermal engine with external rib turbulators

Abstract

The concept of extracting energy from ocean thermal gradients for the long-term deployment of underwater profiling vehicles is promising. One potential method to capture ocean thermal energy is leveraging the volume change of phase-change materials (PCMs). However, the low thermal conductivity of PCMs and the low heat transfer coefficient between heat exchangers and seawater limit the performance of PCM-based ocean thermal engines (OTEngs). In this paper, a new type of heat exchanger structure with external rib turbulators is proposed. Detailed computational fluid dynamics analysis is conducted for the external forced convection heat transfer process of the heat exchanger with Reynolds number in the range of 240,000 to 600,000. In addition, a solver for calculating the phase-change process of the PCM inside the heat exchanger was developed based on OpenFOAM. The influence of the ribs on the heat transfer and energy conversion processes of the OTEng was analyzed. Results show that (i) the heat transfer coefficient of the ribbed heat-exchanger wall significantly improves, especially at low Reynolds numbers; (ii) 2.5–5 mm high ribs can effectively enhance heat transfer through the wall. When rib weight exceeds 5 mm, the heat transfer coefficient does not increase with the height; (iii) increasing the rib spacing reduces the water resistance and heat transfer coefficient of the wall; and (iv) the ribbed OTEng has a higher net power output compared with the conventional engine.