Heat transfer performance of pillow plate heat exchanger with molten salt and supercritical carbon dioxide

Molten salt (MS) and supercritical carbon dioxide (SCO2) are important heat transfer fluids (HTFs) in many energy systems as concentrated solar power. In this study, novel pillow plate heat exchanger (PPHE) with periodically staggered convex spots and concave spots is first proposed for heat transfer between MS and SCO2, and associated flow and thermal performance is numerically analyzed. Because of periodical convex/concave spots, flow pattern, temperature and heat transfer coefficient periodically vary by the interaction of fluid scouring effect and vortices. In downstream region of convex spot or upstream region of concave spot, the fluid temperature changes very quickly and the thermal boundary layer is reduced by fluid scouring effect, so associated heat transfer coefficient is relatively higher. In upstream region of convex spot or downstream region of concave spot, vortices appear and thermal boundary layer is relatively thicker, so heat transfer coefficient will be lower. In flat plate region, its heat transfer coefficient is lower than that in convex spot region and is higher than that in concave spot region, while its heat flow percentage in total system is more than 50% for large surface area ratio. The heat transfer coefficient of PPHE is remarkably influenced by its structures, it increases 28.1% with channel height rising from 2.4 mm to 4.8 mm and increases 40.4% with spot longitudinal distance dropping from 42 mm to 18 mm. In addition, flow friction factor and heat transfer correlations of HTFs in present PPHE from laminar to turbulent flow are respectively predicted with different channel height and spot longitudinal distance.