MOLTEN SALT FLOW AND HEAT TRANSFER IN PADDLE HEAT EXCHANGERS

Constructal law has inspired a large amount of work in fluid flow and heat transfer analysis, design and optimization. In this paper the authors will investigate high temperature (similar to 500 degrees C) molten salt flow and heat transfer characteristics in paddle heat exchangers (more general name of paddle dryers) by invoking constructal law idea and CFD tools. The research shows that among three typical paddle-shaft structures, open hollow paddle-shaft structure transfers more heat than solid paddle-shaft structure and closed hollow paddle-shaft structure for specified inclined angle of paddles and inlet flow rate. The performance superiority of open hollow paddle-shaft structure demonstrates the design optimization direction or constructal evolving direction. Next, under the constraint of the outside surface shape which is determined by the flow of outside materials, the effects of the open hole position, size, non-finned angle, as well as the pressure drop (in dimensionless form, Bejan number) on flow and heat transfer for open hollow paddles are calculated. There exists an open hole position at which pressure drop and heat transfer rate approach maximum simultaneously for specified flow rates. The flow rate and heat transfer rate increase significantly as the hole diameter increases. When the non-finned angle decreases, the heat transfer rate increases whereas the change of the flow rate is not monotonous. Finally, the comparison of original and modified rotary joints shows that internal heat insulation combined with external cooling reduces the average temperature in the sealing packing domain greatly which helps improve the sealing performance under high molten salt temperature conditions. The present work provides more insight in molten salt flow and heat transfer in paddle heat exchangers and provides reference data for design engineers.