Performance evaluation of laminar mixed convection of molten salt in a horizontal circular tube with non-uniform heat flux

Abstract

Molten salt is a prevalent heat transfer medium in thermal energy storage systems due to its excellent thermal properties. During engineering design, the laminar mixed convection often occurs in high-heat-flux tubes, thereby altering the flow and heat transfer characteristics. However, the use of traditional correlations in the design of thermal energy storage system can lead to substantial errors. To address this issue, the laminar mixed convection of molten salt in horizontal tubes under the non-uniform heat flux condition is investigated. Furthermore, the effects of tube diameter, inlet temperature, and heat flux are analyzed. Results showed that increasing heat flux and tube diameter boosts Nusselt number and thermal entropy production rate, yet reduces flow entropy production rate and exergy efficiency. Increasing heat flux and decreasing tube diameter decreases friction factor. Increasing inlet temperature decreases friction factor, flow and thermal entropy production rates, but increases exergy efficiency. Compared to forced convection, the friction factor, Nusselt number, flow entropy production rate and exergy efficiency increases by 47.03 %, 114.43 %, 62.90 %, 21.06 % at most for mixed convection under non-uniform heat flux conditions, respectively. However, the thermal entropy production declines by up to 51.82 %. By increasing heat flux, inlet temperature and tube diameter, the location, at where the buoyancy force takes effect, decreases. Finally, a new friction factor correlation with 3.75 % uncertainty and Nusselt number correlation with 15.00 % uncertainty are proposed. The investigation finding will not only help improve the design of thermal energy storage systems, but also will help to optimize heat transfer, minimize energy loss and improve safety.