Abstract
Molten salts are now widely used as heat transfer fluids in solar thermal plants and molten salt nuclear reactors. In order to explore its heat transfer enhancement method, new coaxial cross twisted tapes (CCTTs) are applied to plain tube with molten salt FLiBe as working fluid, and the characteristics of pressure drop and heat transfer are numerically investigated by the CFD software STAR-CCM+. Simulations are performed in a laminar flow regime where Reynolds number ranges from 100 to 1100, and the velocity, temperature profiles, and enhanced performance evaluation criteria (PEC) are analyzed to investigate the heat transfer performance of FLiBe in the tube fitted with CCTTs and typical twisted tape (TT). Water and lubricating oil are also researched to investigate the effects of fluid thermal-physical properties on the performance of CCTTs. The results show that CCTTs can produce stronger swirl flow and greatly enhance heat transfer. For overall heat transfer performance, the maximum PEC of FLiBe with CCTTs reaches 2.37. The comparison of three different fluids also indicates that CCTTs have better heat transfer performance for higher Prandtl number fluids such as molten salt. The correlations of CCTTs and TT are developed into a unified form for the prediction of friction factors and Nusselt numbers for various Prandtl number fluids.
Highlights
The results showed that twisted tape (TT) is effective in heat transfer enhancement of laminar flow and viscous fluid, which indicates its potential to improve the laminar convective heat transfer of high viscosity molten salts
The streamline, velocity profiles, temperature profiles, and performance evaluation criteria are all presented for analysis and the conclusion are summarized as follows: (1) The velocity and temperature profiles of the tube fitted with different twisted tape reveal that the coaxial cross twisted tapes (CCTTs) can produce stronger swirl flow and improve the near-wall gradient of velocity and temperature
(2) The numerical results show that the CCTTs can greatly enhance the heat transfer, though the pressure drop increases
Summary
During the last few decades, molten salts have been widely applied in various industries, including alloy production, thermal storage, and power plants (Kearney et al, 2004; Serrano-López et al, 2013; Sabharwall et al, 2014; Vignarooban et al, 2015; Garbrecht et al, 2017; Romatoski and Hu, 2017). For the last two applications, interest is growing rapidly because of their desirable thermal-physical properties (Serrano-López et al, 2013) Their advantages include higher volumetric heat capacity and thermal stability at high temperatures, and molten salts do not need to be pressurized as heat transfer fluids and would significantly reduce the cost of heat exchangers and pumps for a required volume (Sabharwall et al, 2014). These attributes enable high-temperature operation which raises the efficiency of thermal conversion. Research conducted using molten salt FLiBe (LiF-BeF2) as a coolant in fluoride-saltcooled high-temperature reactors (FHR) is a highly focused area (Scarlat and Peterson, 2014)
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