CALCULATION OF FRICTION FACTORS AND NUSSELT NUMBERS FOR TWISTED ELLIPTICAL TUBE HEAT EXCHANGERS USING NEK5000

The development of fluoride salt-cooled high-temperature reactors (FHRs) for nuclear power generation relies on the development of new technologies. Of the potential options being explored, twisted elliptical tube geometries for heat exchanger design are promising based on usage in other industries. They are expected to offer significant enhancement in heat transfer with only a marginal increase in frictional losses. This allows them to be deployed in relatively compact designs that are well suited for FHRs. The presented work focuses on the computational fluid dynamics (CFD) simulations of heated molten salt flows through various twisted elliptical tube geometries at low modified Froude numbers. The objectives of this work are to evaluate the available correlations at lower Froude numbers and to determine the impact of using non-zero tube to tube spacing to resolve contact points or numerical singularities for future CFD simulations efforts. The spectral element CFD code Nek5000 was used for all simulations, which were performed in periodic domains of triangular (hexagonal) and square unit cells surrounding a single tube through a complete twist using an explicit filtering large eddy simulation (LES) method. Simulations were used to parametrically test the effects of tube-to-tube spacing for laminar and turbulent flow regimes on frictional pressure drop and heat transfer. The tested Reynolds numbers covered both laminar flow and fully developed turbulent flow (90 < Re < 12200). The tested S-L/d(max) ratios cover the range of 1.02 to 1.08 for both unit cell types. At moderate Reynolds number and comparitively high modified Froude number, excellent agreement for the Nusselt number was observed between simulations and the applicable correlation. As Froude number was decreased towards the bounds of the correlation, the agreement worsened. Cases were then simulated at low Froude number, testing the effects of tube spacing. It was determined that the laminar case for the square unit cell is the most affected by increasing S-L/d(max) and the gap size should be minimized to mitigate this. Whereas in the triangular unit cell the laminar flow regime is more significantly impacted by increasing S-L/d(max) compared to the turbulent flow regime which was only marginally impacted.