Researching On The Equivalent Pressure Drop And Convection Heat Transfer In Molten Salt Pebble-bed Reactor

Pebbles packed bed reactor is one of the solid fuel Molten Salt Reactor. Because of its complexity in structure, it's difficult to simulate the thermal-hydraulic characters of the flow in packed bed by directly modeling the bed. Thus, the packed bed is often simplified using Porous Media Model, of which some formulas are induced to calculate the pressure drop and heat transfer coefficient. For example, most of the Computational Fluid Dynamics codes use the Ergun Equation for pressure drop and Wakao Formula for convective heat transfer coefficient, and KTA Formulas is available for the Gas Cooled Packed Bed Reactors. While, for lacking of experiment data, it has no relevant formulas for Molten Salt Packed Bed Reactor.In this paper, the real structure of packed bed was built and computational models were conducted using the Ansys_Fluent code in 14.5 Version. Uniform energy source was loaded to all the 60 mm diameter sphere surfaces to heat the fluid flowing by. The SST turbulent model was chosen. Finally, the linear pressure drop and convective heat transfer coefficient were acquired from the simulation. Values predicted by the existent porous media model formulas was contrasted with the simulation results to test their applicability for the molten salt packed bed. Modified the inapplicable formulas and finally created the suitable Porous Media Model formulas for the Molten Salt Packed Bed Reactor.Firstly, several details about the bed simulation were analyzed, including the turbulence model?energy source and treatment of the contact point. For the contact point treatment, the gap method and the bridge method were both available. The model were built to analyze the influence of gap size or bridge size on pressure drop and heat transfer. Results show that, for 60 mm diameter spheres which contacted with each other, bridge in cylinder shape with radius of 6mm can be used to treat the contact points to get higher qualified mesh. While, when the spheres had a distant beyond 1mm with each other, no bridges were allowed.Then, a structured packed bed model with 11 cells was built. Gaps in 2mm size were induced in case of the contact points between spheres. Results of pressure drop from the molten salt flow simulation showed about 50 percent of deviation with the existent formulas. While for the heat transfer coefficient, deviation were blow 25 percent. So modification was needed to be done on the existent porous media model formulas. Finally, the modified formulas based on the simulation results were acquired, both for the pressure drop and convective heat transfer coefficient calculation.Meanwhile, molten salt flowing in the random packed bed was simulated, too. The bridges of 6mm radius cylinder were used to handle the contact points. The pressure drop result from simulation showed little deviation with the KTA Formula, while the Ergun Equation had maximum deviation of 20 percent and the modification was needed. For the convective heat transfer coefficient, it was well-distributed in most of the bed but it has much higher heat transfer coefficient in near bed wall zone while much lower in central zone. Average Nusselt number values were acquired from the well-distributed area, and result showed a maximum deviation of 20% comparing with the existent formulas. Modification was done and the modified KTA heat transfer formula showed lower fitting error.