CFD investigation of fluid and heat transfer in a single-phase natural circulation toroidal loop

Natural circulation occurs due to a temperature-induced density gradient, typically observed within closed loops where the fluid is heated in the upper section and cooled in the lower section. This phenomenon finds numerous engineering applications, particularly as a passive heat removal mechanism in nuclear reactors, significantly enhancing their safety. However, the computational modeling of natural circulation poses challenges due to its complex, multidimensional, and nonlinear characteristics. Furthermore, in such systems, fluid dynamics and thermal aspects are intricately intertwined. Hence, the objective of this study is to evaluate the fluid and thermal behavior of a toroidal loop under natural circulation using the Computational Fluid Dynamics (CFD) ANSYS CFX package. A 2D model is employed, along with the Boussinesq approximations, as they are well-suited for toroidal systems. Turbulence is calculated using the k - e. Various modified Grashof Numbers (Gr) normalized by the geometric parameter (NG) are utilized, ranging from 5.4 x103 to 4.7 x105, for two different Prandtl numbers. The lowest Grm/NG value in each simulation represents the convection lower limit (flow initiation), while the highest value indicates the convergence limit, beyond which a steady state cannot be achieved. Temperature and velocity profiles are presented, and their variations with the power level are discussed. The resulting Reynolds of Steady State (Ress) are plotted against Grm/NG values, demonstrating good agreement with experimental data. As the Prandtl Number increases, the system is observed to enter an unstable zone at an earlier stage. Finally, it is observed that multiple steady states exist for Pr = 0.7, and their implications are discussed.