CFD Modeling and Simulation of Natural Circulation in Helium- Cooled Very High Temperature Reactors
This thesis investigates the thermal-hydraulic behavior of natural circulation in helium-cooled Very High Temperature Reactor (VHTR) systems, focusing on the performance and validation of passive safety features under Pressurized or Depressurized Loss of Forced Cooling (P-LOFC or D-LOFC) events. An experimental facility inspired by the General Atomics 350 MWt Modular High Temperature Gas-Cooled Reactor (MHTGR) was developed at The City College of New York. The setup consists of a plenum-to-plenum flow loop with four riser and four downcomer tubes, each equipped with heating coils, thermocouples, and pressure sensors. Natural circulation is driven by density gradients induced by asymmetric heating of the riser and downcomer tubes, simulating decay heat removal during LOFC scenarios. To measure gas velocity and validate flow dynamics, the facility employs advanced Thermal Time-of-Flight sensors. Computational Fluid Dynamics (CFD) simulations were conducted in ANSYS Fluent using a 2D model for 4 Riser and 4 downcomer tubes connected to an Upper Plenum and Lower Plenum, as well as a simplified single-tube heating configuration. Mesh sensitivity and grid independence studies were performed to ensure numerical stability and solution accuracy, with fine resolution applied near heated surfaces. Helium thermophysical properties were implemented using pressure and temperature-dependent polynomial equations to realistically simulate buoyancy-driven convection. Three heat input scenarios (230W, 330W, and 580W) were analyzed, revealing a direct correlation between increased thermal input and enhanced flow velocity and temperature gradients. Comparative analysis showed good agreement in temperature profiles between experimental and simulated results, with outlet deviations within 1.7–3.5%. However, the 2-D model consistently underpredicted flow velocities by up to 70–80%, highlighting the limitations of a 2-D model for a 3-D flow facility and idealized boundary conditions. The study concludes that CFD is a powerful tool for capturing the key dynamics of passive natural circulation systems in high-temperature gas reactors, particularly for thermal analysis and early-stage design. The simulation framework developed here provides critical insight into how asymmetric heating influences system performance and can be scaled for broader reactor applications. Despite some discrepancies in velocity prediction, the validated temperature results affirm the reliability of the modeling approach and reinforce the value of passive safety strategies in next-generation nuclear energy systems.
Heat Transfer, Combustion, Mechanical Engineering, Nuclear Engineering
City University of New York - City College
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