Investigation into different 1D/3D thermal fluid co-simulation methodologies as applied to HTR system configurations

Abstract

The current PhD research focuses on the investigation into different 1D/3D thermal fluid co-simulation methodologies as applied to HTR system configurations. For the modelling and simulation of complex systems, 1D/3D co-simulation is a promising approach used to aid in reducing the computational expense of the system under investigation. The required computational resources are optimized while maintaining an acceptable level of accuracy, by combining the desired features of a one-dimensional (1D) systems code with a three-dimensional (3D) Computational Fluid Dynamics (CFD) code. Within this study the development and validation of different co-simulation strategies that couple the 3D CFD code, Ansys Fluent with the thermal-hydraulic systems code, Flownex were investigated. To analyse radiative and convective heat transport as well as the natural circulation phenomena in a Reactor Cavity Cooling System facility, four different 1D/3D co-simulation methodologies were investigated. The 1D systems code, Flownex, and the 3D CFD code Ansys Fluent were used to perform the different 1D/3D co-simulations. Each code modelled a relevant part of the Reactor Cavity Cooling System (RCCS) facility and the models were coupled by explicitly exchanging boundary conditions or relevant data between the 1D and 3D regions. The corresponding temperatures, mass flow rates, and riser power of all four 1D/3D co-simulation approaches were found to be in good agreement with the results obtained from the validated full 1D simulation. The directly coupled- parallel model is the preferred strategy/approach as it requires less computational resources and reaches a converged solution in the least number of iterations and amount of solution time. Within the directly coupled- parallel approach, Ansys Fluent (3D code) provides the 1D code (Flownex) with the heat flux/heat transfer rate at the inside of the riser tubes, whilst the 1D code provides the 3D code with the temperatures of the inside surfaces of the riser tubes. Using the directly coupled-parallel 1D/3D CFD approach (transferring surface temperatures and heat flux/heat transfer rates between the different codes/solvers) the heat transfer and fluid flow within a single-channel fuel module of a prismatic block VHTR was simulated. Validation of the coupled 1D/3D methodology was done using corresponding values obtained from published work and full explicit 3D CFD. When compared to an explicit 3D CFD analysis of the same system, the coupled 1D/3D projections suggest errors between the calculated fuel- and coolant temperatures of less than 1.5% for both a (i) uniform, and (ii) cosine power profile. When compared to explicit 3D CFD, the computational expense is greatly reduced by explicitly coupling the 1D and 3D solvers, whilst still retaining an acceptable level of accuracy. As a result, the 1D/3D co-simulation methodologies investigated within this study may be implemented for the thermal-hydraulic analysis of HTRs and accompanying systems.