Experimental and Computational Study on Flow Measurement and Mixing Characterization in High-Temperature Gas-Cooled Reactors

Abstract

The High-Temperature Gas-Cooled Reactor (HTGR) has received great attention due to its enhanced safety, high thermodynamic efficiency, and suitability as energy source for high-temperature industrial processes such as hydrogen production. However, despite extensive advances in the development of this design, there remain thermal-hydraulic challenges that need to be addressed before commercialization.

One challenge is the measurement of the primary coolant flow. Currently, the primary helium coolant flow is not directly measured but rather inferred from either the energy balance of the turbine compressor or the helium circulator rotational speed. Though functional, both methods introduce large uncertainties; therefore, HGTR operation would benefit greatly from the development of a suitable gas flow meter. Such development is challenging due to the high flow rates (10 − 15 m/s at the nominal operating conditions), high operating temperatures (> 700oC), and high radiation fields. No commercially available flow meters are suitable for flow rate measurements in such a harsh environment.

Another challenge is related to the accurate prediction of the plant behavior during an extended Loss of Force Circulation (LOFC) accident. During LOFC scenarios, the flow in the reactor pressure vessel reverts its direction, moving upwards. The extent to which the hot coolant jets exiting the core mix in the upper plenum will determine the heat removal rate, the occurrence of local hot spots, and the potential for thermal striping. While 3D Computational Fluid Dynamic (CFD) models can be useful for an accurate prediction of thermal mixing and the occurrence of hot spots in the HTGR upper plenum, a shortage of high-resolution experimental data has hampered the validation of such models.

This thesis addresses the challenges mentioned above by 1): designing a novel acoustic flow meter and 2): conducting high-resolution experiments for mixing in large enclosures to establish a high-resolution database specifically designed for the validation of high-fidelity CFD models. The novel acoustic flow meter relies upon the principle of vortex-induced acoustics. Within the present dissertation, a combination of experiments and simulations was conducted to support the development and optimization of the flow meter design and to successfully demonstrate its proof of principle. To investigate the flow conditions relevant to mixing in the upper plenum and establish a database for CFD model validation, two experimental facilities were designed and constructed. Special care was dedicated to guaranteeing well-characterized boundary conditions so that the data could be used for the validation of CFD codes. First, the Michigan Multi-jet Gas-mixture Dome (MiGaDome) facility, a 1/12th scaled down from the upper plenum geometry of the Modular HTGRs design, was designed and built to investigate multi-jet interactions in a domed enclosure using optical measurements such as Laser Doppler Velocimetry (LDV) and Particle Image Velocity (PIV). Second, the High-Resolution Jet (HiRJet) facility was used to investigate the propagation of stratified fronts in a body of fluid. Wire-mesh sensor (WMS) units were used for high-resolution measurements of the transient density field and characterization of the stratified front. High-resolution experimental data measured at these two facilities were used to shed light on the flow mixing in large. Extensive code validation has been conducted for RANS-based turbulence models to assess and improve the performance of the computational models. The work in this thesis is expected to benefit the advancement of HTGR designs by providing practical solutions and through enhanced computational tools.