Numerical modeling of turbulent hydrogen combustion for nuclear safety management

Abstract

During a severe accident in a water­-cooled nuclear power plant, large quantities of hydrogen can be generated in the reactor core. The hydrogen mixed with air presents a potential risk of combustion when the hydrogen concentration reaches flammability limits. If combustion occurs, pressure loads can damage safety systems and even compromise the structural integrity of the nuclear reactor walls. Thus, predicting the pressure loads is an important safety issue to ensure the reliability of critical structures in the event of a severe accident.

Computational Fluid Dynamics (CFD) codes can be used as numerical tools to assess the risks of hy­drogen combustion and predict detailed transients of the pressure loads. The scenarios simulated in nuclear safety engineering can be analyzed with low fidelity models based on reduction assumptions of the physical phenomena. Specifically, the flame gradient models are chosen to model the com­bustion phenomena based on a turbulent flame speed to which the combustion propagates. Different approaches to compute the turbulent flame speed are implemented and compared in this study, in­cluding the Turbulent Flame Closure (TFC) and the Extended Turbulent Flame Closure (ETFC). New models are proposed based on new experimental correlations specifically derived for lean mixtures of hydrogen. Other required libraries as flame radius calculation, radiative heat transfer properties determination, and axisymmetric Adaptive Mesh Refinement are implemented and evaluated. A study on the influence of the ignition source term is also carried out.

The implementation of the solver and the models is verified with the analytical solution of a planar one­-dimensional premixed flame moving into frozen turbulence. Moreover, three different experimen­tal cases, relevant to nuclear safety, are selected to perform the validation of the solver. The first one is a spherical combustion chamber which was experimentally tested with lean hydrogen mixtures and controlled measured homogeneous isotropic turbulence. The second one is the Thermal-hydraulics Hydrogen Aerosol and Iodine (THAI) facility, which is a larger vessel where effects as buoyancy be­ come more relevant. Finally, the third one is a flame acceleration enclosure called ENACCEF­2, where obstacles are placed along a cylindrical tube to promote flame acceleration and the transition from de­flagration to detonation.

Numerical results are presented in terms of the flame front location, the flame front velocity, and the pressure rise. The sensitivity of the results to mesh resolution, time discretization, and initial turbu­lence levels is assessed. The implemented combustion models are simulated with multiple turbulence models, analyzing the results and the influence on one each other. In general, the proposed mod­els represent an approach that provides good predictions in both flame development and pressure dynamics, being more robust than other models previously available