Research On Simulation For Mixing And Stratification In Large Enclosures

Mixing and stratification in large enclosures can significant affect nuclear systems, building HVAC systems, chemical processing, waste storage, and pollutant dispersal in atmosphere and natural bodies such as lakes, rivers, and oceans. Thermal mixing and stratification often occur in passive containment cooling system (PCCS), which is an important part of passive safety system. Thus, it is important to accurately predict the temperature and density distribution both for design optimization and accident analysis.However, most current reactor system analysis codes such as RELAP5, only provide lumped parameter models which can not give precise results. The traditional2-D or3-D CFD methods require very fine grid resolution to resolve thin substructures. Previous scaling analysis has shown that the mixing processes in large stably stratified enclosures can be modeled with one-dimensional differential conservation equations, with the vertical transport by free and wall jets modeled using standard integral techniques. This allows very large reductions in computational effort compared to three-dimensional numerical modeling. A new computation code was developed to implement such ideas.This paper summarizes the major computation models in the new codes. The codes use a Lagrangian approach to solve1-D transient governing equations for the ambient fluid and uses1-D integral model or Schatzmann model to compute substructures. To run and debug the new computation codes, the new models are applied on mixing and heat transfer experiments in a large rectangular enclosure with an injected buoyant jet and a cooled vertical surface, which simulates a break flow for passive reactor containment systems and studies the change of important parameters such as air temperature and velocity in different condition. The code can simulate the stratification of room fire and water tank. The comparison was made between the new computation code and ANSYS CFX. The new computation codes were vertified by these CFX results.