Qualification of simulation tools for two-phase flows

According to the state of the art nuclear safety analyses related to thermohydraulics are mostly done using so-called system codes. Such one-dimensional codes base on empirical or semi-empirical correlations, which are generally geometry dependent and valid only within a limited range of scales. In principle the validation of such codes requires experimental data obtained for the same geometries, similar scales and flow conditions. In the past such codes were validated and successfully applied to many relevant flow situations. But the transferability to differing flow situations as well as the reliability for flows with pronounced 3D effects as they are observed e.g. in Reactor Pressure Vessel (RPV) is questionably. For this reason there is an interest to apply beside system codes also Computational Fluid Dynamics (CFD) codes for special analyses in future. Presently CFD codes are frequently used in practical applications for single phase flows, e.g. in automobile or aviation industries. Two-phase flow simulations using CFD codes are not yet mature due to the complex interactions between the phases. Closure models are needed to describe mass, momentum and energy transfer between the phases. Such models should consider only local flow parameters, i.e. correlations available for system codes cannot be transferred in general for the use in CFD codes. Such models have to be developed and validated basing on new experimental data with high resolution in space and time. Examples are poly-dispersed bubbly flows which require a multi bubble size modelling or models for separated flows in horizontal or near horizontal channels. TOPFLOW is a unique thermal hydraulic test facility for such two-phase flow studies. Experiments can be carried out for air-water or steam-water two phase flows at pressures up to 7 MPa. For steam production up to 4 MW heating power are available. The TOPFLOW facility is currently prepared for different types of flow experiments in vertical test sections and a large pressure chamber. Unique measurement devices, such as high-pressure wire-mesh sensors and an X-ray scanner are available. They provide CFD like data, which means data in high resolution in space and time. The lecture gives a general overview on the CFD model development and validation for two-phase flows, which is illustrated on the examples of poly-dispersed bubbly flows and Pressurized Thermal Shock (PTS). The experimental capabilities of the TOPFLOW facility and the applied measuring techniques are discussed.