Experimental examinations of the 2D-thermal-hydraulic code FRECON for single phase natural convection flows in complex structures

An experimental program for the verification of the 2D-thermal-hydraulic code FRECON is being carried out. This code serves for the simulation of single phase natural convective flows in complex structures. As a special application it has already been used in the analyses of severe LWR accidents. For this, the code has been coupled with the KESS code system describing the core behaviour during such an accident. While strong improvements have been reached meanwhile concerning the FRECON code itself as well as its application to the reactor problem, experimental verification has not yet been performed. Such a verification is strongly required because of the basic assumption of a quasi-continuum approach for the solid structures in FRECON and the problem of describing local characteristic properties of the structures within this approach.In contrast to integral experiments aiming to simulate the real reactor situation as closely as possible, the present experiments aim to check specific structures and aspects in detail. Thus, in a first step, natural convective flows in a rod bundle with imposed solid temperature profiles are being analysed. In addition to temperature measurements in the solid as well as in the gas, the flow patterns are visualized by means of light-weight glass spheres as tracers, illuminated with a laser-light-sheet. After recording the tracers by a video system the analysis of the flow patterns is made either directly at the TV-monitor or by using the digital image processing technique PIDV (Particle Image Displacement Velocimetry). First comparisons between code calculations and experimental results support the code development in general, but need further refinement.