Messung von instationären Strömungskräften in fluchtenden und versetzten Rohrbündeln bei starren und schwingenden Rohren

Unsteady flow forces arising in heat exchangers with cross-flow my lead to serious vibrations of the tubes. These vibrations can destroy the tubes in the end supports or in the baffles, which would require expensive repairs. The flow forces reach unexpectedly by high values if the vibration of the tube intensifies these forces. To clear up this coupling mechanism the flow forces and the vibration amplitude were measured simultaneously in a staggered and in an inline tube bundle. Considering the tube as a one-mass oscillator excited by the flow force, the min parameters can be derived, i.e. dynamic pressure, reduced mass, eigenfrequency and damping. These parameters form a dimensionless model number describing the coherence of the vibration amplitude and the force coefficient. The validity of this number has been confirmed by varying the test conditions. With the aid of this model number, the expected force coefficient can be calculated and then using a finite-element program information can be obtained about mechanical tensions and the lifetime of the heat exchanger tubes. With this model number the results of other authors, who measured the vibration amplitude only, could be confirmed in good agreement. The experiments were carried out in air with Reynolds numbers 10$^{4}$$\leq$Re$\leq$10$^{5}$

Theoretische Untersuchungen zur Spaltproduktfreisetzung aus dem Core eines Hochtemperaturreaktors bei hypothetischen Coreaufheizungsstörfällen am Beispiel von Cäsium

The fission product release out of the core of a high temperature reactor during hypothetical heat up accidents has been investigated. This has been attained on support to a physical model, taking into account the micro- and macro-structures of the pyrolytical and graphitical reactor components as well as renouncing an introduction of effektive diffusion coefficients by the description of the fission products transport through the coated particle layers and the fuel elements and renouncing an assuption of the spontaneously adsorption-desorption equilibrium on the surface of the fuel elements. The solving method and the respective computer codes were also developped. In addition the theoretically calculated and the experimentally determined results regarding the caesium release from single coated particles as well as fuel elements at accident temperatures were compared. Finally the caesium release from the core of the PNP-500 reactor during a heat up accident has been estimated and discussed