Visualisation of boiling processes in a 3x3 rod bundle using ultrafast X-ray tomography

Boiling processes are an effective way of heat transfer in thermo hydraulic facilities, such as heat exchangers and reactor cores of nuclear power plants. To run these processes safe and efficient comprehensive studies of flow physics within boiling fluids are needed. Within the frame of the coordinated project “Modeling, simulation and experiments for boiling phenomena in pressurized water reactors” (BMBF FZK 02NUK010) HZDR developed and built up an experimental facility for the study of boiling flows in 3 × 3 rod bundle geometry. While representing a fuel element segment of a pressurized water reactor, the bundle depicts every main geometrical dimension. The facility is operated with RC318 refrigerant fluid. The bundle is electrically heated by a dc power supply, which allows heat flux values of up to 0.2MW/m². Ultrafast electron beam X-ray tomography has been used at this experiment. Therefore the bundle was designed for a coaxial electrical current to flow through, to minimize the residual magnetic field, which would interfere with the electron beam deflection. Thus, the center rod - outer rod heat flux ratio is 64:1. The boiling processes will be expected around the centre rod. Beside standard instrumentation for fluid temperature, pressure and flow rate an extensive thermo-instrumentation of centre rod allows axial rod wall temperature measurements and safely shut down when exceeding a certain surface temperature. The goal of these experiments is to gain Access to very transient boiling phenomena, such as quenching or pulsating of vaporized fluid in bündle geometries with previously unseen temporal resolution. First experimental results have been achieved and will be introduced and discussed in this article.