The Thermal Hydraulic Test Facility Topflow: Overview on Experimental Capabilities and Instrumentation

TOPFLOW stands for Transient TwO Phase FLOW. The new thermal-fluiddynamic test facility of FZR was built for generic and applied studies of transient two phase flow phenom-ena in power and process industries. It is used as experimental basis to develop and validate three-dimensional CFD, in particular for an application to safety relevant flow simulations in the field of nuclear reactor safety. The work is carried out in a close and very fruitful coopera-tion with the code developer ANSYS-CFX. Object of the experimental studies is a gas-liquid two-phase flow. In the field of nuclear technology, the introduction of CFD is connected with high expectations concerning the quality of the predictions compared to the established one-dimensional thermal hydraulic analyses, since CFD allows to substitute geometry-dependent empirical closure relations by more physically justified closure laws that are formulated on the scale of the structures of the gas-liquid interface. In this way, modelling becomes much more independent from geometrical and thermodynamic boundary conditions and the scale-up to the real reactor scale becomes more reliable than in case of traditional thermal hydraulic codes.

TOPFLOW is not a dedicated integral test modelling a specific reactor type. It was rather de-signed as a multi-purpose facility for different single-effect experiments. This offers numer-ous possibilities for experiments to study basic two-phase flow phenomena as well as to per-form applied research for the industries. The latter includes, for instance, investigations of innovative and passive safety systems for nuclear reactors, like the emergency condenser for boiling water reactors, a model of which is a major component of TOPFLOW. A carefully designed instrumentation including advanced two-phase flow sensors of own development delivers experimental data of high quality, that reflect the addressed phenomena and processes in the necessary detail.

An important role plays the availability of measuring information from wire-mesh sensors with a high spatial resolution of 3 mm, which deliver sequences of complete two-dimensional gas fraction distributions from the entire cross section of both pipes DN50 and DN200 with a frame rate of 2500 Hz. These sensors were upgraded to operational parameters of 280 °C and 65 bar. First results on void profiles and bubble size distributions as well as the evolution of the flow structure along a vertical pipe obtained in this parameter range will be presented.

Another new method used at TOPFLOW is the strategy to install a test facility inside a tank pressurized with air of up to 50 bar and operate if in pressure equilibrium with the inner at-mosphere of the tank. The advantages can be summarised as follows: (1) the test facility itself can have strange shapes that are unfeasible for pressurised components, (2) the manufacture of the test itself will be cheap, since no pressure carrying components are needed, (3) thin walls make it easy to apply instrumentation, for instance optical measurements through glass walls or temperature field measurements by directing an infrared camera to a thin metal wall, (4) no expensive and time consuming licensing procedures are necessary, because the test is housed in the tank designed for the maximum overpressure.