Flow maps and models for transient two-phase flows
Experiments with novel measurement instrumentation delivered data on the structure of transient gas-liquid flows used for the development and validation of microscopic, i.e. geometry-independent constitutive equations for the description of momentum exchange between the phases as well as for characterising the frequency of bubble coalescence and fragmentation. For this purpose a vertical test section of the two-phase facility MTLoop in Rossendorf was used, whereas wire-mesh sensors with a resolution of 2-3 mm and a sampling frequency of up to 10 kHz were applied for the first time. This allowed to record the evolution of gas fraction and velocity profiles as well as bubble size distributions along the flow path and during fast transients, i.e. to obtain the data necessary for the modelling. For the test of the model equations, a simplified solver for the fluid-dynamic equations along the test pipe was developed, which considers a large number of bubble size classes. The numerical solutions have shown that the transition from bubble flow with a wall peak of the gas fraction to a flow with central peak and further to a slug flow at increasing gas velocities can be modelled on basis of a unified set of physically reasonable and geometry-independent constitutive laws. These model equations have proven to be generalising for a certain region of superficial velocities and are ready for an implementation in CFD codes. Furthermore, experiments on steam condensation were performed, which have a direct relation to condensation models in thermal hydraulic system codes. The investigations gained experimental data for the model validation, including the behaviour and the effect of non-condensible gases. A special kind of probes for the detection of the non-condensible gas and the replacement of plugs of it were developed and applied in transient condensation tests in slightly inclined heat exchanger pipes.
This work was carried out in the frame of a research project funded by the German Federal Ministry of Economics and Technology, project number 150 1215.