Studies of droplet entrainment and dispersion in two-phase flows
Abstract
The present research is motivated by the need to understand droplet entrainment and dispersion phenomena in the postulated Direct Containment Heating (DCH) scenario in severe nuclear reactor accidents. It consists of three closely related subjects, instrumentation development, accident simulation experiments and analytical modeling. A novel system for droplet size measurement is developed for the experimental investigation of entrained droplets in DCH. It consists of an isokinetic probe, a droplet collection mechanism, an image processing unit and a presently developed software. Some problems associated with the system are studied and a simple criterion for minimizing measurement distortion is obtained. The present system is suitable for measurements of both water and liquid metal droplets. Simulated DCH experiments are conducted in a 1/10 scaled test facility, using water or woods metal to simulate molten core materials (corium) and air to simulate steam. The physical processes of liquid entrainment and dispersion are observed by flow visualization. Size distribution and mass flux of entrained droplets are measured by the present droplet size measurement system. A mechanistic model for liquid (corium) dispersion into the reactor containment is developed based on mechanisms observed from flow visualization. The model compares satisfactorily with experimental results from both Purdue separate effects test and counterpart integral effects tests. Entrance and channel size effects on droplet size distribution are studied in view of the discrepancy between DCH data and previous data from others. A combined model for droplet deposition, covering both trajectory dependent deposition and deposition due to gas turbulent eddies, is developed to analyze the change of droplet size distribution along the flow channel. The analysis shows the existence of two different regions, the entrance and quasi-equilibrium regions. Mean droplet sizes in the two regions are significantly different and are strongly influenced by the channel size. The Kotaoka-Ishii correlation is extended to predict droplet size at the time of entrainment, and the extended correlation and the combined deposition model form a unified model for droplet size distribution in the entire annular flow region. The unified model compares well with both DCH and previous droplet size data, and it is also consistent with the Kotaoka-Ishii correlation in the quasi-equilibrium region.
Degree
Ph.D.
Advisors
Ishii, Purdue University.
Subject Area
Nuclear physics
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