A Numerical Analysis on the Hydrodynamics of a Single Droplet in a Water Pool
2011 (English)In: Proceedings of the international congress on advances in nuclear power plants: ICAPP2011, American Nuclear Society, 2011Conference paper (Refereed)
The droplet behavior in coolant is one of the key phenomena that govern the dynamics and energetics of molten fuel coolant interactions (FCI) during a hypothetical severe accident of a light water reactor (LWR). The violent interaction between molten fuel droplets and coolant may lead to a steam explosion that shall challenge the containment integrity. Although an extensive research on steam explosion was conducted to investigate, the understanding of steam explosion energetics is still limited because of the complexity of the phenomenology involving intense thermal-hydrodynamics. The recent work at KTH [1-2] for was focused on the micro interactions of a single droplet with coolant by using a delicate test setup and a novel visualization system (simultaneous high-speed acquisition using x-ray radiography and photography), and it was found that the preconditioning (deformation/pre-fragmentation) of a droplet is instrumental to the resulting steam explosion. With the high-temperature molten droplet falling into the volatile liquid coolant, the droplet surface experiences intensive film boiling, and the formation of the vapor film on the droplet surface also contributes to the preconditioning of the droplet. In one of hypothetical mechanisms of steam explosion, for example, it is believed that the collapse of the vapor film, triggered by a shock wave, is the cause of steam explosion due to the direct contact between melt and coolant. The present study is to simulate and reproduce the preconditioning (deformation/pre-fragmentation) of the droplet by using the commercial CFD code ANSYS Fluent through the Volume of Fluid (VOF) method and User Defined Function (UDF). Since the existence of the intense heat and mass transfer in the multiphase systems presents considerable difficulties in the accurate simulation of the processes, the current simulation is only limited to hydrodynamics of the droplet and its vapor film in a water pool, without consideration of further phase change after film boiling. Due to the focus of the study being placed on the preconditioning whose time scale is quite short (in milliseconds), such a simplification does not lose the key physics of the process. To save the commutating time, a two-dimensional domain of 20mm×50mm is chosen in the computational analysis with a droplet of 3mm in initial diameter covered by a vapor film. The droplet is falling at a certain initial velocity (selected from 0 to 5 m/s). The initial thickness of the vapor film is also varied for a parametric study to see its effect on the deformation of the droplet. For further separate-effect study, analyses are also carried out with different surface tensions and viscosity values to gain insights of the effects of the melt material properties on steam explosion in hypothetical severe accidents of a LWR. As a part of the analysis, a shock wave is introduced inside the domain from one of the boundaries, to investigate the interfacial instability which is of importance to the triggering phase of a steam explosion.
Place, publisher, year, edition, pages
American Nuclear Society, 2011.
IdentifiersURN: urn:nbn:se:kth:diva-53587OAI: oai:DiVA.org:kth-53587DiVA: diva2:470383
2011 International congress on advances in nuclear power plants - ICAPP2011, Nice, France, 2-5 May, 2011
QC 201201032011-12-282011-12-282012-01-03Bibliographically approved