Characterizing Erosion Mixing of a Confined Stratified Layer by Turbulent Impinging Jet

Abstract

This thesis addresses the characterization of the erosion mixing of a confined stratified layer caused by a turbulent impinging jet. The erosion-mixing process determines the changes in a concentration and distribution of a stratified hydrogen layer in a containment building at a nuclear power plant during a severe accident. The dilution of hydrogen gas can mitigate the possibility of a hydrogen explosion, which can seriously affect the integrity of the containment building. To predict or assess the hydrogen risk, it is necessary to analyze the transient erosion-mixing process of a stratified hydrogen layer over a long time period. This is inherently a complex problem with time, flow and geometric conditions, and it is difficult to characterize the complexity because of the high cost of experimental and numerical studies. Hence, this thesis focuses on theoretical modeling of the transient erosion-mixing process, taking into account the effects of flow and geometric conditions. First, this thesis proposes a novel theoretical model of the transient interface displacement for evaluating quantitatively the transient behavior of a stratified layer due to erosion mixing caused by a turbulent impinging jet. We show that the interface displacement has a nonlinear and time-dependent relationship with flow conditions such as the competitive force between the impinging jet and stratified layer, as well as with geometric conditions such as scale difference between the vessel radius and jet radius. We develop a theoretical model for long-term interface displacement by using incrementally a previous model for the quasi-steady entrainment rate into the mass conservation law based on the entrainment of a stratified layer fluid. As a result, we show that the theoretical model characterizes the dimensionless interface displacement in terms of dimensionless parameters for the elapsed time, scale difference, and competitive force. Experimental results obtained from previously reported studies, as well as from the present study, are in good agreement with the interface displacement predicted by the theoretical model. Finally, this thesis proposes a novel theoretical model for the transient behavior of a turbulent jet impinging on a stratified layer, which is the direct cause of the erosion-mixing process. We develop the transient model for a jet flow impinging on a linearly stratified layer over a long period using a previous quasi-steady model for a fountain flow in a uniformly stratified layer. As a result, we show that the theoretical model can successfully predict the transient erosion-mixing process caused by an impinging jet under the effects of a negative buoyant force and a negative pressure force. The predictions of the theoretical model are in good agreement with the experimental results obtained in the present study. The agreements of theoretical models for the interface displacement and impinging jet flow demonstrate that proposed models would be useful for predicting and evaluating the transient erosion-mixing process of a stratified layer caused by an impinging jet flow under various conditions.