Nuclear Energy for New Europe 2009

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No: 207

Conference: Nuclear Energy for New Europe 2009

Title: The Inhomogeneous MUSIG Model for Upward Polydisperse Flows with New Constitutive Models for Bubble Coalescence and Breakup

Theme: Thermal Hydraulics

Author(s): Yixiang Liao, Dirk Lucas, Eckhard Krepper, Martin Schmidtke

Contact : Yixiang Liao

E-mail: y.liao@fzd.de

Address: Forschungszentrum Dresden-Rossendorf
D-01328 Dresden

Country: Germany

Flow fields in the safety research of nuclear reactors are usually complex and often involve two-phase flows, where one of the phases is continuous and the other phase consists of disperse bubbles. Interfacial heat and mass transfer within such flows is directly related to
the interfacial area density, and to the residence time of the disperse phase. Conventional methods are inadequate to resolve the local distribution and movement of disperse phase. Computational fluid dynamics (CFD) has been identified as a viable tool for the analysis of
such cases. In many CFD simulations of two-phase flows, the disperse phases, i.e. the bubbles in gas-liquid flows, are assumed to have an equal size and shape. In the reality, due to bubble coalescence and breakup a wide spectrum of sizes and shapes may exist throughout the flow, which is called polydisperse flow. The inhomogeneous Multiple Size Group (MUSIG) model has led to notable success in the prediction of polydisperse flows by taking the bubble size distribution into account. This model was developed with the cooperation of ANSYS-CFX and Forschungszentrum Dresden-Rossendorf and implemented into the CFD code CFX. During the validation process, the default constitutive models [1, 2] for bubble coalescence and breakup were diagnosed as the weakest points, hich must be further investigated. Details can be found in previous work [3, 4, 5, 6]. Thus, a series of work was done for the purpose of developing more fundamental models for bubble coalescence and breakup in polydisperse bubbly flows, which would apply
to a large range of flow conditions. Based on an extensive literature study on breakup and coalescence models available for fluid particles, a basis model has been proposed, which considers coalescence and breakup due to different mechanisms, including coalescence due to turbulence, laminar shear, wake entrainment and eddy capture, and breakup due to turbulent fluctuation, laminar shear and interfacial slip velocity. In the previous work, the basis model was implemented in a 1D Test Solver and compared with the experiments of TOPFLOW test facility of Forschungszentrum Dresden-Rossendorf. The new model was shown to be capable of tracing the evolution of bubble size distribution due to coalescence and breakup, even for large gas volume fraction [7, 8, 9, 10].
In this work, the basis model is implemented into the CFD code CFX through user FORTRAN routines, and serves as new constitutive models for the inhomogeneous MUSIG NNN.2 Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, Sept. 8-11, 2008 approach. Its validation for several vertical pipe flow situations is presented. Predictions for the evolution of radial gas volume fractions and bubble size distribution as well as separation of small and large bubbles along the vertical pipe are compared to the experimental TOPFLOW data and the default constitutive models of Luo & Svendsen and Prince & Blanch.

No:	207
Conference:	Nuclear Energy for New Europe 2009
Title:	The Inhomogeneous MUSIG Model for Upward Polydisperse Flows with New Constitutive Models for Bubble Coalescence and Breakup
Theme:	Thermal Hydraulics
Author(s):	Yixiang Liao, Dirk Lucas, Eckhard Krepper, Martin Schmidtke
Contact :	Yixiang Liao
E-mail:	y.liao@fzd.de
Address:	Forschungszentrum Dresden-Rossendorf D-01328 Dresden
Country:	Germany

Flow fields in the safety research of nuclear reactors are usually complex and often involve two-phase flows, where one of the phases is continuous and the other phase consists of disperse bubbles. Interfacial heat and mass transfer within such flows is directly related to the interfacial area density, and to the residence time of the disperse phase. Conventional methods are inadequate to resolve the local distribution and movement of disperse phase. Computational fluid dynamics (CFD) has been identified as a viable tool for the analysis of such cases. In many CFD simulations of two-phase flows, the disperse phases, i.e. the bubbles in gas-liquid flows, are assumed to have an equal size and shape. In the reality, due to bubble coalescence and breakup a wide spectrum of sizes and shapes may exist throughout the flow, which is called polydisperse flow. The inhomogeneous Multiple Size Group (MUSIG) model has led to notable success in the prediction of polydisperse flows by taking the bubble size distribution into account. This model was developed with the cooperation of ANSYS-CFX and Forschungszentrum Dresden-Rossendorf and implemented into the CFD code CFX. During the validation process, the default constitutive models [1, 2] for bubble coalescence and breakup were diagnosed as the weakest points, hich must be further investigated. Details can be found in previous work [3, 4, 5, 6]. Thus, a series of work was done for the purpose of developing more fundamental models for bubble coalescence and breakup in polydisperse bubbly flows, which would apply to a large range of flow conditions. Based on an extensive literature study on breakup and coalescence models available for fluid particles, a basis model has been proposed, which considers coalescence and breakup due to different mechanisms, including coalescence due to turbulence, laminar shear, wake entrainment and eddy capture, and breakup due to turbulent fluctuation, laminar shear and interfacial slip velocity. In the previous work, the basis model was implemented in a 1D Test Solver and compared with the experiments of TOPFLOW test facility of Forschungszentrum Dresden-Rossendorf. The new model was shown to be capable of tracing the evolution of bubble size distribution due to coalescence and breakup, even for large gas volume fraction [7, 8, 9, 10]. In this work, the basis model is implemented into the CFD code CFX through user FORTRAN routines, and serves as new constitutive models for the inhomogeneous MUSIG NNN.2 Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, Sept. 8-11, 2008 approach. Its validation for several vertical pipe flow situations is presented. Predictions for the evolution of radial gas volume fractions and bubble size distribution as well as separation of small and large bubbles along the vertical pipe are compared to the experimental TOPFLOW data and the default constitutive models of Luo & Svendsen and Prince & Blanch.