Nuclear Energy for New Europe 2009

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No: 203

Conference: Nuclear Energy for New Europe 2009

Title: Numerical simulations of basic interfacial instabilities with improved two-fluid model

Theme: Thermal Hydraulics

Author(s): Luka Štrubelj, Iztok Tiselj

Contact : Luka Štrubelj

E-mail: luka.strubelj@ijs.si

Address: Institut "Jožef Stefan"
1001 Ljubljana

Country: Slovenia

Two-fluid models are based on averaged Navier-Stokes equations and are frequently used for simulations of industrial two-phase flows. The main disadvantage of the two-fluid models used for simulations of free surface flows is their poor accuracy in simulations of free surface flows due to numerical diffusion of the interface.
In the present paper the interface of the two-phase flow was successfully recognized and sharpened with the conservative level set method within the two-fluid model. After the advection step of volume fraction the numerical diffusion of the interface was reduced in such a way that the thickness of the interface is kept constant during the simulation.
The two basic instabilities of free surface flows: the Rayleigh-Taylor and Kelvin-Helmholtz instability were used to validate the drag force, the interface sharpening and the surface tension force implementation within the two-fluid model.
The Rayleigh-Taylor instability occurs in the system, where the fluid with higher density is located above the fluid with lower density. Due to the gravity the fluid with higher density moves below the fluid with smaller density. The surface tension defines the most unstable wave-number. The most unstable wavelength that appears in the simulation of Rayleigh-Taylor instability and its growth are in good agreement with theoretical prediction.
The tilted tube experiment, where two immiscible fluids are present in the channel, suddenly tilted for a small angle, was simulated. Due to density difference countercurrent flow evolves in the channel. After certain time, when the velocity difference is high enough the Kelvin-Helmholtz instability appears. The onset of instability, critical wavelength, growth of amplitude and wave speed in the simulation with two-fluid model with interface sharpening are in good agreement with experimental measurements and theoretical predictions.
The improved two-fluid model presents a step towards the simulations of mixed flows, where free surface flow is present at one location and dispersed at other location of the computational domain. Both flows can be simulated within two-fluid model: the free surface flow, simulated with two-fluid model with interface sharpening and dispersed flow, simulated with two-fluid model for dispersed flow. Such mixed flow appears during loss of coolant accident, where emergency core cooling system injects cold water into partially uncovered cold leg.

No:	203
Conference:	Nuclear Energy for New Europe 2009
Title:	Numerical simulations of basic interfacial instabilities with improved two-fluid model
Theme:	Thermal Hydraulics
Author(s):	Luka Štrubelj, Iztok Tiselj
Contact :	Luka Štrubelj
E-mail:	luka.strubelj@ijs.si
Address:	Institut "Jožef Stefan" 1001 Ljubljana
Country:	Slovenia

Two-fluid models are based on averaged Navier-Stokes equations and are frequently used for simulations of industrial two-phase flows. The main disadvantage of the two-fluid models used for simulations of free surface flows is their poor accuracy in simulations of free surface flows due to numerical diffusion of the interface. In the present paper the interface of the two-phase flow was successfully recognized and sharpened with the conservative level set method within the two-fluid model. After the advection step of volume fraction the numerical diffusion of the interface was reduced in such a way that the thickness of the interface is kept constant during the simulation. The two basic instabilities of free surface flows: the Rayleigh-Taylor and Kelvin-Helmholtz instability were used to validate the drag force, the interface sharpening and the surface tension force implementation within the two-fluid model. The Rayleigh-Taylor instability occurs in the system, where the fluid with higher density is located above the fluid with lower density. Due to the gravity the fluid with higher density moves below the fluid with smaller density. The surface tension defines the most unstable wave-number. The most unstable wavelength that appears in the simulation of Rayleigh-Taylor instability and its growth are in good agreement with theoretical prediction. The tilted tube experiment, where two immiscible fluids are present in the channel, suddenly tilted for a small angle, was simulated. Due to density difference countercurrent flow evolves in the channel. After certain time, when the velocity difference is high enough the Kelvin-Helmholtz instability appears. The onset of instability, critical wavelength, growth of amplitude and wave speed in the simulation with two-fluid model with interface sharpening are in good agreement with experimental measurements and theoretical predictions. The improved two-fluid model presents a step towards the simulations of mixed flows, where free surface flow is present at one location and dispersed at other location of the computational domain. Both flows can be simulated within two-fluid model: the free surface flow, simulated with two-fluid model with interface sharpening and dispersed flow, simulated with two-fluid model for dispersed flow. Such mixed flow appears during loss of coolant accident, where emergency core cooling system injects cold water into partially uncovered cold leg.