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Simulation of Two-Phase Flow in Complex Systems

Wolfgang Wulff

Nuclear Technology

Volume 159 / Number 3 / September 2007 / Pages 292-309


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The paper presents integral methods for simulating two-phase flow transients in complex cooling systems, such as those in nuclear power plants. The methods are designed to simplify presently prevailing thermohydraulics simulation methods without a loss in simulation fidelity. The paper describes the inherent, but unnecessary, complexity of currently used simulation models, explains their inherent shortcomings, and foretells the impact of current code development trends on future capabilities to resolve safety issues in light of growing code complexities and inflexibility. The purpose of the paper is to present simpler alternatives.

Integral methods described in the paper facilitate flexibility via computer-automated modularity and simplicity. They provide transparency through analytical methods. Integral methods replace partial by ordinary differential equations and thereby simplify the mathematical model formulation and achieve numerical integration with minimal numerical damping. The models connect important physical characteristic response times with the time step for numerical integration. The mixture model of nonhomogeneous, nonequilibrium two-phase flow, the integral of the volumetric flux divergence equation, and the integrals of the system of coupled loop momentum balances for interconnected loops in complex thermohydraulic systems play central modeling roles. A new and compact formulation of these equations facilitates a computerized system assembly of component models, each one being judiciously selected from a model library to impose the minimum necessary complexity. The method of assembly is based on linear algebra and accommodates any combination of phasic flow directions anywhere in the hydraulic system and at any time.

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