All issues

Represented study considers numerical simulation of corium cooling driven by natural convection within a horizontal hemicylindrical cavity, boundaries of which are assumed isothermal. Corium is a melt of ceramic fuel of a nuclear reactor and oxides of construction materials.

Corium cooling is a process occurring during severe accident associated with core melt. According to invessel retention conception, the accident may be restrained and localized, if the corium is contained within the vessel, only if it is cooled externally. This conception has a clear advantage over the melt trap, it can be implemented at already operating nuclear power plants. Thereby proper numerical analysis of the corium cooling has become such a relevant area of studies.

In the research, we assume the corium is contained within a horizontal semitube. The corium initially has temperature of the walls. In spite of reactor shutdown, the corium still generates heat owing to radioactive decays, and the amount of heat released decreases with time accordingly to Way–Wigner formula. The system of equations in Boussinesq approximation including momentum equation, continuity equation and energy equation, describes the natural convection within the cavity. Convective flows are taken to be laminar and two-dimensional.

The boundary-value problem of mathematical physics is formulated using the non-dimensional nonprimitive variables «stream function – vorticity». The obtained differential equations are solved numerically using the finite difference method and locally one-dimensional Samarskii scheme for the equations of parabolic type.

As a result of the present research, we have obtained the time behavior of mean Nusselt number at top and bottom walls for Rayleigh number ranged from 10³ to 10⁶. These mentioned dependences have been analyzed for various dimensionless operation periods before the accident. Investigations have been performed using streamlines and isotherms as well as time dependences for convective flow and heat transfer rates.

A large number of methods have been developed to solve the Navier – Stokes equations in the case of incompressible flows, the most popular of which are methods with velocity correction by the SIMPLE algorithm and its analogue — the method of splitting by physical variables. These methods, developed more than 40 years ago, were used to solve rather simple problems — simulating both stationary flows and non-stationary flows, in which the boundaries of the calculation domain were stationary. At present, the problems of computational fluid dynamics have become significantly more complicated. CFD problems are involving the motion of bodies in the computational domain, the motion of contact boundaries, cavitation and tasks with dynamic local adaptation of the computational mesh. In this case the computational mesh changes resulting in violation of the velocity divergence condition on it. Since divergent velocities are used not only for Navier – Stokes equations, but also for all other equations of the mathematical model of fluid motion — turbulence, mass transfer and energy conservation models, violation of this condition leads to numerical errors and, often, to undivergence of the computational algorithm.

This article presents an implicit method of splitting by physical variables that uses divergent velocities from a given time step to solve the incompressible Navier – Stokes equations. The method is developed to simulate flows in the case of movable and contact boundaries treated in the Euler paradigm. The method allows to perform computations with the integration step exceeding the explicit time step by orders of magnitude (Courant – Friedrichs – Levy number $CFL\gg1$). This article presents a variant of the method for incompressible flows. A variant of the method that allows to calculate the motion of liquid and gas at any Mach numbers will be published shortly. The method for fully compressible flows is implemented in the software package FlowVision.

Numerical simulating classical fluid flow around circular cylinder at low Reynolds numbers ($50 < Re < 140$), when laminar flow is unsteady and the Karman vortex street is formed, are presented in the article. Good agreement of calculations with the experimental data published in the classical works of Van Dyke and Taneda is demonstrated.