Nikolay Ivanov Kolev

The influence of mutual bubble interaction on the bubble departure diameter

Abstract The tangential shear due to the volume and time-averaged pulsation velocity caused by thermally controlled bubble growth and successive cyclic departure is found to be responsible for the natural switch from the regime of isolated bubble growth and departure into the regime of mutual interaction bubble growth and departure as the temperature difference between the bubble interface and sorrounding liquid increases. The proposed theoretical model, based only on first principles. agrees well with the experimental data for boiling water to which it was compared. The new model, which successfully predicts the isolated and mutual interaction bubble departure size, and the quantitative description of the natural transition between the two regimes are the essential new features of this work.

External Cooling: The SWR 1000 Severe Accident Management Strategy

This paper provides the description of the basics behind design features for the severe accident management strategy of the SWR 1000. The hydrogen detonation/deflagration problem is avoided by containment inertization. In-vessel retention of molten core debris via water cooling of the external surface of the reactor vessel is the severe accident management concept of the SWR 1000 passive plant. During postulated bounding severe accidents, the accident management strategy is to flood the reactor cavity with Core Flooding Pool water and to submerge the reactor vessel, thus preventing vessel failure in the SWR 1000. Considerable safety margins have been determined by using state of the art experiment and analysis: regarding (a) strength of the vessel during the melt relocation and its interaction with water; (b) the heat flux at the external vessel wall; (c) the structural resistance of the hot structures during the long term period. Ex-vessel events are prevented by preserving the integrity of the vessel and its penetrations and by assuring positive external pressure at the predominant part of the external vessel in the region of the molten corium pool.Copyright

Modeling the mechanical interaction between the velocity fields in three-phase flow

Abstract The predictions of IVA3 computer code models describing the mechanical interactions between the velocity fields in liquid-gas, liquid-solid, and liquid-solid-gas bubble flows are systematically compared with Sakaguchis experimental data for upward pipe flows. For better understanding of the physics of the experiments, a simple model, describing especially the conditions observed in the experiments, was derived by simplifying the theory on which the IVA3 code relies. The model describes an adiabatic, steady-state, three-phase, three-component flow with thermal equilibrium, mechanical nonequilibrium between the velocity fields, and the condition for critical flow. Ishii and Chawlas correlations for calculation of drag forces for bubbles or solid particles in liquid are checked against the experimental data obtained by Sakaguchi et al The model developed by Kolev for computation of the drag forces when solid particles are free in the flow and the volume fraction of the space among the particles if they were closely packed is larger than the liquid volume fraction was checked against data obtained by Sakaguchi et al for three-phase bubble flow. The agreement obtained between the IVA3 predictions and the data for two- and three-phase flow shows the ability of the Ishii-Chawla correlation and the method proposed by Kolev to predict successfully the drag forces for two- and three-phase flows and the correctness of the mathematical modeling technique incorporated in the IVA3 computer code, the so-called partial resolution of local velocity coupling.

Bubble Growth in Superheated Liquid

This chapter presents the achievements of the theory of bubble growth in superheated liquids. The thermally controlled bubble growth solutions are summarized and the derivation of the Mikic equation is discussed in some detail. Then the link between the solutions for the bubble growth and the mass source terms for the averaged conservation equations for two-phase flow are presented. The way to derive nonaveraged mass source terms and timeaveraged mass source terms is given. The effect of the steam superheating is discussed. A brief discussion of diffusion-controlled bubble growth initially containing noncondensable gases is also given.

Thermo-Physical Properties for Severe Accident Analysis

Several modern aspects of the severe accident analysis can not be understood if the engineer does not have accurate information of the material properties for the participating structural materials in solid, in liquid and in some cases in gaseous states. Chapter 17 contains valuable sets of thermo-physical and transport properties for severe accident analysis for the following materials: uranium dioxide, zirconium dioxide, stainless steel, zirconium, aluminum, aluminum oxide, silicon dioxide, iron oxide, molybdenum, boron oxide, reactor corium, sodium, lead, bismuth and lead-bismuth alloys. The emphasis is on the complete and consistent thermo dynamical sets of analytical approximations appropriate for computational analysis.

Nucleation in liquids

After reviewing the literature for description of the nucleation in superheated liquids the following conclusions and recommendations have been drawn. The maximum superheating in technical systems is a function of the depressurization velocity and of the produced turbulence. The maximum superheating can be predicted by the Algamir and Lienhard and by the Bartak correlations within an error band of 48.5%. Flashing in short pipes and nozzles leads to critical flows driven by the pressure difference equal to the entrance pressure minus the flashing inception pressure. For the prediction of the maximum achievable superheating, which represents the spinoidal line the Skripov correlation is recommended. The wetting angle is an important property of the polished surface characterizing its capability to activate nucleation sites. For the prediction of the activated nucleation sites the correlation obtained by Wang and Dhir is recommended. The establishing of a vapor film around a heated surface having temperature larger than the minimum film boiling temperature takes a finite time. The availability of small bubbles of noncondensing gases reduces the superheating required to initiate evaporation. Evaporation at lower than the saturation temperature is possible.

25-Years Three-Fluid Modeling Experience: Successes and Limits

The paper presents many examples of successful use of the three fluid processes modeling in 1D-networks, 3D-single volumes and 3D-boundary fitted volumes in the nuclear engineering. The modeling techniques, which have proof to be useful, are underlined and recommended for future use. The limits of the method are elaborated and recommended for future investigation.Copyright

Entrainment in Annular Two-Phase Flow

Entrainment is a process defined as mechanical mass transfer from the continuous liquid velocity field into the droplet field. Therefore, entrainment is only possible if there is a wall in the flow, that is in channel flow (see Fig. 5.1) or from the surfaces in pool flows. The surface instability on the film caused by the film-gas relative velocity is the reason for droplet formation and their entrainment.

Heterogeneous nucleation and flashing in adiabatic pipes

A new model of heterogeneous nucleation at walls during flashing and boiling is presented. The model is based on a new method for computation of the bubble departure diameter in flows accounting for mutual bubble interactions and the limitation of the bubble production due to the limited amount of energy supplied into the boundary layer by turbulence induced by bubble growth and departure. The final model together with fragmentation and coalescence models is incorporated into the system code IVA and verified by comparison with data. This chapter is a short version of the primary published work in Kolev (1995).

Acceleration Induced Droplet and Bubble Fragmentation

Consider pool flow, that is, flow without any wall influence. Fluid particles in multiphase mixtures experience forces acting to destroy them and forces acting to retain their initial form. The hydrodynamic stability limit is usually described by the ratio of the forces acting to destroy the particles, the shear forces t \(\pi D^2_d\), where t is the tangential force per unit surface, and the forces acting to retain the particle form, for example, surface tension forces σ d πD d (see Figs. 8.1 and 8.2),