


Boiiling heat transfer is defined as a mode of heat transfer that occurs with a change in phase from liquid
to vapour. The various flow patterns exert different effects on the hydrodynamics conditions near the heating wall. Thus, they produce different modes of
heat transfer. The most dangerous heat transfer mode is the boiling crisis occurrence. Following this Critical Heat Flux (CHF) condition, the produced vapour forms
an insulating layer over the heating surface and raises the surface temperature. As the cooling water reenters the part where the steam surrounds the heating surface
the increase in heat removal takes place, followed by complex hydrodynamics and heat transfer processes related to rewetting (quenching), which corresponds to
Minimum Film Boiling (MFB) temperature. A functional form presenting heat flux transferred from the heating surface to the coolant versus superheating of the two
phase flow mixture wetting the surface is known as a boiling curve.
Upward twophase flow in vertical heated channels or around rods or tubes bundle can take a variety of flow
patterns which are determined by the phases mass fluxes, inlet conditions, intensity and profile of the heat flux, and presence of the obstacles. Prediction of
these flow regimes are important for the proper calculation of heat transfer from the heated walls to the fluid streams, as well as for the indication of Critical
Heat Flux (CHF) occurrence. A methodology for the 




schematization of the multifluid flow is introduced, together with the multifluid models application,
solution of the mass, momentum and energy governing equations and various criteria for fluids flow existence and transformation. The possible multifluid patterns
are described with up to six fluid streams. Multifluid field equations are given as conservation of mass, momentum and energy for each fluid. Mechanistic low
and highquality CHF models coupled with selection criteria for a number and components of fluids are implemented into multifluid concept. This methodology
is applied to the prediction of the CHF conditions in the complex geometry. The results of onedimensional transient calculations are the input boundary conditions
for further numerical investigations with the multi dimensional Computer Fluid Dynamics (CFD).
A threedimensional twofluid model for twophase flow across tube or rod bundles is developed and presented.
The porous media concept is applied in model statement.
The positions and dimensions of the rods/ tubes determine the porosity and flow resistance within the bundle. The model implies nonequilibrium thermal
and flow conditions. The mass, momentum and energy equations are written and solved for both vapour and liquid phase.Closure laws for interfacial mass,
momentum and energy transfer, bundle flow resistance and heat transfer are presented. New correlations for the interfacial drag force are proposed. Developed
model is suitable for the simulation and analyses of





complex multidimensional thermalhydraulics of rod and/or tube bundles or shellandtube heat exchangers
with vapour generation within a tube bundle on the shell side. Mechanistic model of liquid film dryout is applied for occurrence of Critical Heat Flux (CHF).
Spacers are built into rod/tube bundles to support structure. They also have positive effects on
enhancement of the heat transfer and increase of the Critical Heat Flux (CHF). Besides design characteristics, the axial position and distance
between spacers play a significant role in thermal performance of bundles. The evaluation of these effects can be efficiently supported with the numerical
simulation of the turbulent multidimensional coolant flow around the spacers. A numerical procedure is developed for this purpose, and presented.It is based on
the numerical solution of the Reynolds Averaged Navies Stokes Equations in two or three dimensions, with the application of the modified ke turbulence model. Coolant twophase flow is simulated with the application
of the multidimensional twofluid model, where turbulence viscosity is predicted for the continuous phase. Boundary turbulent flow parameters
are predicted with the "wall functions" at the flow channel walls and spacers. The SIMPLE numerical method is applied and the transient form of mass, momentum, k and e balance equations are solved in
Cartesian coordinates. Numerical solution algorithm is based on the control volume approach. 
