Copyright © 2009 Dirk Lucas et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Computational fluid dynamics
(CFD) codes are widely used in industrial applications for single-phase flows (e.g., in the automotive or
aircraft industries). On the other hand, the application of CFD for multiphase
systems is not yet mature. Safety analyses related to nuclear light water
reactors require reliable simulations for different scenarios including
two-phase flow situations. Prominent examples for pressurized water reactor (PWR)
analyses are the prevention from departure from nucleate boiling (DNB) which is
related to critical heat flux (CHF) or the pressurized thermal shock (PTS)
problem which has to be considered in connection with some hypothetical loss of
coolant accident (LOCA) scenarios and may also lead
to two-phase flow situations in the cold leg and in the downcomer. For example, in case of boiling
water reactors (BWR) analyses, the prevention from Dryout is an important
issue.
The currently applied
system codes based on correlations are valid for special geometries, scales,
and flow patterns. This limits the transferability of small-scale experimental
findings to real plant scales. On the other hand, CFD-type models depend only
on local flow parameters and are for this reason much more flexible regarding
geometry and scale. The increased computer power now in principle permits CFD
simulations for multiphase flows and many investigations have been done in the
recent years.
The problems in
modeling of such gas-liquid flows using CFD codes arise from the fact that the
mass, momentum, and heat transfer among the phases are strongly coupled with the complex interfacial structure. The order of magnitudes lies between the size of
the smallest structures of these interfaces and the size of the typical
components of nuclear reactors which finally have to be modeled. For this
reason, averaging procedures are required which lead, for example, to the
well-known two- or multifluid model. Due to this averaging, the primary
information on the structure of the interface gets lost and has to be introduced again by the so-called
closure models. Some of the physical phenomena on microscale are not yet well
understood. Also, CFD-grade experimental data (i.e., data with high resolution
in space and time) are often not available. Despite these open problems, there
is a step-by-step progress in the simulation of gas-liquid flows in geometries
and scales relevant to nuclear reactor safety (NRS).
In view of the above,
it has been decided to bring out the special issue “Computational Fluid
Dynamics for Gas-Liquid Flows.” Two papers review and discuss the state-of-the-art
of modeling and the available experimental database for the CHF and the
two-phase PTS issue, respectively. Research articles focus on important topics
like turbulence modeling in two-phase flows, modeling of polydispersed flows,
mixing problems (including single-phase coolant flows, addressed by one paper), and
jet impingement connected with bubble entrainment. Thus, this special issue
provides the readers with useful information on the progress of CFD modeling
for reactor-specific two-phase flows, and also on open questions, requirements for
further research, modeling, and experimental data.
Dirk Lucas
Iztok Tiselj
Yassin A. Hassan
Fabio Moretti