Countercurrent flow limitations in horizontal stratified flows of air and water [Elektronische Ressource] / Forschungszentrum Karlsruhe GmbH, Karlsruhe. Mireia Gargallo Gallego

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• Publié par: universitat_stuttgart
• Publié le: 01 janvier 2004
• Nombre de lectures: 22
• Poids de l'ouvrage: 4 Mo

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Forschungszentrum Karlsruhe
in der Helmholtz-Gemeinschaft
Wissenschaftliche Berichte
FZKA 7018









Countercurrent Flow
Limitations in Horizontal
Stratified Flows of Air
and Water



M. Gargallo Gallego
Institut für Kern- und Energietechnik
Programm Nukleare Sicherheitsforschung


















September 2004 Forschungszentrum Karlsruhe
in der Helmholtz-Gemeinschaft
Wissenschaftliche Berichte
FZKA 7018
Countercurrent Flow Limitations in Horizontal
Stratified Flows of Air and Water
Mireia Gargallo Gallego




Institut für Kern- und Energietechnik
Programm Nukleare Sicherheitsforschung


Von der Fakultät für Maschinenbau der Universität Stuttgart
genehmigte Dissertation (D93)


Forschungszentrum Karlsruhe GmbH, Karlsruhe
2004
















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Countercurrent Flow Limitations in Horizontal
Stratified Flows of Air and Water
A dissertation accepted by the Faculty of Mechanical Engineering (University of
Stuttgart) for the award of the academic degree of “Doctor of Engineering (Dr.-Ing.)”


Performed by
Dipl.-Ing. Mireia Gargallo Gallego
from Barcelona (Spain)


Referent: Prof. Dr. -Ing. habil. E. Laurien
Faculty of Mechanical Engineering,
Chair of Thermofluid Dynamics,
University of Stuttgart

Co-Referent: Prof. Dr. -Ing. T. Schulenberg
Institute for Nuclear and Energy Technologies,
Forschungszentrum Karlsruhe GmbH
Exam: 8.12.2003


Fakultät für Maschinenbau, Universität Stuttgart, D93
Institut für Kernenergetik und Energiesysteme (IKE)
Abteilung Thermofluiddynamik (TFD)
Forschungszentrum Karlsruhe GmbH, Karlsruhe
Institut für Kern- und Energietechnik
2004
Abstract
During a postulated Loss-Of-Coolant-Accident (LOCA) in a Pressurized Water Reac-
tor (PWR) it is of vital importance that the reactor core remains properly cooled. The Emer-
gency Core Cooling System (ECCS) in German PWRs compensates the loss of coolant with
injection of additional coolant into the cold legs as well as into the hot legs. While the coolant
is injected in the cold legs through nozzles, the hot leg injection is performed by means of a
secondary pipe placed at the bottom of the pipe of the primary circuit. The subject of this the-
sis concerns the latter case.

The liquid injected into the hot leg flows directly into the core from its upper part and
constitutes a rapid delivery of coolant into the reactor core at high mass flow rate. However,
saturated steam is generated in the reactor core due to depressurization of the primary sys-
tem and flows out of the Reactor Pressure Vessel (RPV) into the hot leg. Therefore, a coun-
tercurrent stratified flow of injected coolant and saturated steam occurs along one and a half
meter inside the hot leg before the coolant reaches the RPV. This horizontal stratified coun-
tercurrent flow of coolant and steam is only stable for a certain range of coolant and steam
mass flow rates. Even if the coolant is injected at very high velocities and high Froude num-
bers, there is always a threshold steam velocity above which the cooling of the reactor core
can be reduced or complete interrupted. This phenomenon is known in two-phase flow sci-
ence as Countercurrent Flow Limitation (CCFL), since there is a limitation of liquid delivery
due to the presence of a gas phase flowing countercurrently to the liquid phase.

CCFL in reflux condensation cooling was more investigated than in ECC in the hot leg.
For this purpose, the test facility WENKA was built at Forschungszentrum Karlsruhe GmbH
(Germany) to investigate for which flow conditions CCFL poses a safety risk during hot leg
injection and to provide experimental data to support the analysis of such an accident sce-
nario with CFD - Codes.

The WENKA test facility models a simplified PWR hot leg geometry including the secon-
dary pipeline placed at the bottom of the main coolant line. The countercurrent flow of cool-
ant and saturated steam during injection by means of the ECCS was investigated with air
and water in a rectangular test section. The fluid dynamics of the injection process was re-
produced for a wide range of flow conditions to identify flow regimes and to derive
1-dimensional models to predict the limits of coolant delivery. On the other hand, a data base
of local flow parameters was established to enhance CFD - Codes performance. Experimen-
tal local velocities of the liquid film were obtained by means of Particle Image Velocimetry
and the liquid film morphology was analyzed depending on the flow regimes.

Flow regime maps were obtained for inlet liquid depths ranging from 3 to 15 mm. De-
pending on the superficial velocities of liquid and gas and on the Froude number of the liquid
film, a stratified countercurrent flow, a partially reversed flow or a totally reversed flow were
experimentally observed. The CCFL occurred as a breakdown of the stable countercurrent
stratified two-phase flow: The liquid begun to be carried over by the gas and to flow partially
or totally in the air flow direction. The new flow regime that set in the hot leg and caused the
CCFL was defined as reversed flow, because the coolant flowed “reversely”. It was distin-
guished between partially reversed flow and totally reversed flow, depending on whether only
a fraction of the liquid flowed in the air flow direction or the entire injected liquid did. Experi-
mental observations showed also that a subcritical flow, i.e. a liquid film with Froude numbers
less than one, is a necessary condition for the onset of flow reversal. Flow reversal with an
initially supercritical flow, i.e. a Froude number of the liquid film greater than one, was only
obtained if there was a transition from supercritical to subcritical flow, which did not occur
continuously, but in the form of a hydraulic jump in the channel. Therefore, some experi-
ments were focused to study the occurrence of a hydraulic jump with countercurrent flow of

air, and to investigate the influence of the initial water height. A theoretical approach for the
onset of a hydraulic jump was derived and experimental data were obtained and compared
with the theory.

A new theoretical model to predict CCFL was proposed. This novel criterion predicts
the occurrence of reversed flow in the hot leg and is based on the well known Wallis correla-
tion. The Wallis correlation assesses that the occurrence of reversed flow coincides with the
flow conditions, for which the sum of the squared root of the dimensionless superficial veloci-
ties of liquid and gas is equal to a constant value C. However, a supercritical liquid flow was
reversed in the test facility for greater values of C. There, flow reversal coincided with the
transition from supercritical to subcritical flow. Discrepancies with the values predicted by
Wallis are due to the fact, that the Wallis correlation neglected the inertia of the fluids. The
new theoretical model includes the inertial term and presents two criteria that are fulfilled at
onset reversed flow. The first is a criterion to predict the transition to subcritical flow. This cri-
terion was developed analytically and includes the influence of the Froude number of the liq-
uid at the injection point, the channel length, the inlet liquid depth, the liquid Reynolds num-
ber and the slip velocity ratio between gas and liquid. The second criterion is based on ex-
perimental observations and postulates that reversed flow occurs always a values of C larger
than 0.7. The new model predicted with good accuracy the occurrence of reversed flow in the
WENKA test facility.

And last but not least, liquid delivery rates were measured for a wide range of condi-
tions and an empirical correlation to predict back flow ratios was presented.

Gegenstrombegrenzung in horizontalen
geschichteten Strömungen von Luft und Wasser
Kurzfassung
Diese Arbeit beschäftigt sich mit Schichtenströmungen, die bei schweren Unfällen in
Leichtwasserreaktoren auftreten können. Diese Form der zweiphasigen Strömung ist auch in
vielen anderen industriellen Anwendungen zu finden.
Beim Bruch einer Hauptkühlmittelleitung eines Druckwasserreaktors (in der engli-
schen Literatur als „Loss of Coolant Accident“ bezeichnet) übernimmt das Noteinspeisungs-
system die Aufgabe, unterkühltes Wasser in den Reaktordruckbehälter einzuspeisen. In
deutschen Reaktoren wird Kühlmittel sowohl in den kalten als auch in den hei&