VGB POWERTECH 11 (2019)

VGB PowerTech 11 l 2019
Sub-cooled boiling of natural circulation in narrow rectangular channels
Experimental study on sub-cooled
boiling of natural circulation in
narrow rectangular channels
Zhou Tao, Li Zichao, Li Bing, Qi Shi and Huang Yanping
Kurzfassung
Experimentelle Studie zum unterkühlten
Sieden bei Naturkonvektion in engen
rechteckigen Kanälen
Unterkühltes Sieden unter der Bedingung der
Naturkonvektion wurde experimentell in einem
Versuchsstand mit engen rechteckigen Kanälen
untersucht. Wird die Heizleistung für
den Wärmeübergang auf ein bestimmtes Niveau
erhöht, können das Phänomen des unterkühlten
Siedens sowie die Blasenbewegung
über ein Sichtfenster beobachtet werden. Die
Ergebnisse der Untersuchungen zeigen, dass
der Wärmeübergangskoeffizient des unterkühlten
Siedens mit zunehmender Heizleistung
zunimmt und mit zunehmender Unterkühlung
am Kanaleintritt und Größe der engen rechteckigen
Kanäle abnimmt. Die Wärmeübertragung
des unterkühlten Siedens wird hauptsächlich
durch die Bildung und Abtrennung
von Blasen beeinflusst, begleitet von Strömungsoszillationen.
Beobachtet wurden 3 Stufen
des unterkühlten Siedens in engen rechteckigen
Kanälen. Schließlich wurde eine empirische
Korrelation für den Wärmeübergangskoeffizienten
der natürlichen Konvektion in
engen rechteckigen Kanälen ermittelt, basierend
auf dimensionslosen Kennzahlen. l
Authors
Zhou Tao 1,2,3*
Li Zichao 1,2,3
Li Bing 1,2,3
Qi Shi1 ,2,3
Huang Yanping4
Affiliations:
1 School of Nuclear science and
Engineering
North China Electric Power University
Beijing, 102206, China
2 Institute of nuclear thermal-hydraulic safety
and standardization
North China Electric Power University
Beijing, 102206, China
3 Beijing Key Laboratory of Passive Safety
Technology for Nuclear Energy
Beijing, 102206, China
4 CNNC Key Laboratory on Nuclear
Reactor Thermal Hydraulics Technology
Chengdu, 610041, China
Sub-cooled boiling of natural circulation has
been experimentally investigated based on a
natural circulation device with narrow
rectangular channels. When the heating
power is increased to a certain level, the
phenomenon of sub-cooled boiling and
bubbles movement can be observed through
the visual channel. The results show that the
heat transfer coefficient of sub-cooled boiling
increases with the increasing of heating
power and decreases with the increasing of
inlet sub-cooling and size of narrow
rectangular channels. The heat transfer
process of sub-cooled boiling is mainly
affected by the generation and departure of
bubbles, accompanied with flow oscillation.
It is discovered that there are 3 stages of subcooled
boiling in the narrow rectangular
channels. Finally, an empirical correlation
has been proposed for the heat transfer
coefficient of natural circulation sub-cooled
boiling in narrow rectangular channels,
based on dimensionless analysis method and
the errors fall in the range of ±15 %.
Introduction
The usage of narrow channels is a new
technique for enhancing heat transfer,
having high heat transfer coefficients at low
heat flux [1]. This direction is being
adopted in the heat transfer systems of
new-type reactor design. Therefore, it is
necessary to study heat transfer characteristics
of narrow channels. By investigating
the saturated boiling phenomenon in
vertical narrow channels, Ishibashi [2]
found that there is an obvious improvement
in the heat transfer coefficient, accompanied
with periodic change of bubbles. Sun
Licheng [3] evaluated thirteen different
prediction methods of saturated boiling by
a database and proposed a modified
correlation by introducing the Weber
number. The above studies placed much
focus on saturated boiling, whereas the
flow and heat transfer region [4] of
pressurized water reactors (PWR) is at subcooled
boiling. As for sub-cooled boiling
in narrow channels, Chen [5] investigated
the effect of channel size on subcooled
flow boiling and associated bubble
charac-teristics. Wang [6] investigated
the onset of nucleate boiling (ONB) in narrow
channels at 1.0 to 4.5 MPa pressure
range and a new correlation was obtained
by considering the bilateral heating
factor.
For narrow rectangular channels, the heat
transfer capacity is 1.3 to 2.1 times of that
in conventional channels [7]. Due to above
characteristic, the advanced reactors and
research reactors [8] have adopted this
kind of channel. As for sub-cooled boiling
in narrow rectangular channels, Pan
[9, 10] investigated the effect of mass flux,
sub-cooled temperature of heated section,
pressure and heating modes on the heat
transfer coefficient and the behavior of
bubbles. Al-Yahia O S [11] investigated the
effect of transverse power distribution on
ONB and developed a sub-cooled boiling
model with uniform and non-uniform heat
flux distribution for narrow vertical
rectangular channels. Xu [12] established
a physical model which can explain the
mechanism of bubbles movement in
narrow rectangular channels. Natural circulation
[13, 14] does well in mitigating
reactor accidents, which can improve
the inherent safety of the nuclear reactors.
Such a heat transfer mode can be
used as the main cooling method for
small modular reactors [15]. It [16-19] has
been investi-gated in theory and experiment.
For sub-cooled boiling of natural circulation
in narrow rectangular channels, some
preliminary research has already been
presented [20, 21]. However, sub-cooled
boiling of natural circulation in narrow
rectangular channel which can be
influenced by size of channel and bubbles is
complex. Hence, it is necessary to further
investigate the characteristics of sub-cooled
boiling in narrow rectangular channels
during natural circulation based on
experiments.
Experiment system
Experiment facility
F i g u r e 1 shows the experiment facility,
which is used to investigate the sub-cooled
boiling of natural circulation. It consists of
a preheater channel, a rectangular heater
channel with a visible window, a condenser
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