Study of quench cooling is very important in nuclear reactor safety for limiting
the extent of core damage during the early stages of severe accidents after Loss of
Coolant Accidents (LOCA). Quench of a hot dry surface involves the rapid
decrease in surface temperature resulting from bringing the hot surface into
sudden contact with a coolant at a lower temperature. The quench temperature is
the onset of the rapid decrease in the surface temperature and corresponds to the
onset of destabilization of a vapor film that exists between the hot surface and the
coolant. Re-wetting the surface is the establishment of direct contact between the
surface and the liquid at the so-called re-wetting temperature. Re-wetting is
characterized by the formation of a wet patch on the surface which then spreads to
cover the entire surface. Situations involving quench and re-wetting heat transfer
are encountered in a number of postulated accidents in Canada Deuterium
Uranium (CANDU) reactors, such as re-wetting of a hot dry calandria tube in a
critical break LOCA. This accident results in high heat transfer from the calandria
tube to the surrounding moderator liquid which can cause the calandria tube
surface to experience dryout and a subsequent escalation in the surface
temperature. If the calandria tube temperature is not reduced by initiation of
quench heat transfer, then this may lead to subsequent fuel channel failure. In
literature very limited knowledge is available on quench and re-wetting of hot
curved surfaces like the calandria tubes. In this study, a Water Quench Facility (WQF) has been constructed and a series of
experiments were conducted to investigate the quench and re-wetting of hot
horizontal tubes by a vertical rectangular water multi-jet system. The tubes were
heated to a temperature between 380-800°C in a controlled temperature furnace
then cooled to the jet temperature. The temperature variation with time in the
circumferential and the axial directions of the tubes has been measured. The twophase
flow behavior and the propagation of the re-wetting front around and along
the tubes were simultaneously observed by using a high-speed camera. The
effects of several parameters on the cooling process have been investigated. These
parameters include: initial surface temperature, water subcooling (in the range 15-
800C), jet velocity (in the range 0.15-1.60 m/s), tube solid material (brass, steel
and Alumina), surface curvature, tube wall thickness, jet orientation and number
of jets. The variables studied include the re-wetting delay time (time to quench
after initiating the cooling process), there-wetting front propagation velocity, the
quench and re-wetting temperatures, the quench cooling rates and the boiling
region size. The quench and the re-wetting temperatures as well as the re-wetting
delay time were found to be a strong function of water subcooling. The quench
and re-wetting temperatures increase with increasing water subcooling. The rewetting
delay time decreases with increasing the water subcooling, decreasing
initial surface temperature, increasing liquid velocity and decreasing the surface
curvature. There-wetting front velocity is mainly dependent on the initial surface temperature and water subcooling. The re-wetting velocity increases by
decreasing the initial surface temperature and by increasing the water subcooling.
Decreasing the surface curvature was found to also increase the re-wetting front
velocity. Correlations of the phenomena studied have been developed and
provided good prediction of the experimental data collected in this study and data
available from literature. The. results of this study provide novel knowledge and
an experimental database for mechanistic modeling of quench heat transfer on
calandria tube surfaces that experience dryout and film boiling. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/21157 |
| Date | January 2011 |
| Creators | Takrouri, Kifah |
| Contributors | Luxat, John, Engineering Physics |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
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