Rewetting processes during PWR reflood
Author(s)
Ilyas, Muhammad
Type
Thesis
Abstract
Rewetting of heated surfaces is important in many physical processes and has
important technological applications. Understanding of this phenomenon is required
in many engineering and scientific fields. It is one of the most crucial phenomena to
be considered for the safety analysis of the design basis Loss‐of‐Coolant Accident
(LOCA) in light water reactors (Pressurized Water and Boiling Water Reactors). To
mitigate the consequences of LOCA, water is fed into the reactor core via an
emergency core cooling system; in the PWR, this water is fed to the core via the
lower plenum (“bottom reflooding”) and in the BWR, this water is sprayed onto the
top of the core (“top reflooding”). In both the cases, a quench front is formed which
moves rather slowly. Ahead of quench front, complex and chaotic processes are
occurring over a very small axial region where high temperature gradient exists. The
heat transfer mechanism is not well known in this region. In this work, the detailed
physics of the rewetting processes has been investigated both theoretically and
experimentally.
The thermal hydraulic behaviour of hot vertical channels during emergency core
cooling conditions would be expected to be flow direction‐dependent, it was
important to consider the two cases (top reflooding and bottom reflooding)
separately. It was possible for the first time, to the author’s knowledge, to apply the
fast response infra‐red thermal imaging system to study the rewetting process
during top and bottom reflooding of heated vertical surfaces. The important
contribution of this work was the use of this new technology to sense the variation of
temperature with time at multiple nearby locations at the quench front.
In the top reflood experiments, a heated stainless steel pate was quenched by a
falling film flow. Through an infrared‐transparent substrate embedded in the plate
and coated with platinum, temperature measurements at a location near the
rewetting front were achieved using infrared thermal imaging system. The
temperature/time traces showed fluctuations in temperature indicating occurrence of
intermittent contacts at the quench front. A high speed video camera was also
employed to capture rewetting processes by the visual observation of the rewetting
front from the top surface. In the visual observations, the liquid film has been seen
making intermittent contacts with the hot surface. In these experiments, the effect of
the flow rate and the degree of sub‐cooling of the feed liquid has been studied. The
rewetting temperature and the characteristic length of the intermittent contact region
have been deduced from the experimental results.
Experiments were also done to measure temperature changes at the rewetting front
for the case of bottom reflooding of a heated tube using a similar technique to that
employed for the studies of top reflooding. The results suggested that the rewetting
behaviour was different depending on whether the reflood rate was high or low.
For high reflood rate, the observations are consistent with the regime above the
rewetting front being of the inverted annular type and, for lower reflooding rates,
the results are consistent with the rewetting front corresponding to a film dryout in
annular flow. An important finding from these experiments is the identification of
transient temperature fluctuations in the transition region for the high flooding rate
case. These are similar to those observed in the top reflood case and it seems very
likely that these fluctuations are associated with intermittent wetting of the surface
in this region.
An attempt has been made to model rewetting phenomena in which the mechanism
of heat transfer at the quench has been proposed. The postulated mechanism is
transient near‐surface cooling resulting from intermittent solid‐liquid contacts,
followed by recovery of the surface temperature of the metal substrate, with
explosive vaporization occurring when the homogeneous nucleation temperature is
restored at the metal‐water interface. A one‐dimensional rewetting model was
constructed to explain the cyclical process; this model predicted the cyclical
behaviour, with the expected qualitative dependence on system parameters. Its
predictions are quantitatively consistent with experimental observation, in that the
unsteady model analysis brackets the experimentally observed periodicity of the
quasi‐steady actual process.
The one‐dimensional model of the process has been complemented by twodimensional
simulations using a commercial finite element code (ANSYS). In these
simulations, an intermittent contact region has been modelled by imposing a heat
transfer coefficient over a certain length between dry and wet regions. A parametric
study was performed to see the effect of the rewetting velocity, the wet side heat
transfer coefficient, intermittent contact heat transfer coefficient, and the length of
intermittent contact region.
important technological applications. Understanding of this phenomenon is required
in many engineering and scientific fields. It is one of the most crucial phenomena to
be considered for the safety analysis of the design basis Loss‐of‐Coolant Accident
(LOCA) in light water reactors (Pressurized Water and Boiling Water Reactors). To
mitigate the consequences of LOCA, water is fed into the reactor core via an
emergency core cooling system; in the PWR, this water is fed to the core via the
lower plenum (“bottom reflooding”) and in the BWR, this water is sprayed onto the
top of the core (“top reflooding”). In both the cases, a quench front is formed which
moves rather slowly. Ahead of quench front, complex and chaotic processes are
occurring over a very small axial region where high temperature gradient exists. The
heat transfer mechanism is not well known in this region. In this work, the detailed
physics of the rewetting processes has been investigated both theoretically and
experimentally.
The thermal hydraulic behaviour of hot vertical channels during emergency core
cooling conditions would be expected to be flow direction‐dependent, it was
important to consider the two cases (top reflooding and bottom reflooding)
separately. It was possible for the first time, to the author’s knowledge, to apply the
fast response infra‐red thermal imaging system to study the rewetting process
during top and bottom reflooding of heated vertical surfaces. The important
contribution of this work was the use of this new technology to sense the variation of
temperature with time at multiple nearby locations at the quench front.
In the top reflood experiments, a heated stainless steel pate was quenched by a
falling film flow. Through an infrared‐transparent substrate embedded in the plate
and coated with platinum, temperature measurements at a location near the
rewetting front were achieved using infrared thermal imaging system. The
temperature/time traces showed fluctuations in temperature indicating occurrence of
intermittent contacts at the quench front. A high speed video camera was also
employed to capture rewetting processes by the visual observation of the rewetting
front from the top surface. In the visual observations, the liquid film has been seen
making intermittent contacts with the hot surface. In these experiments, the effect of
the flow rate and the degree of sub‐cooling of the feed liquid has been studied. The
rewetting temperature and the characteristic length of the intermittent contact region
have been deduced from the experimental results.
Experiments were also done to measure temperature changes at the rewetting front
for the case of bottom reflooding of a heated tube using a similar technique to that
employed for the studies of top reflooding. The results suggested that the rewetting
behaviour was different depending on whether the reflood rate was high or low.
For high reflood rate, the observations are consistent with the regime above the
rewetting front being of the inverted annular type and, for lower reflooding rates,
the results are consistent with the rewetting front corresponding to a film dryout in
annular flow. An important finding from these experiments is the identification of
transient temperature fluctuations in the transition region for the high flooding rate
case. These are similar to those observed in the top reflood case and it seems very
likely that these fluctuations are associated with intermittent wetting of the surface
in this region.
An attempt has been made to model rewetting phenomena in which the mechanism
of heat transfer at the quench has been proposed. The postulated mechanism is
transient near‐surface cooling resulting from intermittent solid‐liquid contacts,
followed by recovery of the surface temperature of the metal substrate, with
explosive vaporization occurring when the homogeneous nucleation temperature is
restored at the metal‐water interface. A one‐dimensional rewetting model was
constructed to explain the cyclical process; this model predicted the cyclical
behaviour, with the expected qualitative dependence on system parameters. Its
predictions are quantitatively consistent with experimental observation, in that the
unsteady model analysis brackets the experimentally observed periodicity of the
quasi‐steady actual process.
The one‐dimensional model of the process has been complemented by twodimensional
simulations using a commercial finite element code (ANSYS). In these
simulations, an intermittent contact region has been modelled by imposing a heat
transfer coefficient over a certain length between dry and wet regions. A parametric
study was performed to see the effect of the rewetting velocity, the wet side heat
transfer coefficient, intermittent contact heat transfer coefficient, and the length of
intermittent contact region.
Date Issued
2011-01
Date Awarded
2011-02
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Walker, Simon
Hewitt, Geoffrey
Sponsor
PIEAS, Higher Education Commission of Pakistan and EPSRC
Creator
Ilyas, Muhammad
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)