Experimental investigations of two-phase mixture level swell and axial void fraction distribution under high pressure, low heat flux conditions in rod bundle geometry Page: 3 of 22
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Mixture level swell is of importance in the study of small break
accidents because, for a given geometry and coolant loss, the mixture
swell directly affects the amount of core that remains covered. Mathe-
matically this is illustrated by rearranging Eq. (1) and noting that the
coolant inventory in the boiling length is given by
M = PfAFZCLL . (3)
The result shows the mixture level to be directly related to the mixture
swell
Zmix = 2M (S + 1) (4)
m AF
It was the objective of the subject test series to characterize both
mixture level swell and axial void fraction distribution under conditions
of varying pressure and heat flux.
Experimental Procedures
Experimental testing was performed in the Thermal Hydraulic Test
Facility (THTF), located at ORNL (Ref. 1). The THTF is a high pressure
test loop containing a 3.66-1,, 8 x 8 electrically heated rod bundle (Fig. 2).
The axial power profile is uniform and bundle configuration is typical of a
Westinghouse 17 x 17 fuel assembly. The fuel rod simulator (FRS) bundle
is instrumented with FRS sheath thermocouples at 25 elevations. Most of
the FRS thermocouple "levels" are in the upper 30% of the bundle. This
allows the determination of dryout elevation to within +0.08 m. Axial
void fraction distributions were determinci from the outputs of a set of
stacked differential pressure cells (Fig. 3,.* Because of the low liquid
and vapor velocities extant in these tests, friction and form pressure
drops are negligible and the differential pressure cell output is directly
related to the hydrostatic head of the liquid and steam between the instru-
ment taps. Then, as described in the appendix, the hydrostatic head can be
used to compute a volume average void fraction.
The experimental procedure used in these tests was designed to allow
the acquisition of void distribution data under quasi-steady-state condi-
tiois. The test began by preheating the THTF to the desired test section
inlet temperature, reducing inlet flow to the desired value and applying
bundle power. The flow-power combination was such that most of the heated
length would experience saturated boiling and only the uppermost portion
of the core would experience dryout. After a period of stabilization the
test section flow was trimmed to place the mixture level in the uppermost
0.6 m of heated length. Finally, when the system had stabilized and makeup
flow was just sufficient to compensate for liquid being vaporized,
quasi-steady-state data was taken.*PdE-189 failed prior to testing.
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Anklam, T. M. & White, M. D. Experimental investigations of two-phase mixture level swell and axial void fraction distribution under high pressure, low heat flux conditions in rod bundle geometry, article, January 1, 1981; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1113391/m1/3/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.