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: 9 of 22
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8
Mean Velocity vs Flux Density Plots
The drift-flux model expresses void fraction in terms of a concentra-
tion parameter Co and a mean-weighted drift-velocity Vgj. Generally these
quantities are determined from experimental data. Rearrangement of Eq. (7)
yields
<jg>/<a> = Co<j> + Vgj (10)
where <jg>/<a> is the mean vapor velocity. It is clear from Eq. (10) that,
for a system in which void fraction can be expressed in terms of Eq. (7), Co
and Vgj can be derived from a plot of local void fraction data in the mean-
velocity, flux density plane. The concentration parameter is the slope of
the mean-velocity with respect to flux density and Vgj is simply the zero
flux density intercept of the data.
Figures 8 and 9 are the mean-velocity vs flux density plots for the
4- and 8-MPa data sets respectively. Void fraction is derived as a volume
average quantity and the vapor and mixture flux densities are evaluated at
the midpoint between the differential pressure cell taps.
Most of the data in Fig. 8 is fit reasonably well by the line
<j >/<a> = 0.91<j> + 0.478 ,
where the mean-velocity is in m/s. However, the 0.33 kW/m and, to some
extent, the 0.65 kW/m data are not fit very well. The e are a number of
reasons why this is the case. First, low power tests are characterized
by low void fractions over most of the heated length. Errors inherent in
void fraction measurements can be magnified when propagated through the
ratio <j >/<a>. This is particularly so when void fraction is low.
Therefore, the large scatter in the low power data may be due, at least
in part, to uncertainties in experimental measurements. A second possi-
bility is that a drift-flux model with a constant drift velocity and
concentration parameter may not be sufficiently accurate under very low
power conditions. In either case, further work is recommended as low
power level data is of prime importance under small break conditions.
The 8-NPa data in Fig. 9 shows some scatter but is fit reasonably
well by the line
<j >
= 0.85<j> + 0.338
This indicates a pressure dependency of both the mean weighted drift-
velocity and concentration parameter. Close examination of Fig. 9 shows
that, as with the 4-MPa data, considerable scatter exists at low power
levels (0.33 kW/m).
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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/m1/9/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.