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uncooled wall temperature that would arise if a
hyperbolic cosine power density distribution
existed rather than a uniform one was determined.
The uncooled wal I temperature increment, above
the fluid temperatures, for this nonuniform power
density case was found to be more than twice
as great as the corresponding increment for a
uniform power density case. The experimental
measurements obtained for the uniform power
density case have been modified by this factor
and are plotted in Fig. 4.1.11, along with the
mathematical prediction for an idealized ART
core with a hyperbolic cosine power distribution.
As may be seen, temperatures as high as 1800OF
might occur if the walls are not cooled properly
for this very high Reynolds number case.
Elastic thermal stress calculations were also
made for the core shell and heat exchanger tubes
on the basis of the experimental temperature
fluctuations that were observed in this experiment
2.4
2.2
W = POWER DENSITY ANYWHERE IN FUEL CHAMBER
- -
W, = POWER DENSITY IN CENTER OF FUEL CHAMBER
2.0 - -
KO = RECIPROCAL OF THERMAL DIFFWON LENGTH IN FUEL
- -
r = RADIAL DISTANCE FROM CENTERLINE OF CHANNEL
'" - -
f = WALL OR FLUID TEMPERATURE
- h, - INLET MIXED MEAN FLUID TEMPERATURE
4.6 - h- = OUTLET MIXED MEAN FLUID TEMPERATURE
0 2 4 6 8 q0 12 44 46 18
AXIAL DISTANCE FROM INLET (In.)
mperature Profiles of
Uncooled Outer Core Shell, Island Core Shell Wall,
and Fluid of the One-HaIf-Scale ART Core Model
Adjusted for the Nonuniform Volume Heat Source.
I
I
PERIOD ENDING JUNE 10, 1956
for the uniform-power-density case. For the
uniform-power-density system the results indicated
that cyclic stresses exist that are similar in
magnitude to the endurance limit or fatigue stress
of Inconel. It is believed that the stresses in the
actual ART system may be higher than those
calculated for the uniform-power-density case.
A third set of volume-heat-source experiments
is under way. For these experiments entrance
vanes that will reduce the rotational velocity
component are located in the core throat.
THE RMAL-CYCL ING EXPERIMENT
H. W. Hoffman D. P. Gregory9
The volume-heat-source experiments with the
one-half-scale model of the ART core have verified
that the hydrodynamic instabilities that exist in
some regions of the ART core, as presently
designed, would result in rapid, high-temperaturedifferential
cycling of the lnconel core-shell
surfaces. These studies have also indicated that
the tube bends at the fuel inlet end of the fuel-to-
NaK heat exchanger will be subjected to this
temperature cycling. The volume-heat-source
experiments indicate that the temperature fluctuation
at the lnconel surface may be of the order of
S6OoF. The thermal diffusivity of lnconel is
poor, and therefore these large temperature fluctuations
will occur, chiefly, in a region close to the
metal-fuel interface. For example, the amplitude
of a temperature fluctuation with a -sec period
will be reduced to 40% of its origina 4 value at a
position 35 mils below the metal surface. A
possible result of this cyclic thermal expansion
within the metal will be cracking of the surface
because of metal fatigue. The possibilities of
accelerated creep and corrosion under these
circumstances also arise.
The effect of thermal cycling on material strength
must be determined experimentally, and a system
for accomplishing this study at reactor temperatures
with a fuel environment is currently being
constructed and tested. The apparatus is shown
schematically in Fig. 4.1.12. The fuel mixture
NoF-ZrF,-UF, (5b46-4 mole %) will flow through
theheater under gas pressure and will be subjected
to cyclic heating. The surface of the unheated
test section will experience the resulting periodic
'On assignment from Pratt & Whitney Aircraft.
227