atw - International Journal for Nuclear Power | 08/09.2019

atw Vol. 64 (2019) | Issue 8/9 ı August/September
428
AMNT 2019
30 % more heat is transferred by the d=20 mm TPCT
(q=30 kW/m 2 ) than by the d=45 mm TPCT (q=22.5 kW/
m 2 ). Although the filling ratio is 70 % for all measurements
in Figure 2, the mass of the fluid inventory of the d=45 mm
TPCT is 4.5 times higher compared to the d=20 mm TPCT
and thus the thermal inertia is increased for the larger diameter.
Therefore, it takes higher heat input and increased
operating temperatures to stabilize the heat transfer. The
smaller cross section (d=20 mm) also promotes a better
mixing of the working fluid in the evaporation section with
the cooled backflow from the condenser section, which
results in lower temperatures measured in the evaporation
section.
| | Fig. 2.
Boiling temperature measured inside the TPCT’s evaporation section vs. calorimetrically determined
heat flux for pipe diameters d=20 mm, 32 mm and 45 mm at heat sink temperature 20 °C and filling
ratio 70 %.
| | Fig. 3.
Temperature difference between heat source and heat sink vs. calorimetrically determined transferred
heat for indirect water-heated experiments with various filling ratios of TPCTs.
pressure in both pipe ends. The TPCT operates in a heat
flux driven operation with significant wall overheating in
the evaporation section. The heat sink temperatures were
set to 10 °C, 20 °C and 30 °C. Previous results reveal the
70 % filling ratio with the highest experimental heat
transfer coefficient in the evaporation section [6].
Figure 2 gives the results of the direct electric heating
experiments for different inner pipe diameters (20, 32 and
45 mm). 70 % of the evaporator section volume was filled
with water as working fluid and the experiments were
performed with a predefined heat sink temperature of
20 °C. The points in Figure 2 give the average temperatures
measured in the evaporation section vs. the calorimetrically
transferred heat flux in different colors for each
pipe diameter. The corresponding colored dashed lines
above and below the average points visualize the temperature
fluctuations during the measurements. It is known
from previous results [5, 6], that the experiments with
water as working fluid tend to unstable operation with
pulsating temperature fluctuations for low heat flux and
low driving temperature difference. These fluctuations
stabilize into isothermal operation with increasing heat
input in dependency on the heat sink temperature and
pipe diameter. At similar boiling temperatures, the heat
flux is increasing with decreasing pipe diameter. For
example, at 55 °C evaporator temperature approximately
Temperature driven operation
In the next step of investigations on a passive TPCT cooling
system for spent fuel pools, the heating of the laboratory
test pipe is converted to an indirect temperature driven
operation. Therefore, the tubular cartridge heaters are
replaced by a double-pipe arrangement connected to a
process thermostat in a secondary heating circuit. The
inlet and outlet heat flux is determined calorimetrically by
the mass flow and the inlet and outlet temperatures of the
water jacket heater and cooler. The charged working fluid
in the TPCTs is water in all presented experiments.
Figure 3 shows the results of the indirect water- heating
experiments for heat source temperatures of 45 °C (green),
55 °C (yellow) and 60 °C (red). The filling ratios 100 %,
70 %, 50 % and 30 % were measured for each heating
temperature. The experiments are performed with a
temperature ramp of the heat sink between 0 °C and 30 °C.
At a defined heat source temperature the heat sink is
adjusted at start temperature and increased by 10 K
approximately every hour. The investigated temperature
ramps are performed both ways, upwards with increasing
and downwards with decreasing heat sink temperature to
observe possible effects depending on the operation mode.
In fact, the heat sink temperature direction had no influence
on the results for water-charged TPCT. The blue-colored
area in Figure 3, which mainly covers the experiments at
45 °C heat source temperature, presents single-phase heat
transfer region. The transferred heat is in the range of
measurement uncertainties and most likely, the heat is
transferred only by natural convection inside the test pipe.
The results covered by the yellow- colored area represent a
meta-stable operation mode of the TPCT. The experiments
are not always reproducible and the operation is in a
transition between single-phase convection and irregular
nucleate boiling. With increasing heat source temperature
the two-phase heat transfer stabilizes (non-colored area)
and the operation temperature pulsates regularly as already
known from the electric heating experiments. The blue
lines present the isothermal heat sink temperatures for a
better comparability between the different heat source
temperature ramps. The influence of the filling ratio
increases with increasing heat sink temperature and
increasing temperature difference. For temperature
differences below 35 K, the influence of the filling ratio is
negligible. At 60 °C heat source temperature and 0 °C heat
sink temperature the heat flux for 50 % and 30 % filling
ratio is similar and approximately 30 % improved compared
to 100 % filling ratio. A stabilization of the pulsating
operation temperature with increasing heat flux like in the
results of the electrically heated experiments is not
observed yet. Overall, the temperature driven experiments
result in lower heat transfer coefficients and without wall
overheating due to thermal inertia of the water- heating.
AMNT 2019
Atmospheric Spent Fuel Pool Cooling by Passive Two-Phase Closed Thermo syphons ı Claudia Graß, Rudi Kulenovic and Jörg Starflinger