atw 2017-07

atw Vol. 62 (2017) | Issue 7 ı July
484
AMNT 2017 ı COMPETENCE PRIZE
| | Fig. 3.
Thermal non-steady-state operating thermosiphon.
and transports the heat by sensitive
instead of latent heat transfer. Superheated
vapor bubbles grow to the
dimension of the pipe diameter and
suddenly push the liquid above in the
condenser section (geyser boiling).
At this moment the evaporator
temperature drops to its initial value
and the temperature in the adiabatic
section and condenser rises abruptly.
Similar fluctuations depending on
the overheated pool can also be
observed in the pressure measurements.
Besides these periodical
temperature and pressure fluctuations
of geyser boiling, this boiling regime
is also recognizable by an audible
cracking sound within the pipe.
The cooling circuit expeditiously
removes the heat from the condenser
and the initial temperatures are
reached again. The procedure of overheating
and flashing starts all over
again. This pulsating operation of the
thermosiphon could be the result of
multiple parameters and influences,
which limit the performance and
prevent the pipe from stable operation.
For instance, the surface of the
inner pipe wall of the thermosiphon is
possibly not rough enough and the
resulting lack of evaporation nuclei
effects the overheating of the liquid
pool. Furthermore, the high driving
temperature difference seems to
demand high heat transfer rates for a
steady-state operation. Each driving
temperature difference is related to a
certain heat transport performance.
This oscillating geyser operation
mode is known to occur especially
for high filling ratios and high density
differences [7, 10].
difference between the heat source
i.e. an average evaporator wall
temperature and the heat sink by
means of the inlet temperature of
the double-pipe chiller is shown.
A vertical pipe with 32 mm inner
diameter and a filling ratio of 100 %
is investigated for varying heating
power from 1.3 kW to 2 kW and
different coolant inlet temperatures
from 10 °C to 30 °C. For each coolant
temperature, a linear trend between
the transferred heat and the driving
temperature is observed. An increased
heat transfer rate leads to an increase
in the overall temperature difference.
By comparing the overheated evaporator
wall, the temperature difference
between evaporator and condenser in
the pipe and the temperature difference
in the chiller it becomes apparent
that the increase of the driving
temperature difference is mainly due
to an increase in the wall overheating
of the evaporator. The temperatures
in evaporator and condenser section
in the pipe even converge with
increasing heat transfer rate.
Another apparent trend shown in
Figure 4, is that an increase in the
coolant inlet temperature in connected
to a decrease in the overall
temperature difference at same heat
transfer rate. At a constant driving
temperature difference, the measurements
at higher coolant temperatures
and therefore higher heat transfer
rates showed an increase in the
stability of temperatures and pressures.
It is assumed, that low operation
temperature in combination with
low heat transfer rates promote
fluctuations in the temperatures and
geyser boiling.
The spreading of measurements
shown in Figure 4 is presumably
provoked by variations in the volume
flow of the coolant. The volume flow
rate in the secondary cooling circuit is
manually adjusted and for all shown
results approximately 4.8 l/min.
Conclusion and future work
The thermal operating characteristic
of long thermosiphons (10 m) were
investigated varying their filling ratio
(100 %, 70 %) and their thermal
boundary conditions considering the
operating conditions for passive heat
removal in spent fuel pools.
So far the experimental results
imply a tendency that the filling ratio
of 70 % operates at higher performances
than the 100 % FR for same
diameters. With larger pipe diameter,
the transferred heat increases by
trend, according to the increase of the
transfer area. Overall, the heat
transfer performance increases with
higher driving temperatures.
Especially at low operating temperatures,
the thermosiphon operates
in a periodic geyser boiling. In this
context, the minimum driving temperature
difference has to be
Influence of the heat sink
In Figure 4 the influence of the heat
sink on the heat transfer rate and the
corresponding driving temperature
| | Fig. 4.
Transferred heat vs. temperature difference between heat source and heat sink for varying inlet
temperatures in the double-pipe cooler.
AMNT 2017
Experimental Investigation on Passive Heat Transfer by Long Closed Two-Phase Thermosiphons ı Claudia Graß, Rudi Kulenovic and Jörg Starflinger