VGB POWERTECH 11 (2019)

VGB PowerTech 11 l 2019
Sub-cooled boiling of natural circulation in narrow rectangular channels
––
At the beginning of the experiments,
deionized water is added into the whole
loop, and the pressure is regulated using
a pressure stabilizer.
––
A preheater and a rectangular heater are
used to heat the fluid medium. The power
of preheater is maintained so that the
fluid enters the rectangular heater at a
certain temperature.
––
Once natural circulation begins, the power
of rectangular heater is increased by a
certain amount of time step. In fact, the
power should be added gradually, in order
to allow the system to balance itself.
––
All requisite operational parameters are
recorded. The above procedure is repeated
by changing the fluid temperature at
the inlet of the rectangular heater, the
heat flux and size of experiment channel.
In practice, the heating power is lost due to
the glass wall of the experiment channel
and a heat transfer efficiency of 0.75 is
used. This is calculated using a heat balance
experiment.
Heat transfer coefficient kW/(m 2 K)
4.0
3.5
3.0
2.5
2.0
1.5
80 90 100 110 120 130 140
Heat flux kW/m 2
Experiment calculations
Wall temperature calculation
The effective heating power of the experiment
channel is defined as equation (1).
(1)
In this equation, q is the effective heating
power of experiment channel measured in
kW/m 2 . U is the voltage of experiment
channel measured in V. I is the electric current
of experiment channel measured in A.
eff is the heat transfer efficiency. b is the
width of rectangular heating surface measured
in m. L is the length of rectangular
heating surface measured in m.
In addition, there is an offset between the
position of thermocouples and the inner
wall surface of the experiment channel.
Because the offset is small, it can be
assumed that the temperature varies
linearly along the thickness of inner wall.
This temperature can be calculated by
Fourier heat conduction law, which is
showed as equation (2).
(2)
Fig. 4. The influence of heat flux on heat transfer coefficient.
In the above equation, T wi is the inner wall
temperature of test position i measured
in K. T i is the thermocouple measuring
temperature of i test position measured
in K. λ
w is thermal conductivity of heating
plate measured in kW/(m K). δ is the gap
w
between the thermocouple and inner wall
of experimental channel measured in m.
Heat transfer coefficient of sub-cooled
boiling calculation
Through observation and experiment data
analysis, it is found that the ONB occurs at
the lower part in the mid-section of experiment
channel. Hence, the average heat
transfer coefficient of 9-12 test positions
which are in the mid-section of experiment
channel is regard as the standard heat
transfer coefficient of sub-cooled boiling
shown as equation (3).
(3)
Heat transfer coefficient kW/(m 2 K)
2.6
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
In the above equation, h is the heat transfer
coefficient of sub-cooled boiling measured
in kW/(m 2 K). T fi is the temperature of fluid
at i th test position measured in K. It has
been assumed [22] that the fluid’s temperature
varies linearly along the axial direction
from inlet to the outlet of the experiment
channel.
Experiment results and analysis
The influence of heat flux on heat transfer
coefficient
F i g u r e 4 shows the influence of heat flux
on heat transfer coefficient in a 3 mm experiment
channel. The inlet sub-cooling is
50 o C.
As it is depicted in F i g u r e 4 , the heat
transfer coefficient of sub-cooled boiling in
natural circulation is 1.9 kW/(m 2 K) when
the heat flux is at 80 kW/m 2 , and which in-
2.0 2.5 3.0 3.5 4.0 4.5 5.0
Experiment channel size mm
Fig. 5. The influence of experiment channel on heat transfer coefficient.
65