VGB POWERTECH 11 (2019)

Sub-cooled boiling of natural circulation in narrow rectangular channels VGB PowerTech 11 l 2019
Calculation results kW/m 2 K
6
5
4
3
2
1
+30 %
Cao correlation
Hong correlation
Rohsenow correlation
-30 %
ature, as well as the size of narrow rectangular
channels.
––
For natural circulation systems, the generation
and detachment of bubbles have
an influence on heat transfer coefficient
during sub-cooled boiling. This
process is accompanied by flow oscillation.
It is discovered that there are 3
stages during the sub-cooled boiling
phenomenon.
––
The empirical correlation has been proposed
for the heat transfer coefficient of
sub-cooled boiling, during natural circulation
through narrow rectangular channels.
It has been derived using dimensionless
analysis method. All experiment
results fall within ±15 % of the proposed
correlation.
apart from just the heat flux. In addition, as
seen from F i g u r e 7, the generation and
disappearance of bubbles which has a great
influence on the volume flow rate leads to
a instability in the process of sub-cooled
boiling. This decreases the heat transfer coefficient.
Although Cao and Hong correlations
fit well with the experiment results,
they can’t reflect the process of sub-cooled
boiling. In this paper, dimensional analysis
method has been performed in order to realize
an empirical correlation for the heat
transfer coefficient of sub-cooled boiling in
natural circulation.
According to the results of previous studies
[10, 21, 25] and experiment results based
on F i g u r e 1 , the governing factors which
influence the heat transfer coefficient for
natural circulation sub-cooled boiling phenomenon
have been identified. Ta b l e 3
gives a comprehensive list of such factors.
According to the π theorem [26], D e , , f ,
are selected as the fundamental variables
to be analyzed and the equation (7) describing
the heat transfer coefficient of subcooled
boiling in natural circulation can be
obtained.
(7)
The equation (7) is fitted based on the experiment
results of the natural circulation
system. The resulting empirical correlation
is shown as Equation (8)-(9).
(8)
0
0 1 2 3 4 5 6
Experiment results kW/m 2 K
Fig. 9. Comparison between calculation and experiment results.
(9)
F i g u r e 10 shows the calculation results
using above correlation, as compared with
Tab. 3. Dimensional parameters.
the experiment results for the natural circulation
system.
As depicted in F i g u r e 10 , the calculation
results show a good fit with the experiment
results within an accuracy of ±15 %. In
contrast to the previous studies, dimensional
analysis method has been used to
formulate the empirical correlation, which
can describe the physical process about
sub-cooled boiling of natural circulation in
narrow rectangular channels.
Conclusions
Based on the experiments of sub-cooled
boiling in natural circulation, different factors
have been identified and investigated,
which have an effect on heat transfer coefficient.
The following conclusions have
been drawn from this study:
––
For sub-cooled boiling, the heat transfer
coefficient increases with an increase in
the heating power and decreases with an
increase in the inlet sub-cooling temper-
Acknowledgments
The research was funded by National Natural
Science Foundation of China
(No.50976033), Beijing Natural Science
Foundation (No.3172032) and
Nomenclature Meaning of nomenclature Unit Dimension
h Heat transfer coefficient kW(/m 2 K) MT –3 –1
D e Hydraulic diameter of heating channel m L
Liquid thermal conductivity kW/(m K) MLT –3 –1
V Thermal expansion coefficient K - 1 –1
g Gravitational acceleration m/s 2 LT –2
∆T Sub-cooling degree K
q Heat flux kW/m 2 MT –3
C p Constant specific heat capacity kJ/(kg K) L 2 T –2 –1
Fluid density Kg/m 3 ML –3
f Dynamic viscosity of fluid Pa˙s ML –1 T –1
w Dynamic viscosity of fluid near wall Pa˙s ML –1 T –1
Length-width ratio of experiment cross-section Dimensionless N/A
Fundamental Research Funds for Central
Universities (No.2017XS086). Finally, the
authors would also like to thank the
researchers of Institute of Nuclear Thermal
Safety and Standardization for their
contribution.
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