7th International Symposium on Fluid Machinery and Fluids Engineering, ISFMFE 2016
October 18 - 22, 2016, Jeju, Korea
Oral Presentation ( O ) Poster ( )
INVESTIGATIONS ON CRITICAL FLOW OF SUPERCRITICAL CARBON DIOXIDE
WITH IMPLICATIONS IN TURBOMACHINERY SEAL DESIGN
Min Seok Kim1, Hwa-Young Jung1, Jinsu Kwon1, Jeong Ik Lee1*
1
Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology
291 Daehak-ro, (373-1, Guseong-dong), Yuseong-gu, Daejeon 305-701, Republic of KOREA
E-mail: [email protected], [email protected], [email protected], [email protected]
Introduction
The supercritical CO2 (S-CO2) cycle can improve
the safety of the Sodium-cooled Fast Reactor
(SFR) by substituting violent sodium-water
reaction with benign sodium-CO2 reaction. The
additional benefits of the S-CO2 cycle are
achieving relatively high efficiency under the mild
turbine inlet temperature condition, having a
simple layout and compact physical size [1]. Since
the S-CO2 power cycle is a highly pressurized
system, certain amount of leakage flow is
inevitable in the rotating turbo-machinery via seals.
The parasitic loss caused by the leakage flow
should be minimized since this greatly influences
the cycle efficiency. Thus, a simple model for
estimating the critical flow in a turbo-machinery
seal is essential to predict the leakage flow rate and
calculate the required total mass of working fluid
in a S-CO2 power system to minimize the parasitic
loss. This paper presents both numerical and
experimental study of critical flow of S-CO2 while
special attention is given to the turbo-machinery
seal design. A computational critical flow model is
described and experiments were conducted to
validate the critical flow model. Various conditions
have been tested to study the flow characteristic
and provide validation data for the model. The
comparison of numerical and experimental results
of S-CO2 critical flow will be presented.
expansion process of CO2 at the nozzle exit
although the CO2 pressure at the exit is higher than
that of CO2 in the low pressure tank when the flow
is choked. Based on the CO2 critical flow model,
the sensitivity study of the transient response
during the leak process was performed while
varying nozzle diameter and initial conditions
(temperature and pressure).
Experimental Result
A critical flow test facility was constructed to
validate the S-CO2 critical flow model. Fig.1
shows the designed experimental facility for the
CO2 critical flow simulation and the design
specifications are shown in Table.1. Initial
conditions of the low-pressure tank is maintained
at room condition (about 15ºC, 0.101MPa) to
maximize the pressure difference and have a long
depressurization time for stable measurement of
the CO2 critical flow.
Developed Model
To simplify the expected CO2 leak flow in a turbomachinery, a simplified model for CO2 leak flow
simulation was constructed. The previous study
described the governing equations and calculation
process [2]. To simplify the model, it was assumed
that temperature and pressure of CO2 at the seal
exit are at equilibrium with CO2 in the low
pressure tank. This assumption actually neglects
Fig.1 Conceptual diagram of the simplified model
for a numerical analysis
1
Table.1 Design specifications for experimental
facility
Design Parameters
Pressure (MPa)
22
150
Temperature ( )
47
Volume (L)
(I.D.:200 mm, H:
1600mm)
Internal diameter
57
(mm)
Length (mm)
1090
Electric capacity
5
(kW)
Ball valve
High/Low-pressure
tank
Pipe connecting two
tanks
Heater
(Jacket-type)
Valve type
The initial conditions of the high-pressure tank
are set at 10MPa and 100ºC. The nozzle has the
diameter of 1.5 mm and length of 5.0 mm as shown
in Fig. 2.
The comparisons of mass flux between the result
from the experiment and the numerical model are
shown in Fig.3. And Fig.4 shows the Mach number
of experimental result which indicates whether the
flow is choked or not. It is shown that the S-CO2
critical flow model based on an isentropic flow
assumption has a good accuracy for predicting the
S-CO2 critical and subcritical flows and the critical
flow model predictions are all within the
uncertainty band of the experimental data. Thus, it
is reasonable to conclude that the isentropic critical
flow model is sufficient to predict the critical and
subcritical flows of S-CO2 to the low pressure side.
Fig.2 Designed experimental facility for the CO2
leak test
40000
Mass flux (kg/m2-sec)
Experiment (with uncertainty)
Numerical model
30000
Fig.5 Labyrinth seal geometry nozzle
20000
10000
0
-50
0
50
100
150
200
250
300
Time (sec)
Fig.3 Comparison of mass flux between the
experimental result and CO2 critical flow model
Mach number (-)
1.0
Fig.6 Comparison of results with labyrinth seal
geometry nozzles
0.5
0.0
-100
0
100
200
Time (sec)
Fig.4 Mach number of experimental result
300
The additional experiments with changes of
labyrinth seal geometry nozzle were performed to
verify the labyrinth seal effect. The labyrinth seal
geometry nozzle are shown in Fig.5, and detail
2
information of three nozzles and experiment results
are shown in Fig.6. As increasing the teeth number,
the reaching time to the equilibrium state is
delayed about 230s. Also, it has been observed that
the reaching time increases as the cavity length
increases.
Conclusions
In order to simulate the transient response of the
CO2 leak, the S-CO2 critical flow model based on
an isentropic ideal gas flow assumption was
evaluated. Through a simple experiment it was
confirmed that even though there is a phase change
still a simple critical flow model based on an
isentropic ideal gas flow assumption provides a
reasonable result. Therefore, it can be concluded
that the developed isentropic critical flow model
can estimate the behavior of the CO2 critical flow
in a S-CO2 turbo-machinery. Also, the additional
experiments with labyrinth seal geometry nozzle
were performed to verify the labyrinth seal effect.
It was re-confirmed that the reaching time
increases as the teeth number and cavity length
increase.
The developed CO2 critical flow model does not
correctly reflect a labyrinth seal geometry effect.
The real labyrinth seal has multiple tooth to further
minimize the leak. Therefore, to upgrade the
numerical model, applying the labyrinth seal
geometry effect and conducting additional
experiment of a real labyrinth seal geometry nozzle
will be performed in the near future.
Acknowledgement
This research was supported by the Ministry of
Trade, Industry and Energy (MOTIE) [2015-67110063187]
References
[1] V. Dostal, M.J. Driscoll, P. Hejzlar, A
Supercritical Carbon Dioxide Cycle for Next
Generation Nuclear Reactors, Thesis, MITANP-TR-100 (2004).
[2] M.S. Kim, H. Jung, Y. Ahn, J. Lee, J.I. Lee,
"Experimental and Numerical Analysis of SCO2 Critical Flow for SFR Recovery System
Design", Korean Nuclear Society Spring
Meeting, Jeju, Korea, May 12-13, 2016
[3] Cenay, S., 2002, “Paper title,” in Edited book,
editor(s), Driscal, J.E., Yener, M.F., 591-98,
State, Country.
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