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3 
Herium-Air Exchange Flow Rate Measurement 
Through a Narrow Flow Path 
Motoo Fumizawa 
Shonan Institute of Technology 
 Japan 
1. Introduction 
Buoyancy-driven exchange flows of helium-air were investigated through horizontal and 
inclined small openings. Exchange flows may occur following a window opening as 
ventilation, fire in the room, over the escalator in the underground shopping center as well 
as a pipe rupture accident in a modular high temperature gas-cooled nuclear reactor. Fuel 
loading pipe is located in the inclined position in the pebble bed reactor such as Modular 
reactor (Fumizawa, 2005, Kiso, 1999) and AVR(El-Wakil, 1982, Juni-1965, 1965).  
In safety studies of High Temperature Gas-Cooled Reactor (HTGR), a failure of a standpipe 
at the top of the reactor vessel or a fuel loading pipe may be one of the most critical design-
base accidents. Once the pipe rupture accident occurs, helium blows up through the breach 
immediately. After the pressure between the inside and outside of he pressure vessel has 
balanced, helium flows upward and air flows downward through he breach into the 
pressure vessel. This means that buoyancy-driven exchange flow occurs through the breach, 
caused by density difference of the gases in the unstably stratified field. Since an air stream 
corrodes graphite structures in the reactor, it is important to evaluate and reduce the air 
ingress flow rate during the standpipe rupture accident.  
Some studies have been performed so far on the exchange flow of two fluids with different 
densities through vertical and inclined short tubes. Epstein(Epstein, 1988) experimentally 
and theoretically studied the exchange flow of water and brine through the various vertical 
tubes. Mercer et al. (Mercer, 1975) experimentally studied an exchange flow through 
inclined tubes with water and brine. He performed the experiment in the range of 3.5 <L/D 
< 18 and  0 deg < θ < 90 deg, and pointed out that the length-to-diameter ratio L/D, and the 
inclination angle θof the tube are the important parameter for the exchange flow rate. Most 
of these studies were performed on the exchange flow with a relatively small difference of 
the densities of the two fluids (up to 10 per cent). However, in the case of HTGR standpipe 
rupture accident, the density of the outside gas is at least three times larger than that of the 
gas inside the pressure vessel. Few studies have been performed so far in such a large range 
of density difference. Kang et al. (Kang, 1992) studied experimentally the exchange flow 
through a round tube with a partition plate. Although we may think that the partitioned 
plate, a kind of obstacle in the tube, decrease the exchange flow rate, he found that the 
exchange flow rate is increased by the partition plate because of separation of an upward 
and downward flow.  
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 Nuclear Reactors 
 
48
The objectives of the present study are to investigate the behavior of the exchange flow, i.e., 
exchange flow through the round long tube by several flow visualization method, then to 
evaluate the exchange flow rate by the PTV and PIV methods and mass increment with 
helium-air system. Therefore the following methods are investigated in the present study.   
1. Smoke wire method 
2. The optical system of the Mach-Zehnder interferometer 
3. The method of the mass increment 
2. Smoke wire method 
2.1 Experimental apparatus and procedure 
The smoke wire method was used for the present investigations. Figure 1 shows a typical 
sketch of the apparatus. It consists of a smoke pulse generator, thin Nichrome wire with oil 
and a test chamber. This figure also shows the high-speed camera system, and it transfers 
the visual digital data to the personal computer for data acquisition. The experimental 
procedure is as follows. The test chamber is filled with pure helium. By removing the cover 
plate placed on the top of the tube, exchange flow, i.e., exchange flow of helium and air is 
initiated. At such condition, the smoke pulse generator ignites the high voltage. 
Immediately a smoke appeared and visualized the helium up flow and the air down flow in 
the flow path in the long tube. Test chamber diameter and height are 350 mm, the long tube 
on the test chamber diameter(=D) are 17.4 and 20mm, length of it (=L) are 200 and 319mm. 
They denote L/D=10 and L/D=18.3 respectively. It simulates a typical long tube. The 
inclination angle θis 30 deg. The smoke wire conditions are as follows. The voltage is around 
250 (V), current duration is 30 m sec, and the oil of thin wire is CRC-556. The high-speed 
camera system using D-file records the visual data up to 1600 flames in a second. Upward 
flows peak velocity measured by PTV method. 
Test 
chamber
High speed 
camera
VCC-H1600C
Control computer
D-file
Smoke pulse 
generator
Helium gas
Air
θ
Flow 
path
Smoke wire L
D
L:length of flow path
D: diameter of flow path
 
Fig. 1. Exchange Flow Apparatus of Smoke Wire Method High speed camera system 
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 Herium-Air Exchange Flow Rate Measurement Through a Narrow Flow Path 
 
49 
 
Fig. 2. Mechanism of exchange flow 
 
Fig. 3. The visualized data listed along the elapsed time (L/D=18.3, flame rate=200 f/s, 
θ=30 deg) 
2.2 Results 
The typical exchange flow in the tube was visualized in the figure 2. The visualized 
exchange flow resembles to the S-shape. The flames are detected 200 and 500 flames in a 
second by the high-speed camera. The visualized data listed along the elapsed time in the 
figures 3 and figure 4, respectively. It is clearly visualized that helium up-flow along left 
hand side and air down-flow along right hand side. Figure 5 shows time history of the 
upward flow peak velocity measured by PTV. In the case of L/D=18.3, the average velocity 
value U0  is evaluated as 0.315 m/s. In the case of L/d=10, the average velocity value U0  is 
Heliumas
Air
Exchange flow path
“S-shape smoke line
(a) Ignition: 0 sec (b)Elapsed time:0.050 sec
www.intechopen.com
 Nuclear Reactors 
 
50
evaluated as 0.662 m/s. It means that high exchange velocity detects in low L/D ratio. 
Figue.6 shows the visualization data of  PIV measurement of 0.050sec after ignition 
(L/d=18.3 and flame rate=200 f/s ). The adopted mesh size is 40x10 points in the flow area. 
The upward peak velocity was measured to 0.517 m/s. It means the value is higher than 
average velocity in this case. The exchange flow rate Q is derived as follows assuming 
parabolic flow profile, where r is the radius of flow path of the horizontal direction. 
 
2
02
9
U r
Q
  (1) 
 
(a) Elapsed time 0.062 sec (b) Elapsed time 0.082 sec
 
Fig. 4. The visualized data listed along the elapsed time  (L/D=10, flame rate=500 f/s, θ=30 
deg) 
 
Fig. 5. Upward flow peak velocity measured by PTV method (L/D=18.3) 
-100
100
300
500
700
900
1100
1300
1500
0 0.1 0.2 0.3
Elapsed time (sec)
Up
w
ar
d 
flo
w
 
pe
ak
 v
el
o
ci
ty
(m
m
/s
)
θ＝30゜
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 Herium-Air Exchange Flow Rate Measurement Through a Narrow Flow Path 
 
51 
 
 
 
 
 
Fig. 6. The visualization data of PIV measurement of 0.050sec after ignition (L/D=18.3 and 
flame rate=200 f/s, θ=30 deg ) 
In PTV method, the exchange flow rate Q is calculated as 1.47x10-5 m3/s under the conditon 
of . L/D=18.3. Therefore, the densimetric Froude number Fr is evaluated as 0.202, in this 
condition Reynords number Re is 79.2. In the condition of L/D=10, the densimetric Froude 
number Fr is evaluated as 0.287. 
 
 
 
Mach-Zehnder 
Interferometer
Air
Electronic balance
Helium 
Gas Mixture
Inclined 
angle 
Exchange flow path
Computer
Computer
High speed camera
D
Test chamber
L
Cross section of 
Exchange flow path
 
 
Fig. 7. Experimental apparatus of optical system and mass increment 
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 Nuclear Reactors 
 
52
 
Fig. 8. The mechanism of optical system of the Mach-Zehnder interferometer, MZC-60S 
(a) (b) (c)
(d) (e) (f)  
Fig. 9. The typical interference fringes for the inclined long round tube (L/D=5) 
3. Optical system of mach-zehnderinterferometer 
3.1 Experimental apparatus and procedure 
The optical system of the Mach-Zehnder interferometer, MZC-60S is shown in figure 7 and 
figure 8 to visualize the exchange flow. After being rejoined behind the splitter, the test and 
reference laser beams interfere, and the pattern of interference fringes appears on the screen. 
www.intechopen.com
 Herium-Air Exchange Flow Rate Measurement Through a Narrow Flow Path 
 
53 
If the density of the test section is homogeneous, the interference fringes are parallel and 
equidistant (Keulegan, 1958). If it is not homogeneous, the interference fringes are curved. 
An inhomogeneity in the test section produces a certain disturbance of the non-flow fringe 
pattern. The digital camera and high-speed camera using D-file is able to attach to the 
interferometer. 
3.2 Results 
Figure 9 shows the typical interference fringes for the inclined long round tube (L/D=5). 
The curved interference fringes indicate that the lighter helium flows in the upper passage 
of the tube. The straight fringes indicate that the heavier air flows in the bottom of the tube. 
It is clearly visualized that the exchange flows take place smoothly and stable in the 
separated passages of the tube. This leads to less resistance for the exchange flow in the 
inclined tubes compared to he vertical ones. In the case of 30 deg, the curvature of the 
interference fringes is larger than that at other angles, indicating that the exchange flow rate 
and the densimetric Froude number are the largest at 30 deg. 
L/D
 
Fig. 10. The relationship between Fr and inclination angle θ with L/D as a parameter 
4. Method of mass increment 
4.1 Experimental apparatus and procedure 
The method of the mass increment was used for the investigations. Figure 7 shows a rough 
sketch of the apparatus. It consists of a test chamber, an electronic balance and a personal 
computer for data acquisition. The experimental procedure is mentioned in Sec. 2.1. Air 
enters the test chamber and the mass of the gas mixture in the test chamber increases.  
The mass increment  m is automatically measured by the electronic balance with high 
accuracy. From mass increment data, the density increment of the gas mixture  ρL = 
 m/V is calculated. The density increment means the difference of densities of the gas 
mixture from the density of pure helium in the test chamber. Then, volumetric exchange 
flow rate is evaluated by the following equation:   
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 Nuclear Reactors 
 
54
 
( )L
H L
V d
Q
dt

 
   (2) 
The densimetric Froude number is defined by the following equation derived from the 
dimensional analysis suggested by Keulegan (Merzkirch, 1974):   
 
Q
Fr
A gD

   (3) 
In the above equations, V is the volume of test chamber, ρH the density of air, ρL the density 
of gas mixture in the test chamber,  ρL ( =ρH -ρHe) = the density increment of the gas 
mixture, t the elapsed time, U(=Q/A) the exchange-velocity, ρ( =ρH +ρHe)/ 2, D the diameter 
and g the acceleration of gravity. The experiments are performed under atmospheric 
pressure and room temperature using the vertical and inclined round tubes, and using the 
vertical annular tube. The density of the gas mixture is close to that of helium in the present 
experiment. The sizes of the tubes are as follows. The diameter of the round tube D is 20 
mm, which is much smaller than that of the test chamber. The inclination angle θ ranges 
from 15 to 90 deg and the height L ranges from 0.5 to 200 mm. 
4.2 Results and discussion 
It is already known that it is regarded as constant within a time duration when the gas in the 
upward flow can be assumed helium (Fumizawa, 1989). Figure 10 shows the relationship 
between Fr and inclination angle θ with L/D as a parameter. For inclined tubes, Fr is larger 
than that for vertical tubes. The black circles show the experimental data for the orifice (i.e. 
L/D =0.05) and the black rhombuses for the long tube (i.e. L/D = 5). Densimetric Froude 
number reaches the maximum at 60 deg for the orifice and 30 deg for the long tube. It is 
found that the angle for the maximum Fr decreases with increasing L/D in the helium-air 
system. On the other hand, Mercer's experiments with water and brine indicated that the 
inclination angle for the maximum Fr was about 80 deg in the several long tubes 
investigated. It may depend on the difference of dynamic viscosity between the gas and the 
liquid.  
5. Conclusion 
1. Flow visualization results indicate that the exchange flows through the inclined round 
tube take place smoothly and stable in the separated passages of the tube.  
2. The visualized inclined exchange flow resembles to the S-shape. 
3. In the inclined round long tube, the inclination angle for the maximum densimetric 
Froude number decreases with increasing length-to-diameter ratio for the helium-air 
system. On the other hand, this angle remains almost constant for the water-brine 
system.  
6. Acknowledgements 
The authors are deeply indebted to Dr. Makoto Hishida, who is professor of Chiba 
University in Japan, and Mr. Akira Furumoto who is manager of Digimo CO.,LTD for their 
unfailing interest and many helpful corporations to this study.  
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 Herium-Air Exchange Flow Rate Measurement Through a Narrow Flow Path 
 
55 
7. Nomenclature 
A: flow passage area (m2) 
D: inner diameter of the tube of the flow path (m) 
Dc: inner diameter of test chamber (m) 
Fr: densimetric Froude number defined by eq.(3) 
g :acceleration of gravity (m/s ) 
Hc: inner height of test chamber (m) 
L: height of the tube of the flow path (m) 
Q :volumetric exchange flow rate defined by eq.(l) (m/s) 
r: radius of flow path of the horizontal direction (m) 
T: elapsed time (s) 
U: exchange-velocity (=Q/A) (m/s) 
U0: maximum exchange-velocity (m/s) 
V: volume of test chamber (m ) 
Greek 
m: mass increment in test chamber (kg) 
 ρL : density increment (=  m/V)  (kg/m3)  
θ: inclination angle if flow path from perpendicular line (deg) 
ρ: mean density ( =ρH +ρHe)/ 2  (kg/m3) 
Subscripts 
L : lighter fluid (gas mixture) 
H : heavier fluid (air) 
He: helium 
8. References 
El-Wakil, M.M., Nuclear Energy Conversion, Thomas Y. Crowell Company Inc., USA (El-
Wakil, 1982) 
Epstein,M., Trans. ASME J. Heat Transfer, 110, pp885 -893 (Epstein, 1988) 
Fumizawa,M. et. al., J. At. Energy Soc. Japan, Vol.31, pp1127-1128 (Fumizawa, 1989)   
Fumizawa,M.; Proc. HT2005 ASME Summer Heat Transfer Conference, HT2005-72131, pp.1-
7, Track 1-7-1 (Fumizawa, 2005)  
Juni-1965, “Sicherheitsbericht fuer das Atom-Versuchskraftwerk Juelich”, Arbeitsgemein-
schaft Versuchs-Reactor AVR (Juni-1965, 1965)  
Kang,T. et al., NURETH-5, pp541-546 (Kang, 1992) 
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 Nuclear Reactors 
 
56
Keulegan,G.H., U.S.N.B.S.Report 5831 (Keulegan, 1958) 
Kiso et.al.; JSME Annual MTG, pp.339-340 (Kiso, 1999) 
Mercer, A. and Thompson.H., J. Br. Nucl. Energy Soc., 14, pp327-340 (Mercer, 1975) 
Merzkirch,W., "Flow Visualization”, Academic Press (Merzkirch, 1974)   
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Nuclear Reactors
Edited by Prof. Amir Mesquita
ISBN 978-953-51-0018-8
Hard cover, 338 pages
Publisher InTech
Published online 10, February, 2012
Published in print edition February, 2012
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This book presents a comprehensive review of studies in nuclear reactors technology from authors across the
globe. Topics discussed in this compilation include: thermal hydraulic investigation of TRIGA type research
reactor, materials testing reactor and high temperature gas-cooled reactor; the use of radiogenic lead
recovered from ores as a coolant for fast reactors; decay heat in reactors and spent-fuel pools; present status
of two-phase flow studies in reactor components; thermal aspects of conventional and alternative fuels in
supercritical water?cooled reactor; two-phase flow coolant behavior in boiling water reactors under earthquake
condition; simulation of nuclear reactors core; fuel life control in light-water reactors; methods for monitoring
and controlling power in nuclear reactors; structural materials modeling for the next generation of nuclear
reactors; application of the results of finite group theory in reactor physics; and the usability of vermiculite as a
shield for nuclear reactor.
How to reference
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Motoo Fumizawa (2012). Herium-Air Exchange Flow Rate Measurement Through a Narrow Flow Path,
Nuclear Reactors, Prof. Amir Mesquita (Ed.), ISBN: 978-953-51-0018-8, InTech, Available from:
http://www.intechopen.com/books/nuclear-reactors/the-thermal-hydraulic-feasibility-design-study-and-safety-
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