**7. Nomenclature**

54 Nuclear Reactors

*H L V d <sup>Q</sup> dt*

The densimetric Froude number is defined by the following equation derived from the

*A gD* 

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

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

1. Flow visualization results indicate that the exchange flows through the inclined round

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

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

tube take place smoothly and stable in the separated passages of the tube.

2. The visualized inclined exchange flow resembles to the S-shape.

unfailing interest and many helpful corporations to this study.

 

*<sup>Q</sup> Fr*

dimensional analysis suggested by Keulegan (Merzkirch, 1974):

from 15 to 90 deg and the height L ranges from 0.5 to 200 mm.

**4.2 Results and discussion** 

liquid.

**5. Conclusion** 

system.

**6. Acknowledgements** 

( ) *<sup>L</sup>*

(2)

(3)

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 )
