The turbulence intensity measured in this study could be divided into wall turbu­lence and bubble-induced turbulence. However, turbulent production from bubbles is dominant at the pipe center. Thus, the turbulence intensity at r/R = 0 was plotted against the void fraction measured by the four-sensor probe (Fig. 11.6). In addition, the present results were compared with the previous experimental data of the bubble-induced turbulence in an air-water two-phase flow system. The solid line in this figure denotes the calculated value by the following semi-theoretical equation [5].

u’ = ur a0’5. (11.2)

In this equation, the velocity field around the bubble is assumed as potential flow and the rotational component of the wake is ignored. In addition, the value calculated by the empirical equation for air-water two-phase flow [6] is also drawn as the dashed line in Fig. 11.6; the equation is represented as follows:

u’ = 0.85a0’8. (11.3)

Although the fluid properties are different with the air-water two-phase flow, the measured turbulence intensity agrees with Eq. (11.3) and the previous data [7—9], except the result at z/D = 3.2. However, Eq. (11.3) was derived for an air-water flow system and its applicability to liquid metal flow was not clear. Therefore, the mechanism of turbulence production in liquid metal two-phase flow should be investigated in more detail. On the other hand, the turbulence intensity at z/ D = 3.2 was slightly larger than other plots and Eq. (11.3). The measurement

Void fraction, a [-]

position at z/D = 3.2 was relatively close to the gas injector, so it is expected that the flow was not fully developed.

11.2 Conclusions

A liquid metal two-phase flow was investigated by using a four-sensor probe and an electromagnetic probe. From the measurement results of two-phase flow structure and turbulence characteristics, the following knowledge was obtained.

• Radial profile of void fraction changes from wall peak to core peak along the flow direction.

• Axial development of the liquid velocity field shows different tendency for the void fraction profiles.

• Existing correlations for interfacial area concentration overestimate interfacial area concentrations at present experimental conditions, which might be attrib­uted to the difference in bubble size. A new correlation should be modeled with further consideration of bubble size and the wall conditions.

• Bubble-induced turbulence at the pipe center in lead-bismuth two-phase flow agrees well with the previous experimental data for air-water flows. However, the mechanism should be clarified by measuring the liquid-metal two-phase flow in a wide range of flow conditions.

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