2.1 Test of tube-bundle channel

High-speed camera is adopted to carry out experiments of the two-phase boiling flow in the tube-bundle channel for different heat flux and different flow rate. Several representative pictures obtained are displayed in Fig.12.

Fig. 12. Flow patterns in a vertical tube-bundle channel

Fig. 12 shows that there are four main flow patterns, bubbly flow, bubbly-churn flow, churn flow and annular flow, which is different from flow patterns of two-phase boiling flow in a circular tube. Through the analyses, it is shown that there may be two reasons for these differences. Firstly, the geometric dimensions cause the different flow patterns. The tube — bundle channel is divided into several sub-channels by the tubes, as shown in Fig. 12. And the inner tubes divide the large bubble and make disturbance on two-phase flow. Furthermore, flows in the sub-channels interact and enhance the complexity of two-phase flow in the tube-bundle channel. Secondly, the heating mode of boiling flow in the tube — bundle channel and that in the circular tubes are different. When flowing in a circular tube, the fluid is surrounded and heated by the wall of the tube. On the other hand, in a tube — bundle channel, the fluid surrounds the tube bundle, which acts as the heating source. Then, all of these might cause differences between flow patterns in vertical tube-bundle channels and that in vertical circular tubes (Hewitt & Roberts, 1969).

Two-phase boiling flow in tube-bundle channels exhibits several main flow patterns, bubbly flow, bubbly-churn flow, churn flow and annular flow, as shown in Fig. 13. There are differences from the results gained by Grant & Chisolm (1979) and Ma (1992). These differences might be caused by the different test conditions. The characteristics of flow patterns and transitions in the present experiments are analyzed as follows.

(1) Bubbly flow: In the experiments, bubbles begin to come into being in the liquid when the heat flux is small. The mainstream is liquid and discrete bubbles are dispersed in the mainstream, which indicates that bubbly flow in circular tubes is similar to that in the tube — bundle channel, as shown in Fig. 13(a). With the sustaining heating, the dimension and the quantity of bubbles increase gradually along the flow direction, and discrete bubbles begin to aggregate to combine to be large bubbles. Confined by the narrow space in the tube — bundle channel, large bubbles transfigure to be oval or crescent.

(2) Bubbly-churn flow: When the little discrete bubbles aggregate and combine to be large bubbles, the tubes agitate and divide these large bubbles. This makes there be no slug flow in the tube-bundle channel. And the flow pattern begins to be transformed from bubbly flow to churn flow.

(3) Churn flow: With the augmentation of heat flux, discrete bubbles continue to aggregate and combine and the aggregation bubbles become bigger in size. These aggregation bubbles present unstable state, on account of the agitation and division of the tubes. And the aggregation bubbles begin to burst into many little discrete bubbles with unequal geometric dimensions. Then, the flow pattern is transformed to churn flow.

When churn flow occurs in the tube-bundle channel, there might be many bubbles with unequal geometric dimensions. The liquid moves up and down in the channel and the two — phase flow exhibits surge state. [8]

In the experiments, there is no mist flow due to the limit of the experiment condition, which is detected by Grant & Chisolm (1979).

There may be several differences between flow patterns and their transition in circular tubes and that in tube-bundle channels. Firstly, slug flow is one of the main flow patterns for two-phase flow in a circular tube. But the tube-bundle channels, the inner tubes divide the large bubbles and flows in the sub-channels interact, which makes bubbles can not aggregate and combine to be a slug. Thus there is a lack of slug flow in the tube-bundle channel and the flow pattern is transform directly from bubbly flow to churn flow. Secondly, churn flow, as the transition from slug flow to annular flow, exists transitory in circular tubes. Under some circumstances, there might be a lack of churn flow in circular tubes. But in the tube-bundle channel, churn flow presents itself as one of the main flow patterns. And churn flow exists in many experimental conditions and present a long time in the tube-bundle channel, as shown in Fig. 12.

With the same flow rate, there may be different flow patterns in the tube-bundle channel due to the different heat flux. For example, when the flow rate is 281.8 (kg/m2s), bubbly flow occurs in the tube-bundle channel where the heat flux is comparatively small. But the amount of bubbles begin to increase with the enhanced heat flux. And the flow pattern changes from bubbly flow to churn flow when the heat flux is equal to 5.1X104 (w/m2). This difference in flow patterns will be more obvious along with the decrease of the flow rate, which is shown in Fig. 12. When the flow rate is 133.9 (kg/m2s), there will appear three flow patterns, which are bubbly flow, bubbly-churn flow and churn flow, under different heat flux in the tube-bundle channel. And when the flow rate is getting smaller, the flow patterns with differnet heat flux will be differentiated more distinctly. As shown in Fig. 12, there appear four different flow patterns, which are bubbly flow, churn flow, churn-annular flow and annular flow, when the flow rate is equal to 63.3 (kg/m2s). From above all, it is shown that heat flux will affect the flow pattern more remarkably under the small flow rate condition.

Furthermore, it is found that there may be two different flow patterns in the same cross­section when the experiments run. The flow pattern transitions exhibit unsynchronized in different sub-channels. This unsynchronized phenomenon is caused by the different geometric dimensions, different heat flux and different quantity of discrete bubbles generated in different sub-channels. It was found the same phenomenon in a tube-bundle channel by adopting microprobe to detect the flow pattern transitions (Bergles 1981).

The flow pattern map obtained in the experiments is shown in Fig. 14. Comparisons are made with the results gained by Hewitt & Roberts (1969), which is figured by dashed line. Two-phase flow patterns and their transition in vertical circular tubes were experimentally studied by Hewitt & Roberts. The mass flow rate in the present experiments is less than that of Hewitt & Roberts owing to the experimental condition limits. In Fig. 6, the abscissa p’ j ^ and the ordinate p" jg are calculated by (Hewitt & Roberts, 1969),

G2 x2

P" Jg =—IT

s p"

It is shown in Fig. 14 that there are great differences between flow pattern transitions in a tube-bundle channel and that in a circular tube. The generation regions of bubbly flow and churn flow in a tube-bundle channel move left in Fig. 14, compared with the region in a circular tube. And it is shown that the regions of bubbly flow and churn flow are larger in a tube-bundle channel, which is caused by that the inner tubes have effects of disturbance and division on the bubbles and these effects make it impossible for discrete bubbles to converge and unite to be a slug. In addition, on account of being heated by the inner tubes, the fluid generates bubbles in the core of the channel, which makes the probability increase for bubbles to converge and unit to be a continuous axle center in the core of the tube-bundle channel, and then the flow pattern transform to annular flow in a tube-bundle channel is earlier than that in a circular tube.