Experimental measurement of bubbly two-phase flow in a 4x4 rod bundle with wire mesh sensor

In order to investigate two-phase bubbly flow through fuel assembly of pressurized water reactor (PWR), the wire mesh sensor (WMS) is employed to measure air-water flow in a up-scaled 4x4 rod bundle based on PWR fuel assembly. Measurement is carried out in the bare rod bundle and downstream of mixing-vane spacer grid. Air bubbles are injected equally into 16 subchannels at the inlet with a standalone bubble generator which adjusts the injected bubble size. Distribution of void fraction and bubble size across the rod bundle are derived based on the WMS data. In the bare rod bundle, small-size bubbles (<3 mm) are inclined to coalesce and medium-size bubbles (3-8 mm) tends to break up at low gas superficial velocity, while at high gas superficial velocities bubble coalescence produces large-size bubbles (>8mm). High liquid superficial velocity reduces the bubble size. Void fraction distribution of bubble size groups shows that bubbles with diameter less than 4 mm mainly reside near the solid walls, while large bubbles with diameter larger than 6 mm mainly appear in the subchannel center, which is consistent with Tomiyama's lift force correlation. The effect of injected bubble size can hardly be eliminated along with flow development in bare rod bundles. When installed with mixing-vane spacer grid, the vortices produced at the tips of mixing vanes induce bubble entrapment into the swirl core, which significantly enhances the interaction between bubbles, as well as the interphase interaction. Such enhancement is even more remarkable in the low-void-fraction cases. As a consequence, the bubble size distribution downstream of the spacer grid shows relatively weak dependence on the injected bubble size. Besides, the vortices lead to strong inter-subchannel void drift of small-size bubbles, which results in void fraction difference between subchannels. The effects of mixing-vane spacer grid decay comparatively fast in the high void fraction cases.