An experimental study of boiling two-phase flow in a vertical rod bundle with a spacer grid-Part 2: Effects of vane angle

We performed boiling flow experiments and measured the void fraction in a 3 x 3 rod bundle including a spacer grid with split type vanes using X-ray computed tomography, which provides high-resolution time-averaged void data without disturbing the flow. We studied the effects of mixing vanes with different vane angles, namely, 20 degrees, 29 degrees and 40 degrees, for a mass flux between 535 and 1950 kg/m2 s and the central rod being heated giving a heat flux of 85.7 kW/m2. The experiments were conducted using octafluorocyclobutane (RC318) as the working fluid. The presence of vanes leads to an increase of the cross-sectional averaged void fraction up to an axial position of Z & AP; 0.8Dh. After that, the void fraction decreases until 3Dh < Z < 4Dh due to the induced swirl flow, before it increases again. It was further found that the vanes cause a high local void fraction near the spacer, which in-creases the possibility of DNB occurrence. From this study, it can be concluded that a vane angle of 29 degrees is optimal for two-phase flow, which is consistent with the findings in the literature for single-phase flow.