The effects of rolling and heaving on flow boiling heat transfer in a 3 x 3 rod bundle channel in a natural circulation system

Floating nuclear power plants are always influenced by sea waves. Rolling and heaving are two common conditions the plants will be subjected to. The previous rod bundle flow boiling experiments are mostly conducted at low pressure, and the results cannot be directly utilized in the nuclear power plants. Flow boiling experiments at 13-13.5 MPa and visualization experiments in a 3 x 3 rod bundle channel for rolling and heaving conditions were conducted in this research. The effects of rolling and heaving on flow boiling heat transfer coefficients (HTCs) and bubble dynamics in the rod bundle were analyzed for both subcooled and saturated boiling. The visualization results reveal that when the rolling angle is large, the void fraction of the fluid is much larger than that in the vertical static channel for most of the rolling period, and in a heaving period, the void fraction is larger than that in the static vertical channel for half the period. The experimental results show that the fluctuation amplitude of HTCs will become larger when rolling amplitude increases. When the maximum rolling angle is 10 degrees, the average HTC during rolling process is the same with the HTC in the static vertical channel. Only when the rolling angle is 30 degrees, the average HTC during the whole rolling process is 6.15 % higher than the static HTC. In the heaving process, the mass flux and flow boiling HTCs vary in a sine plot with the periodical movement. When the heaving amplitude becomes larger or heaving period becomes shorter, the additional driving force will increase and the mass flux fluctuation amplitude and HTC fluctuation amplitude become larger. Heaving has little effect on the time-averaged flow boiling HTCs in the rod bundle. This research widens the experimental data of high pressure flow boiling for motion conditions, and the results can be useful to the design of floating nuclear power plants.