抄録
To improve the understanding on the mechanisms of phase distribution and bubble interaction for cap bubbly flow in rod bundles, the adiabatic upward cap bubbly flow has been investigated experimentally in 5×5 rod bundles with prototypical mixing vane spacer grids (MVSGs). Miniaturized four-sensor conductivity probe (MFSCP) has been developed to measure the local two phase flow parameters, such as void fraction, interfacial area concentration (IAC), bubble velocity, and bubble chord length. The detailed local parameters in cap bubbly flow has been measured and analyzed, based on which the phase distribution characteristics have been analyzed for Group I and II bubbles in detail. Moreover, the axial development and the effect of MVSG have also been analyzed. Some interesting phenomenon have been observed: Group I void fraction profile is almost flat at the measuring line while Group II void fraction profile has the skewed peaks near the sub-channel center resulted for the limitation of rod walls and the strong secondary flow; Group I IAC is the main contribution to the total IAC, whose profile has the peaks near the rod gap due to the smaller bubble size near the rod walls. For current cap bubbly flow condition, the bubble velocity and size are the two determined factors for Group I IAC for the similar void fraction, while the void fraction is the key factor for Group II IAC for the larger bubble size. Furthermore, because of the shear force of MVSG and induced stronger turbulence, the large Group II bubbles break up into Group I bubbles and smaller Group II bubbles, which results in the smaller bubble size and larger Group I void fraction and IAC. According to the analysis on the axial development of local parameter profiles, the MVSG dissipation length might be larger than 19.4 L/Dh, but smaller than 32.1 L/Dh for current cap bubbly flow condition.
