Sub-channel flow regime maps in vertical rod bundles with spacer grids

The accurate prediction of thermal-hydraulic parameters is based on the flow regime maps for rod bundle, which is important for the safety of nuclear reactor. An air-water two phase flow experiment has been performed to study the sub-channel flow regime (SCFR) maps in 5 × 5 rod bundles section with two distinct spacer grids, including simplified spacer grid (SSG) and mixing vane spacer grid (MVSG). To obtain the objective SCFR maps in rod bundles, a sub-channel impedance void meter is newly developed to measure the time series void fraction in sub-channels. Besides, the random forest clustering algorithm has been adopted to identify the sub-channel flow regimes objectively based on a training sample and 13 selected feature values of time series void fraction. The feature values include mean value, standard deviation value, sample entropy value and 10 proportion values. In this way, the objective sub-channel flow regime maps are obtained at four different locations with different spacer grids. Distinct features have been observed for different SCFRs. As for the SCFR transitions in different sub-channels over the same cross-section, almost all of them arise in the inner sub-channel firstly for the effect of casing tube. Moreover, the dissipation length of spacer grid is larger than 19.5 L/D. In the influencing region of spacer grid, the transition from cap bubbly to cap turbulent flow occurs in the corner sub-channel at 19.5 L/D downstream of spacer grid firstly and then at 6.8 L/D for low liquid velocity, while firstly occurs at 6.8 L/D for high liquid velocity. The magnitude of the spacer grid effect on SCFR transition depends on the superficial liquid and gas velocity, as well as the structure of spacer grid for current flow conditions. Only Liu and Hibiki's model is applicable for the transition from bubbly to cap bubbly flow in sub-channel. Therefore, new transition models or correlations should be developed for other sub-channel flow regime transitions.