Axial development of gas-liquid flow regime maps in a vertical 5 × 5 rod bundle with prototypic spacer grids

In relation to the development of flow regime criteria for rod bundle geometry, this paper provides several experimental flow regime maps at different axial locations with simple and prototypic spacer grids, respectively. Experiments with vertical adiabatic air-water flow were conducted under an atmospheric pressure condition. The experiment facility was composed of 5 × 5 rod arranged on a square pitch in a square casing with 9.5 mm outside diameter and 12.6 mm pitch. The inlet superficial gas velocities range from 0.01 to 8.69 m/s, while inlet superficial liquid velocities range from 0.06 to 2.51 m/s. New flow regime maps were acquired at three axial locations of z/D_H = 21.9, 58.4 and 107 through high-speed camera images. The experimental maps obtained were compared with the existing analytical models of flow regime transition criteria for vertical rod bundles. A reasonable agreement has been obtained between measured flow regime maps and models of the existing flow regime transition criteria for rod bundles at fully developed flow conditions. The different effects of two types of spacer grids on the flow regime transition were also discussed in this paper.