Flow Characteristics in a 3×3 Rod Bundle under Flooding Conditions

Experiments on countercurrent flows of air and water in a 3×3 rod bundle were carried out, and countercurrent flow limitation (CCFL), pressure gradients dP/dz and void fractions α were measured under flooding conditions. Flooding with smooth and thin liquid film occurred at the top when the gas volume flux JG was small, whereas flooding with rough and thick liquid film took place at the bottom for medium and large JG. This behavior was similar to flooding in a vertical pipe. CCFL characteristics were expressed by Kutateladze parameters and agreed with those for the upper tie plates in BWR fuel bundles. The volume flux JL of falling water in the bundle was larger than JL in a vertical pipe, but it was smaller than JL through horizontal perforated plates without rods. α under flooding conditions were lower than α under stagnant water (JL = 0) conditions in the region of JG > 4 m/s. The wall friction and interfacial friction factors were obtained from the data and discussed.