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During the refill stage of a Loss of Coolant Accident (LOCA) in Pressurised Water Reactor (PWR) the effectiveness with which the emergency coolant penetrates to the lower plenum, and hence to the core, is of paramount importance. This thesis presents the results of experimental and theoretical work carried out at the University of Strathclyde on two-1/10 scale planar test sections of a PWR downcomer annulus. The experiments involved the countercurrent flows of air and water and the data were compared with existing flooding correlations for tubes. The comparison revealed the inadequacy of the existing correlations. In the Phase 1 test section arrangement, it was found experimentally that, as the inlet air flowed upwards against two opposing waterfalls, an increase in air flowrate caused the waterfalls to move closer together until a critical air flowrate was reached where the waterfalls collapsed. A theoretical model was also developed to define this collapse condition which was shown to be-analogous to the choked flow of air through a nozzle whose cross sectional area varied with pressure. Whilst this phenomenon was not directly related to the PWR refill problem, it was an interesting feature in its own right. Use was made of previous experimental results for steam-water mixtures on similar test sections, in conjunction with the present air-water data, to study condensation effects. Here the non-equilibrium effects were isolated and correlated against the dependent parameters of inlet water flowrate, inlet subcooling and downcomer wall temperature. A theoretical model for the condensation process was developed which gave good qualitative and quantitative agreement with experiment. Its superiority over a current BCL correlation is demonstrated.