[en] The Westinghouse flow mixer (W-4) air-water experiment is the ninth in a series designed to improve understanding of flow behavior in the upper end box, UCSP, and upper plenum regions of a PWR during the reflood phase of a loss-of-coolant accident. The test apparatus consists of a Plexiglas vessel fitted with internals simulating the upper fuel, upper end box, UCSP, and upper plenum regions corresponding to a single fuel assembly in a PWR. Vertical dimensions are preserved but since there is only one fuel assembly simulated the flow field is primarily vertical. Air is injected into the bottom of the test section and a froth is allowed to build up and reach steady state in the upper plenum region by injecting water into the upper plenum and core regions of the test vessel. Measurements of the injection flows and water fallback and carryout flows are then made to provide CCFL data. Motion pictures are also taken at selected data points to provide visual documentation of the flow patterns obtained. Also, at selected data points, measurements of the water storage in the upper plenum are made. For the Westinghouse flow mixer configuration, a total of 31 steady state data points were taken during September 1978. The tests were performed at ambient temperature and approximately 48 kPa pressure