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A series of seven rack storage fire tests was conducted, using four pendent fast-response prototype sprinklers to determine the sprinkler discharge characteristics necessary for suppressing four-tier array rack storage fires under a 9.14-m high ceiling. The sprinklers had a nominal K-factor of either 11 or 14, and the sprinkler discharge pressure was maintained at 345 kPa. The commodities used as fuel consisted of polystyrene cups packaged in compartmented cartons. The commodities were arranged in double-row steel racks, two pallet-loads wide, two pallet-loads deep and four tiers high. Three different ignition locations with respect to sprinklers were employed. Sprinkler water distribution under a no-fire condition and spray center-core thrust force were identified as key parameters of sprinkler discharge characteristics pertaining to sprinkler fire-suppression ability. The relationships among the fire size at first sprinkler actuation, plume momentum flux, Required Delivered Density, sprinkler water distribution and spray center-core thrust force, with regard to suppression of the rack storage fires, were explored. For fire tests with ignition directly under a sprinkler, it was highly desirable for the sprinkler spray to overpower the fire plume to achieve efficient delivery of sprinkler water to the fire source. In the fire tests when the spray center-core thrust force was greater than the plume momentum flux and the average water flux over the top surface of the f u e l array under a no-fire condition was greater than the Required Delivered Density, fire suppression was achieved. For fire tests with ignition centered below either two or four sprinklers, the fire plume was largely confined to the center flue of the fuel array, and most of the sprinkler water projected toward the top surface of the fuel array reached the top surface without passing through the fire plume. Therefore, the measured water flux over the top surface under a no-fire condition was expected to be close to that delivered during a fire. Fire suppression in these tests resulted from contributions of both the sprinkler water reaching the top surface and the side exposed surfaces of the fuel array. When the average water flux over the top surface under a no-fire condition was greater than the Required Delivered Density, fire suppression was achieved.