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TRANSPORT OF HIGH BOILING POINT FIRE SUPPRESSANTS IN A DROPLET-LADEN HOMOGENEOUS TURBULENT FLOW PAST A CYLINDER

Liquid agent transport was investigated around unheated and heated horizontal cylinders (to a near-surface temperature of approximately 423 K, i.e., well above the water boiling point) under ambient conditions. Experimental results are presented for a well-characterized, droplet-laden homogenous turbulent flow field, using water, methoxy-nonafluorobutane (i.e., HFE-7100, C4F9OCH3, with a boiling point of 334 K), and 1-methoxyheptafluoropropane (i.e., HFE-7000, C3F7OCH3, with a boiling point of 307 K). Phase Doppler interferometry and visualization techniques were used to explore the thermal effects on spray surface impingement, vaporization, and transport around and downstream behind the cylinder by providing information on droplet size and velocity in the vicinity of the cylinder. For water, results indicated that impinging droplets larger than about 35 µm generally coat the unheated cylinder surface, with few droplets rebounding back into the free stream. Downstream, in the wake region of the cylinder, smaller size droplets (generally, of less than 35 µm) are entrained into the recirculation zone. Heat transfer reduces droplet mean size and velocity significantly in the vicinity of the heated cylinder. For the two HFE agents, liquid coating and dripping (observed for water) were eliminated due to vaporization. Droplet mean size increases and velocity decreases with increasing agent boiling point. These variations may also be explained by the changes in agent physical properties. It is improbable that shattering occurs for the droplet sizes and velocities encountered for the given operating conditions, although it could conceivably occur for a few individual impinging droplets.