Nonboiling Heat Transfer and Friction of Air/Water Mist Flow in a Square Duct With Orthogonal Ribs

Abstract

Heat transfer of air/water mist flow in a single-side heated vertical duct was experimentally investigated. The mist flow was produced by introducing fine dispersed water droplets into the air stream, and the water-air mass flow ratios were up to 15%. The Reynolds numbers of the air flow were 7900, 16,000, and 24,000. The rib spacing-to-height ratios were 10 and 20 in the current study. Mist flow cooling achieved higher heat transfer rates mainly because of the droplet deposition and liquid film formation on the heated surface. The heat transfer enhancement on the smooth surface by the mist flow was 4-6 times as high as the air flow. On the ribbed surface, a smaller rib spacing of 10 was preferred for air cooling, since the heat transfer enhancement by the flow reattachment was better utilized. However, the rib-induced secondary flow blew away the liquid films on the surface, and the heat transfer enhancement was degraded near the reattachment region for the mist cooling. A larger rib spacing-to-height ratio of 20 thus achieved higher heat transfer because of the liquid film formation beyond the reattachment region. The heat transfer enhancement on the ribbed surface using mist flow was 2.5-3.5 times as high as the air flow. The friction factor of the mist flow was two times as high as the air flow in the ribbed duct.