낮은 Dean number의 만곡 채널에서 표면 열전달에 관한 연구
A Study of surface heat transfer in a curved channel at low Dean numbers
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Heat transfer in transitional curved channel flow is investigated over a range of Dean numbers from 50 to 600. New mixed convection experimental data are presented for a channel with buoyancy induced motions orthogonal to the forced flow. The channel aspect ratio is 40, radius ratio is 0.979, and the ratio of half channel width to channel inner radius is 0.011 which indicates mild curvature. Measured Nusselt numbers illustrate behavior of four different flow regimes: pure natural convection, mixed convection with strong buoyancy, mixed convection with weak buoyancy, and pure forced convection. With fully developed thermal boundary layers, two regimes of mixed convection are observed. Differences in behavior in the curved and straight portions of the channel are described, along Nusselt number correlations for the two mixed convection regimes. The effects of various phenomena on the forced convection Nusselt numbers, and the influences of Dean vortex pairs and other important transitional phenomena are discussed. Forced convection Nusselt numbers in transitional curved channel flow without the influences of natural convection are determined from mixed convection measurements when buoyancy is relatively weak using a new procedure. Just after the influences of centrifugal instabilities impose themselves on the flow, Nusselt numbers on the concave surface become significantly higher˙ than ones measured at the same streamwise station on the convex surface. The most important of these occur due to the twisting secondary instability which causes drastic increases of Nusselt numbers on concave and convex surfaces as the Dean number increase from 150 to 200, and as x/d increases above 158.
Abstract
Heat transfer in transitional curved channel flow is investigated over a range of Dean numbers from 50 to 600. New mixed convection experimental data are presented for a channel with buoyancy induced motions orthogonal to the forced flow. The channel aspect ratio is 40, radius ratio is 0.979, and the ratio of half channel width to channel inner radius is 0.011 which indicates mild curvature. Measured Nusselt numbers illustrate behavior of four different flow regimes: pure natural convection, mixed convection with strong buoyancy, mixed convection with weak buoyancy, and pure forced convection. With fully developed thermal boundary layers, two regimes of mixed convection are observed. Differences in behavior in the curved and straight portions of the channel are described, along Nusselt number correlations for the two mixed convection regimes. The effects of various phenomena on the forced convection Nusselt numbers, and the influences of Dean vortex pairs and other important transitional phenomena are discussed. Forced convection Nusselt numbers in transitional curved channel flow without the influences of natural convection are determined from mixed convection measurements when buoyancy is relatively weak using a new procedure. Just after the influences of centrifugal instabilities impose themselves on the flow, Nusselt numbers on the concave surface become significantly higher˙ than ones measured at the same streamwise station on the convex surface. The most important of these occur due to the twisting secondary instability which causes drastic increases of Nusselt numbers on concave and convex surfaces as the Dean number increase from 150 to 200, and as x/d increases above 158.