| Budget Amount *help |
¥6,600,000 (Direct Cost: ¥6,600,000)
Fiscal Year 1986: ¥3,000,000 (Direct Cost: ¥3,000,000)
Fiscal Year 1985: ¥3,600,000 (Direct Cost: ¥3,600,000)
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| Research Abstract |
When forced and natural convection appear combined together, it is of prime interest to discriminate which convection regime is dominant as well as to resolve how much the heat-transfer coefficient contributes. This research deals with the uniformly heated upward flow in a tube, which seems to be important from theoretical as well as practical viewpoints.
In experiments, the Grashof number was varied over a wide range, by using pressurized nitrogen gas as a test fluid. First, the fully developed heat transfer in a downstream enough region was investigated. As the Grashof number increased from zero under a constant Reynolds number, the heat-transfer coefficient decreased in the mixed-convection region and then turned to increase in the natural convection region. A regime map for forced, mixed, and natural convection was drawn in the plane of Grashof number vs. Reynolds number. In the mixed-convection regime, laminarization of the flow at a Reynolds number of 3000 was demonstrated by a ho
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t-wire anemometry. Secondly, streamwise variations of the heat-transfer coefficient were measured in the thermal-entry region which was following the fully developed isothermal flow. In the mixed-convection regime, the local impairment of the heat-transfer coefficient at a certain streamwise position was demonstrated. This position with the minimum heat-transfer coefficient was quantitatively grasped.
Two-equation models of turbulence were applied to these problems. The predicted regime map, including discrimination between turbulent and laminar flow, as well as the fully-developed heat-transfer coefficient in each regime agreed well with the experimental results. Two-dimensional calculations in the thermal-entry region could reproduce the local impairment of heat-transfer coefficient in the mixed-convection regime as well as the longitudinal variations of heat-transfer coefficient quantitatively in good agreement with the experimental results except for the immediately downstream region of the minimum heat-transfer point.
In the turbulent mixed convection, the buoyancy force makes the shear stress rapidly decrease near the wall so that the turbulence-energy production is suppressed. Resultant decrease in the turbulence energy causes the decrease in the heat-transfer coefficient and eventually the complete laminarization of the flow. Less
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