NUMERICAL SIMULATION OF LAMINAR MIXED CONVECTION IN THE ENTRANCE REGION OF HORIZONTAL RECTANGULAR DUCTS

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Abstract
A laminar, three-dimensional, steady-state model has been solved to determine the nature and effect of thermally driven secondary flows in, a horizontal channel with heated top and bottom surfaces and insulated side walls. The secondary flow is characterized by ascending and descending thermals that form longitudinal vortex rolls and enhance the bottom surface heat transfer and channel friction factor by as much as 400 and 30%, respectively. Parametric calculations have been performed to determine the effect on hydrodynamic and thermal conditions of Reynolds number (100 ≤ Re ≤. 1000), Grashof number (2.5 × 105 ≤ Gr∗ ≤ 6.5 × 104), Prandtl number (Pr = 6.5 and 0.7), top-to-bottom surface heat flux ratio (−1.0 ≤ q1/qb ≤ 5.0), aspect ratio (1 ≤ A ≤ 10), and entrance velocity profile. Conditions near the bottom surface are characterized by mixed convection and are unaffected by heating at the top surface. Conversely, conditions at the top surface are dominated by forced convection and are unaffected by heating at the bottom surface.

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