Lecture 13

# Lecture 13 - Free Convection Free Convection General...

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Unformatted text preview: Free Convection: Free Convection: General Considerations General Considerations and Results for and Results for Vertical and Horizontal Plates Vertical and Horizontal Plates Chapter 9 Chapter 9 Sections 9.1 through 9.6.2, 9.9 Sections 9.1 through 9.6.2, 9.9 General Considerations General Considerations General Considerations • Free convection refers to fluid motion induced by buoyancy forces . • Buoyancy forces may arise in a fluid for which there are density gradients and a body force that is proportional to density . • In heat transfer, density gradients are due to temperature gradients and the body force is gravitational. • Stable and Unstable Temperature Gradients General Considerations (cont) General Considerations (cont) • Free Boundary Flows Occur in an extensive (in principle, infinite), quiescent (motionless at locations far from the source of buoyancy) fluid. Plumes and Buoyant Jets : • Free Convection Boundary Layers Boundary layer flow on a heated or cooled surface induced by buoyancy forces . ( 29 s T T T T General Considerations (cont) General Considerations (cont) • Pertinent Dimensionless Parameters Grashof Number: ( 29 3 2 Buoyancy Force Viscous Force s L g T T L Gr β ν- = : Rayleigh Number : ( 29 3 Pr s L L g T T L Ra Gr β να- = = characteristic length of surface L T (a ther ther mody mal expa namic pr nsion coefficien operty of the fl d) t ui β 1 ρ β ρ = - ? p T Liquids: Tables A.5, A.6 β ( 29 Perfect Gas: =1/ K T β General Considerations (cont) General Considerations (cont) • Mixed Convection A condition for which forced and free convection effects are comparable....
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Lecture 13 - Free Convection Free Convection General...

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