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 - = : Rayleigh Number : ( 29 3 Pr s L L g T T L Ra Gr - = = characteristic length of surface L T (a ther ther mody mal expa namic pr nsion coefficien operty of the fl d) t ui 1 = - ? p T Liquids: Tables A.5, A.6 ( 29 Perfect Gas: =1/ K T 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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