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Natural Convection

Natural Convection. New terms Volumetric thermal expansion coefficient Grashof number Rayleigh number Buoyancy is the driving force Stable versus unstable conditions Nusselt number relationship for laminar free convection on hot or cold surface

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Natural Convection

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  1. Natural Convection • New terms • Volumetric thermal expansion coefficient • Grashof number • Rayleigh number • Buoyancy is the driving force • Stable versus unstable conditions • Nusselt number relationship for laminar free convection on hot or cold surface • Boundary layer impacts: laminar  turbulent

  2. Natural Convection Boundary Layer : Governing Equations • The difference between the two flows (forced flow and free flow) is that, in free convection, a major role is played by buoyancy forces. Very important • Consider the x-momentum equation. • As we know, , hence the x-pressure gradient in the boundary layer must equal that in the quiescent region outside the boundary layer.

  3. Pascal Law : Buoyancy force

  4. Dimensionless Similarity Parameter • Define new dimensionless parameter, • Grashof numberin natural convection is analogous to the Reynolds number in forced convection. • Grashof numberindicates the ratio of the buoyancy force to the viscous force. • Higher Gr number means increased natural convection flow natural forced

  5. u(x,y) g x y u v Laminar Free Convection on Vertical Surface • As y   : u = 0, T = T • As y  0 : u = 0, T = Ts • With little or no external driving flow, Re  0 and forced convection effects can be safely neglects

  6. Empirical Correlations Typical correlations for heat transfer coefficient developed from experimental data are expressed as: For Turbulent For Laminar

  7. Vertical Plate at constant Ts

  8. Alternative applicable to entire Rayleigh number range (for constant Ts) Vertical Cylinders D D • Use same correlations for vertical flat plate if: L

  9. Horizontal Plate Cold Plate (Ts < T) Hot Plate (Ts > T) Active Upper Surface Active Lower Surface

  10. Empirical Correlations : Horizontal Plate • Define the characteristic length, L as • Upper surface of heated plate, or Lower surface of cooled plate : • Lower surface of heated plate, or Upper surface of cooled plate : Note: Use fluid properties at the film temperature

  11. Empirical Correlations : Long Horizontal Cylinder • Very common geometry (pipes, wires) • For isothermal cylinder surface, use general form equation for computing Nusselt #

  12. RaD C n Constants for general Nusselt number Equation

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