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Unformatted text preview: Chapter 20 Natural Convection Chapter 20 NATURAL CONVECTION Physical Mechanisms of Natural Convection 20-1C Natural convection is the mode of heat transfer that occurs between a solid and a fluid which moves under the influence of natural means. Natural convection differs from forced convection in that fluid motion in natural convection is caused by natural effects such as buoyancy. 20-2C The convection heat transfer coefficient is usually higher in forced convection because of the higher fluid velocities involved. 20-3C The hot boiled egg in a spacecraft will cool faster when the spacecraft is on the ground since there is no gravity in space, and thus there will be no natural convection currents which is due to the buoyancy force. 20-4C The upward force exerted by a fluid on a body completely or partially immersed in it is called the buoyancy or “lifting” force. The buoyancy force is proportional to the density of the medium. Therefore, the buoyancy force is the largest in mercury, followed by in water, air, and the evacuated chamber. Note that in an evacuated chamber there will be no buoyancy force because of absence of any fluid in the medium. 20-5C The buoyancy force is proportional to the density of the medium, and thus is larger in sea water than it is in fresh water. Therefore, the hull of a ship will sink deeper in fresh water because of the smaller buoyancy force acting upwards. 20-6C A spring scale measures the “weight” force acting on it, and the person will weigh less in water because of the upward buoyancy force acting on the person’s body. 20-7C The greater the volume expansion coefficient, the greater the change in density with temperature, the greater the buoyancy force, and thus the greater the natural convection currents. 20-8C There cannot be any natural convection heat transfer in a medium that experiences no change in volume with temperature. 20-9C The lines on an interferometer photograph represent isotherms (constant temperature lines) for a gas, which correspond to the lines of constant density. Closely packed lines on a photograph represent a large temperature gradient. 20-10C The Grashof number represents the ratio of the buoyancy force to the viscous force acting on a fluid. The inertial forces in Reynolds number is replaced by the buoyancy forces in Grashof number. 20-1 Chapter 20 Natural Convection 20-11 The volume expansion coefficient is defined as P T - = ∂ ∂ρ ρ β 1 . For an ideal gas, P RT = ρ or ρ = P RT , and thus ( 29 ( 29 T T RT P T RT P T RT P P 1 1 1 1 / 1 2 = = = -- = - = ρ ρ ρ ρ ∂ ∂ ρ β Natural Convection Over Surfaces 20-12C Rayleigh number is the product of the Grashof and Prandtl numbers....
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This homework help was uploaded on 04/18/2008 for the course EML 3007 taught by Professor Chung during the Spring '08 term at University of Florida.
- Spring '08
- Heat Transfer