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Unformatted text preview: Chapter 9 Natural Convection Chapter 9 NATURAL CONVECTION Physical Mechanisms of Natural Convection 91C 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. 92C The convection heat transfer coefficient is usually higher in forced convection because of the higher fluid velocities involved. 93C 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. 94C 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. 95C 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. 96C 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 persons body. 97C 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. 98C There cannot be any natural convection heat transfer in a medium that experiences no change in volume with temperature. 99C 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. 910C 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. 91 Chapter 9 Natural Convection 911 The volume expansion coefficient is defined as P T  = 1 . For an ideal gas, P RT = or = P RT , and thus ( 29 ( 29 T T RT P T RT P T RT P P 1 1 1 1 / 1 2 = = =  =  = Natural Convection Over Surfaces 912C Rayleigh number is the product of the Grashof and Prandtl numbers....
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 Spring '08
 BENARD
 Heat Transfer

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