Page 1 NATURAL CONVECTION Page 2 NATURAL CONVECTION 2 Objectives Understand the physical mechanism of natural convection Derive the governing equations of natural convection, and obtain the dimensionless Grashof number by nondimensionalizing them Evaluate the Nusselt number for natural convection associated with vertical, horizontal, and inclined plates as well as cylinders and spheres Examine natural convection from finned surfaces, and determine the optimum fin spacing Analyze natural convection inside enclosures such as double-pane windows Consider combined natural and forced convection, and assess the relative importance of each mode. Page 3 NATURAL CONVECTION 2 Objectives Understand the physical mechanism of natural convection Derive the governing equations of natural convection, and obtain the dimensionless Grashof number by nondimensionalizing them Evaluate the Nusselt number for natural convection associated with vertical, horizontal, and inclined plates as well as cylinders and spheres Examine natural convection from finned surfaces, and determine the optimum fin spacing Analyze natural convection inside enclosures such as double-pane windows Consider combined natural and forced convection, and assess the relative importance of each mode. 3 PHYSICAL MECHANISM OF NATURAL CONVECTION Many familiar heat transfer applications involve natural convection as the primary mechanism of heat transfer. Examples? Natural convection in gases is usually accompanied by radiation of comparable magnitude except for low-emissivity surfaces. The motion that results from the continual replacement of the heated air in the vicinity of the egg by the cooler air nearby is called a natural convection current, and the heat transfer that is enhanced as a result of this current is called natural convection heat transfer. The cooling of a boiled egg in a cooler environment by natural convection. The warming up of a cold drink in a warmer environment by natural convection. Page 4 NATURAL CONVECTION 2 Objectives Understand the physical mechanism of natural convection Derive the governing equations of natural convection, and obtain the dimensionless Grashof number by nondimensionalizing them Evaluate the Nusselt number for natural convection associated with vertical, horizontal, and inclined plates as well as cylinders and spheres Examine natural convection from finned surfaces, and determine the optimum fin spacing Analyze natural convection inside enclosures such as double-pane windows Consider combined natural and forced convection, and assess the relative importance of each mode. 3 PHYSICAL MECHANISM OF NATURAL CONVECTION Many familiar heat transfer applications involve natural convection as the primary mechanism of heat transfer. Examples? Natural convection in gases is usually accompanied by radiation of comparable magnitude except for low-emissivity surfaces. The motion that results from the continual replacement of the heated air in the vicinity of the egg by the cooler air nearby is called a natural convection current, and the heat transfer that is enhanced as a result of this current is called natural convection heat transfer. The cooling of a boiled egg in a cooler environment by natural convection. The warming up of a cold drink in a warmer environment by natural convection. 4 Buoyancy force: The upward force exerted by a fluid on a body completely or partially immersed in it in a gravitational field. The magnitude of the buoyancy force is equal to the weight of the fluid displaced by the body. The net vertical force acting on a body Archimedes’ principle: A body immersed in a fluid will experience a “weight loss” in an amount equal to the weight of the fluid it displaces. The “chimney effect” that induces the upward flow of hot combustion gases through a chimney is due to the buoyancy effect. Page 5 NATURAL CONVECTION 2 Objectives Understand the physical mechanism of natural convection Derive the governing equations of natural convection, and obtain the dimensionless Grashof number by nondimensionalizing them Evaluate the Nusselt number for natural convection associated with vertical, horizontal, and inclined plates as well as cylinders and spheres Examine natural convection from finned surfaces, and determine the optimum fin spacing Analyze natural convection inside enclosures such as double-pane windows Consider combined natural and forced convection, and assess the relative importance of each mode. 3 PHYSICAL MECHANISM OF NATURAL CONVECTION Many familiar heat transfer applications involve natural convection as the primary mechanism of heat transfer. Examples? Natural convection in gases is usually accompanied by radiation of comparable magnitude except for low-emissivity surfaces. The motion that results from the continual replacement of the heated air in the vicinity of the egg by the cooler air nearby is called a natural convection current, and the heat transfer that is enhanced as a result of this current is called natural convection heat transfer. The cooling of a boiled egg in a cooler environment by natural convection. The warming up of a cold drink in a warmer environment by natural convection. 4 Buoyancy force: The upward force exerted by a fluid on a body completely or partially immersed in it in a gravitational field. The magnitude of the buoyancy force is equal to the weight of the fluid displaced by the body. The net vertical force acting on a body Archimedes’ principle: A body immersed in a fluid will experience a “weight loss” in an amount equal to the weight of the fluid it displaces. The “chimney effect” that induces the upward flow of hot combustion gases through a chimney is due to the buoyancy effect. 5 The coefficient of volume expansion is a measure of the change in volume of a substance with temperature at constant pressure. Volume expansion coefficient: Variation of the density of a fluid with temperature at constant pressure. ideal gas The larger the temperature difference between the fluid adjacent to a hot (or cold) surface and the fluid away from it, the larger the buoyancy force and the stronger the natural convection currents, and thus the higher the heat transfer rate.Read More

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