PPT - Natural Convection | Heat Transfer - Mechanical Engineering PDF Download

 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.