9 - Natural Convection Flashcards
What is natural convection in gases usually accompanied by?
Radiation of comparable magnitude (except for low-emissivity surfaces)
Define buoyancy force
The upward force exerted by a fluid on a body completely or partially immersed in it in a gravitational field
What is the magnitude of the buoyancy force equal to?
Weight of fluid displaced by the 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
Define volume expansion coefficient
Variation of the density of a fluid with temperature at constant pressure
What is flow rate in natural convection determined by?
Dynamic balance of buoyancy and friction
Where is the fluid velocity zero on a velocity profile for natural convection?
Outer edge of boundary layer
Surface of plate
What is the Grashof number?
Represents the natural convection effects in momentum equation
Provides main criterion in determining whether the fluid flow is laminar or turbulent
Measure of relative magnitudes of the buoyancy force and the opposing viscous force acting on the fluid
When are natural convection effects negligible?
Gr/Re^2 «_space;1
When does free convection dominate and forced convection effects become negligible?
Gr/Re^2»_space; 1
When are both free and forced convection effects significant and must be considered?
Gr/Re^2 = 1
Grashof number for vertical plates
Around 10^9
What does natural convection heat transfer on a surface depend on?
Surface geometry
Orientation
Variation of temperature on surface
Thermophysical properties of fluid involved
What do the constants C and n depend on?
Surface geometry
Flow regime
What is the usual value of n for laminar and turbulent flows?
1/4 for laminar
1/3 for turbulent
