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

 Page 1


WHAT IS 
CONVECTION??
• Convection is the transfer of heat by the 
movement of a fluid (liquid or gas) between 
of different temperature. Warm air is less dense 
than cold air, and so convection currents can 
form in the presence of a temperature gradient
• Newton’s law of Cooling:
Q = h·A·(Ts- T) 
• Type of convection.
1. Free/natural convection
2. Force convection
Page 2


WHAT IS 
CONVECTION??
• Convection is the transfer of heat by the 
movement of a fluid (liquid or gas) between 
of different temperature. Warm air is less dense 
than cold air, and so convection currents can 
form in the presence of a temperature gradient
• Newton’s law of Cooling:
Q = h·A·(Ts- T) 
• Type of convection.
1. Free/natural convection
2. Force convection
? WHAT IS 
FREE/NATURAL 
CONVECTION?
• Natural convection is a mechanism, 
or type of heat transport, in which 
fluid motion is generated only by 
density differences in the fluid 
occurring due to temperature 
gradients, not by any external source 
(like a pump, fan, suction device, 
Page 3


WHAT IS 
CONVECTION??
• Convection is the transfer of heat by the 
movement of a fluid (liquid or gas) between 
of different temperature. Warm air is less dense 
than cold air, and so convection currents can 
form in the presence of a temperature gradient
• Newton’s law of Cooling:
Q = h·A·(Ts- T) 
• Type of convection.
1. Free/natural convection
2. Force convection
? WHAT IS 
FREE/NATURAL 
CONVECTION?
• Natural convection is a mechanism, 
or type of heat transport, in which 
fluid motion is generated only by 
density differences in the fluid 
occurring due to temperature 
gradients, not by any external source 
(like a pump, fan, suction device, 
DAILY LIFE EXAMPLE OF NATURAL CONVECTION 
Boiling water - The heat passes from 
the burner into the pot, heating the 
water at the bottom. ...
Radiator - Puts warm air out at the 
top and draws in cooler air at the 
bottom.
Steaming cup of hot tea - The steam 
is showing heat being transfered 
into the air.
Ice melting - Heat moves to the ice 
from the air.
Page 4


WHAT IS 
CONVECTION??
• Convection is the transfer of heat by the 
movement of a fluid (liquid or gas) between 
of different temperature. Warm air is less dense 
than cold air, and so convection currents can 
form in the presence of a temperature gradient
• Newton’s law of Cooling:
Q = h·A·(Ts- T) 
• Type of convection.
1. Free/natural convection
2. Force convection
? WHAT IS 
FREE/NATURAL 
CONVECTION?
• Natural convection is a mechanism, 
or type of heat transport, in which 
fluid motion is generated only by 
density differences in the fluid 
occurring due to temperature 
gradients, not by any external source 
(like a pump, fan, suction device, 
DAILY LIFE EXAMPLE OF NATURAL CONVECTION 
Boiling water - The heat passes from 
the burner into the pot, heating the 
water at the bottom. ...
Radiator - Puts warm air out at the 
top and draws in cooler air at the 
bottom.
Steaming cup of hot tea - The steam 
is showing heat being transfered 
into the air.
Ice melting - Heat moves to the ice 
from the air.
Hot air balloon - A heater inside the 
balloon heats the air and so the air 
moves upward. This causes the 
balloon to rise because the hot air 
gets trapped inside.
Frozen material thawing - Frozen food 
thaws more quickly under cold 
running water that if it is placed in 
water. The action of the running water 
transfers heat into the food faster.
Page 5


WHAT IS 
CONVECTION??
• Convection is the transfer of heat by the 
movement of a fluid (liquid or gas) between 
of different temperature. Warm air is less dense 
than cold air, and so convection currents can 
form in the presence of a temperature gradient
• Newton’s law of Cooling:
Q = h·A·(Ts- T) 
• Type of convection.
1. Free/natural convection
2. Force convection
? WHAT IS 
FREE/NATURAL 
CONVECTION?
• Natural convection is a mechanism, 
or type of heat transport, in which 
fluid motion is generated only by 
density differences in the fluid 
occurring due to temperature 
gradients, not by any external source 
(like a pump, fan, suction device, 
DAILY LIFE EXAMPLE OF NATURAL CONVECTION 
Boiling water - The heat passes from 
the burner into the pot, heating the 
water at the bottom. ...
Radiator - Puts warm air out at the 
top and draws in cooler air at the 
bottom.
Steaming cup of hot tea - The steam 
is showing heat being transfered 
into the air.
Ice melting - Heat moves to the ice 
from the air.
Hot air balloon - A heater inside the 
balloon heats the air and so the air 
moves upward. This causes the 
balloon to rise because the hot air 
gets trapped inside.
Frozen material thawing - Frozen food 
thaws more quickly under cold 
running water that if it is placed in 
water. The action of the running water 
transfers heat into the food faster.
NATURAL CONVECTION 
(THEORY)
• Here, The hot egg is exposed to 
cooler air.
• 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.
• the heat transfer that is enhanced as 
a result of this natural convection 
current is called natural convection 
heat transfer.