convection-why hot body becomes colder after some time?-how convection happens?-difference between conduction and convection-types of convection-
Convection:
Did you ever notice when we leave a hot cup of tea for some time it did not remains hot as before? Why?
It is due to heat being transferred from the cup to the surrounding. As we already know if the temperature difference between two bodies is present then heat transfers from the hot to the cold body. Heat can transfer by different modes. In the case of a hot cup, heat is transferred by convection. Due to the motion of air, warm air rises, and cold air replaced it that’s why after some time hot cup of tea becomes relatively cold.
What is convection?
Convection is a mode of heat transfer. In convection heat transfer between a solid surface and the adjacent liquid or gas that is in motion.
Convection involves the combined effect of conduction and fluid motion. That’s why convection is also known as conduction with fluid flow.
How convection happens?
 |
| cooling of block by convection. |
Why does warm air rise and be replaced by cool air?
Warm or hot air is less dense than cool air
because molecules in hot air move faster and they are moving apart so there is
more space between the molecules that’s why warm air (less dense) floats upward
and is replaced by cool air (denser).
 |
| warm air rises and replaced by cool air. |
Heat transfer through a solid is always by conduction since the molecules of the solid remains at a fixed position but the heat transfer through the liquid or gas can be by conduction or convection depending on the presence of any bulk fluid motion. If the bulk fluid motion is present, then heat is transferred by convection however in the absence of bulk fluid motion heat transferred by conduction.
Why heat transfer between the block surface and surrounding air (in absence of any bulk fluid motion) is by conduction?
If the temperature difference between the
block surface and surrounding air is not large enough to overcome the
resistance of air to move and thus initiate natural convection (that means air
did not move) then the heat transfer between the block and surrounding air is
by conduction.
Difference between conduction and convection?
Conduction and convection are similar in
that both mechanisms require the presence of a material medium.
But they are different in that convection
requires the presence of fluid motion.
Example of convection:
An example of convection is land and sea
breezes.
 |
| Sea breeze and land breeze. |
Sea breeze occurs during the daytime. The sun heats both land and sea surface. Water has greater heat capacity that’s why it absorbs much of the sun’s energy and gets warmed up much slower than land. That’s why daytime the temperature on land is higher than at sea. At seashores, the warm air having low pressure expands and rises and is replaced by cooler denser air from the sea surface.
Land breeze occurs during the night when
the situation reverses. As the sun sets the sea and land start to cool down.
Due to the capacity, the land cools quickly so the sea temperature is
relatively high and has high-pressure air (denser) and the sea’s air expands
and rises this cause the flow of the cool breeze offshore.
Types of convection
Based on how the fluid motion is initiated,
convection is classified into four types.
1. Natural
(or free) convection
2. Forced
convection
3. External
convection
4. Internal
convection
Natural or free convection
Fluid motion is caused by natural means, such as the buoyancy effect, the rise of warm fluid, and the fall of the cooler fluid due to density difference.
For example, oceanic winds.
Forced convection
The fluid is forced to flow over the
surface of the hot body by external means.
For example, cooling any hot surface with a
fan.
 |
| difference between natural and forced convection. |
External convection
Fluid is forced to flow over the surface.
Internal convection
Fluid is forced to flow in the pipe.
 |
| internal convection. |
Newton’s law of cooling
In convective heat transfer, the forced
convection is related to newtons law of cooling where properties of fluid do
not vary strongly with temperature.
Newton’s law of cooling states that,
“The rate of convective heat transfer is
proportional to temperature difference.”
 |
| Newton's law of cooling. |
Convection heat transfer coefficient (h)
Convective heat transfer depends upon
1. properties of fluid-like,
• Fluid
density
• Fluid
dynamic viscosity
• Fluid
velocity
• Fluid
thermal conductivity
• Specific
heat capacity
2. And it also depends on the nature or type
of fluid flow and geometry of the surface.
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