Radiation, Conduction and Convection, As A Continuity to Heat Transfer
|Radiation heat transfer, conduction heat transfer, and convective heat transfer. These are the three ways of how heat transfer could occur.|
Heat transfer can either,
- occur with radiation, conduction, or convection
- or, any possible combination between those three
There is no other mode of heat transfer that may occur, apart from those three independently, or in combination.
Say, what does each of those heat transfer modes mean?
Let us touch briefly on temperature:
Temperature page has explained that the absolute temperature of any matter, is proportional to kinetic energy of its particles.
That is, when the absolute temperature in Kelvin’s scale is 0, there will be no particle motion.
Please tell me where you’re getting at:
Now, if you recall the explanation on atoms basic building blocks, every atom is build by:
- electron/s with negative charge
- proton/s with positive charge
- and a neutron with no net charge
You would also remember that the electron/s is/are continuously orbiting the nucleus that contains proton/s and neutron/s – if the temperature is above absolute 0.
What does this suggest? It suggests that,
- there is an electric field within the particles
- and these electrically charged particles will create a magnetic field
As a result, these particles will emit two waves, consisting of an electrical wave, and a magnetic wave. The fields face will be perpendicular to each other.
This combined wave is also known as, the electro-magnetic wave.
The emitted wave, if exposed to another body of a lower temperature, will excite the particles and cause the temperature to raise, until equilibrium is reached. More on heat transfer concept here.
Radiant heat (or radiation) is born:
|Yep, radiation heat transfer do occur at temperatures as low as 1 oK. Reason being,|
- heat is a function of absolute temperature, mass and the mass’ specific heat value. More than 0 oK is some heat already
- radiation heat occurs due to presence of electric wave, and magnetic wave, with fields perpendicular to each other. Simply said, as long as there is particle motion, radiant heat is existent
What? Radiant heat starts at as low as 1 Kelvin? Even an ice at 0 oC (32 oF) is freezing cold.
Yep, cold to you, but most refrigerants will boil instantaneously even with ice present. This is due to the lower than 0 oC (32 oF) boiling temperatures of most refrigerants, at our atmospheric conditions.
Again, feeling of hot or cold is relative.
Some people would say. There is no cold, only absence of heat. That is true. However, our daily conversation will be quite boring wouldn’t it?
A: We have just come from the Himalayas. The "lower heat" (cool) atmosphere is really nice.
B: Wow, that is so "heat in absence" (cool)!
Just a very crude example.
The fact is, radiation heat do occur at temperatures above 0 Kelvin. The only difference is the magnitude.
|Stephan-Boltzmann has stated that radiant heat is proportional on the, absolute temperature of a body, to the power of 4 (T4).|
Since radiant heat is transmitted in wave form, heat transfer may occur through vacuum, and mostly in straight line.
That. Is how the sun transfers its heat. Through vacuum, to earth.
In straight line? Yes. Face you palm towards the sun. You’ll feel the heat as hand cells start to be excited.
Face that palm away, you will feel the heat on the back side of your hand.
The principle is, radiant heat uses electromagnetic wave to excite the particles, and raising the temperature.
It can raise the temperature of steel, air, water, anything, as long as the temperature difference exists.
It is a very effective way to get an almost instantaneous heat, from camp fire, or radiators.
Instant heat? There is such thing as relatively slower heat transfer?
Try conductive heat transfer (or conduction):
|Consider you’re in your house, and heat from the sun is almost unbearable outside.|
Initially you won’t feel the heat from the sun, but your house walls will be subject to heat transfer.
Your house’s external wall temperature will rise. Particles’ kinetic energy from external will be transferred towards inside.
|Eventually, the kinetic energy will be transferred to internal wall. Kinetic energy transferred, and wall temperature will rise.|
Radiant heat from the sun raises the external wall temperature, and that heat is transferred through a solid matter, towards an area of lower temperature, due to excitation of adjacent solid particles.
This type of heat transfer is known as the conduction heat transfer. It can only occur through a solid object.
You’ll feel the heat when you touch the wall.
How about for liquids and gases?
Convective heat transfer (or convection), is for fluids.
If radiation can heat surface of a body, and pass through vacuum, conduction can transfer heat through solid matter.
However, when liquid and gases are concerned, convection rules.
Continuing from our example of the wall.
|Now, the temperature of the wall inside the house has already went up. But it won't stop there, as there is temperature difference between the wall, and the air.|
The air particles will be excited and heated up. Hence, the expanding air will reduce in density. Reduction in density will cause it to move up.
The rising air will draw in cooler air from bottom. Now, heating of the ‘new’ air will start.
The cycle will continue, and continuous air movement will happen. This is known as free convection.
However, free convection is quite a slow process. Which is why human created a thing called fan to encourage rapid fluid circulation, and cooling. Therefore, forced convection comes into picture.
Application in air conditioners:
The knowledge of radiation heat transfer, conduction heat transfer and convection heat transfer needs to be used in designing and sizing air conditioners.
Knowledge of radiation is used to design tinted windows to reduce heat transfer from the sun.
Knowledge of conduction heat transfer is used in designing air conditioner tubes with high conductivity rate, for effective heat transfer with air.
Knowledge of convection heat transfer is used in designing fans to cool down the condenser, and deliver cool air to you, from the evaporator.
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