Heat is the form of energy that can be transferred from one system to another due to a temperature difference or gradient. The science which deals with the rates of such energy transfer is known as “heat transfer”. Heat is a vector quantity, flowing in the direction of decreasing temp, with a negative temp. gradient.
Basics of Heat Transfer
In the simplest of terms, the discipline of heat transfer is concerned with only two things: temperature, and the flow of heat. Temperature represents the amount of thermal energy available, whereas heat flow represents the movement of thermal energy from place to place.
On a microscopic scale, thermal energy is related to the kinetic energy of molecules. The greater a material’s temperature, the greater the thermal agitation of its constituent molecules (manifested both in linear motion and vibrational modes). It is natural for regions containing greater molecular kinetic energy to pass this energy to regions with less kinetic energy.
Several material properties serve to modulate the heat tranfered between two regions at differing temperatures. Examples include thermal conductivities, specific heats, material densities, fluid velocities, fluid viscosities, surface emissivities, and more. Taken together, these properties serve to make the solution of many heat transfer problems an involved process.
Heat Transfer
According to thermodynamic systems, heat transfer is defined as
The motion of heat across the border of the system due to a difference in temperature among the device and its surroundings. Interestingly, the difference in temperature is said to be a ‘potential’ that causes the transfer of heat from one point to another. Besides, heat is also known as flux.
Heat Transfer Methods
Heat can travel shift from one place to another in several ways. The different modes of heat transfer include:
- Conduction
- Convection
- Radiation
Meanwhile, if the temperature difference exists between the two systems, heat will find a way to transfer from the higher to the lower system.
Conduction of Heat Transfer
The process in which heat flows from objects with higher temperature to objects with lower temperature.
An area of higher kinetic energy transfers thermal energy towards the decrease kinetic energy area. High-speed particles clash with particles transferring at a slow speed, as a result, slow speed particles increase their kinetic energy. This is a typical form of heat transfer and takes location through physical contact. Conduction is also known as thermal conduction or heat conduction.
Conduction Equation
The coefficient of thermal conductivity shows that a metal body conducts heat better when it comes to conduction. The rate of conduction can be calculated by the following equation:
Q = [K.A.(Thot−Tcold)]d
Where,
- Q is the transfer of heat per unit time
- K is the thermal conductivity of the body
- A is the area of heat transfer
- Thot is the temperature of the hot region
- Tcold is the temperature of the cold region
- d is the thickness of the body
Conduction Examples
Following are the examples of conduction:
- Ironing of clothes is an example of conduction wherein the heat is conducted from the iron to the clothes.
- Heat is transferred from hands to ice cube resulting in the melting of an ice dice while held in hands.
- Heat conduction through the sand on the beaches. This can be experienced during summers. Sand is a good conductor of heat.
Convection of Heat Transfer
The movement of fluid molecules from higher temperature regions to lower temperature regions.
Convection Equation
As the temperature of the liquid increases, the liquid’s volume also has to increase by the same factor and this effect is known as displacement. The equation to calculate the rate of convection is as follows:
Q = hc ∙ A ∙ (Ts – Tf)
Where,
- Q is the heat transferred per unit time
- Hc is the coefficient of convective heat transfer
- A is the area of heat transfer
- Ts is the surface temperature
- Tf is the fluid temperature
Convection Examples
Examples of convection include:
- Boiling of water, that is molecules that are denser move at the bottom while the molecules which are less dense move upwards resulting in the circular motion of the molecules so that water gets heated.
- Warm water around the equator moves towards the poles while cooler water at the poles moves towards the equator.
- Blood circulation in warm-blooded animals takes place with the help of convection, thereby regulating the body temperature.
Radiation of Heat Transfer
Thermal radiation is generated by the emission of electromagnetic waves. These waves carry away the energy from the emitting body. Radiation takes place via a vacuum or transparent medium which can be either solid or liquid. Thermal radiation is the result of the random movement of molecules in the matter. The motion of charged electrons and protons is responsible for the emission of electromagnetic radiation.
Radiation Equation
As temperature rises, the wavelengths in the spectra of the radiation emitted decreases and shorter wavelengths radiations are emitted. Thermal radiation can be calculated by Stefan-Boltzmann law:
P = e ∙ σ ∙ A· (Tr – Tc)4
Where,
- P is the net power of radiation
- A is the area of radiation
- Tr is the radiator temperature
- Tc is the surrounding temperature
- e is emissivity and σ is Stefan’s constant
Radiation Example
Following are the examples of radiation:
- Microwave radiation emitted in the oven is an example of radiation.
- UV rays coming from the sun is an example of radiation.
- The release of alpha particles during the decaying of Uranium-238 into Thorium-234 is an example of radiation.
Unit of Heat Transfer
| SI system | Joule |
| MKS system | cal |
| Rate of transfer of heat | KW |
