Heat Transfer || Definition, Modes, Conduction, Convection & Radiation

Heat Transfer

  • The term heat is taken as synonymous to warm vitality.
  • This utilization has its birthplace in the recorded understanding of warmth as a liquid ( caloric ) that can be moved by different causes, and that is additionally regular in the language of laymen and regular daily existence.
  • Heat transfer consistently moves  from a region of high temperature to another region of lower temperature.
  • It changes the inside vitality of the two frameworks required by the principal law of thermodynamics.
  • The trading of dynamic vitality of particles through the limit between two frameworks which are at various temperatures from one another or from their environment.
  • The second law of thermodynamics characterizes the idea of thermodynamic entropy, by quantifiable warmth move.
  • Thermal equilibrium is reached when all involves bodies and the surroundings reach the same temperature.

Definition of Heat Transfer

  • It is defined as the transmission of energy from one region to another region to temperature difference.

Modes of Heat Transfer

  • There are three modes of heat transfer namely :-

1. Conduction

2. Convection

3. Radiation

1. Conduction

  • Conduction is the flow of heat in a substance due to the exchange of energy between molecules having more energy and less energy.
  • In Conduction, energy exchange takes place by the kinematics motion or direct impact of molecules.
  • Pure conduction is found only in solids.

Mechanism 

  • Lattic vibration ( 70 % ).
  • Due to the free electron ( 30 % ).

Fourier’s Law of Heat Conduction

  • It states that the rate of heat conduction through plain layer to the solid body is proportion to the temperature difference across the layer & heat transfer area inversely proportional to the thickness of layer. 

             Q = – KA × ( dt / dx )

            K = Thermal conductivity

        A = Area Square meter

      ( dt / dx ) = Temperature Gradient

Thermal conductivity

  • Thermal conductivity is the ability of the material to conduct heat.
  • Its unit is W / m K or °C.

Representative values of some thermal conductivity material

  • For Solid
Metal K ( W / m K ) State
Siliver 429 20°C
Copper 401 20°C
Pure Cu 380 20°C
Brass 110 20°C
Steel 54 20°C
Stainless Steel 16 20°C
Non-metal
Asbestos 0.23 20°C
Plastic 0.58 20°C
Wood 0.17 20°C
  • For Liquid
Liquid K ( W / m K ) State
Water 0.60 20°C
Light Oil 0.14 20°C
  • For Gases
Gases K ( W / m K ) State
Dry air 0.026 20°C
Steam 0.025 20°C

 

2. Convection

  • The transfer of energy from one region to another due to the macroscopic bulk motion in a fluid, aeeded onto the energy transfer by conduction is called heat transfer by convection.
  • It is possible only in the pressure of fluid medium.

Newton’s Law of Cooling or Convection Law

  • It states that the rate of heat flux transfer from surface to fluid is directly proportional to the difference in temperature.

             q = hA ( Ts – T∞ )

Ts = Temperature of fluid

T∞ = Temperature of the surface

3. Radiation

  • The heat transfer from one body to another without any transmitting medium is known as Radiation.
  • It is an electromagnetic wave phenomenon.

Properties of Radiation / Characteristics

  • It doesn’t require any medium.
  • Rate of emission increase with temperature level.
  • It travel with the speed of light.

Fundamental Law of Heat Transfer

  • There are two law of heat transfer :

1. Fundamental law

i. Law of conversation of mass

ii. Newton Law of motion

iii. Law of Thermodynamics

2. Subsidiary law

i. Fourier Law of Heat Conduction

ii. Newton’s Law of Cooling

iv. Law of Radiation

 

Leave a Reply

Your email address will not be published. Required fields are marked *