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Renin Angiotensin Mechanism.

Mechanisms of Heat Transfer.

 

  • Heat is a form of energy.

  • As per the law of thermodynamics whenever a physical or chemical transformation takes place the heat transfer occurs.

  • The transfer of heat takes place by many ways as follows.

  • When two objects at different temperatures are brought into thermal contact, heat flows from the object at higher temperature to the object at lower temperature. 

  • The mechanisms by which the heat may flow are 

    • conduction, 

    • convection and 

    • Radiation.

  1. Conduction

  • When heat flows through a body without any observable motion of matter, the type of heat flow is called conduction.

  • Mechanism :

    • In metallic solids, thermal conduction results from the unbound electrons (which is similar to the electrical conductivity).

    • In solids that are poor conductors of heat and in liquids the heat is conducted by the transport of momentum of the individual molecules along the temperature gradient.

    • In gases the conduction occurs by the random motion of the molecules, so that heat is “diffused” from hotter regions to the colder ones.

    • Examples:

      • Heat flows through the brick wall of a furnace or the metal wall of a tube.

  1. Convection

  • When heat flows by the transfer of matter, the type of heat flow is called convection. 

  • In this case the heat flows by actual mixing of warmer portions with the cooler portions of the same material. 

  • Convection is restricted to the flow of heat in fluids (i.e. gas and liquids). 

  • Heat flows through fluid by both conduction and convection and it is difficult to separate the two methods because of the eddies set up by the change of density with temperature.

  • Examples

    • Transfer of heat by the eddies of turbulent flow and by the current of warm air from a room heater flowing across the room.

    • Natural and forced convection

  • The forces used to create convection currents in fluids are two types.

  1. Natural convection:

  • When a fluid is heated the warmer part becomes lighter than the cooler part. 

  • Due to this difference in density the cooler (higher density) fluid moves downwards and the warmer (lighter density) moves upwards and thus forming convection current. 

  • Thus heat is transferred with mass. 

  • This method of heat transfer is called natural convection.

  1. Forced convection:

  • If the current (or movement of fluid) is caused not by the density difference but by some agitator or by some mechanical devices then the type of heat flow associated with it is called forced convection.

  1. Radiation

  • When heat is transferred through space by electromagnetic waves the type of flow of heat is called radiation heat flow.

  • Characteristics

    • Any solid body at any temperature above absolute zero radiates energy.

    • This radiation is an electromagnetic phenomenon and takes place without the necessity of any medium.

    • When radiation is passing through empty space, it is not transformed into heat or any other form of energy nor is it diverted from its path.  

    • If the radiation falls on a matter the radiation energy will be transmitted (i.e. pass through the matter), reflected or absorbed. 

    • Only the energy that is absorbed is converted into heat energy.

    • The approximate range of wavelengths for infra-red radiation (or heat rays) is 0.8 to 400 m.

    • In most of the industries, the thermal radiation corresponds to wavelengths from 0.8 to 25 m.

Commonly Asked Questions.

  1. Write a short note on “Mechanisms of Heat Transfer”.
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