Introduction to heat transfer

HEAT TRANSFER                                                                                       Heat transfer, also referred to simply as heat, is the movement of thermal energy from one thing to another thing of different temperature. The rate of heat transfer is dependent on the temperatures of the systems and the properties of the intervening medium through which the heat is transferred. Modes of heat transfer:

  1. Conduction
  2. Convection
  3. Radiation

Conduction or diffusion

Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. Conduction may occur in the liquid or gas medium if the velocity of the particles are about to zero.

Convection

The transfer of energy between an object and its environment, due to fluid motion. In convection, fluid particles absorb heat from a heated body and change position .The particles of liquid and gas are in motion.

Radiation

The transfer of energy by the emission of electromagnetic radiation. It may occur in no medium.

HEAT TRANSFER IN DIFFERENT MEDIA

                         MODES                                         MEDIA   Conduction    Convection     Radiation
    SOLID           √          ˗˗˗          ˗˗˗
    LIQUID           √           √          ˗˗˗
    GAS           √           √          ˗˗˗

Why there is no energy transfer in same temperature substances?   Consider two objects at different temperatures that are brought together. Energy is transferred from the hotter object to the cooler one, until both objects reach thermal equilibrium . How is this energy transferred? No work is done by either object, because no force acts through a distance. The transfer of energy is caused by the temperature difference, and ceases once the temperatures are equal. This observation leads to the following definition of heat: Heat is the spontaneous transfer of energy due to a temperature difference.                    Fig. 1: Substances in different temperature If there is no temperature difference then there will be no energy transfer. Because the substances are in equilibrium state. What is the driving force for heat transfer? Ans.: Temperature difference or temperature gradient. Without difference in temperature there will be no transfer of heat or energy. Short notes:

  1. In 1800-1900 heat was supposed to be a substance.
  2. Heat is the quantity of kinetic energy in a substance.
  3. Specific heat for water 4200 J.
  4. Specific heat for air 1 J.
  5. Temperature is a measuring scale in terms of vibration.

6.Heat = Intermolecular vibration

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