Heat Transfer Processes

Heat Transfer Processes

• Heat transfer processes that involve a change of phase of a fluid are considered as a distinct type.

Conduction

• Conduction is the transfer of heat energy between particles in physical contact with each other.
• It is the passing of heat energy from molecule to molecule through a substance.

Convection

• Convection is the transmission of heat from one place to another by fluid circulation between spots of different temperatures.
• In natural convection, a heated portion of a fluid moves because of a density differential.

Radiation

• Radiation is the transfer of heat energy through electromagnetic waves.
• It is the mechanism of heat transfer that does not require a medium (e.g., water, air, solid concrete) for the heat energy to travel.

Heat Exchangers

• A heat exchanger is a device that transfers heat from one fluid to another.
• The function of a heat exchanger is to transfer heat from one fluid to another.

Thermodynamic Properties

• Internal energy is a measure of the total energy of a system.
• Enthalpy is the energy of a body that can be transmitted in the form of heat.

Material Properties

• Metals are good conductors of heat because they contain free electrons.
• The effectiveness of a body as a thermal radiator at a given temperature is known as emissivity.

Heat Transfer Coefficients

• The heat transfer coefficient is a measure of the rate of heat transfer between a surface and a fluid.
• If the viscosity of the fluid increases, the heat transfer coefficient will decrease.

Nusselt's Number

• Nusselt's number is a measure of the relative effectiveness of momentum and energy transport by diffusion in the velocity and thermal boundary layer.

Biot Number

• The Biot number is the ratio of the rate of heat transferred by conduction to the rate of energy stored.