Heat transfer comes under the purview of thermodynamics. To put it simply, Heat transfer deals with the amount of heat transfer that takes place in a system from one state of equilibrium to another. This might not take into account the time taken for the heat transfer or the amount of heat transferred over a period of time.
Why does heat transfer take place?
Heat is a form of energy that moves along temperature difference. Heat transfer is energy transfer from one region to another because of the difference in temperature that exists between them. If in a closed system, if there is a temperature difference, then heat transfer must take place.
Engineers, especially chemical engineers are concerned with the amount of heat transferred over a period of time as well as the distribution of temperature in a system over a period of time. This is evident in most of the appliances at work. For example, in a room heater or in a refrigerator the amount of heat transferred over a period of time is of much importance. In both these cases, the higher the heat transferred in a shorter period of time means efficiency.
Types of heat transfer
The heat that transfers can be differentiated into two types. One is Steady-state heat transfer and the other is unsteady-state heat transfer.
In steady-state heat transfer, the amount of heat transferred will be the same over a period of time and the temperature in any location will not change with time. The formula is T = f (x, y, z).
In unsteady-state heat transfer, the amount of heat transferred will not be the same as the passage of time. The formula is T = f (x, y, z; t).
Modes of Heat Transfer
Conduction: Heat gets transferred when a difference of temperature exists in the medium. The medium can either be a solid, liquid or a gas. This type of heat transfer is called conduction. In heat transfer through conduction, the molecules in high energy state or higher temperature transfer their energy to the molecules of low energy state or lower temperature. Thermal energy in solids gets transferred both through electrons as well as by vibrations. The heat transfer through electrons is very powerful compared to heat transfer through vibrations. In most cases, metals are good conductors of electricity as well as heat, due to the presence of a large number of electrons. These electrons carry heat from one state of temperature to another state of temperature in a similar fashion to carrying an electrical charge. The heat transfer rate gets deteriorated from solids to liquids to gases. In gases, unless and until high pressure is applied, there is a negligible amount of heat transfer taking place.
Convection: When heat is transferred from the surface to a moving medium with a difference in temperature, then it is called convection. For heat transfer to occur in convection mode, there must a presence of fluid particles. The movement of fluids between the high-temperature region to the low-temperature region is the basic governing law in convection heat transfer. The faster the motion of fluids through the medium the greater will be the heat transfer. In the convection model, there are further two types of heat transfer. One is the diffusion method and the other is the advection method.
Thermal Radiation: The third mode is called thermal radiation. According to thermal radiation, any object with finite temperature will emit energy in the form of electromagnetic waves. Thermal radiation does not require any medium for transfer. According to Maxwell, the radiation emitted by a body is transferred in space in the form of electromagnetic waves. According to Max Planck, the radiation emitted by a body is transferred in the form of photon particles. Thermal radiation occurs due to a change in the electronic configuration in the molecules.
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