Mode of Heat Transfer
Heat transfer which is defined as the transmission of energy from one region to another as a result of temperature gradient takes place by the following three modes:
Heat transmission, in majority of real situations, occurs as a result of combinations of these modes of heat transfer. Example: The water in a boiler shell receives its heat from the fire-bed by conducted, convected and radiated heat from the fire to the shell, conducted heat through the shell and conducted and convected heat from the inner shell wall, to the water. Heat always lows in the direction of lower temperature.
The above three modes are similar in that a temperature dierential must exist and the heat exchange is in the direction of decreasing temperature; each method, however, has different controlling laws.
“Conduction” is the transfer of heat from one part of a substance to another part of the same substance, or from one substance to another in physical contact with it, without appreciable displacement of molecule forming, the substance.
In solids, the heat is conducted by the following two mechanisms:
i. By lattice vibration (the aster moving molecules or atoms in the hottest part of a body transfer heat by impacts some of their energy to adjacent molecules).
ii. By transport of free electrons (Free electrons provide an energy flux in the direction of decreasing temperature – For metals, especially good electrical conductors, the electronic mechanism is responsible for the major portion of the heat flux except at low temperature).
In case of gases, the mechanisam of heat conduction is simple. The kinetic energy of a molecule is a function of temperature. These molecules are in a continuous random motion exchanging energy and momentum. When a molecule from the high temperature region collides with a molecule from the low temperature region, it losses energy by collisions.
In liquids, the mechanism of heat is nearer to that o gases. However, the molecules are more closely spaced and intermolecular forces come into play.
Fourier’s law of heat conduction:
Fourier’s law of heat conduction is an empirical law based on observation and state as follows:
“The rate of low of heat through a simple homogeneous solid is directly proportional to the area of the section at right angles to the direction of heat flow, and to change of temperature with respect to the length of the path of the heat flow”.
Thermal conductivity (a property of material) depends essentially upon the following factors:
i. Material structure
ii. Moisture content
iii. Density of the material
iv. Pressure and temperature(operating conditions)
“Convection” is the transfer of heat within a luid by mixing of one portion of the fluid with another.
· Convection is possible only in a fluid medium and is directly linked with the transport of medium itself.
· Convection constitutes the macroform of the heat transfer since macroscopic particles of a fluid moving in space cause the heat exchange.
· The effectiveness of heat transfer by convection depends largely upon the mixing motion of the fluid.
This mode of heat transfer is met with in situations where energy is transferred as heat to a flowing fluid at any surface over which flow occurs. This mode is basically conduction in a very thin fluid layer at the surface and then mixing caused by the flow. The heat flow depends on the properties of fluid and is independent of the properties of the material of the surface. However, the shape o the surface will influence the low and hence the heat transfer.
Free or normal convection. Free or natural convection occurs when the fluid circulates by virtue of the natural differences in densities of hot and cold fluids; the denser portions of the fluid move downward because of the greater force of gravity, as compared with the force on the less dense.
Forced convection. When the work is done to blow or pump the fluid, it is said to be forced convection.
The rate equation or the convective heat transfer (regardless of particular nature) between a surface and an adjacent fluid is prescribed by Newton’s law of cooling.
Where Q = Rate of conductive heat transfer
A = Area exposed to heat transfer
ts= Surface temperature,
tf = Fluid temperature, and
h = Coefficient of conductive heat transfer
“Radiation” is the transfer of heat through space or matter by means other than conduction or convection.
Radiation heat is though o s electromagnetic waves or quanta (as convenient) an emanation of the same nature as light and radio waves. All bodies radiate heat; so a transfer o heat by radiation occurs because hot body emits more heat than it receives and a cold body receives more heat than it emits. Radiant energy (being electromagnetic radiation) requires no medium for propagation and will pass through vacuum.
Note. The rapidly oscillating molecules of the hot body produce electromagnetic waves in hypothetical medium called ether. These waves are identical with light waves, radio waves carry energy with them and transfer it to the relatively slow-moving molecules of the cold body on which they happen to fall. The molecular energy of the later increases and results in a rise of its temperature. Heat travelling by radiation is known as radiant heat.
The properties of radiant heat in general, are similar to those o light. Some o the properties are:
i. It does not require the presence of a material medium or its transmission.
ii. Radiant heat can be reflected from the surfaces and obeys the ordinary laws of reflection.
iii. It travels with velocity of light.
iv. Like light, it shows interference, direction and polarisation etc.
v. It follows the law of inverse square.