Heat transfer occurs between states of matter which are made up of molecules and atoms whenever a temperature difference exists across a well-defined boundary around a thermodynamic system. Heat transfer is the process of transfer of heat from a high-temperature body to a low-temperature one.
How is Heat Transferred?
Heat can be travel from one place to another in three ways
- Conduction
- Convection
- Radiation
Conduction:-
Conduction is the transfer of energy from one molecule to another by direct contact. This transfer occurs when molecules hit against each other, Conduction takes place in solids, liquids, and gases, but works best in materials that have simple molecules that are located close to each other. For example, metal is a better Heat conductor than wood or plastic.
A common example of conduction is the process of heating a pan on a stove. The heat from
the burner transfers directly to the surface of the pan.
Conduction heat transfers can be quantified
using Fourier’s Law, which includes the important thermal physical
property of thermal conductivity. The thermal conductivity of a
material is an indication of how effectively the material transfers heat.
The thermal conductivities of materials vary
widely, thermal conductivity of
Metals> Nonmetals> liquids > gases.
“Fourier’s
law of thermal conduction states that the rate of heat transfer through a
material is proportional to the negative gradient in the temperature and the
area (perpendicular to the gradient) of the surface through which the heat
flows.”
The differential form of Fourier’s law can be represented as:
q = - k▽T
where,
- ∇T is temperature gradient (K. m-1)
- k is
the conductivity of the materials (W. m-1. K-1)
- q is the heat flux density vector (W. m-2)
Convection is the movement of
heat by a fluid such as water or air. The
fluid (liquid or gas) moves from one location to another, transferring heat
along with it. When water boils, the heat passes from the
burner into the pot, heating the water at the bottom. This hot water rises and
cooler water moves down to replace it, causing a circular motion. Air
conditioners and refrigerators are also examples of convection.
when a fluid is in contact with a surface at a
temperature different from the fluid temperature, conduction will transfer heat
from the hot surface to the cold fluid (or from a cold surface to a hot fluid).
The motion of the fluid will then transfer the heat within the fluid.
Convection is quantified using Newton’s Law of Cooling, which includes
the heat transfer coefficient.
The equation for convection can be
expressed as:
q = hc A
dT
where
q = heat transferred per unit time
(W, Btu/hr)
A = heat transfer area of the
surface (m2, ft2)
hc = convective heat
transfer coefficient of the process (W/(m2oC, Btu/(ft2 h oF))
dT = temperature difference
between the surface and the bulk fluid (oC, F)
Radiation:-
Radiation is the transfer of heat
by electromagnetic waves. When you stand in the sun, you
are warmed by electromagnetic waves, mainly infrared radiation that travels
from the sun to Earth.
Radiative heat transfer is the transfer of energy via Thermal radiation. Thermal radiation is generated by the emission of electromagnetic waves. It occurs across a vacuum or any transparent medium. Thermal radiation is emitted by all objects at temperatures above absolute zero, due to random movements of atoms and molecules in matter.
Sun,
light bulbs, irons, and toasters also transfer heat via radiation. Note that,
unlike conduction or convection, heat transfer by radiation does not need any
matter to help with the transfer.
Radiation Equation
As temperature rises, the wavelengths in the spectra of the
radiation emitted decrease, and shorter wavelengths of radiation are
emitted. Thermal
radiation can be calculated by the Stefan-Boltzmann law:
P = e ∙ σ ∙ A· (Tr – Tc)4
Where,
- P is the net power of radiation
- A is the area of radiation
- Tr is the radiator temperature
- Tc is the surrounding temperature
- e is emissivity and σ is Stefan’s constant
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