Modes of Heat Transfer
Heat transfer is the study of transmission of thermal energy from a high temperature region / body to a low temperature region / body on account of temperature difference. The rate of heat transfer is directly proportional to the temperature difference between the heat exchanging regions / bodies. Once the process of heat energy is complete, it is stored in one or more forms of energy such as potential, kinetic and internal energy. It is pertinent to mention that energy in transition as heat can never be measured; however, it is determined in terms of observed changes in other forms of energy. Transfer of heat between two regions / bodies maintained at different temperatures can occur in three different modes namely:
Conduction
Convection
Radiation
In the conduction and convection modes, heat flows from high temperature to low temperature region / body whereas in radiation mode, transfer of heat takes place from both the bodies towards each other. However, net transfer of heat is always from high temperature body to low temperature body. Mechanism of heat transfer in each mode is different and controlled by different laws.
Conduction
Conduction is a process of heat transfer from a high temperature region to a low temperature region with in a body or between different bodies which are in direct physical contact. In heat conduction, energy is transferred due to exchange of molecular kinetic energy. According to kinetic theory, temperature of body is proportional to the mean kinetic energy of its constituent molecules. As the temperature in one region of a body increases, kinetic energy of molecules in that region also increases as compared to that of the molecules of adjacent low temperature region. High energy molecules transfer a part of their energy by impact in case of fluids or by diffusion in case of metals to low energy molecules, thereby resulting in increase in their energy levels, hence temperature. Likewise, this process of energy transfer by molecular activity continues till temperature along the entire length of the body becomes equal and has been depicted i fig. below.
Heat transfer by conduction in solids, liquids and gases is determined by the thermal conductivity and temperature difference. The basic law of heat transfer by conduction was proposed by the French Scientist J. B. J. Fourier in 1822 and one dimensional Conduction rate
equation described by the Fourier Law is written as:
Where, Qx – Heat Flow, (W)
k – Thermal conductivity of the material, ( W/(m-K)
A – Cross-sectional area in the direction of heat flow, (square metre)
dT/dx – Temperature gradient, (K/m)
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| conduction |
Convection
Heat transfer by convection occurs when a fluid (Liquid and gas) comes in contact with a solid through direct contact and a temperature difference exists between them. Heat transfer by Convection occurs under the combined action of heat conduction and mixing motion. When a fluid comes in contact with a hot surface, energy in form of heat flows by conduction from hot surface to the adjacent stagnant layer of fluid particles, thereby increasing their temperature and internal energy. Due to increase in temperature, density of the fluid particles decreases and they become lighter as compared to the surrounding fluid particles. The lighter fluid particles move up to a region of lower temperature with in the fluid where they mix and exchange a part of their energy with colder fluid particles. Simultaneously, the cold fluid particles move downwards to occupy the space vacated by hot fluid particles. This upward and downward movement of hot and cold fluid particles continues till temperature of the fluid and the surface becomes equal. The convection heat transfer process has been shown in Fig. below.
The upwards movement of hot fluid particles and downward movement of cold fluid particles is called convectional currents. If the convectional currents are set up only due to density differences, then the heat transfer process is termed as natural or free convection.
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| Convection |
However, if the convectional currents are caused by some external means such as blower, fan, pump etc. then heat transfer process is called forced convection.
It is virtually impossible to observe pure heat conduction in a fluid because as soon as a temperature difference is imposed in a fluid, natural convection currents will occur due to resulting density differences.
Convective heat transfer rate is governed by Newton’s law of cooling and is expressed as
Q = h As (Ts – Tf)
Where, ‘h’ is convective heat transfer coefficient in W/ (m2 – K)
As is heat transferring area, square metre
Ts and Tf are temperatures of surface and the fluid respectively, K
Radiation
Heat exchanged between two bodies or mediums, which are separated and are not in contact with each other, is called radiation heat transfer. Radiation heat transfer does not require presence of an intervening medium between the two bodies as in case of conduction and convection and it takes place most effectively in a vacuum.
Example of radiation heat transfer
Example of radiation heat transferis the energy received on the earth from the Sun and has been shown in Fig. below.
Thermal radiation is the energy emitted by a body in the form of electromagnetic waves due to changes in the electronic configuration of the constituent atoms or molecules of the body. The electromagnetic waves travel through the intervening medium between two bodies with a speed that is related to speed of light in vacuum by the following equation
c= co / n.
Where c is the speed of propagation in a medium,
Co is the speed of light in vacuum and is equal to the light speed in m/sec,
‘n’ is an index of refraction of a medium which is unity for air and most of the gases, 1.5 for glass and 1.33 for water.
When electromagnetic waves come in contact with a body, energy is transferred to the body as thermal energy which is partly absorbed, reflected and transmitted.
Energy emitted per unit area as thermal radiation is called emissive power of a body and the maximum energy emitted as radiation by a body at a particular temperature is governed by
Stefan-Boltzmann law which is expressed as
Eb = σ AT4
Where, Eb is the energy emitted per unit time, W
A is the surface area, m2
T is the absolute temperature of the body, K
σ is Stefan-Boltzmann constant which is equal to 5.67 Χ 10 -8 W/(m2 – K4)
At a given temperature, maximum radiations are emitted by an ideal emitter called black body. The energy emitted by non-black bodies are less as compared to that of the ideal body when both the bodies are maintained at same temperature. Energy emitted by a non-black body maintained at temperature ‘T’ is given as
E = ε σ AT4
Where, ε is called emissivity of non-black bodyand is defined as ratio of emissive power of a non- black body to that of a black body. Emissivity is a radiative property of the body and its value depends upon surface characteristics and temperature of the body.
All the bodies radiate energy and receive energy emitted by other bodies simultaneously.
Consider heat exchange between two black bodies maintained at temperatures T1 and T2 respectively and body 1 is completely enclosed by body 2. The net heat transfer by radiation from body 1 to body 2 is given as
Q1-2 = σ A1(T14 – T24)
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| Radiation |
