Modes of Heat Transfer

Introduction:

In everyday life, we find multiple instances where heat transfer is felt and observed. From feeling warmth by holding a cup of tea during winters to reheating a cold cup of coffee in microwave oven, each of them happened because of the basic principles of heat transfers.

Each and every matter has the ability to transfer heat due to presence of atoms and molecules within them. The motion of these atoms and molecules is responsible for transfer of thermal energy and every matter has its own thermal energy. Greater the motion of molecules, higher will be the heat energy of the matter.

The molecules themselves do not necessarily have to change their positions, the molecules can simply vibrate more or less quickly against each other to transfer the heat. Therefore, heat transfer is nothing but the process of transfer of heat from a high temperature body to a low temperature one.

In order to grasp the concept of heat flow completely, we must understand few terms related to it. These terms are discussed below:

i) Heat:

Heat is a form of energy denoted by Q and it is measured in terms of joules and calories. Unit joule (J) is the SI unit of heat in the International System of Units. It produces the sensation of warmth and is able to flow between the bodies through a variety of mediums. The heat energy is inter-convertible between forms of energy, like kinetic energy, mechanical energy, etc. Heat not only help us to sustain our life, but also make us comfortable and help the organisms to prepare their food, by understanding the properties of heat, many applications are found in fields of scientific research.

The common formulae for calculating heat used in the fields of science:

Q= m.c.ΔT

Where, Q = the quantity of heat transferred to or from the object,

m = the mass of the object,

C = the specific heat capacity of the material of which the object is made

ΔT = the resulting temperature change of the object.

ii) Heat Transfer:

Heat is a form of energy which is transferred from one body to another in accordance with the differences in temperature between them. If two bodies at different temperatures are brought together, energy get transferred by means of heat flow. The direction of this heat flow is from the hotter body to the colder ones.

How Does Heat get Transferred?

Modes of Heat Transfer:

Heat can be transferred in four different ways: by conduction, by convection, by radiation and advection. These methods are discussed below with examples and specials facts about each of them.

i) By Conduction:

Conduction is by means of touch or contact between two parts. It is a type of transfer of heat through physical contact. It is the process by which heat is transferred from the one part to the other part of the body due to difference in temperature, such that the heat flow is from the hotter region to colder region. This heat flow is through the transfer of energy from one particle to another particle of the body without the actual movement of the particles from their equilibrium positions. The mode of heat transfer by conduction is a static process.

For Example:

Conduction is perhaps the most common of all the forms, it occurs regularly in nature. It can be observed even when you press your hand onto a window pane, when you place a pot of water on an active element, and when you place an iron in the fire.

Metals are very good conductors of heat, in case of metals, there is very high amount of free electrons, they are attributed as free-flowing conduction electrons. In metal molecules have bonded atoms that share their valence electrons. This sharing of electrons creates a sea of freely moving conduction electrons which carry the heat actively. Hence, heating the metal at one end primarily leads to thermal conduction. In metals, the free electrons can move freely throughout the solid and thus transfers of the thermal energy is at a very high rate as compared to any other non-metallic substance. Even the simplest electrical metal conductors also exhibit the best thermal conductivity, when compared with non-metals.

ii) By Convection:

It is the process of heat transfer from one part of a fluid to another part of it by the actual movement of the particles of the fluid. This heat flow is through the transfer of energy from one particle to another particle of the body with the actual movement of the particles from their equilibrium positions. Liquid and gases are heated by the process of convection.

The process of convection gives information about the heat transfer between a surface and a liquid or a gas in motion. As the fluid or gas travels faster, the rate heat transfer by convection increases.

There are two types of convection:

• Natural Convection:

Natural convection is a mechanism of heat transportation in which the fluid motion is not generated by an external source. Instead the fluid motion is caused by buoyancy which is the result of difference in fluid density occurring due to temperature gradients.

This occurs by a basic principle of density, such that, when the fluid is allowed to come in contact with any hot surface or atmosphere, the molecules are separated and get scattered, this thereby reduces the fluid density. Through this, a difference in densities is created which results in generation of buoyant forces that causes the heat flow.

Few examples of convection in daily life are explained below:

1. Hot air around fire: It creates warm atmosphere. When a fluid such as air or water touches any hot object, it can heat up by convection and rise above the surface, thereby carrying the heat from object quickly to its surroundings.
2. Melting of Ice is a good example of natural convection. The water particles near the ice surface receives heat through convection from surrounding atmosphere to the ice and in turn melts down. These "warmer" water particles are now less denser than colder ones and thus they will sink. New, colder particles will take their place, ready to transfer more heat energy to the ice surface and repeat the process. In this way a cube of ice gets melted completely naturally by means of convection.
3. Generation and movement of Sea breeze or Land breeze: It is caused by a difference in pressure. Since, sea water radiates comparatively less heat than the heated land surface, it consequently remain hotter than the land part. Hence the cold air from the land goes through the convection process of air to the seawater and this flow of air is called the Land Breeze. As explained earlier, the land surface is bound to absorb more heat from the sun than the sea surface. And in the day time the land becomes hotter than the water part due to the specific heat of water being greater than the land part. Hence, the air along the land surface rises above the ground, being hot and light. To fill these blank spaces over the ground, the cold air from the sea goes through the convection process of air to the land surface and this flow of air is called the Sea Breeze. The sea breeze and the land breeze are therefore caused by the natural process of convection current of air.
4. Blood circulation in animals: It helps reduce heat loss. This example illustrates how the convection principle can be put to use to conserve the metabolic heat. In this type of circulation, there is counter current mechanism, in which the blood vessels surrounding lungs are arranged in a spatially organized fashion in close proximity to each other forming net like structure. In these net, blood in arteries and veins flows in opposite direction, as a result of which the outward flowing arterial blood loses most of its heat to the colder inward-flowing venous blood. The heat transfer between arterial and venous blood is very efficient due to the naturally developed convection which maintains temperature gradient between arteries and veins. Therefore, when arterial blood reaches body surface, it has cooled down so that the temperature gradient between the blood and the environment is much lower than between the core body temperature and the environment, and the heat loss due to heat transfer from body to environment is reduced.

• Forced Convection:

It is an artificial process. In the process of forced convection, the heat transfer occurs in the fluid such that the fluid is forced to flow over a surface or in a tube by external means such as a pump, water heater or a fan.

For example:

• Water Heater: During course of water heater, the cold wateris fed to the bottom of the heater tank via a tubeand is then heated by electrical resistors. As the water is heated, it rises to the top of the tank due to convection and is discharged through the hot-water outlet for further application.
• Car Radiators: The coolant liquid present in car radiators flows through the tubes inside the combustion engine and absorbs the engine's heat and in-turn gets heated itself. This flowing air picks up heat from the coolant by means of convection, as a result of which the heated air is blown away by the fan, and colder air replaces it.
• Air Conditioning: Air conditioners are used to allow the circulation of air by setting up of convection currents in the room it is being installed. They release cool dry air into the room, this cool air is denser and thus it will easily sink in the room. The warm air already present in room, being less dense, will rise. In this way the air gets circulated through convection being created and the temperature of the air will eventually fall to the desired value.
• Convection Oven: In these ovens the force required to create convection comes from a fan. This circulation of heat created by fan forms an even distribution of heat and uniform temperature inside the cavity of oven. This helps to cook food faster and better.

iii) By Radiation:

All objects, whether hot or cold, emit some type of radiations continuously. Radiation is the process of the transfer of the heat from one place to another place without heating the intervening medium, which means there is no point of direct contact between the two bodies for which the heat is transferred. Radiation is the fastest among all modes of heat transfer. A body's surface temperature affects the wavelength at which it radiates. This heat flow is through the transfer of energy from one particle to another particle of the body without any medium.

For Example:

• It is commonly seen when the Sun's rays, which are electromagnetic waves by nature, travel through space in a vacuum and fall on the surface of the Earth. Photon particles in the radiation collide with the air molecules in the atmosphere and transfer energy.

iv) By Advection:

The flow of heat through the horizontal movement of air is called as advection. The horizontal movement of the air is comparatively more significant than the vertical movement. Its significance is prominently observed during seasonal changes, most of the diurnal variation in weather gets created by advection (only in the middle latitudes). Also in tropical regions during summer season, predominantly in Northern India, local winds called 'Loo' arises, it is the result of the advection process.

Difference between Conduction, Convection, and Radiation:

These three modes of heat transfer are most common, but there are certain basic differences between them, these are discussed briefly in the table given below:

Factors affecting Heat Transfer by Conduction Method:

There are four factors on which process of heat conduction basically depends. These are discussed below:

1. The Temperature Gradient:

Coldness is nothing but the absence of heat energy, heat always flows from the hotter region to colder region, this is due to the development of temperature gradient between the two positions. A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes in a specific location. Heat transfer between bodies continues until the temperature difference is removed, and a state known as thermal equilibrium occurs.

2. Cross-Section and Path Length:

The greater the size of the material involved in the transfer process, the more heat is required to warm it. Larger the surface area that is exposed to open air, greater the chances for heat loss. Thus, shorter objects with a smaller cross-section area are the best means of heat transfer as they minimize the loss of the heat.

3. Physical Properties of Conducting Device's Material:

When it comes to conduction of heat, not all substances act similarly. Metals and stones are considered as good conductors as they can readily transfer heat, whereas materials like wood, paper, air, and cloth are poor conductors of heat as they may take very long to transfer heat.

The conductive property of materials are rated on the bases of the "Coefficient of conductivity of heat", which is measured with respect to silver metal. The thermal conductivity coefficient of silver is known to be 100 which is a standard value, whereas other materials are ranked lower to this. For example, the heating coefficient of conductivity of copper is 92, for iron it is 11, for water it is 0.12, and for wood it is the lowest 0.03, etc.

Materials that are poor conductors of heat are often called as insulators. Air, which has a conduction coefficient of heat as 0.006, is an exceptional insulator because it is capable of being contained within an enclosed space. This is why artificial insulators make use of air compartments, such as double-pane glass windows which are used for cutting heating bills. Basically, they act as buffers against any heat loss. Plastic and rubber are some of the good examples of thermal insulators.

Applications of Heat Transfer in Daily Life:

1. Applications of Conduction:

• The basic method of cooking involves heating food over a metal cookware. Conduction is applied in cooking food in metallic pots, Aluminium pots are most commonly used for this purpose.
• The handles of metallic cookware are generally made up of plastic, high quality rubber and even wood is also used, because these objects cannot conduct heat as a result of which it gets easier and safer to hold the handles of cookware while cooking.
• Ironing of clothes is done by pressing with a heated metallic iron is also based on conduction of heat.
• The process of welding of iron most commonly used in construction is also based on the general principles of conduction.
• Thatched roof rooms are cooler inside as compared to those rooms which have an iron roof, this is because iron conducts heat easily and thatch is an insulator, which prevents heat to fall inside the room.
• In winters, woolen clothes are found to prevent heat loss from body because of poor conductivity as they are good insulators of heat.

2. Applications of Convection:

Natural convection as a means of transfer of heat is extensively used in numerous areas of application where thermal transfer largely depends on the difference in density of the thermal fluid. Thermal convection has been employed in the industries, solar energy collectors, agriculture, nuclear energy plants, telecommunications, aviation, geophysics, power generation, electronics cooling, and many more fields of production. Some of the examples are as follows:

• Convectionis used to cool down the fins of integrated circuits, engine combustion chamber and all industrial heat exchanges including, boiler tubes used in factories and even at home; in the household refrigerator role of convection is broadly observed.
• It is used in cooking food as we see in case of convection oven, in such devices a convection mode is present in which there is a fan inside the oven that circulates the hot air, which helps to cook food more quickly by distributing hot air around the top, bottom and sides of your meal without even bringing the hot coil in direct contact of the food. The hot air circulation around the cavity of the oven also help to reduce hot and cool spots and for more even results with multi-rack cooking.

3. Applications of Radiation:

Radiation is used in medicine, academics, and industry, as well as for generating electricity. In addition, radiation has useful applications in such areas as agriculture, archaeology (carbon dating), space exploration, law enforcement, geology (including mining), and many others. Few of these applications are listed below:

• The most common of medical procedures involve the use of X-Rays, it is a type of radiation that can pass through our skin. When radiography through X-ray is performed, our bones and other solid structures cast shadows because they are denser than our skin, and those shadows can be detected on photographic film. This helps the doctor to understand better about the injury.
• Electricity produced by nuclear fission which is actually the process of splitting of the atom is one of the greatest uses of radiation. This is one of the most reliable and sustainable form of electricity generation.