What is heat transfer?

Heat transfer is an essential discipline in thermal engineering that deals with the transfer, conversion, usability, and creation of heat or thermal energy. Transfer of heat occurs through various mechanisms like conduction, convection, radiation, and change of phases. All these mechanisms have distinct characteristics, but these mechanisms can simultaneously occur in an identical system.

Heat transfer rate is reliant on the temperature difference. The rate of heat transfer is proportional to the difference in the temperatures between two mediums; such that, if the difference rises, then the transfer rate increases, and if temperature difference decreases, then simultaneously heat transfer rate decreases.

There is a class of applications of the rate of the temperature difference and heat transfer rate in automobile engineering, structural engineering, thermal engineering, metallurgical engineering, energy-saving, and so on.

What are the mechanisms of heat transfer?

There are the following three mechanisms of heat transfer:

• Conduction
• Convection
• Radiation

Conduction

Heat conduction is a mechanism in which heat is shifted from the higher temperature zone to the lower temperature zone. Conduction occurs because of the interaction of the adjacent atoms, electrons, or ions existing in the body. In the conduction mechanism, the collision between molecules is the only reason for the heat transfer. The conduction process typically occurs in solids as heat is supplied to one end and molecules travel faster to the farther end. The area which has lower kinetic energy will try to obtain the energy from the higher kinetic energy area. This indicates that particles that would be moving with low velocity will collide with the higher velocity particles and thus gain velocity from them.

Types of conduction materials

Conduction materials consist of metals, plasmas, superconductors, electrolytes, semiconductors, and non-metallic conductors such as graphite and conductive polymers. But the conduction properties in each of the above mentioned elements are different. The conductivity in metals is generally high as they have free electrons through which electricity can pass easily. Semiconductors also conduct electricity, but they will not always conduct electricity. They will show conduction only when they meet particular conditions.

Metals and semiconductors as conduction material

Metals

Metals usually have high conduction properties. All metals have different conducting powers based on their conductivity applied in electrical appliances as they provide a smooth flow of electricity. The metals have electrons in their outer shell, which get readily dissociated from the parent atoms making the conductance of electricity possible through the metals.

Semiconductors

The bandgap between the conductance band and valence is more in semiconductors than metals. Still, it is less than non-metals, so there are no free electrons for conductance in semiconductors. But semiconductors conduct electricity through them at specific temperature or pressure. When the temperature is increased, few of the electrons get free, thus conducting electricity quickly. Electronic appliances used are made with semiconductors.

Convection

Convection is the mechanism of heat transfer from higher temperature to lower temperature zone by the flow of material particles. The convection mode of heat transfer is used by only fluids, that is, liquids and gases. Mathematically, the convection heat transfer can be represented as follows:

$H=hA∆T$

Here, H is the heat transfer through the convection process, h is the convective heat transfer coefficient, A is the surface area, and ∆T is the difference in temperature. The convection heat transfer coefficient depends upon density, viscosity, thermal conductivity, and specific heat capacity of the fluid.

Convection heat transfer process

In the convection heat transfer method, a part of the fluid in contact with heat gets expanded when the fluid is being warmed up. Due to thermal expansion of the fluid, the fluid becomes less dense with respect to the other regions of the fluid. This heated part of the fluid rises, and moves towards the colder part of the fluid; this cycle of heating and replacing sets up convectional current, which occurs in the transfer of energy.

In the convection process, both buoyancy and gravitational forces working on the fluid are necessary. For instance, the lower part of a liquid near the heat source is expanded when the liquid is warmed up. Due to this thermal expansion of liquid, the density of this part reduces, and the liquid becomes lighter with respect to the upper part. Due to buoyancy and gravitational force, the liquid of the lower region rises, and the liquid of the upper region (heavier) moves down. Hence, convection heat transfer occurs.

Types of convection process

There are the following two convection processes:

• Natural or free convection
• Forced convection

Natural or free convection

In this kind of convection, particles of a liquid flow due to differences in density or thermal expansion. Like trade winds, land and sea breeze occurs due to natural convection.

Forced convection

In this kind of convection, particles of a liquid are forced to flow from one region to another by some external force. Example-exhaust fan, air heat system, human heart, etc.

Radiation

Radiation can be described as a release of vibrations, ions, or rays. Radiation does not need an energy storage tool. The subject is either reflected or consumed. The force consumed increases the object's kinetic strength and so raises the surface temperature.

Electromagnetic radiation is generated when particles in matter begin to flow and generate heat. The heat that dissipates is recognized as thermal radiation. Some instances of thermal radiation are infrared radiation, which is emitted by fauna, and cosmic microwave background radiation.

When thermal energy is transformed into electrical energy, the emission of electromagnetic wave occurs. It is known as thermal radiation. Radiation is viable for all kinds of matter, which is at a temperature above absolute zero. In radiation, the heat gets transferred in the form of infrared waves. As electromagnetic radiation can flow through vacuum, the radiation moves through space unless obstructed.

Theories about radiation

• Thermal radiation is different from the conduction and convection heat flow processes in the context of the medium of travel. Thermal radiation can flow without a medium; that is, it can travel through vacuum. Some characteristics of electromagnetic radiation rely on the wavelength and frequency of the same.
• Thermal radiation possesses the properties of electromagnetic radiation.
• Radiation waves can move through a non-absorbing medium to reach higher temperatures.

The concept of black body radiation

A black body is an opaque body with a surface that is perfectly non-reflective. Black body radiation is the radiation that is emitted by a black body and surrounds a body in a situation of absolute thermodynamic equilibrium. It is an ideal situation to understand and construct theories on the nature of radiation.

Emitted thermal energy is supposed to approximate black body radiation, and various everyday objects emit this kind of energy. An enclosure with perfect insulation, put in thermal equilibrium, includes internal black body radiation, which can get emitted if a very tiny hole is punctured on the surface in a way that the equilibrium is not disturbed. The black body radiation will then be seen to get emitted through the tiny hole. In normal room temperatures, a black body seems to be black, as most of the energy delivered lies in the infrared spectrum, which is invisible to the human eye.

Common Mistakes

Students get confused between conduction and convection. Conduction is the method of heat transfer in which the heat is transferred by direct physical contact of two objects, while in the convection heat transfer process, heat is transferred by the movement of the fluid.

Students get confused between the conductors and semiconductors. The conductors are the material that has low resistivity, while the semiconductors are the material that has resistivity lying between the conductor and insulator.

Context and Applications

The topic of the mechanisms of heat transfer is very much significant in the several professional exams and courses for undergraduate, diploma level, graduate, and postgraduate. For example:

• Bachelors in Technology
• Bachelors of Science
• Masters of Technology
• Masters in Science

Related Concepts

• Heat capacity
• Heat transfer enhancement
• Thermal resistance in electronics
• Emissivity
• Thermal conductivity

Practice Problems

Q1: The process of the heat transfer occurs according to:

1. Second law of thermodynamics
2. Third law of thermodynamics
3. Stefan’s law
4. None of these

Correct option: (a)

Explanation: The second law of thermodynamics governs the heat transfer direction and entropy. The third law of thermodynamics indicates the entropy of a pure crystalline substance at absolute zero temperature is equal to zero.

Q2: The insulation capability of an insulator with the presence of moisture will:

1. Remain unchanged
2. Deceases
3. Increase
4. None of these

Correct option: (b)

Explanation: Whenever a specific amount of moisture is present in an insulator, then the insulating capacity of the insulator would decrease because the properties of moisture would not be the same as insulating material.

Q3: The transfer of heat through the collision of molecules is known as:

1. Scattering
2. Convection
3. Radiation
4. Conduction

Correct option: (b)

Explanation: Whenever heat transfer through a fluid medium occurs due to the collision of fluid molecules with each other, it refers to the convection mode of heat transfer.

Q4: Heat is intimately linked to:

1. Entropy
2. Enthalpy
3. Temperature
4. Liquid

Correct option: (c)

Explanation: The heat transfer always takes place due to temperature difference, which means temperature difference works as the main cause for heat transfer.

Q5: Among the four options, which of the following has the least conductivity value?

1. Air
2. Water
3. Rubber
4. Plastic

Correct option: (a)

Explanation: The term thermal conductivity decides the rate of heat transfer through a material. The value of thermal conductivity of air is less than the thermal conductivity of plastic.