What is meant by heat transfer?

Whenever there is a flow of heat flux resulting from a temperature gradient within the material or between two materials, there is heat transfer. Heat transfer primarily arises due to temperature differences between two bodies or within bodies. The term temperature gradient denotes temperature differences or variation of temperature due to change in some quantity. The fundamental use of the concept of temperature is found in the zeroth law of thermodynamics.

The concept of heat transfer is utilized in different areas of engineering, mechanical engineering, civil engineering, electrical engineering, and chemical engineering. These are the major areas that witness the phenomenon of heat transfer. In this article, a brief concept of the topic of heat transfer is introduced and its various practical uses have also been introduced.

Conduction heat transfer

Conduction is the mode of heat transfer that occurs between a solid body to another solid body that is in contact with each other, assuming that there is complete contact between the two bodies without any air gaps. Conduction can also occur within the same solid body. The phenomenon of conduction occurs in the microscopic range rather than in a macroscopic range. The fundamental reason for conduction is the presence of free electrons and atomic vibrations. If the outer orbit of the material has free valency electrons, these electrons jump to a higher energy level on receiving the heat energy and transfer that energy to the adjacent particle. Through a series of free electron transfers from one level to another, thermal energy is transferred from one end of the material to the other. These can be generally seen in metals that have plenty of free electrons. In the other case, the rise in molecular vibrations, lattice vibrations, or thermal kinetic energy results in the heat transfer process. According to the first law of thermodynamics, the temperature of a body is directly proportional to the internal energy of the body. This internal energy of the body directly accounts for the rise in the sensible energy of the body. Due to vibrations, energy interactions occur between constituent atoms and molecules, which results in heat transfer. Fourier law of heat conduction governs the heat flow through the conduction process.

The Fourier law of heat conduction equation is given by $Q=-KA\frac{∆T}{∆x}$

where

•  $Q$ is heat transfer by conduction whose unit is Joule, J,
• $∆T$ is the temperature difference between higher-temperature and lower-temperature,
• $∆x$ represents the change of distance,
• $A$ is the cross-sectional area perpendicular to the heat flux,
• $K$ is the conductive heat transfer coefficient.

Convection heat transfer

Convection is the mode of heat transfer between a fluid and a solid body. For instance, if a hot plate is kept in an open area, the heat from the plate will flow from the hot surface of the plate to the surrounding fluid. The law that governs this phenomenon is given by Newton's law of heat convection or Newton's law of cooling. The heat transfer through convection is proportional to the area of the solid body and the temperature difference between the solid body and the flowing fluid.

Mathematically, it can be expressed by an equation as $Q=hA(T_s-T_f)$ 

where

• $h$ is the convective coefficient of heat transfer,
• $A$ is the surface area of the body,
• $T_s$ denotes the temperature of the solid body,
• $T_f$ represents the temperature of the fluid. 

There are mainly two processes of heat transfer through convection. They are:

1. Forced convection: In forced convection, the flow of fluid over a hot body is caused by an external source such as a fan or a turbine.
2. Natural convection: In natural convection, the flow of fluid occurs due to the density differences of the hot-temperature fluid and the low-temperature fluid.

Radiative heat transfer

When the energy transfer occurs between two bodies in the form of photons and thermal radiation, such types of heat transfer are so-called radiative heat transfer. Thermal radiation is the spectrum of electromagnetic radiation falling in the range of  $0.1-100 \mu m$.

Heat exchangers

Heat exchangers are the devices that are used to transfer thermal energy from one fluid to another fluid, either by direct contact or by separating the two fluids. For a perfect heat exchange between two fluids having temperature differences, the heat loss from the higher-temperature fluid equals the heat gain from the lower-temperature fluid. Mathematically, $Q_h=Q_c$, where $Q_h$ is heat loss from high-temperature fluid, $Q_C$ is heat gained by low-temperature fluid. 

The equation can be further expanded as, $m_h{C}_h∆T=m_C{C}_C∆T$, where $m_h$and $m_C$ are the masses of high-temperature and low-temperature fluids, respectively. $C_h$ and $C_c$ are the specific heats of high-temperature and low-temperature fluids, respectively. Depending on the fluids used, the specific heat would vary. 

Context and Applications

The topic is primarily taught in the third and fourth engineering degree courses of bachelor of science specialized in mechanical engineering and chemical engineering.

The topic has a wide number of applications in the industrial sectors such as thermal energy plants, oil and gas industries, and chemical industries. Besides, this topic is also taught in different polytechnic courses.

• Bachelors in Technology in Mechanical Engineering
• Bachelors in Technology in Chemical Engineering
• Masters in Technology in Mechanical Engineering
• Masters in Technology in Chemical Engineering

Practice Problems

1. In which of the following modes of heat transfer, thermal energy interaction occurs by electromagnetic radiation?

1. Convection
2. Conduction
3. Radiation
4. Forced convection

Correct option- c

Explanation: During heat transfer through radiation, the thermal energy interaction occurs by electromagnetic radiation. Radiation energy is thermal energy, which is a part of the electromagnetic spectrum. Different ranges in the electromagnetic spectrum signify different waves characteristics.

2. In which of the following heat transfer, the fluid motion occurs due to density differences?

1. Forced convection
2. Conduction
3. Natural convection
4. None of these

Correct option- c

Explanation: The motion of fluid occurs due to density differences in the natural convection process. A heated plate for instance will exchange heat with its surroundings through the process of natural convection. The hot air near to the plate moves up as it is less dense, to fill the void space created by the movement of hot air, it is filled by cool air which has high density.

3. Which of the following is the unit of heat interaction?

1. Candela
2. Joule
3. Watt
4. Kelvin

Correct option- b

Explanation: The unit of heat interaction is Joule 'J'. Heat is a form of energy interaction between a system that is at high temperature with its surroundings. Heat is also characterized by the number of thermal energy interactions between a system and a surrounding, hence it possesses the unit of energy.

4. Which of the following is the thermal radiation range in the electromagnetic spectrum?

1. $0.01-100 \mu m$
2. $0.1-100 \mu m$
3. $1 - 100 \mu m$
4. None of these

Correct option- b

Explanation: In the electromagnetic spectrum, thermal radiation falls in the range of $0.1-100 \mu m$. Ranges lower than this or higher than this will represent different wave characteristics and nature like microwaves, X-rays, radio waves, cosmic rays, etc. 

5. Which of the following expressions represents Newton's law of cooling?

1. $Q=KA∆T$
2. $Q=hA\frac{∆T}{∆x}$
3. $Q=-KA\frac{∆T}{∆x}$
4. $Q=hA∆T$

Correct option- d

Explanation: The expression $Q=hA∆T$ correctly represents Newton's law of cooling. Newton's law of cooling emphasizes the forced convection of the heat transfer mechanism. The forced convection mechanism requires an external agency or mechanism to induce heat interactions, unlike natural convection which occurs due to density differences.