Heat Transfer

Introduction

Thermodynamics is the study of transfer of energy by the interaction of system and surrounding. These interactions are heat and work. In thermodynamics we are only interested with the amount of heat transfer.

But in engineering we extend the thermodynamics analysis through the study of modes of heat transfer and through the development of relations to calculate heat transfer rates.

In industries we are often interested in the rate of heat transfer.

In simple words, heat transfer is the transit of thermal energy due to a temperature difference in certain time.

Historical Background:

In past, we considered that the heat is something that give us some sense of warmth. This is the first thing understood by the mankind about heat.

Kinetic theory is the first theory which help us in the study of the nature of heat. According to this theory the molecules are treated as tiny balls which are in motion and thus possesses kinetic energy.

In 1789: Caloric theory was proposed by French chemist Antoine Lavoisier. According to this theory the heat is treated as a fluid-like substance which is known as caloric.

The caloric is massless, colourless, odourless and tasteless substance which can be poured from one body to another. When caloric was added to the body its temperature is increased, and when the caloric is removed from the body the temperature is decreased. When a body could not contain any more calorie then the body is said to be saturated with caloric.

For Example: When salt is added to a glass of water after some limit it cannot dissolved more in it that point is known as saturation point. By using the same concept, two new terms were arisen saturated liquid and saturated vapour that are still in use today.

In 1798: caloric theory was challenged by Benjamin Thompson, he stated that heat can be generated by rubbing two things or in simple words by friction.

In 1843: James P. Joule proved by his experimental work that heat is not a substance and hence this theory is abandoned.

Application Areas of Heat Transfer:

Application of heat transfer are not only in engineering but also in our daily life

Human body is also an example of it. We constantly reject heat from our body to surrounding and this thing is related to human comfort. We try to control heat transfer of our body by clothing.

It has a wide range of application in our daily household appliances. Some examples are refrigerator, freezer, geyser, iron, TV, Computer etc

In the designing of many devices such as car radiator, solar collector and various components of power plant, heat transfer plays an important role

Roofs and walls of houses are designed on the basis of heat transfer analysis.

Modes of Heat Transfer:

Heat transfer from one place to another when there is a temperature gradient existing between both points/mediums.

Usually heat is transferred from one place to other by following three modes:

Conduction:

   When there is a temperature gradient exists in a stationary medium, which may be a solid or a fluid then we use the term conduction for the transfer of heat.

Example: Heat transfer from one part of metal rod to another by heading.

Convection:

   Convection occurs when there is a temperature difference between surface and a moving fluid.

Example: Cooling of a boiled egg.

Radiation:

   In radiation no medium is required to transfer heat, all surface of finite temperature emit energy in the form of electromagnetic waves. There is net heat transfer by radiation between two surfaces at different medium.

Example: Radiations from sun.