Principles of Heat Transfer

PRINCIPLES OF HEAT TRANSFER

Heat transfer is a discipline of thermal engineering that concerns with exchange of thermal energy or heat between physical systems.

When the temperature of a body increases, the energy supplied to the body is being stored in the form of thermal or heat energy.

In the normal process, the transmission of heat takes place from a region of higher temperature to a region of lower temperature. There are three modes of transmission of heat (Fig 1.1).

They are

1. Conduction

2. Convection

3. Radiation

Thermal Conduction, Convection and Radiation

Thermal Conduction:

It is the process in which heat is transferred from one point to another through the substance without the actual motion of the particles.

Thermal Convection:

It is the process in which heat is transmitted from one place to another by the actual motion of the heated particles.

Example: Let us consider a beaker of water heated by a flame as shown in figure 1.2. The water in the central portion at the bottom of beaker gets heated first.

It rises up and the water from the top comes down along the sides to get heated. This upward and downward motion can be made visible by placing a crystal of potassium permanganate at the bottom of the beaker.

The hot air furnace, hot water heating system and the flow of blood in the body are examples of convection.

This takes places in case of liquids or gases only.

Types of convection

There are two types of convection.

(i) Natural convection

 (ii) Forced convection

Natural convection

If the convection is induced by density differences, resulting from temperature differences within the fluid, then it is termed as natural convection.

Forced convection

If the fluid motion is caused by external mechanical means, e.g. by a pump, fan, etc., the process is known 'forced convection'.

Amount of heat transfer

The amount of heat transfer by convection is calculated on the basis of the Newton's law of cooling.

Amount of heat transferred

Q = h A ∆t

where A  - area of the surface from which heat is transferred

∆t temperature difference between the temperature of the surface, from which heat is transferred and the temperature of the fluid to which heat is transferred.

h- proportionality constant is called 'film coefficient of heat transfer' or simply 'convective heat transfer coefficient'.

Thermal Radiation

It is the process in which heat is transmitted from one place to the other directly, without any material medium.

Example: The heat radiation from the sun reaches us with an enormous velocity of light without any intervening medium as shown in fig. 1.3.

Though the sun is millions of miles away from the earth and there is no material medium for the greater part of the distance, the heat reaches us with the velocity of light.

Thus, heat radiations can pass through vacuum. The properties of heat radiations are similar to light radiation.

Properties of Thermal Radiation

• They travel through vacuum just like light.

• Like light, they travel in straight lines.

• They travel with the same velocity as light. aud eluselor re

• Radiant energy follows the law of inverse square as light.

• They exhibit the phenomenon of reflection and refraction as light.

Heat Conductions in Solids

It is a well known fact that heat is conducted through the material of a body. E

The heat is transmitted from a body of higher temperature to that of lower temperature.

As an example, when a metal rod is heated at one end, heat gradually flows along the length of the rod and other end of the rod also becomes hot after some time. (Fig. 1.4)

This shows that heat has travelled through the molelcules of the rod from one end to other. The molecules in the rod remain fixed in their mean positions.

On heating, the energy of molecules increases and they start vibrating more about their mean positions. They collide with the neighbouring molecules. Because of this collision, the neighbouring molecules are set into vibration.

Each molecule thus transfers some of heat it receives from its predecessor to its successor. Thus, the transmission of heat takes place by molecular vibrations in the case of conduction.

Definition

It is the process of transmission of heat from one point to another point through a solid substance (or some medium) without the actual motion of the particles (molecules or atoms) of the subtance.

Note

Heat conduction always requires some material medium. The material medium must be solid. As it requires material medium, the heat  conduction process never takes place in vacuum