Heat Transfer Basics

Heat is a form of energy. HEAT TRANSFER is part of thermal engineering in which heat is a transfer from one body to other body and both are at different temperatures. Heat is transferred from the body at higher temperatures to lower temperatures. For transferring heat it requires a medium.

Representation 

SI Unit -Joule, denoted as ' J ' 

CGS Unit - Calorie

 Unit Conversion -1 Cal = 4.186 J

There are three ways for heat transfer

-Conduction

-Convection

-Radiation

Conduction

It is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. It occurs above hot because hot air expands, becomes less dense, and rises.

In Pic. You can clearly understand how heat is flowing. In this one paper of rod is heated but after some time the other part will become hot.

In this when one part is heated, its molecule vibrates and transfer heat to the adjacent molecules and some occur with other molecule and in this way, heat is transferred.

Heat is conducted through solid, liquid , gases.

It is governed by Fourier's law

                              Q = -kA(dT/dx)

                                   ......where K is thermal conductivity and Q is heat transfer rate

                                                    A is area of heat flow normal to direction of flow 

                                                     dT/dx is temperature gradient

Convection

When a fluid, such as air or liquid is heated and then travel away from the source, it carries the thermal energy along with the movement. This type of flow called convection.

It cannot take place in solid.

In this Pic water is heated in a bowl,

water at the bottom part gets heated and its density reduced. Due to that, it moves upwards

and the same process occurs multiple times and in this way, heat is transferred. This heat is transferred due to density differences.

It is governed by Newton's law of cooling

                      Q=hA(Tw-Tf)

                                    ......where Q is the heat transfer rate 

                                                     A Surface area 

                                                     h is co-efficient of convective heat transfer or film conductance

Convective heat transfer is defined as the ratio of thermal conductivity to its thickness.

Radiation

Anybody above absolute zero emits radiation.

All the bodies radiate heat energy.

Heat transfer through radiation takes place in the form of electromagnetic wave mainly in the infrared region. Radiation emitted by a body is a consequence of thermal agitation of its composing molecule.

If two bodies are placed at a different temperature, then both the bodies send radiation to each other as both the body are above absolute zero temperature 

But hotter body send more radiation as compare to a colder body 

which result in 

Hot body temperature reduced and cold body temperature increased.

It is Governed by the Stefan-Boltzmann law of radiation

 P is the rate of heat emission

 A is radiating surface

T is the absolute temperature 

   σ   is Stefan-Boltzmann constant

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