Introduction To Heat Transfer- Conduction, Convection, Radiation

Introduction Heat Transfer/ What is Heat Transfer

Heat will always be transferred from higher temperature to
lower temperature independent of the mode. The energy
transferred is measured in Joules (kcal or Btu). The rate of
energy transfer, more commonly called heat
transfer, is measured in Joules/second  (kcal/hr or
Btu/hr). Heat is transferred by three primary

  • Conduction (Energy transfer in a solid)
  • Convection (Energy transfer in a fluid)
  • Radiation (Does not need a material to travel through)

Three Modes Of Heat Transfer
Three Modes Of Heat Transfer


The conduction of heat takes place, when two bodies are in
contact with one another. If one body is at a higher
temperature than the other, the motion of the molecules in
the hotter body will vibrate the molecules at the point
of contact in the cooler body and consequently result
in increase in temperature.   The amount of
heat transferred by conduction depends upon the temperature
difference, the properties of the material involved, the
thickness of the material, the surface contact area,
and the duration of the transfer. Good conductors
of heat are typically substances that are dense as they have
molecules close together. This allows the molecular
agitation process to permeate the substance easily. 

So,metals are good conductors of heat, while gaseous
substance, having low densities or widely spaced
molecules, are poor conductors of heat. Poor conductors of
heat are usually called insulators. The measure of the
ability of a substance to insulate is its thermal resistance.
This is commonly referred to as the R-value (RSI in
metric).  The R-value is generally the inverse of
the thermal conductivity, the ability to
conduct heat.

Typical units of measure for conductive heat transfer

Per unit area (for a given thickness)

Metric (SI) :  Watt per square meter (W/m)


Metric (SI) :  Watt (W)  or kilowatts (kW)


The transfer of heat by convection involves the movement of a
fluid such as a gas or liquid from the hot to the cold
portion. There are two types of convection: natural and
forced. In case of natural convection, the fluid in
contact with or adjacent to a high temperature body is
heated by conduction. As it is heated, it expands, becomes
less dense and consequently rises. This begins a
fluid motion process in which a circulating current of fluid
moves past the heated body, continuously
transferring heat away from it. In the case of forced
convection, the movement of the fluid is forced by a fan,
pump or other external means.  A centralized hot
air heating system is a good example of forced convection.

Convection depends on the thermal properties of the fluid as
well as surface conditions at the body and other factors
that affect the ability of the fluid to flow. With a low
conductivity fluid such as air, a rough surface can trap
air against the surface reducing the conductive
heat transfer and consequently reducing the convective

Units of measure for rate of convective heat transfer

Metric (SI) : Watt (W) or kilowatts (kW)

Thermal Radiation

Example Of Heat Transfer
Fig. Example Of Heat Transfer

Thermal radiation is a process in which energy is
transferred by electromagnetic waves similar to light
waves. These waves may be both visible (light) and invisible.
A very common example of thermal radiation is a heating
element on a heater. When the heater element is first
switched on, the radiation is invisible, but you can
feel the warmth it radiates.  As the element heats,
it will glow orange and some of the radiation is now
visible.  The hotter the element, the brighter it
glows and the more radiant energy it emits.

 The key processes in the interaction of a substance
with thermal radiation are: 

  • Absorption- the process by which radiation enters a
    body and becomes heat.
  • Transmission -the process by which radiation passes
    through a body
  • Reflection -the process by which radiation is
    neither absorbed or transmitted through the body; rather it
    bounces off

Objects receive thermal radiation when they are struck by
electromagnetic waves, thereby agitating the molecules
and atoms. More agitation means more energy and a higher
temperature.Energy is transferred to one body from
another without contact or transporting
medium such as air or water.In fact, thermal
radiation heat transfer is the only form of heat transfer
possible in a vacuum.

All bodies emit a certain amount of radiation. The amount
depends upon the body’s temperature and nature of its
surface. Some bodies only emit a small amount of radiant
energy for their temperature, commonly called
low emissivity materials (abbreviated low-E).  Low-E
windows are used to control the heat radiation in and
out of buildings. Windows can be
designed to reflect, absorb and transmit
different parts of the sun’s radiant energy.

The condition of a body’s surface will determine the amount
of thermal radiation that is absorbed, reflected or
re-emitted. Surfaces that are black and rough, such as black
iron, will absorb and re-emit almost all the energy that
strikes them. Polished and smooth surfaces will not
absorb, but reflect, a large part of the incoming radiant

Typical units of measure for rate of radiant heat

Metric (SI) ——Watt per square meter (W/m