CHAPTER (1)
INTRODUCTION AND DIFINITIONS
Heat transfer is the area of engineering dealing with the energy
transport from one place to another one as a resultant of temperature
difference. The subject of heat transfer has a great important in many
applications. It helps in air conditioning, refrigerations, power
stations, combustion engines and many other fields. In air
conditioning, it enables the designer in calculating the energy
transmitted through any building. This heat gained should be
removed to maintain that place at the comfort conditions. In the
refrigeration field, it helps in estimating the energy, which must be
removed from such a space to preserve foods or any product at the
suitable condition. In power stations and combustion engines, it is
important in designing the different components of the power plant.
Heat has three main forms as follows:
1- Heat conduction,
2- Heat convection, and
3- heat radiation.
The concept of heat conduction embrace the process of heat
propagation through direct contactis between particles of a substance.
In gases, heat conduction occurs by molecular and atomic
interactions. In fluid and solids, dielectrics by elastic molecular
collision. In metals, the flow of energy is mainly due to the diffusion
of free electronics. We can say that, conduction is the mechanism by
which heat energy is transmitted between the solids or the particles
of any solid element. The early development of heat conduction is
largely due to the efforts of the French mathimatician, Fourier
(1922), who first proposed the law that is known todays as Fourier’s
law of heat conduction. It predicts how heat is conducted through a
medium from a region of high temperature to a region of low
temperature. Let us consider a wall of cross section area (A) and
thickness (L) as shown in Fig. 1. 1,
and the two surfaces of the wall are at
temperatures of (T1) and (T2) and that
2
1
T1  T2. As a resultant of temperature
difference, heat is transferred between
the two surfaces. The amount of heat
T1
T2
A
7
transfer is proportional to the wall
normal area, temperature difference
(T1-T2) and opposite to the wall
thickness L. Thus we can say that:
Q
A( T1  T2 )
L
or:
Q
kA( T1  T2 )
L
L
Fig. 1. 1 Heat conduction
through a plane wall
(1. 1)
Where: k is the proportionality constant which is called the thermal
conductivity of the wall material.
Equation (1. 1) can be rewritten in a differential form as:
Q   kA
dT
dx
(1. 2)
Observing that, the quantity (dT/dx) is the change of temperature
with respect to the increase in the x coordinate. Since we want
Convection heat transfer problems are considerably more
difficult than those encountered in conduction, and analytical
solutions are frequently impossible. This difficulty arises from the
fact that, the basic mechanism for convection is a combination of
conduction and fluid motion. Convection occurs whenever a surface
comes in contact with a fluid at a temperature that is different from
its own of transfer. When it is transferred between the particles of
any solid fluid, it is known as heat conduction. When heat is
transferred between fluid and solid surfaces it is called heat
convection. The amount of heat transfer by convection between any
surface and a fluid can be obtained by the following relation:
Q = h A (Tw - Tf)
(1. 3)
where: h is called the heat convection coefficient, A is the area of
heat transfer. In equation (1. 3) the wall is consedered at higher
temperature than that of fluid.
The third form of heat transfer is called thermal radiations. This
method of heat transfer occurs between non-contact bodies if they
are at different temperatures.