Civil Engineering Reference
In-Depth Information
to electrical appliances, lighting and people (
Q
iGain
). In the following, a detailed
description of the procedure to estimate each of the terms which appear in Eq.
4.20
is given.
1.
Heat Gain by means of Natural Convection
(
Q
conv
)
Thermal convection can be defined as a process which allows to transfer heat
between a surface and a fluid in movement which is in contact with this sur-
face (Incropera and De Witt
2002
). In addition, convection can be classified as a
function of how the fluid motion is initiated (Cengel
2006
):
•
Natural or free convection
. Any fluid motion is originated by buoyancy
forces, which, at the same time, are derived from density differences caused
by temperature changes in the fluid.
•
Forced convection
. The fluidmotion is produced bymeans of external factors,
such as a fan or outside winds.
In addition, as a consequence of the interaction between the surface and the fluid,
a region known as
velocity boundary layer
is originated. In this region, it can be
observed that the velocity of the fluid varies from zero at the surface to a finite
value associated with the flow at a certain distance of it (Incropera and De Witt
2002
). Moreover, inside this layer the fluid motion can be classified as:
•
Laminar flow
. Its main characteristics are smooth streamlines and highly
ordered motion.
•
Turbulent flow
. It is characterised by velocity fluctuations and highly disor-
dered motion.
Therefore, heat transfer by means of convection depends on the nature of the fluid
(density, thermal conductivity, specific heat and viscosity), the type of fluid flow
(laminar or turbulent), the surface geometry, the existing temperature differences
between the surface and the fluid and the characteristic dimension. However,
despite the complex nature of heat transfer by convection processes, they can be
expressed through
Newton's law of cooling
(Incropera and De Witt
2002
), see
Eq.
4.21
.
Q
conv
=
h
c
A
(
T
s
−
T
∞
)
(4.21)
where
Q
conv
, the convective heat flux (
W
), is proportional to
A
, the surface area
(m
2
),
h
c
which is known as the convection heat transfer coefficient (W/m
2
K) and
the temperature difference between the surface temperature and the fluid temper-
ature in a point far enough from the surface,
T
s
and
T
∞
, respectively (
K
).
Therefore, to estimate the heat gain obtained by means of natural convection
inside a room, it is necessary to calculate the heat gain added by each surface
of the room, i.e. the walls, the floor and the ceiling, see Fig.
4.12
a. Furthermore,
the convection flux between each surface of the room an the air is assumed to
be
positive
if heat is transferred
from
the wall
(
T
s
>
T
∞
)
, and it is considered
negative
if heat is transferred
to
the wall
(
T
∞
>
T
s
)
(Incropera and De Witt
