Applications Of Heat Transfer Fundamentals In Buildings
INTRODUCTION
The intent
of this blog is to throw light upon the various considerations that one must
make in the domain of heat transfer when it comes to providing thermal
solutions to a building or designing one.Building heat transfer calculations
are performed for different applications like Heat loss and heat gain through
the exterior envelope, conduction through exterior envelope, conduction heat
transfer through basement walls and slab-on-grade floor, short-wavelength (or
solar heat) transmission, absorption, and reflection for fenestration, air
leakage through exterior envelopes as well as the interior partition walls,
ceilings and floors, The calculations of heat transfer also depend on material
or building element-related problems like cold-bridge effect, convection within
porous insulation, moisture condensation due to the simultaneous flow of air,
moisture, and heat.
The fact
that heat transfer mechanisms for buildings are typically ill-defined,
time-dependent, multi-dimensional, and non-linear is fundamental to all of
these applications. As a result, all current solutions for any of these
applications are dependent on multiple simplifying assumptions. Although
refined and sophisticated solutions are sometimes available, they are sometimes
too complicated and of little practical utility. Furthermore, most applications
of sophisticated heat-transfer analysis have typically been studied by the
aerospace and nuclear power sectors, at least in the United States, until
recently. As a result, many difficult problems in building heat transmission
have remained unaddressed.
Fig.1 Modes of heat transfer[9]
The purpose
of a building is to provide safety and thermal comfort for healthy living.
Buildings are essentially made up of an envelope, which serves as a barrier
between the outside and the inside environments. In reaction to exterior
climatic conditions, the building envelope is principally responsible for
managing inside thermal comfort. It generally consists of a combination of
building components that has provided the required structural performance thus
far. However, research into different building-envelope layouts to achieve a
certain thermal performance is restricted. Heat and moisture will be
transferred to the internal environment through the building envelope since it
is exposed to the outside environment. The microstructure and layout of the
building material have an impact on the overall phenomena of heat and moisture
transmission.
Furthermore,
a material's thermal behavior is largely determined by its microstructure,
which is made up of a network of pores and particles organized in a certain
pattern. The thermal conductivities of solid particles, pore microstructure,
and the component fluid (air and/or moisture) all influence the thermal
behavior of construction material. The thermal properties of specific building
materials and their design affect the thermal response of a building envelope.
Understanding how intricate networks of pores and particles impact heat
transmission is relatively new research in the field of building climate
response. The current study examines the heat-transfer mechanisms that govern a
building material's thermal performance as a function of its microstructure.
CONDUCTION:
Conduction heat transfer problems relevant to buildings
include:
1 Exterior
Wall Conduction:
○ transient heat transfer responding
to climatic effects, such as temperature fluctuation, solar radiation, wind and
precipitation; thermal storage ... damping and lag effect; and cold-bridge
effect (two-dimensional and non-linear heat flow path).
2 Interior
Mass Conduction:
○ heat storage in partition walls,
floor/ceiling sandwich.
3 Conversion from heat gain/loss to cooling and heating
load.
4 Ground heat loss from slab-on-grade floor and basement
walls.
CONVECTION
Convection is the flow of heat within a fluid, with
warmer fluids rising and colder fluids falling.
In homes, this fluid is air.
Therefore for our intents and purposes, convection
is a heat transfer mechanism, resulting from the movement of air at different
temperatures.
This is a
very important mechanism in the design of buildings, where air movement is
necessary to
●
Moderate
internal temperatures.
●
Reduce
the accumulation of moisture, odours, and other gases that can build up during
occupied periods.
●
Improve
the comfort of occupants.
In the air, convection is often called the “stack
effect.” As air warms, the molecules move farther apart, and the air becomes
more buoyant, floating upwards. As that air rises, cold air is pulled from
below to replace it.
In a boiler or heat pump, warmed water circulates in
a similar way, and piping systems can be designed to use this “thermosiphon” to
circulate water.
When we account for convective air flows in buildings,
we look at the following variables:
● Difference in
temperature (ΔT): As with all methods of heat transfer, a difference in
temperature from one area to the other is a necessary condition for heat to
flow.
● Time (t): Length of
time the air movement occurs.
● The volume of air (V):
The volume of air within a home can be measured by multiplying the length,
width, and height of interior space. The volume of air in a home remains
constant, although the air itself changes.
● Air changes per hour
(AC/hr): The rate of air movement is measured as air changes. The “change” is
the movement into and out of a defined space, such as the volume of air in a
room (the amount used to balance the airflow in an HVAC system), or in a whole
house (the amount used to measure house leakage).
In buildings, heat is also transported by the
following mechanisms, which basically belong to the convective mode:
● Transfer of latent heat
by transport of water or water vapor.
● Thermal energy
associated with the air being replaced in a building by ventilation or by air
leakage (infiltration).
● Thermal energy
associated with fresh and used domestic water and combustion air (including
flue gases), and fluids feeding heat pumps.
Air movement in buildings can be 'forced' (for
example driven by fans), or 'natural' resulting from pressure differences from
one part of a building to another. Natural air movement can be either
wind-driven or buoyancy-driven.
Accurately predicting the movement of air within
buildings is extremely complicated and can require the use of computational
fluid dynamics (CFD) modelling software.
Fluids can also be used to transfer heat within a
building by 'mass transfer', for example by the flow of a refrigerant, chilled
water, or hot water around a building to provide heating or cooling.
RADIATION
Heat transfer through radiation takes place in the
form of electromagnetic waves mainly in the infrared region. Radiation emitted
by a body is a consequence of thermal agitation of its composing molecules.
Radiation heat transfer is very important in building
application in the following areas:
A) Short-wavelength radiation:
● Solar heat absorption
on opaque exterior surfaces
● Solar heat transmission
through transparent surfaces
● Solar heat absorption
and reflection by interior building surfaces
● Absorption and
reflection of solar heat by window glass
B) Long-wavelength
radiation:
● Heat emission by the
exterior surfaces to the sky
● Heat exchange among
interior surfaces
● Heat exchange between
interior surfaces and occupants
● Heat exchange between
the lighting fixture and interior surfaces
Solar gain through windows exposed to either the
direct sun, or reflected sun (reflected off the particles in the sky, creating
diffuse radiation, or reflected off a surface) can dramatically affect the heat
flow in a building. Hence, the building energy flows must account for the solar
gain through windows. This amount of heat can dominate the performance of a
modern building with relatively high window coverage (i.e., above 20 to 30%
window to wall ratio). The Solar Heat Gain Coefficient (SHGC) is the window
property used to rate the amount of energy allowed through windows. The SHGC is
the fraction of incident solar radiation that passes through a window and
becomes heat inside the building. For
example, if the SHGC of a glazing unit is 0.50, and the sun is shining on the
window with an intensity of 500 W/m2, 250 W/m2 will enter the building.
While extensive work has been reported in the area
of solar energy exchange with exterior surfaces, basic irradiation data are
still insufficient with respect to the diffuse sky radiation component,
particularly for vertical surfaces and for cloudy sky conditions. Perhaps the
most difficult and tedious problem in dealing
with solar heat exchange analysis for building
applications is the analysis of direct or beam radiation that is transmitted
through fenestration, absorbed, reflected and reemitted by the interior
surfaces. Because of the complex geometry of the time-dependent shade and
sunlight patterns, exact solutions to simulate realistic solar heat exchange in
a room are virtually impossible. It is usually assumed that the interior
surface is gray (non-spectral) and diffuse (no specular reflection), and the
solar heat is diffused once it enters through the window and interior shading
devices. The long-wavelength radiant heat emission to the sky from the building
exterior surface has not been well explored except for the clear sky condition,
although some work is going on at CSTB in France.
BUILDING MATERIALS
Principles of Heat Transfer Through Walls: Before we touch upon the microscopic considerations and
factors let's take a look at the Macroscopic section of our topic of discussion
- ‘The Walls’.When it comes to walls the basic factors that need to be looked
into are, the heat flow under steady
conditions, the surface resistance, transfer of heat by air infiltration, and
the flow of heat under variable conditions.
The steady
flow of heat through a wall by reason of a constant temperature difference
between the surroundings on the two sides depends upon four partially
independent processes,
(1) the
transfer of heat to the wall from the
surroundings on the hot side,
(2) the
transfer through the wall,
(3) the
transfer of heat to the surroundings on the cold side, and
(4) the
diffusion or flow of air through the wall in either direction.
The
resistance to heat flow between the surface of a wall and its surroundings is
ordinarily called surface resistance, although it is only partially dependent
upon phenomena taking place at the surface of the wall. The transfer of heat
takes place by all the modes of heat transfer, viz, conduction, convection, and
radiation, and the relative contribution of each depends upon the conditions.
Heat is transferred from the air to the wall, or vice versa, by convective air
currents set up by temperature differences between wall and air, by wind or
forced ventilation.. In addition to the heat transferred by convection and
conduction in the air, an entirely independent transfer takes place by
radiation between the surface of the wall and its surroundings. The magnitude
of such transfer depends on the absolute temperature, the temperature
difference, and the character of the surfaces of the wall and surroundings. If
the wall and its surroundings had reflecting surfaces of clean, bright metal,
the interchange of heat by radiation would be very small at ordinary
temperatures. In all practical cases, however, the surfaces are nonmetallic,
and the transfer of heat by radiation is usually somewhat greater than the
convective transfer, even at ordinary temperatures.
The thermal
resistance of the wall itself is separate and distinct from the surface
resistances. It is a property of the wall and is not influenced by the
surroundings except in certain cases of air leakage which will be noted later.
Heat transfer through solid walls takes place only by conduction in the
direction of temperature gradients. Such transfer is proportional to the
temperature difference between the two surfaces of the wall and further depends
on the materials composing the wall. The thermal conductivities of building
materials in general, increase slightly with increasing temperature;
consequently the resistance of a wall will decrease somewhat with increasing
mean temperature of the wall.
Aside from
conduction, convection, and radiation, an entirely independent transfer of heat
may take place by infiltration of cold air through the wall on the windward
side of a building, with the conse- quent efflux of warmer air on the lee side
of a building. In an extreme case, air infiltration may be so large as to cause
material change in the temperature gradient in the wall, and, therefore,
influence the outward heat flow by conduction.
Heat
Transfer Mechanisms in Building Materials
The heat transfer through the material is a combination of
conductive, convective and radiative heat transfer components. A schematic
representation of the heat transfer mechanism through the porous
building-material microstructure as shown in Fig. below. Conduction involves
heat transfer through excitation of atoms, while convection involves heat
transfer through molecule movement induced by differential temperature
variation; radiative heat transfer involves heat-transfer through
electromagnetic energy.
Fig. 2 heat transfer mechanisms
through the porous building material[8]
Understanding
heat transfers through the building materials is a complex phenomenon due to
the irregularity of the porous building-material microstructure. It is a
challenge to quantify the modes of heat transfer through material solid matrix
and voids. In such materials, heat is propagated by thermal conduction through
the solid phase, thermal conduction through the fluid phase, radiation between
solid particles, and convection in the fluid phase. The heat transfer through
different components is briefly explained below. These complex heat transfer
processes involve many components such as:
- Heat conduction in solid matrix/particles,
- Heat conduction through pore fluid (air or water),
- Heat conduction in micro-gaps that exist between
particles,
- Particle contact heat conduction,
- Heat transfer through pore fluid,
- Radiation from solid surfaces of pores (particle to
particle radiation in pores)
Heat conduction in solid matrix/particles:
Heat
conduction occurs through the solids matrix/particles in a material by
electrons (particle collision)
and photons
conduction (lattice vibration). The thermal conductivity of the material is a
combined influence of these two mechanisms from the following equation:
λ= λl + λe
Where, λl
and λe are the lattices and electronic thermal conductivities
In metals,
heat conduction through electron collision is effective and dominant; in
non-metal and insulators heat conduction through lattice vibration is
effective, which do not have many free electrons.
Heat conduction through pore fluid (air or water):
The
component of heat transfer mainly depends on the type of the fluid present in
pores. Fluids may be air or water; the thermal properties of fluids vary with
respect to their chemical composition and state (phase). For example, water
conducts heat nearly twenty-five times more than air. Scientist Loeb explains
heat conduction through pore fluid, especially through gases, to be dependent
on the mean free path (λl), which is a function of temperature and pressure.
This is valid when pore dimensions are larger than the mean free path. However,
gas conductivity falls below the free gas value if the pore structure is finer
than the mean free path, in which case there will be gas molecule–wall
collisions (Knudsen conduction). From kinetic theory, the mean free path is the
average distance covered by a moving particle between collisions with other
moving particles. It is evident that the mean free path in air varies also with
relative humidity (moisture content in air), along with temperature and
pressure.
Fig. 3 Mechanisms of heat transfer
in porous materials[8]
The
phenomenon of particle contact heat conduction can be neglected at normal
temperatures and pressures but might be very substantial at low pressures and
moderate temperatures.
Fig.4
Particle-contact and other heat transfer paths in a material.[8]
Heat
transfer through pore fluid
The heat
transfer through pore fluid mainly depends on the type or nature of the fluid
present in pores, and primarily occurs through convection. Fluids may be air or
water. Heat exchange or transfer from the particle to fluid in the pore is
eventually by convection.
Radiation from solid surfaces of pores (particle to particle
radiation in pores)
The
radiative component is more significant at very low densities at a high
temperature where the effective coefficient of heat conductivity of porous
building materials will be high. At very high temperatures the pores themselves
offer little resistance leading the porous body to conduct as though the pores
were solid. With increasing temperature, the apparent thermal conductivity of
loose and porous insulating materials become greater, because the
conductivities of the solid substance and of the gas in the pores, and the
inner radiation increase. The scale of the pore structure (and the grain size)
can also affect radiative heat transfer since radiation is effectively
scattered by interfaces (grain boundaries). A fine-scale structure results in
increased scattering and reduced transmission. However, an increase in the
scale increases the possibility of convection within or between pores. Luikov
studied the effect of the radiative heat transfer in porous materials for
different pore sizes at different temperatures, and concluded that radiative
heat transfer can be neglected for a pore diameter smaller than 5mm. Luikov
also showed that with (Gr.Pr) < 1000 convective heat transfer in closed
pores can be neglected and that the heat transfer through the fluid layer in
the gap occurs by conduction. Clyne et al showed the effect of the temperature
and pressure on the Grashof number. Convection is significant if the Grashof number
exceeds 1000. The study concludes that closed-cell convection can be neglected
for most porous materials, under commonly encountered conditions. In open-cell
materials, on the other hand, convective heat transfer may be highly
significant, depending on the connectivity and scale of the pores.
Fig.5 Dependence of Grashof number
on length scale (pore size), for different pressures and temperatures[8]
CHAPTER 4
CONCLUSION
In this blog
we've attempted to bring the reader's attention to the various considerations
and phenomena that need to be taken into account when designing buildings from
a thermal perspective. Basic problems
and difficulties inherent in the comprehensive analysis of building heat
transfer, are due to many parameters which are ill-defined and some of which
are time-dependent and multi-dimensional. We have discussed some of the factors
ranging from conduction convection and radiation considerations to the macro
and microscopic effects of building materials in the building heat transfer.
REFERENCES
[1]Raychaudhuri,
B. c., Transient thermal response of enclosures; the integrated thermal time
constant, International J. Heat Mass Transfer 8 , 1439 (1965).
[2] Laudo n,
A. C., Summertime Temperatures in Buildings, Building Research Studies, 1968.
Building Research Establishment, Building Research Station, Carston, Watford,
WD2 7JR.
[3] Muncey,
R. W., The thermal response of a building to sudden changes of temperatures or
heat Oow, Australian J. Sci. 14, 123,1963.
[4] Mitalas,
G. P., and Arsenault, J. C. Fortran IV program to calculate Z-transfer
functions for the calculation of transient heat transfer through walls and
roofs, use of computers for environmental engineering related to buildings,
NBS-BSS 39, 1971, pp. 663-668.
[5]
Discerning heat transfer in building materials N. C. Balajia *, Monto Manib, B.
V. Venkatarama Reddy
[6] Tamami
Kasuda, Fundamentals of Building Heat Transfer Institute for Applied
Technology, National Bureau of Standards, Washington, D.C. 20234 (July S,
1977).
[7] Muncey,
R. W., The thermal response of a building to sudden changes of temperatures or
heat Oow, Australian J. Sci. 14, 123, 1963.
[8] N. C.
Balajia*, Monto Manib, B. V. Venkatarama Reddyc, Discerning heat transfer in
building materials
4th
International Conference on Advances in Energy Research 2013, ICAER 2013
[9 Insulation Materials DOI:10.13140/RG.2.2.36009.03681 Applications of Renewable Energy by Tawfeeq
Wasmi Mohammed Salih -Al-Mustansiriya
University
Comments
Post a Comment