Canceran, CharleneLlana, Mervin FloydRusiana, Eugene Benito

Title: Comparative analysis of heat transfer and thermal conductivity of construction woods in the Philippines

Chapter 2

REVIEW OF RELATED LITERATURE

HISTORY AND OVERVIEW OF THE LOCAL WOOD INDUSTRY

CURRENT USE AND APPLICATIONS OF LOCALLY AVAILABLE WOODS

MATERIAL SCIENCE OF WOOD: MECHANICAL AND CHEMICAL PROPERTIES

Wood is a natural product with a complex structure, and thus one cannot expect a homegeneous

product for engineering designs such as with an alloy steel bar or an injectiion-molded thermoplasic

part. The strength of wood is highly anisotropic, with its tensile strength being much greater in the

direction parallel to the tree stem (Smith, 1997). The anisotropy of the wood structure is manifested in

almost all wood properties. The structure alone, however, is not the only factor that controls properties.

For example, trees have moisture present in them. The natural moisture content of a newly cut tree the

green condition, which can vary from 30% to 200%, depending on the species, sapwood, or heartwood

and time of season. However, the structure of wood is hygroscopic, so water can also be absorbed

unless treated. Moisture content, whether green or absorbed, affects mechanical, electrical, and

chemical properties. Another factor that affects properties is the density, or specific gravity, that is

affected by the growth structure of the specific wood piece (Colling & Vasilos, 1995).

The cross section of a wood consists of outer bark layer which is composed ofdry, dead tissue and

provides external protection for the tree, inner bark layer which is moist and soft and carries foof from

the leaves to the growing parts of the tree, cambium layer which is the tissue layerbetween the bark

amd wood that forms the wood and bark cells,

Sapwood which is the light colored wood which forms the outer part of the tree stem. The sapwood

contains some living cells which function for food storage and carry sap from the roots to the leaves of

the tree, hartood which is the older inner region of the tree stem which no longerliving. The heartwood

is usually darker than the sapwood and provides strength for the tree, and pith which is the soft tissue at

the center of the tree around which the first growth of the tree takes place.

Trees are classified into two major groups called softwoods (gymnosperms) and hardwoods

(angiosperms). The botanical basis for their classification is that if the tree seed is exposed, the tree is a

softwood type and if the seed is covered, the tree is hardwood type. With a few exceptions, a softwood

tree is one which retains its leaves and a hardwood tree is one which shed its leaves annually. Softwood

trees are often referred to as evergreen trees ad hardwood trees are deciduous trees. Most softwood

trees are physically soft and most hardwoo trees are physically hard but, there are execeptions.

Lumber used for structural purposes is most commonly softwood, such as white pine, which is cut,

dried, and visually inspected or mechanically graded. Both natural seasoning (air drying) and kiln drying

are practiced to reduce the moisture content of the lumber to that whic is desired for use. As drying

takes place, the wood becomes becomes denser (shrinks) and stronger but because wood is also

hygroscopic, this effect on mechanical strength is reversible to some extent. Density is also affected by

the growth rate because of the difference in earlywood and latewood.

People normally think of wood as an electrical and thermal insulator, but this true only when the wood

has low moisture content. In the live or green condition, wood is conductive; this is also true when wood

absorbs moisture. Because of its high electrical resistance when dry,though, wood is used for electric

utility poles (protected from absorbing moisture creosote)a nd for tool handles for use around

electricity. It is a common knowledgethat good electrical resistance is synonymous with good thermal

resistance and wood is a good thermal insulator.

Wood is combustible – it burns. This good thermal resistance is important in stuctural applications of

wood because wood away from the charred or pyrolyzed surface remains relatively cool. Thus load-

bearing members retain their strength for long periods of time under fire conditions. This means that

evacuation proceed safely without conern of building collapse, at lear during early stages of fire.

Wood is also used for some applications because of its resistanceto chemicals. For example, it is used

extensively for cooling towers where he boiler waters being cooled have been treated with chemicals

and algae-destroying chlorine. Although the chemicals do cause swelling of the wood, the reaction is

reversible. There are some reactions that occur, however, that weaken the wood and are not reversible.

If steel nails are not galvanized and are used to join wood in the outdoors, for example, exposure can

produce iron salts that soften ad discolor the wood around the corroded nail (Colling & Vasilos, 1995).

MECHANISMS OF HEAT TRANSFER

The transfer of energy in the form of heat occurs in many chemical and other types of processes. Heat

transfer occurs because of a temperature difference driving force and heat flows from high to low

temperature region.

Heat transfer may occur by any one or more of the three basic mechanisms of heat transfer: conduction,

convection, or radiation.

In conduction, heat can be conducted through solids, liquids, and gases. The heat is conducted by heat

transfer of the energy motion between adjacent molecules. In a gas, the “hotter” molecules, which

have greater energy and motions, impart energy to the adjacent molecules at lower energy levels. His

type of transfer is present to some extent in all solids, gases, or liquids in which a temperature gradient

exists. In conduction, energy can also be transferred by “free” electrons, which is quite important in

metallic solids.

The transfer of heat by convection implies the heat transfer of bulk transport and mixing of macroscopic

elements of warmer portions with cooler portions of a gas or a liquid. It also often involves the energy

exchange between a solid surface and a fluid. A distinction must be made between forced-convection

heat transfer, where a fluid is forced to flow past a solid surface by a pump, a fan or other mechanical

means, and natural or free convection where warmer or cooler fluid next to to the solid surface causes a

circulation because of density difference resulting from the temperature differences in the fluid.

Radiation differs from heat transfer by conduction and convection in that no physical medium is needed

for its propagation . Radiation is the transfer energy through space by means of electromagnetic waves

in much the same way as electromagnetic light waves transfer light.

In order to transfer a property such as heat, a driving force is need to overcome a resistance:

rate of a transfer process=driving forceresistance

The transfer of heat by conduction follows this basic equation and is written as Fourier’s law of heat

conduction in fluids or solids:

qxA

=−k dTdx

Where qx is the heat transfer rate in the x-direction in Watts(W), A is the cross-sectional area normal to

the direction of flow of heat in m2, T is the temperatur ein K, x is the distance in m, and k is the thermal

conductivity in W/m·K in the SI system. The quantity qx/Ais called the heat flux in W/m2. The quantity

dT/dx is the temperature gradient in the x direction. The minus sign is required because if the heat flow

is positive in a given direction, the decreases in this direction (Geankoplis, 1995).

PREVIOUSLY UTILIZED METHODS IN DETERMINING CONDUCTIVITY OF MATERIALS

RELATED ENVIRONMENTAL CONCERNS

SAFETY AND PRECAUTIONS REGARDING LOCALLY AVAILABLE WOODS