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# The Ultimate Guide

written by Stanley Udegbunam || Dec 29, 2020

## What is Thermal Conductivity?

Thermal conductivity refers to how highly or poorly a medium conducts heat.

It is an indication of the rate at which heat energy is transmitted through a medium by conduction process.

The symbol for thermal conductivity is k.

In actual practices, the thermal conductivity varies from point to point within the material.

For instance,  if kx , ky , and kz are thermal conductivities in x, y and z directions respectively, then in some cases,

(kx )1 ≠ (kx)2

(ky )1 ≠ (ky)2

The difference can be of negligible value and in such cases, the thermal conductivity is assumed to have the same value throughout the material. This depicts ideal cases.

In addition, the value of k for heat conduction in a given direction may be different for heat conduction in another direction. i.e.

kX ≠ ky ≠ kZ

Thermal conductivity varies from material to material.

They are generally high for solids, intermediate for liquids and low for gasses, as shown below

Kgas is in the range of 0.008 – 0.08 W/mK                (for gases)

Kliquid is in the range of 0.08 – 0.8 W/mK                (for liquid)

Ksolid is in the range of 0.8W/mK – higher W/mK   (for solids)

Heat is thermally conducted along the rod.

## Thermal Conductivity for Homogenous and Heterogeneous Materials

If k has the same value throughout a material medium, then the material is said to be homogeneous;

but if the thermal conductivity varies from point to point within the material, the material is said to be homogeneous.

i.e. for a given material medium,

k1 = k2        homogeneous material

k1 ≠ k2       heterogeneous material

Solids may consist of a fine mixture of different materials making them heterogeneous.

Also, porous substances like fired clay, cork, foam, etc are heterogeneous.

Thermal conductivity is also a function of temperature.

This means that a material can be heterogeneous if it exists at a non-uniform temperature.

As mentioned earlier, the variation of k in many engineering materials may be so small that the differences are assumed to be negligible.

In such cases, the material is assumed to be homogeneous.

Examples of homogeneous materials are Cu, Zn, H2O, air and aluminum  (at uniform temperature).

## Thermal Conductivity in Isotropic and Anisotropic Material

An Isotropic material is a material whose thermal conductivity at any location is the same in all directions.

But if k at a point varies with direction, the material is said to be anisotropic.

 kX = ky = kZ = k ………… if isotropic

kX1 = ky1 = kZ1 = k1 ——at location 1 (isotropic)

kX2 = ky2 = kZ2 = k2 ——at location 2 (isotropic)

but:

 kX ≠ ky ≠ kZ ………… if anisotropic

Examples of anisotropic materials are wood, glass other fibrous materials.

Many engineering materials can be assumed to be isotropic.

Water, copper, aluminum and materials with directionally invariant structures are isotropic.

copper is an isotropic material

## Variation of Thermal Conductivity with Temperature

Thermal conductivity for crystalline materials and liquids decreases with temperature.

Thermal conductivity for gas and amorphous substances (like glass and rubber) increases with temperature.

## Effect of Alloying on Thermal Conductivity

For alloys, the thermal conductivity may be in-between those of the constituents or even less than that of any of the material.

The alloy nickel-silver contains

Cu – 64%  k = 381  W/mK   at  20C

Ni – 18%  k = 90   W/mK  at  20C

Zn ~ 17%  k = 112.1  W/mK  at  20C

But k for nickel – silver at 20C is 31.3 W/mK, which is below the values for all the constituents.

alloying metals

## Effect of Density and Wetness on Thermal Conductivity

For any material, k increases with increasing density and with increasing wetness.

## Unit of Thermal Conductivity

The unit of thermal conductivity is obtained from Fourier’s law.

Fourier’s law states that:

“The heat flow rate per unit cross-sectional area A, is proportional to the average temperature gradient and it’s in the direction of decreasing temperature”.

Therefore,

Where:

L = length or thickness of the material (m);

Q = heat flow (W)

A = surface area of material(m2),

T2 – T1 = temperature difference (K or C)

Hence Unit of k = W/ mC   or   W/ mK

# The Ultimate Heat Guide

written by Stanley Udegbunam || Dec 29, 2020

## WHAT IS THERMAL CONDUCTION?

Thermal conduction is the mode of heat transfer by which heat is transferred in a material medium through the microscopic movement of the particles (molecules, atoms, electrons) in the medium without physically transporting the material from one position to another.

This means that thermal conduction doesn’t involve macroscopic movement, i.e bulk fluid movement.

Conduction requires the existence of a material medium before the heat can be transferred, unlike radiation where heat can be transferred through a vacuum and doesn’t need a material medium for its propagation.

Heat is thermally conducted in material from high-temperature areas to low-temperature areas.

Heat is thermally conducted along the rod.

## MICROSCOPIC AND MACROSCOPIC TRANSPORT

Let’s differentiate between these two terms to avoid confusion.

Microscopic transport:  This is the movement of micro-particles.

Microparticles are tiny and can’t be seen with the human eyes. They include molecules, atoms and electrons

Macroscopic transport: This is the physical movement of a material from one position to another. It’s also referred to as bulk transport.

It’s important to point out again that conduction involves microscopic transport of particles and not macroscopic transport.

Clear right?

Let’s continue..

## KINETIC THEORY OF MATTER AND CONDUCTION

According to the kinetic theory of matter, molecules vibrate about their mean positions in solids.

These molecules travel at different velocities depending on their kinetic energy.

The mean kinetic energy is proportional to the absolute temperature.

Molecules at higher temperatures have higher kinetic energy and collide faster with molecules that have lower kinetic energy.

As a result of this, heat is transferred from higher K.E – molecules to lower K.E molecules.

The increase in kinetic energy of the lower K.E molecules will lead to a corresponding increase in temperature.

Heat will continue to flow across molecules as long as a thermal difference exists.

A point is reached when the thermally conducted heat across all molecules exists at the same temperature.

This is known as thermal equilibrium.

Stage 1

Stage 2

Stage 3

## THERMAL CONDUCTIVITY

Thermal conductivity refers to how highly or poorly a medium conducts heat. The symbol for thermal conductivity is k.

Thermal conductivity in metals varies depending on the nature of the material.

There are no free electrons in insulators.

Conduction in insulators is governed only by molecular motion and therefore their thermal conductivities are consequently low.

For liquids and gases, the probabilities of collisions are lower than in solids because they have a longer mean free path.

Therefore, liquids and gases possess lower thermal conductivity when compared with solid.

Also, the molecules in liquids have higher chances of collision compared to gasses. This means that gases have lesser thermal conductivity than liquids.

SOLIDS —> LIQUIDS —> GASES

Decreasing order of thermal conductivity

Thermal conductivity provides an indication of the rate at which heat energy is transferred through a medium by the conduction process.

## FOURIER’S LAW OF CONDUCTION

Fourier’s law is a particular law of conduction, formulated by Joseph Fourier in 1922.

It helps us determine the parameters required for heat flow or heat transfer through a conducting medium.

Fourier’s law states that:

“The heat flow rate per unit cross-sectional area A, is proportional to the average temperature gradient and it’s in the direction of decreasing temperature”.

Where k is the constant of proportionality and it’s the Thermal Conductivity of the material.

The heat flow rate per unit area (Q/A) is known as the heat flux.

Obtaining the heat flux involves knowing the individual temperature or temperature difference, the geometry and the thermal conductivity of the object.

If given the heat flux and thermal conductivity of a material, we can determine the temperature difference using Fourier’s law of conduction.

## SUMMARY OF THERMAL CONDUCTION

Thermal conduction is the mode of heat transfer through the microscopic movement of particles.

Conduction requires the existence of a material medium before heat can be transferred.

Thermal conduction doesn’t require the physical movement of the material from one location to another (Macroscopic transport).

Thermal conductivity refers to how highly or poorly a medium conducts heat.

It is an indication of the rate at which heat energy is transferred through a medium by conduction process.

Thermal conductivity highest in solids and lowest in gases.

Fourier’s law of conduction states that “The heat flux, q is proportional to the negative average temperature gradient and it’s in the direction of decreasing temperature”.

Fourier’s law is useful for determining the heat flow rate, heat flux, thermal conductivity and temperature difference if given the necessary parameters.

# Types and Benefits

written by Stanley Udegbunam || Dec 25, 2020

## WHAT IS EVAPORATIVE COOLING?

Evaporative cooling is a rapid cooling phenomenon that involves the evaporation of liquid from a surface or an environment.

Water has a large enthalpy of vaporization. Therefore, it needs a relatively large amount of heat to change phase from liquid to gas.

Evaporation occurs when the water molecules absorb enough heat to break the intermolecular bonds.

Evaporation is an endothermic process; this means that the gas formed takes the heat along while leaving the surface.

This leads to a corresponding temperature drop on the surface hence it feels cool.

The cooling effect caused by surface temperature drop as a result of evaporation is what is referred to as evaporative cooling.

This phenomenon explains why our skin feels cool when the body sweat evaporates.

## TYPES OF EVAPORATIVE COOLING

There are two types of evaporative cooling:

1. Direct Evaporative Cooling and
2. Indirect Evaporative Cooling.

### 1.     Direct evaporative cooling

Direct evaporative cooling is the simplest form of evaporative cooling and is widely applied in dry regions.

Direct evaporative cooling system uses a blower to draw hot air from the outside environment and passes the hot air through a dampened sponge-like pad.

The water content in the pad absorbs the heat from the hot air and it evaporates from the pad.

The air leaves the cooling chambers either directly or through ducts at a much lower temperature suitable enough to cool the surrounding environment.

A direct evaporative cooling system works best in a hot, dry climate where it’s okay to add humidity to air while cooling it.

### 2. Indirect evaporative cooling

Indirect evaporative cooling works on the same principle as direct evaporative cooling lowering air temperature by causing water to evaporate.

The main difference between a direct and indirect evaporative cooling system is that:

A direct cooling system forces air through a dampened sponge-like pad with the help of a blower while an indirect cooling system uses a heat exchanger is to cool the air supplied to the living space.

The evaporative cooling cycle occurs in the heat exchanger therefore the water content of the cooled air remains unchanged in an indirect evaporative cooling system.

Indirect evaporative cooling lowers the air temperature without adding moisture to the air, making it more attractive than direct cooling.

indirect evaporative cooling

Apart from stand-alone types of evaporative cooling systems, there are devices that combine both direct and indirect evaporative cooling system.

## EVAPORATIVE COOLERS

Evaporative coolers, also known as swamp coolers are devices that use the cooling effect of the evaporation of liquid water to cool an air stream directly or indirectly.

The evaporative coolers are energy-efficient, cost-effective, eco-friendly and easily maintained.

## BENEFITS OF EVAPORATIVE COOLING

With a combined effort of cooling and strategically placed ventilation, evaporative cooling can solve these problems with unprecedented energy efficiency and incredible cost reduction.

Evaporative cooling offers:

• Improved efficiency and operational capability by introducing fresh, cool air in your facility
• Elimination of the use of refrigerant gases
• Reduction of carbon emissions; as well as
• Improve your corporate environmental credentials.

The cooling system boasts not only key environmental benefits but delivers incredibly high-efficiency rates due to its remarkably low energy consumption rates.

Evaporative coolers are far more economical than any other conventional air conditioning system.

Theyd can produce 35kw of air cooling for every 1.5kw of electricity consumed.

# The Ultimate Guide

written by Stanley Udegbunam || Dec 22, 2020

### Table of Content

#### AFRILCATE

Radiation is the transport of thermally generated electromagnetic waves.

This mode of heat transfer does not require the existence of an intervening material medium for heat to be transferred from one surface to the other.

Rather, it can be transferred through a vacuum or through a medium that is either transparent or translucent to the radiation.

Radioactive atoms are atoms with unstable nuclei. To attain stability, these atoms emit the excess energy by radioactive decay. These emissions are called radiation.

Contrary to popular opinion, non-radioactive substances also emit radiation because radiation encompasses all forms of energy, not just those produced by radioactive decay.

There are two types of radiation:

1. ionizing and

The word “ionize” refers to the breaking of one or more electrons away from an atom.

Ionizing radiation is radiation traveling as a particle or electromagnetic wave, that carries sufficient energy to knock off electrons from atoms or molecules.

Ionizing radiation comes from radioactive elements, cosmic particles from outer space and x-ray machines.

Typical ionizing subatomic particles found in radioactive decay include alpha particles, beta particles and neutrons.

Ionizing radiation is used in a wide variety of fields such as medicine, nuclear power, research, construction and many other areas.

Exposure to such radiation can cause damage to living tissue, cancer and in intense cases, death.

Non-ionizing radiation has enough energy to move atoms in a molecule around or cause vibration, but not sufficient enough to remove electrons from atoms.

• Alpha Particle
• Beta Particle
• Gamma Ray
• X – Rays

• Alpha Particle

They are positively charged and made of two protons and two neutrons from the atom’s nucleus.

Alpha particles are identical to the nucleus of a normal helium atom.

They are very energetic but only travel short distances.

Alpha particles can be very harmful, but they can’t penetrate the outer layer of the human skin.

They can be stopped by a sheet of paper.

• Beta Particles

They are small, fast-moving particles with a negative electrical charge that are emitted from an atom’s nucleus during the radioactive decay of an atomic nucleus.

A beta particle has the same mass and charges as an electron.

Beta particles travel farther in air than alpha particles.

They are also more penetrating but can be stopped by a thin aluminum plate.

• Gamma Rays

Also called photons, they are similar to visible light but have much higher energy.

Gamma rays consist of the shortest wavelength electromagnetic waves and so imparts the highest photon energy.

They can penetrate the human skin, therefore they are highly hazardous.

Gamma rays require shielding by dense material such as lead, or concrete.

• X – Rays

X-rays wavelengths are shorter than those of U.V rays and typically longer than those of gamma rays.

X-rays and gamma rays have the same basic properties but come from different parts of the atom.

They are generally lower in energy and therefore less penetrating than gamma rays.

X-rays are utilized mostly in the medical field to view images of the bones and other structures in the body.

x-ray

## WHAT IS ELECTROMAGNETIC SPECTRUM?

Electromagnetic Spectrum is the distribution of all electromagnetic waves arranged according to their frequency and wavelength.

The energy of the radiation shown on the spectrum below increases from left to right as the frequency rises.

## WHAT IS THE SYMBOL FOR RADIATION?

The international symbol used to indicate radioactive sources, radioactive materials and radioactive storage area is known as the trefoil.

The trefoil is a black propeller on a yellow background.

The presence of this symbol denotes the need for caution, a warning to avoid contamination with or undue exposure to atomic radiation.

# The Best Learning Guide

written by Stanley Udegbunam || Dec 20, 2020

## WHAT IS CONDUCTION?

Conduction is the transfer of heat or electricity between two matter in contact through direct molecular collision.

It requires a material medium for its propagation and it doesn’t travel through a vacuum.

Conduction is one of the modes of heat transfer, the others been convection and radiation.

For heat to be transferred between two bodies in contact, there must be a temperature difference.

Heat flows from high-temperature regions to low-temperature regions and this will result in either a loss or gain of thermal energy.

## TYPES OF CONDUCTION

They are three major types of conduction:

1. Thermal Conduction
2. Electrical Conduction
3. Sound Conduction

### WHAT IS THERMAL CONDUCTION?

Thermal conduction is a conductive process by which heat is transferred in a material through the microscopic movement of the particles in the medium.

In thermal conduction, heat is transferred without physically transporting the material from one position to another.

This means that thermal conduction does not require the physical movement of the material from one location to another.

The tendency of a substance to conduct heat varies depending on the nature of the material.

Materials with higher thermal conductivities conduct better than those with lower thermal conductivities.

Thermal conductivity refers to how highly or poorly a medium conducts heat.

It’s highest in solids and lowest in gases.

When we iron clothes, the heat is thermally conducted from the iron plate to the cloth in contact with the plate.

### WHAT IS ELECTRICAL CONDUCTION?

Electrical conduction is the movement of electrically charged particles through a transmission medium.

Electrical energy is generated when electrons move from one atom to another through a conducting material.

This movement sets up an electric field around the charged particles.

The flow of electric charge through a conducting material produces electricity.

Electrical Conduction in metals and resistors is governed by Ohm’s Law.

Metals are good conductors because they have a high free – electron density.

A good example of electrical conduction is Lightening.

Lightening is caused by excess charged particles in the cloud.

### WHAT IS SOUND CONDUCTION?

Sound conduction is the conduction of pressured air particles in the form of sound waves to the eardrums through a propagation medium.

A biological example of sound conduction is bone conduction in which sound is conducted as subtle vibrations through the bones of the skull to the inner ear.

A sound is a form of energy, just like electricity and light.

Sound waves are created when air molecules vibrate about their mean position and moves in a pattern like motion.

The stronger the vibrations produced, the louder the sound.

The music speaker and other sound devices utilizes the principle of sound conduction to produce sounds.

# The Best Introductory Guide

written by Stanley Udegbunam || Dec 19, 2020

## WHAT IS HEAT?

Heat is energy that is transferred from one body to another as a result of the difference in temperature.

If two objects with different temperatures are brought in contact, heat will be transferred across the objects.

The heat will flow from the object with a higher temperature to the object with a lower temperature.

Heat can also be defined as the amount of thermal energy in a system.

Thermal Energy is the energy obtained from the vibration of atoms and molecules of a substance due to its rise in temperature.

Heat is an extensive property because it is proportional to the total energy of all atoms in an object.

Heat capacity is the amount of heat required to raise the temperature of a body by 1 Kelvin.

It depends upon the mass of the material so it’s also an extensive property.

If there is a change in temperature of a substance without a change in phase, it is called sensible heat.

## WHAT IS LATENT HEAT?

Latent heat is the energy absorbed by or released from a substance during a phase change.

Latent heat is also called hidden heat because the heat absorbed or lost is used to either break or form bonds.

As a result of this, there’s no increase or decrease in temperature.

In otherwords, a substance might transform from one phase to another without a change in temperature.

A typical example is the melting of ice at 0 degree Celcius to liquid at 0 degree celcius.

Similar to the vaporization of liquid at 100 degree celcius to steam at 100 degree celcius.

Generally, an increase in temperature corresponds to an increase in the kinetic energy of the molecules.

But for an ideal phase change, there is no corresponding increase in kinetic energy of the molecules.

Latent heat of fusion is the thermal energy required for an object to change from the solid-state to the liquid state and vice versa.

While latent heat of vaporization is the thermal energy required for a liquid to vaporize to a gas or the heat released when a gas condenses to liquid.

Heat is transferred either through conduction, convection, radiation or a combination of any.

## UNIT OF HEAT

The S.I unit of heat is Joule.

Calorie is also a unit of heat energy but it is not the S.I unit.

1 cal = 4.186 J

## SUMMARY

Heat is the energy that is transferred from one body to another as a result of temperature differences.

Heat always flows from objects of higher temperature to objects of lower temperature.

While latent heat refers to changes in phases without a change in temperature, sensible heat is the change in temperature without a change in phase.