Introduction - combustion Combustion or burning is the sequence
of exothermic chemical reactions between a fuel and an oxidant
accompanied by the production of heat and conversion of chemical
species. The release of heat can result in the production of light
in the form of either glowing or a flame.
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FLAME A flame is the visible (light- emitting), gaseous part of
a fire. It is caused by a highly exothermic reaction taking place
in a thin zone. If a fire is hot enough to ionize the gaseous
components, it can become a plasma.
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Types Smoldering Rapid Turbulent Microgravity
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Smoldering combustion SSSSmoldering is the slow, low-
temperature, flameless form of combustion, sustained by the heat
evolved when oxygen directly attacks the surface of a
condensed-phase fuel. Solid materials that can sustain a smoldering
reaction include coal, cellulose, wood.
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Rapid combustion Rapid combustion is a form of combustion,
otherwise known as a fire, in which large amounts of heat and light
energy are released, which often results in a flame. This is used
in a form of machinery such as internal combustion engines.
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Turbulent combustion Combustion resulting in a turbulent flame
is the most used for industrial application (e.g. gas turbines,
gasoline engines, etc.) because the turbulence helps the mixing
process between the fuel and oxidizer.
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COMBUSTIBLE SUBSTANCES PAPER MATCHSTICK
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NON-COMBUSTIBLE SUBSTANCES SANDSTONES
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During extreme heat of summer, at some places dry grasses catch
fire. From grasses, it spreads to trees, and very soon the whole
forest is on fire.It is very difficult to control such fires.
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IGNITION TEMPERATURE The lowest temperature at which a
substance catches fire is called its ignition temperature. WHITE
PHOSPOROUS
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Microgravity Combustion processes behave differently in a
microgravity environment than in Earth- gravity conditions due to
the lack of buoyancy. For example, a candle's flame takes the shape
of a sphere.
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Fuels Fuel is any material that stores energy that can later be
extracted to perform mechanical work in a controlled manner. Most
fuels used by humans undergo combustion, a redox reaction in which
a combustible substance releases energy after it ignites and reacts
with the oxygen in the air.
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Types
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LIQUID FUELS Combustion of a liquid fuel in an oxidizing
atmosphere actually happens in the gas phase. It is the vapour that
burns, not the liquid. Therefore, a liquid will normally catch fire
only above a certain temperature: its flash point.
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SOLID FUELS Solid fuel refers to various types of solid
material that are used as fuel to produce energy and provide
heating, oih sually released through combustion.Solid fuels include
wood, charcoal, peat, coal, Hexamine fuel tablets, and pellets made
from wood, corn,wheat, rye and other grains.
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HOW CAN WE CONTROL FIRE?? FIRE EXTINGUISHER The most common
fire extinguisher is water. But water works only when things like
wood and paper are on fire. If electrical equipment is on fire,
water may conduct electricity and harm those trying to douse the
fire. Water is also not suitable for fires involving oil and
petrol. For fires involving electrical equipment and inflammable
materials like petrol, carbon dioxide is the best extinguisher.
CO2, being heavier than oxygen, covers the fire like a blanket.
Since the contact between the fuel and oxygen is cut off, the fire
is controlled. The added advantage of CO2 is that in most cases it
does not harm the electrical equipment.
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Different Zones of Candle flame
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The amount of heat energy produced on complete combustion of 1
kg of a fuel is called its calorific value. The calorific value of
a fuel is expressed in a unit called kilojoule per kg (kJ/kg).
Calorific Values of Different Fuels
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Burning of Fuels Leads to Harmful Products 1. Incomplete
combustion of these fuels gives carbon monoxide gas. It is a very
poisonous gas. It is dangerous to burn coal in a closed room. The
carbon monoxide gas produced can kill persons sleeping in that
room. 2. Combustion of most fuels releases carbon dioxide in the
environment. Increased concentration of carbon dioxide in the air
is believed to cause global warming. 3. Burning of coal and diesel
releases sulphur dioxide gas. It is an extremely suffocating and
corrosive gas. Moreover, petrol engines give off gaseous oxides of
nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and
form acids. Such rain is called acid rain. It is very harmful for
crops, buildings and soil.