4.4 - Flame

In this section, we will study the concept of a flame, its structure, and the different types of flames that can be observed during combustion. Flame is produced when a substance undergoes combustion in the presence of oxygen, releasing energy in the form of light and heat.

Flame is a mixture of hot gases, which may include carbon dioxide, water vapor, and some unburnt particles. The combustion process in flames generally involves the chemical reaction of a fuel with oxygen from the air, resulting in the production of heat, light, and gaseous products.

Structure of a Flame

A typical flame has three distinct zones:

• Innermost Zone (Inner core) : This is the zone closest to the fuel source. It is usually dark and cooler in temperature as insufficient oxygen is present for complete combustion. This zone has unburnt fuel and is known as the reducing zone.
• Middle Zone : This is the hottest region of the flame, where the combustion process is complete. The gas particles are fully oxidized, and light is produced in this area.
• Outer Zone (Outer envelope) : The outermost layer is where oxygen from the surrounding air enters and helps in the complete combustion of the fuel. This zone is typically blue in color and is known as the oxidizing zone.

Types of Flames

There are mainly three types of flames based on the availability of oxygen and the fuel being burned:

• Luminous Flame : This flame produces a yellow color due to incomplete combustion. It is seen when the fuel is not burning completely due to a limited supply of oxygen. A common example is the flame of a candle.
• Non-Luminous Flame : A non-luminous flame burns with a blue color and results from complete combustion. This flame is seen when there is an adequate supply of oxygen, leading to the production of a clean flame with less soot. An example is the flame of a gas stove.
• Smoky Flame : A smoky flame is a result of incomplete combustion, often producing soot particles. It is generally seen when burning substances like kerosene or wood in insufficient oxygen.

Factors Affecting Flame

The type of flame produced depends on several factors, including:

• Amount of Oxygen: Incomplete combustion, which results in a luminous or smoky flame, occurs when there is insufficient oxygen. Complete combustion, which leads to a non-luminous flame, occurs with an adequate supply of oxygen.
• Temperature: The temperature at which combustion occurs influences the color and the nature of the flame. Higher temperatures generally produce blue or non-luminous flames, while lower temperatures can lead to yellow or luminous flames.
• Fuel Type: Different fuels have different combustion properties. For example, natural gas burns with a blue, non-luminous flame, while wood produces a yellow, luminous flame.

Important Formulae

During combustion, the following chemical reaction typically takes place:

Complete Combustion: $C_xH_y + O_2 \rightarrow CO_2 + H_2O + \text{Heat}$

Incomplete Combustion: $C_xH_y + O_2 \rightarrow CO + H_2O + \text{Heat}$

In complete combustion, the carbon in the fuel is fully oxidized to form carbon dioxide, and water vapor is produced as a by-product. In incomplete combustion, carbon monoxide (CO) is produced along with water vapor due to insufficient oxygen.

Understanding the flame is essential to controlling the combustion process for practical uses, such as cooking, heating, and industrial processes.