4.5 Structure of a Flame

In this subtopic, we will learn about the different parts of a flame and their characteristics. A flame is a visible, gaseous part of a fire that occurs when a material undergoes combustion. Combustion is a chemical reaction between a fuel and an oxidizer, usually oxygen, producing heat and light.

The structure of a flame can be understood by studying its various zones, which are the result of the different temperatures and the ongoing chemical reactions at various points in the flame. The flame has three main regions: the outermost zone, the middle zone, and the innermost zone. Let’s explore these zones in detail.

1. Outer Zone (Non-luminous Zone)

The outer zone of the flame is the hottest part and is called the non-luminous zone. In this zone, the combustion is complete, and the gases burn with a blue color. This blue color is due to the presence of oxygen and the high temperature that ensures complete combustion. The temperature in this zone is typically around 1400°C to 1600°C.

Here, the fuel is fully oxidized, and there is a high concentration of oxygen reacting with the fuel. The reaction can be represented by the following equation:

$\text{Fuel (e.g., CH}_4\text{)} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$

2. Middle Zone (Luminous Zone)

The middle zone is the luminous zone, where the flame appears yellow. This part is cooler than the outer zone, with temperatures ranging between 1000°C and 1200°C. The yellow color of the flame is due to the incandescence of small soot particles (carbon) that are heated to glowing temperatures. Incomplete combustion occurs here because oxygen supply is not sufficient to fully oxidize the fuel. This leads to the formation of soot and carbon monoxide.

In this zone, the reaction is less efficient, and the products of combustion include carbon monoxide (CO) and other partial combustion products. The incomplete combustion reaction can be represented as:

$\text{2C}_3\text{H}_8 + 9\text{O}_2 \rightarrow 6\text{CO} + 8\text{H}_2\text{O}$

3. Innermost Zone (Feather Zone)

The innermost part of the flame is called the feather zone or the inner core. This region is where the temperature is lowest, typically between 600°C and 800°C. The flame in this zone is pale and has less visible light. It is also known as the zone where fuel begins to vaporize. In this region, the fuel has not yet fully reacted with oxygen and combustion has not reached completion.

This zone is mostly composed of unburnt fuel and hot gases, and the incomplete combustion continues here. Due to the lower temperature, there is less light emitted, which is why this part of the flame appears faint or transparent.

Flame Colors and Their Significance

The color of the flame depends on the temperature and the type of fuel being burnt. A blue flame indicates complete combustion, while a yellow flame indicates incomplete combustion. The temperature and color of a flame can be used to determine the efficiency of combustion and the amount of oxygen available in the environment.

Factors Affecting the Structure of a Flame

The structure of a flame can be influenced by several factors:

• Type of Fuel: Different fuels burn with different characteristics, leading to variations in flame structure.
• Air Supply: The amount of oxygen present affects the completeness of combustion and the resulting flame color.
• Temperature: The temperature at various points in the flame determines its color and the reactions taking place in each zone.
• Pressure: Atmospheric pressure can influence the flame structure, especially when burning fuels in confined spaces or under high pressure.