Short Answer:

Combustion in a thermal system is the chemical process of burning fuel in the presence of oxygen to produce heat energy. This heat is then used to run engines, heat water, or generate electricity. Combustion mainly involves the reaction of a hydrocarbon fuel with air, producing carbon dioxide, water vapor, and thermal energy.

In thermal systems like boilers, engines, or power plants, combustion takes place inside a combustion chamber where fuel is supplied and ignited. The heat released increases the temperature and pressure of the working fluid, which is then used to perform useful mechanical or electrical work.

Detailed Explanation:

Combustion in a thermal system

Combustion is a rapid oxidation reaction between a fuel and an oxidizer (usually oxygen in the air), which releases a large amount of heat energy. In most thermal systems, this heat energy is used to heat a fluid (like water, air, or gas) which then powers turbines, engines, or heating systems.

Combustion is an essential process in thermal systems such as internal combustion engines, steam power plants, gas turbines, furnaces, and industrial boilers. These systems convert the chemical energy stored in fuels into thermal energy, and then into mechanical or electrical energy.

Process of Combustion in a Thermal System

1. Fuel Supply
   • The system supplies fuel such as petrol, diesel, coal, natural gas, or biomass into the combustion chamber.
2. Air or Oxygen Supply
   • Air is mixed with fuel to provide oxygen needed for combustion.
3. Ignition
   • The air-fuel mixture is ignited using a spark plug, pilot flame, or compressed heat (as in diesel engines).
4. Combustion Reaction
   • The chemical reaction starts:

Fuel+Oxygen→CO₂+H₂O+Heat\text{Fuel} + \text{Oxygen} \rightarrow \text{CO₂} + \text{H₂O} + \text{Heat}Fuel+Oxygen→CO₂+H₂O+Heat

1. Heat Generation
   • The reaction generates heat energy, which is absorbed by a working fluid like steam, water, or air.
2. Utilization
   • The heated fluid is used to:
     • Rotate a turbine (power generation)
     • Drive a piston (automobile engines)
     • Heat a space (heaters and boilers)

Types of Combustion

1. Complete Combustion
   • Enough oxygen is present.
   • Produces CO₂ and H₂O, and releases maximum energy.
   • Example: Clean burning of natural gas in a gas stove.
2. Incomplete Combustion
   • Limited oxygen supply.
   • Produces carbon monoxide (CO), soot, and less heat.
   • Example: Smoke from wood or coal.
3. Internal Combustion
   • Combustion happens inside the engine cylinder (e.g., cars, bikes).
   • Heat directly pushes the piston.
4. External Combustion
   • Combustion happens outside the engine (e.g., steam engines, boilers).
   • Heat is transferred to another fluid to produce motion.

Applications of Combustion in Thermal Systems

• Steam Power Plants – Use combustion of coal/gas to generate steam
• IC Engines – Combust fuel-air mix to move pistons in vehicles
• Gas Turbines – Used in airplanes and power plants
• Domestic Heating – Gas burners, water heaters
• Industrial Boilers – Heating fluids for processing and power

Factors Affecting Combustion

• Fuel-air ratio – Too much or too little air reduces efficiency
• Fuel quality – Poor fuels produce more pollution
• Temperature and pressure – Must be maintained for steady combustion
• Ignition source – Needs to be reliable and safe

Environmental Concerns

• CO₂ emissions contribute to global warming
• Incomplete combustion produces carbon monoxide, a toxic gas
• Use of clean fuels and combustion control systems helps reduce pollution

Conclusion

Combustion in a thermal system is the core process that converts fuel energy into heat, which is then used for mechanical or electrical output. It involves burning fuel with air in a controlled chamber to produce high-temperature gases. This process drives engines, turbines, and heating systems. A clear understanding of combustion helps in designing efficient, safe, and eco-friendly thermal systems in various industries.