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# FACTORS AFFECTING COMBUSTION CHAMBER DESIGN:

Table of Contents:

  • What Are The Factors Affecting Combustion Chamber Design?
  • Temperature Of Gases
  • Temperature Distribution
  • Combustion
  • Carbon Deposits/Coking
  • Combustion Chamber Pressure
  • Pollutants
  • Volume And Weight

What Are The Factors Affecting Combustion Chamber Design?

The design of combustion chamber of gas turbine engine is different than conventional combustion devices. Space, combustion intensity, faster combustion process are major requirements. Following are the factors affecting combustion chamber design.

factors affecting combustion chamber design

1. Temperature Of Gases (Factors Affecting Combustion Chamber Design)

  • The temperature of the burned gases must be comparatively low to suit highly stressed turbine material materials.
  • The temperature may be allowed to rise if improved materials and blade cooling methods are adopted.
  • Permissible combustor outlet temperature is approximately 1850 K for aircraft application.

2. Temperature Distribution (Factors Affecting Combustion Chamber Design):

  • At the end of combustion chamber, the distribution of temperature along the turbine annulus, influences the allowable stress and centrifugal stress from root to tip.
  • Therefore, temperature distribution need not be uniform.
  • It may increase with radius over the turbine annulus as blade stresses decreases from root to tip.

3. Combustion (Factors Affecting Combustion Chamber Design):

  • Combustion is a function of air-fuel ratio. Stable combustion must be maintained over wide range of air-fuel ratio from full load to idling conditions.
  • Design should be such that combustion is maintained in a stream of air moving with a high velocity approximately of 30-60 m/s.
  • If the air-fuel ratio vary from 60:1 to 120:1, the combustor can be of simple design. But if the air fuel ratio is from 100:1 to 200:1, the gas turbine combustor should use heat exchanger.
  • If stoichiometric ratio is considered 15:1, the combustor requires high dilution to lower the temperature of the burnt gases entering the turbine. So that the turbine stresses are avoided.

4. Carbon Deposits/Coking (Factors Affecting Combustion Chamber Design)

  • During the design of combustion chamber, formation of carbon deposits on the wall must be avoided.
  • Carbon deposits are hard brittle materials, may damage turbine blades, block cooling air passage, change the flow pattern, or any severe change may result.
  • So, the necessary design alternations may be employed to prevent “coking”.

5. Combustion Chamber Pressure (Factors Affecting Combustion Chamber Design):

  • Combustion stability is also a function of pressure. Combustion must be stable over wide range of chamber pressure.
  • Also, drop in pressure in the combustion chamber between inlet and outlet, increases specific fuel consumption and reduces specific power output. Therefore, pressure loss should be minimum.

6. Pollutants (Factors Affecting Combustion Chamber Design):

  • Smoke in the exhaust should be minimum.
  • Oxide of nitrogen (NOx), carbon monoxide (CO), unburnt hydrocarbons (UHC) should be minimum.
  • Stringent emission legislation to minimize harmful pollutants led to significant changes in combustor design.

7. Volume And Weight (Factors Affecting Combustion Chamber Design):

  • Volume and weight of the combustor directly affects the overall size and weight of the power plant. 
  • Operational requirement will restrict the size and weight of the engine and it also affects the design of the combustor.
  • For aircraft application for same power out size and weight is comparatively low than ground power plant. 
  • Ground based power plant combustor can allow the use of thicker materials to improve operational life.
The interdependence of compressor air, delivery air and mass flow which leads to the velocity of the air at entry to the combustion system is reasonably constant for the operating range of gas turbine engines. This is the only feature that eases the combustion chamber design. Otherwise, maintaining combustion efficiency is major hurdle in combustion chamber design.
 
For the operating range of the engine, fuel must burn completely and full calorific value is extracted. Any pressure loss results into increase in specific fuel consumption and reduction in specific output power, smaller space availability and shorter chemical reaction time leads to difficulty in meeting all the requirements of maintaining high combustion efficiency with low pressure loss. Design of aircraft gas turbine combustion chamber is a real challenge.

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