A turbine engine combustor operates with an overall fuel-air ratio of roughly 60:1 by weight, far leaner than the chemically correct (stoichiometric) ratio of about 15:1. Why is the combustor designed to run this lean overall?
- ATo reduce compressor discharge pressure requirements
- BTo increase flame temperature and improve fuel economy
- CBecause igniter plugs cannot sustain a flame at richer mixtures
- DBecause only a portion of incoming air enters the primary combustion zone; the remaining air cools the liner and dilutes combustion gases to a temperature the turbine can withstand
Show answer & explanationAnswer & explanation
Correct answer: D. Because only a portion of incoming air enters the primary combustion zone; the remaining air cools the liner and dilutes combustion gases to a temperature the turbine can withstand
Only about a third of compressor discharge air enters the primary combustion zone near stoichiometric proportions to sustain efficient burning; the rest is directed around the liner for cooling and then mixed into the gas stream in the dilution zone to lower the gas temperature to a level the turbine section can safely tolerate.
Why the other options are wrong
- A. Compressor discharge pressure is determined by compressor design, not combustor fuel-air ratio needs.
- B. Running lean overall actually lowers average temperature, it does not increase flame temperature.
- C. Igniters only initiate combustion briefly and are unrelated to the sustained overall fuel-air ratio.
Combustor Fuel-Air Ratio
The overall fuel-to-air ratio in a turbine combustor is much leaner than stoichiometric because most air is used for liner cooling and dilution rather than combustion.
- Primary zone burns near stoichiometric (~15:1) with about a third of the air
- Secondary/dilution zones add the remaining air to cool gases
- Overall ratio may be around 60:1 by weight
Memory trick: Only a third of the air lights the fire; the rest keeps things cool.