Expert: Paul Soderman - 4/2/2010

Question
QUESTION: Sir, the pressure inside the combustion chamber of a jet engine is kept fairly constant just about 3% of pressure loss although the gases expand after combustion .How this pressure is maintained?

ANSWER: Gautam
I may not understand your question completely, because in steady, level flight there is no reason for pressures to change.  Air entering the combustion chamber has passed through the high pressure compressor.  If fuel flow is held constant, and altitude is held constant, the engine will seek the pressure ratio (Pexit/Pinlet) designed for that engine.  So, due to the turbomachinery pressure rise and the pressure developed during combustion, the combustion chamber pressure is kept constant.  With good nozzle design, losses out the nozzle are minimal.
Paul

---------- FOLLOW-UP ----------

QUESTION: why is it so that one combustor design works well and gives good service life and another type will not when installed in the same engine? The answer referenced in 'jeppesen aircraft gas turbine powerplants ' is its a BLACK ART . Why its still a mystery ?

Answer
I don't know where you are getting your information.  An engine design team will optimize each engine component as part of the system after extensive design, simulation and testing.  Changing to another combustor might break the design.  Why would anyone do that ?