Turbine Combustors
The combustion system in a turbine engine receives engine airflow that is highly compressed from the compressor, adds heat energy to this airflow and delivers the hot gases to the turbine. The combustor must deliver uniformly mixed hot gases to the turbine just slightly below stoichiometric fuel-air mixture combustion temperatures. At stoichiometric conditions, the maximum amount of heat is released and all the available fuel and oxygen is consumed. Over stoichiometric, excess fuel acts as a heat sink reducing the heat released.
There are generally two regions in a combustor system that are divided into about equal volumes but perform different functions. The upstream region is the primary combustion zone where nearly stoichiometric burning takes place with the correct fraction of air flow. The downstream region is the secondary or dilution zone where the excess air is mixed with the hot combustion products to provide the desired turbine inlet temperature. The average flow velocities in typical combustors range from 60 to 100 Ft/sec. All in all, it is a very complex system.
Combustor efficiency is a measure of the ratio of actual to theoretical heat release and must be as high as possible over the entire operating range of the engine.
The total pressure loss of the combustion system is defined as the difference between the averaged stream total pressure at the compressor exit station and the turbine inlet station. In general, higher pressure losses result in better combustor performance, but of course the engine cycle performance is reduced. A balance must be found between these opposing factors.
TD