Partially Premixed and Premixed Aero Engine Combustors

Kurzfassung

This chapter reviewed partially premixed and premixed aero engine combustors. Apart from a description of the state of the art, it tried to explain how the design constraints of airborne combustors enforced the breed of premixing combustors that will soon enter service and why the NOx levels emitted from airplanes will continue to be so different from those of industrial gas turbines. The efficiency required from aero engines of long-range aircraft forces exit temperatures that cannot be sustained by pure convective cooling with the airworthy materials available today. The airflow for combustion thus being reduced by the necessary film cooling air results in combustion temperatures above the onset of thermal NOx production, which must be limited by a short residence time. As premixing results in poor stability, enforcing piloted combustion, the trade-off between NOx emission at full power and combustion efficiency at cruise defines combustor size and premixed combustion temperature. This strongly favors single annular combustors. Autoignition times of kerosene at high-power conditions preclude full premixing in premixing channels before the combustor that could be safe from flashback. Premixing is therefore completed in a combustor with lifted flames. Flexible internal piloting has to ensure stability and the appropriate flame lift-off. A multitude of operability requirements, including ignition, absence of combustion oscillations, and thermal management, has to be solved before airworthiness is reached. All of those have a potentially detrimental effect on NOx reduction. Together with the moving target of engine pressures and combustor exit temperatures, this explains why it took so long for the technology to mature enough to be introduced replacing optimized RQL combustion with a step change in NOx emission. Having made the transition into service, it can be expected that further emission reductions will be possible with a further optimization using refined design tools and a penetration of lower thrust classes with premixing combustors can be expected.