PREDICTION OF STABILITY LIMITS FOR LP AND LPP GAS TURBINE COMBUSTORS

ABSTRACT

The prediction and the systematic suppression of self-sustained combustion instabilities in combustors for gas turbine applications still suffer from the lack of physical models for the dynamic flame characteristics of Lean-Premixed natural gas and Lean-Prevaporized-Premixed kerosene flames. Hence, the experimental determination of the flame transfer functions of LPP swirl flames was achieved using a prevaporization unit for kerosene. A time-independent and spatial homogeneous mixture of kerosene and combustion air was generated at the burner exit. By exchangeable fuel nozzles the flame dynamics were as well detected for natural gas LP swirl flames with identical operation conditions. The flame dynamics are strongly affected by the formation and in-phase reaction of coherent vortex structures, well known as drivers of combustion instabilities. These structures have been visualized with a phase-correlated imaging technique.

The results discussed in this paper lead to a basic understanding and quantitative prediction of the frequency-dependent dynamics of LP and LPP swirl flames. Especially, the influence of preheating temperature, air equivalence ratio and the type of fuel on the amplitude responses and phase angle functions were investigated in detail. Based on theoretical considerations concerning the burning velocity of steady-state premixed flames a physical model and – derived from that it–scaling laws for the prediction of unstable operation modes in dependence on main operation parameters of the flame were formulated and validated by the measurements. Thus, it is now possible to scale and quantitatively predict the stability limits for the formation of periodic combustion instabilities of lean-premixed combustors with swirl-stabilized flames.

Keywords:

• combustion instabilities
• stability limits prediction
• scaling of delay time
• flame transfer functions

Notes

The investigations of the kerosene LPP flames were funded by the European Commission whose support is gratefully acknowledged. It was part of the GROWTH programme, research project ICLEAC: Instability Control of Low Emission Aero Engine Combustors (G4RD-CT2000-00215).