Johannes Eckstein

Low-order modeling of low-frequency combustion instabilities in aeroengines

Rumble is a combustion-induced instability occurring in aeroengines during start-up. Characteristically, low-limit-cycle frequencies between 50 and 150 Hz are obtained. Two basic feedback mechanisms are susceptible to promote rumble: entropy waves being reflected as pressure waves at the (nearly) choked combustor outlet and the purely thermoacoustic mode, originating from an in-phase oscillation of the heat release and the combustor pressure. Prior experiments on a generic rich-quench-lean (RQL) combustor have shown that the thermoacoustic mode determines the instability behavior over a wide range of operating conditions. A low-order model is developed for the staged RQL combustor to replicate theoretically the experimental findings and to investigate further the interaction of entropy waves and the purely thermoacoustic mode. The model accounts for dispersion and incorporates a simple flame transfer function, developed for spray combustion with negligible prevaporization. The model results confirm the experimental findings indicating a dominating thermoacoustic mode in the spectrum. The characteristically low-limit frequencies are a result of the characteristic timescales of droplet transport and combustion in the primary zone. It is further shown that the prevailing instability mode is strongly dependent on the dispersive properties of the primary zone and in the dilution sector of the combustor.

FORCED LOW-FREQUENCY SPRAY CHARACTERISTICS OF A GENERIC AIRBLAST SWIRL DIFFUSION BURNER

The low frequency response of the spray from a generic airblast diffusion burner with a design typical of an engine system has been investigated as part of an experimental study to describe the combustion oscillations of aero engine combustors called rumble. The atomization process was separated from the complex instability mechanism of rumble by using sinusoidal forcing of the air mass flow rate without combustion. Pressure drop across the burner and the velocity on the burner exit were found to follow the steady Bernoulli equation. Phase-locked PIV measurements of the forced velocity field of the burner show quasi-steady behavior of the air flow field. The phase-locked spray characteristics were measured for different fuel flow rates. Here again quasi-steady behavior of the atomization process was observed. With combustion, the phase-locked Mie-scattering intensity of the spray cone was found to follow the spray behavior measured in the non-combusting tests. These findings lead to the conclusion that the unsteady droplet SMD mean and amplitude of the air-blast atomizer can be calculated using the steady state atomization correlations with the unsteady burner air velocity.

Experimental Study on Laser-Induced Ignition of Swirl-Stabilized Kerosene Flames

Conventional ignition systems of aeroengines are an integral part of the combustion chambers structure. Due -to this hardware-related constraint, the ignition spark has to be generated in the quench zone of the combustion chamber, which is far from the optimum regarding thermo- and aerodynamics. An improved ignitability of the fuel-air mixture can be found in the central zone of the combustor, where higher local equivalence ratios prevail and where mixing is favorable for a smooth ignition. It would be a major advancement in aeroengine design to position the ignition kernel in these zones. A laser system is able to ignite the fuel-air mixture at almost any location inside of the combustion chamber. Commercial laser systems are under development, which can replace conventional spark plugs in internal combustion engines and gas turbines. This study was conducted to evaluate the applicability of laser ignition in liquid-fueled aeroengines. Ignition tests were performed with premixed natural gas and kerosene to evaluate the different approaches of laser and spark plug ignition. The experiments were carried out on a generic test rig with a well-investigated swirler, allowing sufficient operational flexibility for parametric testing. The possibility of the free choice of the lasers focal point is the main advantage of laser-induced ignition. Placing the ignition kernel at the spray cones shear layer or at favorable locations in the recirculation zone could significantly increase the ignitability of the system. Consequently, the laser ignition of atomized kerosene was successfully tested down to a global equivalence ratio of 0.23. Furthermore, the laser outperformed the spark plug at ignition locations below axial distances of 50 mm from the spray nozzle.