Johannes Peterleithner

Analysis of Measured Flame Transfer Functions With Locally Resolved Density Fluctuation and OH-Chemiluminescence Data

The goal of the study presented in this paper is to analyze flame transfer functions with a new approach based on the combination of-line-of sight OH*-chemiluminescence and density fluctuation data. The OH*-chemiluminescence is acquired with a photomultiplier and an intensified camera, the density fluctuations are measured with a Laser vibrometer on a two axis traverse. In flames with forcing the acoustic fluctuations can be extracted from the data by discrimination of all contributions from combustion noise, because it is not correlated with the excitation device. Assuming rotational symmetry of the fluctuations originating from excitation, planar phase-resolved and pseudo-local OH*-chemiluminescence and density fluctuation data is obtained from the measured line-of-sight integrated signals.In the study this technique is applied to a swirl burner configuration with FTFs from known multi-microphone measurements (MMM). In the first step, the externally premixed mode without equivalence ratio fluctuations is studied and in the second step the fuel is injected in the swirler in order to generate equivalence ratio waves. At selected frequencies the planar fields of the OH*-chemiluminescence and density fluctuations are compared to the FTFs in order to improve the understanding regarding the specific amplitude and phase values. In addition to heat release the vibrometer data reveals the periodic oscillation of the conical annular jet of the cold reactants in the combustor filled with hot products. On the global scale the amplitudes and phases of heat release expected from the MMM are satisfactorily reproduced by both methods for the premixed cases, whereas OH*-chemiluminescence data cannot be used as indicator for heat release if equivalence ratio fluctuations are present, because the amplitude of the FTF is significantly over-predicted due to the sensitivity of OH* on the local fuel-air mixture.Copyright

Interferometric Investigation of the Thermoacoustics in a Swirl Stabilized Methane Flame

In this work, an atmospheric model combustion chamber was characterized employing Laser Vibrometry, chemiluminescence and Particle Image Velocimetry. The test object was a variable geometry burner enclosed with a liner, with the flame optically accessible through four fused silica windows. In this burner with adjustable flame conditions the cavity of the atmospheric model combustion chamber was excited at a frequency around 200Hz. Resonant and non-resonant flame conditions were investigated and compared by laser vibrometer interferometry, schlieren visualization and OH*/CH* chemiluminescence. Additionally, the velocity field was recorded with Particle Image Velocimetry, while the aerodynamics of the burner plenum was analyzed with Computational Fluid Dynamics.Copyright

Laser vibrometry for combustion diagnostics in thermoacoustic research

Abstract A novel technique for time- and space-resolved measurement of density fluctuation is presented. It is nonintrusive and based on laser Doppler vibrometry. The density fluctuations reveal information on dynamic heat release and fluid velocity. The significance of this technique is proven by recording thermo-acoustic oscillations in a model combustor.

Frequency resolved density fluctuations measurements of combustion oscillations in a model combustor

In this work, investigations of thermo-acoustic oscillations by means of density fluctuation measurements, are presented. For this, laser-vibrometers were used. Although these systems were initially developed for highly accurate detection of surface vibration, they are also capable of measuring the integral gas density fluctuation along the line of sight. In order to facilitate a laser based investigation of a thermo-acoustically unstable setup, a variable geometry burner – developed in a previous project was operated in an optically accessible gas turbine model combustion chamber at atmospheric conditions. Thermoacoustic coupling ocured at 165 Hz when the burner was operated at a power of 5 kW. Density fluctuations outside and inside the flame region were measured using vibrometers. Furthermore, the the eigenfrequencies of the combustion chamber were detected by measuring the surface vibration, as well as, the acoustic waves forming within the volume of the chamber without combustion, but with loudspeaker excitation. The spectrum of the cavity resonance was compared to the spectra obtained with combustion under different conditions. The variable geometry burner was helpful in order to easily change flow momentum or flow angular momentum. With a fixed vibrometer as a reference and a traversable laser-vibrometer moving along the flow axis, the phase relation of structures released by the flame, was measured. Consequently the velocity of these structures was estimated.