Holger Konle

A fiber-optical microphone based on a Fabry-Perot interferometer applied for thermo-acoustic measurements

A high-temperature resistant fiber-optical microphone (FOM) was developed and successfully applied in a combustion chamber (~1.2 × 105 Pa, ~1400 K gas temperature) with thermo-acoustic oscillations resulting in a sound pressure level of 154 dB at the dominant frequency. The core of the optical set-up used for the FOM is a Fabry–Perot interferometer. To create an acoustical sensor based on this type of interferometer, a new method of generation and postprocessing of the interference signal was developed. The simple replaceability of the used membrane material allows the adaptation of the sensor sensitivity to the projected field of application.

Measurements of Density Pulsations in the Outlet Nozzle of a Combustion Chamber by Rayleigh-Scattering Searching Entropy Waves

The development of measurement techniques, which enable temporal and spatial highly resolved density investigations even in harsh environments, is essential. Rayleigh scattering is a noninvasive optical measurement technique permitting such investigations. A Rayleigh-scattering measurement system is set up, providing a new insight into fluid mechanical processes in turbomachines. In this paper, Rayleigh scattering is used for the detection of density oscillations in the optical accessible convergent-divergent outlet nozzle of a small scale combustion test rig at various power consumptions and equivalence ratios. Until now, this part of the combustion chamber is sparsely investigated due to the challenging measurement conditions. The temporal density oscillation inside the nozzle can be shown up to 4 kHz as well as its spatial distribution. Systematic errors of the setup are investigated. Spectra of pressure and density oscillations are compared. Measurements with nonreacting air flow are conducted to study flow induced density fluctuations. Entropy noise related correlations between density and pressure fluctuations are found. Therewith, the builtup Rayleigh-scattering system enables investigations of the presumed region of indirect noise generation.

Effect of Fuel/Air Mixing on NOx Emissions and Stability in a Gas Premixed Combustion System

Understanding the mixing properties in lean premixed combustors is of critical importance to realize low NO x emissions and stable combustion over a wide range of operating conditions. This goal can be partially achieved if spatial and temporal homogeneity of the fuel/air distribution is ensured and if the mixing profile remains less sensible to perturbations of the flow field. Fuel/air oscillations are one of the mechanisms leading to thermoacoustic instabilities and should be minimized. In this paper, experimental techniques (Laser Induced Fluorescence, Particle Image Velocimetry, flame transfer function measurements) are used to investigate the response of different mixing profiles of a lean premixed swirl burner to simulated acoustic forcing of different amplitudes. Results for reacting and cold flow investigations are presented. The flame transfer function appeared to be the most reliable tool to predict the impact of the mixing profile on system stability, but the cold flow investigations gave additional information on the mixing mechanisms at the burner outlet.

Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV

Thermo-acoustic investigations require reliable measurement techniques in hot environments for pressure, density fluctuations with a high dynamic range and acoustic particle velocity. This paper presents recent developments of optical measurement techniques in combustion diagnostics. A fibre-optic microphone based on the interferometric detection of membrane deflections was designed to measure acoustic pressure oscillations. Due to the heat resistant design, the sensor has an upper temperature limitation of approximately 970 K. Rayleigh-Scattering measurements, using the density dependent intensity of scattered light were performed in an unconfined flame with approximately 1600 K to study amplitude and phase distribution of the flame pulsation. Acoustic particle velocity can be determined applying acoustic PIV (particle image velocimetry) technique. This paper shows a way to measure simultaneously the acoustic particle velocity and the locally resolved mean flow velocity of a turbulent flow. Together these non-invasive techniques are applicable to study thermo-acoustic processes and sound generation in combustion chambers or turbines.

A new approach for setting up and applying a fiber-based Fabry–Perot interferometer

A new approach using fiber-based Fabry–Perot interferometry for constructing highly resistant and highly accurate vibration sensors is presented. The application of a diode array makes it possible to acquire a multitude of interference signals which can be classified into pairs of signals fulfilling the quadrature condition. This approach reaches the sensitivity of single-mode optical fiber interferometers proposed in the past while providing a much higher signal accuracy.

Design and Application of Fiber-Optic Microphones for Thermo-Acoustic Measurements

For the study of combustion driven pressure fluctuations an adequate measurement technique has to be designed, which enables its application under hot conditions while covering a wide dynamic range. To overcome disadvantages of measurement techniques used up to now for this research topic, a new sensor has been designed based on the optical measurement technique of interferometry. Here, an interferometer is coupled via a glass fiber with a reflecting membrane which is acoustically excited, thus the measured membrane displacements correlate to the acoustic field in the test section. Two dierent interferometer designs were successfully tested to build such an optical sensor and will be presented in this paper. The first design employs a commercial Mach-Zehnder interferometer and has been used for several acoustic measurements under cold conditions, but also for the detection of a combustion induced pressure fluctuation in the exhaust gas duct of an atmospheric combustion test rig. The second interferometric setup is based on a fiber based Fabry-Perot interferometer. Starting from a classical homodyne interferometer this setup has been modified to reach a sensitivity which enables the detection of acoustically induced membrane movements. This setup has been extensively studied under cold conditions and is currently prepared for the application in an atmospheric combustion chamber. Observations of several pre-studies for a safe hot application of this new sensor are discussed.

Development of a novel technique of pressure pulsation measurements in turbomachinery: fibre optic microphone

One way to reduce NOx emissions of power plants is to use lean premixed combustion. This type of combustion, however, leads to combustion instabilities. To investigate the corresponding pressure fluctuations in the combustion chamber, a high temperature resistant measurement technique has to be set up. One possible technique for this application with a high potential is a fiber-optic microphone (FOM). Using a Michelson interferometer, the movement of a high temperature resistant membrane can be measured; these measurements can be used to calculate the pressure fluctuations. In this paper, one possible design of such a FOM is presented. Furthermore, a first prototype and the experimental results obtained until now are shown. Moreover, an analytic model is presented, with which the experimental results can be compared.

Application of Fiber-Optical Microphone for Thermo-Acoustic Measurements

A high temperature resistant fiber optical microphone (FOM) was developed and successfully applied in a combustion chamber at a thermal power of 8.4 kW to measure thermo-acoustic oscillations at a frequency of 85 Hz and a sound pressure level of 154 dB. The sensor head temperature was estimated to ∼ 1000 K. The core of the optical setup used for the FOM is a Fabry-Perot interferometer. To create an acoustical sensor based on this type of interferometer, a new method of generation and postprocessing of the interference signal was developed. The simple replaceability of the sensor membrane reduces the requirements concerning the sensor handling compared to conventional condenser microphones and allows the adaption of the sensor sensitivity to its application case changing the membrane stiffness.Copyright