Development of Optical Measurement Techniques for Thermo-Acoustic Diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV

Abstract

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

“…As reported in previous publications [7,8], a fiberbased interferometer can easily be combined with a reflecting membrane to create an acoustic sensor, whereas the performance of the created sensor can be validated by comparing the acquired data with the signal of conventional acoustic sensors, for example, condenser microphones. The newly designed optical setup was combined with a stainless steel sensor head (see figure 6), based on a stainless steel membrane material with a thickness of 20 μm.…”

“…To reach the nm-sensitivity of interferometer setups proposed in the past (see e.g. [1,8]), a single photodiode, which summarizes the whole intensity over its sensitive area, is not sufficient. Instead of a simple photodiode, a diode array realized by a line scan camera was used to enable the temporally and spatially resolved acquisition of the interferogram dependent on the fiber end conditioning [6].…”

“…One important advantage over conventional Mach-Zehnder interferometers, the most used interferometer type, is that both the measurement beam and the reference beam are guided within the same optical fiber, i.e. fiber vibrations, which change the optical length of the fiber, do not influence the quality of the acquired interference signal [8]. 3 Author to whom any correspondence should be addressed.…”

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

“…First design tests have been performed with a commercial Mach-Zehnder (MZ) interferometer, a type of interferometer often used for vibration measurements in diverse fields of application. This optical set-up, however, has two main disadvantages, which complicate its employment for the desired application for combustion diagnostics (see [4]).…”