Optical Transfer Function Measurements for Technically Premixed Flames

Abstract

This paper deals with a novel approach for measuring thermo-acoustic transfer functions. These transfer functions are essential to predict the acoustic behavior of gas turbine combustion systems. Thermoacoustic prediction has become an essential step in the development process of low-NOx combustion systems. The proposed method is particularly useful in harsh environments. It makes use of simultaneous measurement of the chemiluminescence of different species in order to obtain the heat release fluctuations via an inverse method. Generally, the heat release fluctuation has two contributions: one due to equivalence ratio fluctuations, the other due to modulations of mass flow of mixture entering the reaction zone. Because the chemiluminescence of one single species depends differently on the two contributions, it is not possible to quantitatively estimate the heat based on this information. Measurement of the transfer matrix based on a purely acoustic method provides quantitative results, independent of the nature of the interaction mechanism. However, this method is difficult to apply in industrial full-scale experiments. The method developed in this work uses the chemiluminescence time traces of several species. After calibration, an over-determined inverse method is used to calculate the two heat release contributions from the time traces. The optical method proposed here has the advantage that it does not only provide quantitative heat release fluctuations, but it also quantifies the underlying physical mechanisms that cause the heat release fluctuations: it shows what part of the heat release is caused by equivalence ratio fluctuations and what part by flame front dynamics. The method has been tested on a full scale, swirl stabilized gas turbine burner. Comparison with a purely acoustic method validated the concept.