Entropy Waves Measurement by Tunable Diode Laser Absorption Spectroscopy

Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique has been widely applied to measure the gas temperature and species concentration in combustion environments. In this work, we show the capability of TDLAS in measuring the coherent temperature fluctuations of a turbulent swirled flame. Two distributed feedback grating (DFB) lasers are used to probe the H 2 O absorption transitions near 7185.59 cm − 1 and 6806.03 cm − 1 . Using the Peak-Scanned Wavelength Modulation Spectroscopy (WMS), the 2f/1f absorption spectra are converted into temperature at a rate of 5 kHz. A particular quantity that we highlight is the Entropy Transfer Function (ETF) which relates the upstream acoustic perturbation to the temperature fluctuation downstream of the flame. The ETF is obtained by taking the ratio between the measured temperature fluctuation and the upstream acoustic perturbation input, induced by loudspeakers, reconstructed by multi-microphones-method. It is found that the ETFs of the swirling flame do not decrease monotonically with respect to the forcing frequency and exhibit a minimum response at particular frequencies depending on the operating conditions. The remarkable similarity between the measured ETFs and Flame Transfer Functions (FTFs) indicates a strong link between flame heat release rate response and the temperature fluctuation downstream of the flame. Furthermore, the measurement series demonstrate the strong capability and robustness of the TDLAS-WMS method in uncovering the peculiar entropy response of a turbulent swirled flame.