Entropy transfer function measurement with tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation spectroscopy (WMS) 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 H2O absorption transitions near 7185.59 cm−1 and 6806.03 cm−1. Using WMS, the 2f/1f absorption spectra are converted into temperature at a rate of 5 kHz. The quantity of interest that we measure is the Entropy Transfer Function (ETF) which, for low Mach numbers, 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 downstream of the flame, measured with TDLAS and the upstream acoustic perturbation input, induced by loudspeakers with forcing frequency ranging from 40 to 250 Hz, reconstructed with the multi-microphone method. We show that the TDLAS-WMS technique can measure relatively low coherent temperature fluctuations as low as 5 K. It is found that the gain of ETFs of the swirling flame does not decrease monotonically with respect to the forcing frequency and exhibits a minimum response at particular frequencies depending on the operating conditions. Moreover, the ETFs for different operating conditions collapse on each other when plotted against the Strouhal number. The 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 suitability of the TDLAS-WMS technique for entropy waves measurements, leading to the discovery of peculiar entropy responses at high frequencies.