Experimental study on supersonic combustion fluctuation using thin-film thermocouple and time–frequency analysis

To accurately assess the combustion fluctuation for a scramjet, a new method based on indium tin oxide (ITO) thin-film thermocouples (TFTCs) and time–frequency analysis (TFA) was developed in this study. ITO TFTCs were used to measure the transient wall-temperature change of the combustor, and the combustion fluctuation features were extracted from the temperature signal via TFA. The combustion fluctuation-resolving abilities of different TFA methods, including the short-time Fourier transform (STFT), continuous wavelet transform (CWT), and Hilbert–Huang transform (HHT), were investigated using a reconstruction signal. The results indicated that the CWT can provide an excellent global view and that the HHT is useful for localizing the frequency features in the time domain. Then, the effect of the equivalence ratio (ER) on the supersonic combustion fluctuation was researched. The total energy of the fluctuation was strongly correlated with the increase in the ER. Meanwhile, the frequency of main fluctuation was always concentrated at approximately 395 Hz, just like an intrinsic characteristic of the combustor. Additionally, the fuel type affected the energy of the combustion fluctuation, rather than the frequency.