Parameter Fitting-Based Fast Imaging of Axisymmetric Flame Temperature Field Using Wavelength-Modulated Lateral Shearing Interferometry

A parameter fitting-based fast imaging method for axisymmetric flame temperature field was proposed in a lateral shearing interferometer. In the extinction state, a sawtooth waveform was used to modulate a distributed feedback (DFB) laser to obtain the quantitative mapping from modulated voltage to phase. In the combustion state, the laser was rapidly driven by an irregular waveform of five steps. Linear equations were established, and the phase difference caused by the flame at each pixel was extracted by parameter fitting. Flame phase distribution was obtained to image refractive indexes and temperatures. Compared with the existing waveform fitting-based wavelength modulation for flame monitoring, the proposed method reduced the modulation steps to five and improved the imaging speed up to eight times. In numerical simulations, an axisymmetric counterflow flame was reconstructed to evaluate the performance of the proposed method. Reconstruction errors of phase difference, refractive index, and temperature distributions were 0.0117, 0.0692, and 0.0795, respectively. In experiments, three counterflow flames of different flow conditions were imaged. The flame structures were clearly visualized, and the interferometric temperatures matched well with the readings of a thermocouple. A Bunsen-type flame with 100-Hz acoustic excitation was monitored at 8 kHz. The variation of the temperature distributions well reflected the flame oscillations. The frequencies of fundamental, second, and third harmonics were captured, and the harmonic at 99.43 Hz caused by the acoustic excitation was also precisely identified.