Research On The Measurements Of Equivalence Ratio And Flame Temperature Of Methane/Air Mixture By Laser-Induced Breakdown Spectroscopy

Equivalence ratio and temperature are key parameters in affecting combustion reaction path. The real-time measurement and precise control of them are of great value in scientific research and engineering application. Laser-induced breakdown spectroscopy(LIBS) has been increasingly used in combustion diagnostics as a novel spectral analysis method in recent years. The equivalence ratio and flame temperature in methane/air mixture are measured by LIBS in this thesis.The quantitative local equivalence ratio of methane/air mixture is determined by LIBS using different emission intensity ratios of H/O and H/N. The comparison between calibration curves of H656/O777 and H656/N746 is performed in gated mode, which shows that H656/O777 can achieve better prediction accuracy and higher sensitivity. More spectral intensity ratios(H656/O777、H656/N500+、H656/N567+ and H656/N746) can be used to make calibration measurements in ungated mode and H656/O777 is also tested best among them. The comparison between gated and ungated detection modes shows that gated mode offers better accuracy and precision. In addition, the effects of different laser wavelengths(1064 nm, 532 nm and 355 nm) on LIBS spectra and calibration curves are investigated with laser focal point size and laser fluence kept constant. The results show that with longer laser wavelength, the peak intensity and SNR(Signal Noise Ratio) of H, O and N lines increase, as well as the slope of calibration curve of H656/O777. Among these three wavelengths, 1064 nm laser is best suited to measure the equivalence ratio of CH4/air mixture by LIBS. The experimental results are explained in terms of plasma electron density and temperature, which have a significant impact on the emission intensity and the partition function of hydrogen and oxygen, respectively.The experimental study of flame thermometry by LIBS is conducted on top of heat flux burner, which is based on the heat flux method. Heat flux burner is used as the temperature calibration source and produces 1D adiabatic flames with temperature in the range of 1650~2300 K by controlling the flow velocities and equivalence ratios of methane/air mixture by MFC(Mass Flow Controller). The calibration curve of breakdown threshold and flame temperature(1650~2300 K) is established using the breakdown threshold measured in adiabatic flames generated by 6 equivalence ratios. The predictive capability of the calibration curve is tested in other 5 adiabatic flames and the results show that maximum error is less than 25 K. Therefore, the approach of laser-induced breakdown thermometry can provide a good prediction to flame temperature.