Quantitative measurement method of equivalence ratio based on temperature characterization in actual methane-air premixed flame

Abstract

Background

Laser-induced breakdown spectroscopy (LIBS) has emerged as a powerful technique for equivalence ratio measurement, offering deeper insight into chemical reaction processes in combustion systems. However, in practical flame conditions, variations in the equivalence ratio are inherently coupled with variations in local gas temperature, which significantly affect the accuracy of LIBS measurements by altering spectral line intensity ratios.

Results

To mitigate the impact of local gas temperature, a hybrid correction method comprising two key steps is proposed. First, plasma-based reference signals are exploited to accurately characterize local gas temperature and correct the spectral line intensities accordingly. Second, the modified spectrum standardization method is applied to compensate for deviations in spectral line ratios. This approach significantly reduces deviations in line intensity ratios across different gas temperatures. Specifically, the AS_RSD of H/O ratio decreases from 25.96 % to 3.94 %. The H/O ratio measured at different flame positions can be regressed onto a unified calibration curve, with an R² greater than 0.95. The robustness of the method is demonstrated on various line ratio pairs, such as H/O, C/O, H/N and C/N. This calibration model enables precise equivalence ratio determination throughout the entire flame field.

Significance

The hybrid correction method allows for the simultaneous and accurate measurement of both local gas temperature and equivalence ratio. By effectively mitigating the influence of temperature variations, this method represents a great progress in enhancing the accuracy of LIBS for equivalence ratio measurement in complex combustion environments.