Effects of gas temperature on equivalence ratio measurement in premixed methane–Air flame using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been demonstrated as a promising technique for real-time combustion diagnosis due to its capacity for simultaneous multi-species analysis. The gradient of species concentration in reacting flows often coupled with variations in gas temperature which prevented accurate concentration measurement. The effect of gas temperature on spectral emission intensity and plasma property was comprehensively investigated by employing a Bunsen flame. With the increasing of gas temperature, less laser energy was deposited into the plasma, resulting in a monotonic decline in plasma volume, brightness and atomic emission intensity. Conversely, the plasma temperature improved due to fewer gas molecules being excited. The intensity of ionic lines and electron density were initially increased but subsequently decreased, with the reduction in gas density playing a dominant role at higher gas temperature. Intensity ratio pairs of C/O and H/O were found to be susceptible to gas temperature. The deviation of C/O ratio caused by gas temperature from burner nozzle to Bunsen tip (∼1100 °C) was about 28.4 %. Clear elucidation of the effect of gas temperature provides reliable basis to accurate combustion diagnosis with LIBS.