Instantaneous monitoring of local fuel concentration in a liquid hydrocarbon-fueled flame using a LIBS plug

A portable device composed of photodiodes and bandpass filters was developed to measure local fuel concentration in a liquid hydrocarbon-fueled spray flame. The plasma emission spectra in and around the flame were selectively captured using such simplified device or plug instead of using a laboratory standard laser-induced breakdown spectroscopy (LIBS) system consisting of an ICCD and a spectrometer. The hydrogen (656 nm) and oxygen (777 nm) atomic lines were selected to determine the fuel concentration in atmospheric pressure. The H/O signal intensity ratio was found to be a strong function of the fuel concentration, and thus a calibration curve for the concentration measurements was established and validated using conventional LIBS. The proposed scheme to measure the local equivalence ratio of spray flames using a bundled layout of multiple LIBS plugs alongside the combustor wall may offer simple and highly robust diagnostics, especially under the harsh combustion conditions within air-breathing engines.