Study on Spectral Characteristics and CH* Distribution Characteristics of Diesel Flames in an Entrained-Flow Gasifier

The spontaneous emission spectra of flame are closely related to flame characteristics such as flame structure and temperature distribution. And the combustion state of flame can be reflected clearly by radiation intensity and distributions of excited radicals without being destabilized. Based on a bench-scale opposed multi-burner (OMB) gasification platform, a fiber spectrometer and a high-temperature endoscope coupled with a CCD camera were applied to investigate the two-dimensional distributions of CH* of diesel diffusion flames. The effects of equivalence ratio and impingement on emission spectraand CH* distributions of flame were further compared. The results show that there exists OH* (306. 47, 309. 12 nm) , CH* (431. 42 nm) , Na* (589. 45 nm) as well as K* (766. 91, 770. 06 nm) radicalsin diesel flames. In addition, due to incomplete combustionofdiesel fuel, a lot of black carbon is emitted, which leads to strong continuous black-body radiation in visible wavelengths. The black-body radiation interferes with the detection of the CH* characteristic peak, and the lower the equivalence ratio, the stronger the background radiation, and the greater the interference to the detection. According to Planck's law and interpolation method, background radiation can be subtracted from total radiation in the band around 430 nm. The peak intensity of CH* decreases monotonically with the increase of equivalence ratio. Meanwhile, the contours of CH* radiation appear in the form of three-peak, double-peak, and single-peak along the direction of flame development, and eventually shrink into a circular nucleus centered on the reaction zone. As the equivalence ratio increases, the thresholds of each form continuously decrease, andthe reaction zone gradually shrinks and moves downstream. When the equivalence ratio increases to 1. 0, the diesel fuel burns completely, CH* radiation intensity decreases significantly, and the intensity and distribution of CH* chemiluminescenceof fuel-lean flame remain stable. The flame lift-off length can be evaluated by CH* radiation. For one-burnerjet flame, the flame lift-off length increases significantly and then decreases slightly with the increase of the equivalence ratio. The peak intensity of CH* of impinging flame is always higher than that of jet flame. The lift-off length of impinging flame increases slightly with the increase of the equivalence ratio. More obviously, the confining effect of impingement makes the lift-off length of impinging flame not easy to fluctuatewith the change of the equivalence ratio, which enables the combustion process to be stabler. This provides an intuitive and effective method for quantitatively judging the flame combustion state, as well as an experimental basis for the study of the chemical kinetics of diesel combustion.