IMAGING OF THE REACTION ZONE IN A 100 KW OIL-BURNING FURNACE BY USE OF A BROAD-BAND EXCIMER-LASER

The reaction zone in the hostile combustion environment of a 100 kW oi l-burning furnace has been imaged by laser-induced fluorescence using a broad-band XeCl-excimer laser. Upon excitation, the averaged images obtained by using an interference filter around 320 nm (FWHM of 10 nm) show three distinct areas along the direction of the gas flow. An int ense emission spreads around the spray axis and is attributed to the f luorescence of large hydrocarbons in the unburned fuel. Approximately 12 cm downstream of the nozzle, a narrow dark region is displayed sugg esting the preheat zone of the combustion process where large hydrocar bons are considerably degraded. The third distinct region is character ized by a strong onset of the fluorescence intensity localized downstr eam of the dark region. This feature is strongly suppressed by replaci ng the interference filter by a broad-band transmission filter passing light from 350 to 500 nm, Since OH strongly absorbs at the laser wave length and its fluorescence is significantly lower above 345 nm, the f indings imply that the major contribution to the observed intensity in this region originates from the OH radical. This molecule reaches its maximum concentration immediately downstream of the flame front. Howe ver, a contribution from other flame species fluorescing around 320 nm cannot be ruled out. Nevertheless, the combined spatial and spectral information obtained imply that the reaction zone of the combustion pr ocess can be localized accurately. The results are compared with simul taneously performed numerical simulations of the burner and are in rea sonable agreement.