SOOT FORMATION IN A STEADY, NONPREMIXED, RECIRCULATING FLAME

An inverted step burner has been designed in which a steady ethylene, recirculating flame is established. The burner was housed within a ver tical wind tunnel. Laser extinction was used to determine the soot vol ume fraction in the recirculation zone. Temperatures were determined b y a thermocouple. One-dimensional laser-Doppler velocity (LDV) measure ments were obtained with a frequency shift system to measure the flow field in the recirculating flame. All the measurements were obtained f or a fixed ethylene flow rate; a low and a high velocity in the approa ch flow were investigated. Variation in air velocity changed the struc ture of the flame. At low flow conditions, the soot loading has two di stinct peaks at the lower and upper edge of the flame. At the higher a ir velocity, the upper part of the flame has a much lower relative soo t loading as a result of the shorter residence time. The location of t he peak values of the soot also changed with the residence time. The p eak temperature was of the order of 1600 degrees C. The soot loading w as low in the regions of high temperature and relatively high in regio ns of low temperatures, reflecting the important role of thermal radia tion in these luminous flames. The LDV measurements were used to revea l the nature of the flow field. The local soot loading in the flame in creased as the approach flow velocity increased; this result suggests the possibility that soot may continue to grow when it is recirculated to regions of growth in a flame.