Formaldehyde Visualization Near Lift-off Location in a Diesel Jet

Formaldehyde (HCHO) near the lift-off location in a reacting diesel jet was visualized using planar laser-induced fluorescence (PLIF). Simultaneous imaging of OH chemiluminescence identified the high-temperature combustion region (lift-off). Experiments were performed in a constant-volume combustion vessel at ambient gas conditions (temperature and oxygen concentration) that generate no-soot, low-soot and moderate-soot diesel jets during mixing-controlled combustion. For no-soot conditions, results show that HCHO is formed upstream of the lift-off location and is consumed downstream of the lift-off length in fuel-rich premixed reaction zones at the jet center. Despite the fuel-rich combustion, and downstream regions that are surrounded by a high-temperature diffusion flame, there is no detectable PAH formation in the no-soot condition. For low-soot conditions (achieved by increasing the ambient temperature), HCHO is formed upstream of the lift-off length, consumed downstream of the lift-off length at the jet center, followed by PAH and soot formation. However, there is a distinct separation between the region of HCHO consumption and PAH formation. As ambient temperature is increased further to produce moderate-soot fuel jets, the axial location of HCHO formation is essentially coincident with the lift-off length and it is harder to distinguish between HCHO and PAH downstream. The general trends above apply to different ambient oxygen concentrations, but the axial and radial positions are elongated at low ambient oxygen. Experimental results were analyzed using a two-reactor (TSL) model for detailed chemistry in turbulent jets. A conceptual model of reacting diesel jets is proposed that includes cool-flame chemistry effects upstream of the lift-off length and the rich, premixed reaction zone at the jet center.