Optical diagnostics for soot formation from evaporating fuel films in combustion

Late-evaporating liquid fuel wall-films are considered a major source of soot in spark-ignition direct-injection (SIDI) engines. In this study, a direct-injection model experiment was developed, and optical diagnostics were used to image fuel-film evaporation and soot formation. Fuel is injected by a multi-hole injector into the optically accessible test section of a constant-flow facility. Some of the liquid fuel impinges on a quartz-glass window and forms fuel films. After spark ignition, a turbulent flame front propagates through the chamber, and subsequently sooting combustion arises near the evaporating fuel films.

Laser-induced fluorescence (LIF) of toluene (fluorescent tracer) added to iso-octane (surrogate fuel) in small concentration, excited by laser pulses at 266 nm, is used to image the fuel-film thickness and the air/fuel equivalence ratio in the gas phase. The LIF images of fuel films show that the films remain on the wall surface long after the flame front has passed and that the fuel accumulates to thick fuel blobs in the films throughout the evaporation. Generally, and consistent with results from a Computational Fluid Dynamics (CFD) simulation, the evaporation rate is highest early after start of injection (aSOI) and then decreases and remains on a constant level, with the magnitude strongly depending on the wall temperature. Combustion and thus convective heat transfer show a minor effect on the fuel-film evaporation rate compared to the wall temperature in either CFD or LIF. A low-dimensional model (LDM), developed in this work, shows that the quartz wall only slightly cools down in the impingement region. Consistent with results from the CFD, the fuel-film temperature rapidly approaches the wall temperature, independent of the initial film temperature. Gas-phase LIF finds fuel-rich vapor plumes emerging from the fuel films until late aSOI without combustion. Interestingly, when combustion is initiated fuel vapor plumes are not detected, indicating that the fuel vapor pyrolytically decomposes spatially very close to the fuel films.

The second part of this work deals with the visualization of soot formation from evaporating fuel films by multiple laser-based and high-speed imaging diagnostics. Overlapping laser light sheets at 532 and 1064 nm excited LIF of polycyclic aromatic hydrocarbons (PAH) -potential soot precursors- and laser-induced incandescence (LII) of soot, respectively. For preliminary measurements, the constant-flow facility was replaced with a Santoro or Yale burner, providing a steady, sooting, laminar co-flow diffusion flame. In complementary line-of-sight integrated imaging, the fuel spray, chemiluminescence, and soot incandescence were captured with a high-speed color camera. PAH LIF is found in close vicinity of the evaporating fuel films. Soot is found spatially separated from, but adjacent to the PAH, both with high spatial intermittency. Average images indicate that soot is formed with a much higher spatial intermittency than PAH. Images from the color camera show soot incandescence earlier and in a similar region compared to soot LII. Chemiluminescence downstream of the soot-forming region is thought to indicate the subsequent oxidation of fuel, soot, and PAH. High-speed imaging and the CFD simulation predict the inception of soot pockets to similar times, close to the evaporating fuel films and in consistent spatial extent. Excitation of PAH LIF with 266 and 532 nm and a variation of the detection bandpass-filter show PAH of different size classes systematically in a thin layer between the fuel films and soot.


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