Laser diagnostics in Combustion: •Soot Formation in Internal Combustion Engines
Flickering candle lights, campfires or internal combustion engines produce soot. Formation of soot means a loss of usable energy. Deposits of soot vitiate the thermal and mechanical properties of an engine. The distribution of soot directly affects the heat radiation and the temperature field of a flame. The exhaust gas of Diesel engines contains fine soot particles. These are suspected to cause cancer. The black soot clouds of the Diesel engines in the 1980s are gone but the remaining, invisible, fine particles pose a severe toxicological problem. 
Figure 1: Experimental setup for the simultaneous, spatially and temporally-resolved measurement of soot volume fraction (LII), NO concentration (NO-LIF) and fuel vapor concentration (Rayleigh scattering) in a high-pressure high-temperature Diesel spray chamber under engine-like conditions.
Soot formation in engines fueled by hydrocarbons, especially Diesel engines and aircraft turbines are in the focus of research. Soot and nitric oxide (NO) are generated antagonistically. A small amount of soot is produced at the expense of a large amount of NO formation, and vice versa. However, while the formation of nitric oxides in internal combustion engines is quite well understood the formation of soot is far more complicated and difficult to examine. The formation of soot particles in Diesel sprays (vaporization of sprays) is so fast and complex that it is not sufficiently understood yet. Soot consists of agglomerates with a diameter of up to several hundred nanometers. These have a fine structure consisting of spherical primary particles. Soot formation starts with the pyrolysis of fuel molecules and the formation of polycyclic aromatic hydrocarbons (PAH). One important precursor of the formation of higher hydrocarbons is acetylene (C2H2). Condensation processes follow and lead to two-dimensional structures. Finally, a rearrangement produces spherical primary particles which continue to grow at their surface.
Figure 2: Schematic setup from Fig. 1
Most examinations of soot in combustion processes were based on elastic scattering, extinction or sampling. Latterly, Laser-Induced Incandescence (LII) proved to be an attractive, robust and versatile technique for measuring soot in unstable flows of complex geometry . Fig. 1 and 2 show an experimental setup for the simultaneous measurement of soot volume fraction (LII), NO concentration (NO-LIF) and fuel vapor concentration (Rayleigh scattering) in a burning Diesel spray under engine-like conditions.
Figure 3: Simultane Messung Rußvolumenbruch (LII) und Treibstoffdampfdichte (Rayleigh Streuung) am brennenden Dieselspray. Gastemperatur 720 K, Injektionsdruck 1200 bar, Gasdruck 73 bar, Beobachtungszeit 3,0 ms nach Injektion, Injektionsdauer 4,5 ms, Durchschnitt aus 50 Einzelbildern, Durchmesser Einspritzdüse 0,19 mm, 10,5% Hydrogrinding, Treibstoff n-Dekan. Zu diesem Beobachtungszeitpunkt wird das elastische Streusignal (rechts) ausschließlich von Rußpartikeln erzeugt.
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