Shock-Tube Measurements of Carbon to Oxygen Atom Ratios for Incipient Soot Formation with C2H2, C2H4and C4H6Fuels

The critical atomic carbon to oxygen ratios, φc, for incipient soot formation in shock-heated acetylene, ethylene, ethane/oxygen/argon mixtures have been measured over the temperature range 2000 °K to 2500 °K for reactant partial pressures between 0.1 and 0.4 almos. Absorption of light from a He-Ne laser at 6328 Å was used to detect soot. It was observed that the values ofφ c for all three fuels increased uniformly with temperature such that at the highest temperatures φ c was considerably greater than unity, i.e. greater than the value of about unity at which solid carbon should have been precipitated on a thermochemical equilibrium basis. Observations were made over periods extending up to about one millisecond, which was well in excess of the time required for the major heat release of the combustion reactions. The relevance of these experimental findings to the problem of soot formation in gas turbine combustion chambers is discussed.