Investigation Of Soot Inception And Sooting Limit In Hydrocarbon Flames Based On Carbon-to-oxgen Atom Ratio Space

Hydrocarbon fuels are a major source of energy consumption, and Soot is a kind of particles generated from hydrocarbon fuel combustion under fuel-rich conditions. Soot particles can affect fire propagation and the thermal efficiency of combustion devices. In addition, soot particles have a significant impact on human health and environment. The formation of soot in hydrocarbon diffusion flames is a series of complex physical and chemical processes, including the formation of polycyclic aromatic hydrocarbons, the inception and surface growth of soot particles, and soot oxidation. It is generally considered that the sooting limit is a critical condition of soot particle inception in hydrocarbon flames. The analysis in carbon-to-oxygen atom ratio(C/O) space can not dependent on the concentrations of fuel and oxidizer, and eliminate system geometry dependences.Thus, a deeper studying on flame structure and soot inception in C/O space has important significance on the understanding of soot formation and controlling its effects.Firstly, based on detailed gas-phase chemical reaction mechanism and soot model, a numerical study was conducted for soot formation in C2H4/O2/N2 and C2H4/O2/CO2 counterflow flame with different stoichiometric(Zst). Flame structure, soot inception and the effect of CO2 replacement of N2 in air on soot inception were analyzed in carbon-to-oxygen atom ratio(C/O ratio) space. Results show that in O2/N2 and O2/CO2 atmospheres, the flame sheet locations of ethylene counterflow flame remain unchanged with the increase of stoichiometric mixture fraction in C/O space, the corresponding carbon-to-oxygen atom ratio values are 0.5 and 0.53, respectively. In C/O ratio space, the location of C6H6 onset is independent of Zst in both atmospheres, and the C/O values are approximately 0.47. Replacement of N2 in air with CO2 has a suppression effect on soot inception.Secondly, sooting limits in a laminar ethylene/air coflow diffusion flame with different Zst were studied, and the simulation results were compared with experimental results. Results show that with the increase of Zst, the temperature peak increases, the mole fractions of C2H2 reduce gradually and the mole fractions of OH and O increase.The flame temperature at the sooting limits increase linearly with Zst. In C/O ratio space, the critical values of sooting limits in ethylene diffusion flame are 0.67 and 1300 K. H diminishes near C/O=0.67 in C/O ratio space.Finally, sooting limits in a laminar ethylene/air coflow diffusion flame have been numerically investigated with different oxygen concentration. Results show that with increased oxygen concentration in oxidizer, the peak flame temperature increases, high temperature regions shift from the two sides to the top of flame, the mole fractions of C2H2, OH, O increase, and the peak of soot volume fraction increases at first then decreases. In C/O ratio space, with the oxygen concentration from 21% increased to 70%, the critical values of sooting limits in ethylene diffusion flame are 0.6 and 1700 K.