Effect of Adding N₂/H₂O to Ethylene Laminar Diffusion Flame on Soot Formation

The method of adding exhaust gas to fuel to reduce soot and nitrogen oxide (NO_X) is called exhaust gas recirculation (EGR). This article was carried out to investigate the effect of adding N₂ and H₂O to the fuel side to dilute ethylene on soot generation in laminar diffusion flame by combining experiment and numerical simulation. In the experiment, the flame was optically detected, and the volume fraction of soot and temperature was reconstructed. The numerical simulation adopts a simplified GRI-Mech 3.0 ethylene 23-step combustion reaction mechanism. It introduces virtual species FX (F1H₂O, F2H₂O, F3H₂O) to isolate the effects of H₂O addition on thermal, transport, chemical, and density effects. The results show that the numerical values agree well with the experimental results. At the same dilution ratio, the direct involvement of H₂O in the reaction affects the flame temperature and intermediate products, leading to a more significant suppressing effect on soot than N₂ dilution. After decoupling the effects of H₂O, it was found that there are two main reasons for the decrease of soot caused by the addition of H₂O. The first is the dilution effect and thermal effect, which hinder the hydrogen extraction C₂H₂ addition (HACA) reaction by reducing the concentration of intermediate component C₂H₂, greatly inhibiting the surface growth rate of soot, and playing a decisive role in reducing the formation of soot. The second is the chemical effect, which mainly enhances the oxidation process of soot by increasing the concentration of OH free radicals during combustion through the elementary reaction OH + H₂ ↔ H + H₂O. Additionally, the degree of influence of various effects on soot was qualitatively determined as follows: dilution effect > chemical effect > thermal effect > density effect > transport effect.