Combined Effects of Dilution Gases with Polyoxymethylene Dimethyl Ether (PODE ₃ ) on Physicochemical Properties for Soot Particles in Isooctane Inverse Diffusion Flames

The present study explores the combined effects of diluents (N2, He, and CO2) with polyoxymethylene dimethyl ether 3 (PODE3) addition on soot formation in isooctane inverse diffusion flames to optimize combustion processes. The results indicated that replacing N2 with He diluent in isooctane caused an increase in flame height, temperature, and the mean KL factor (a two-color pyrometry parameter that quantifies soot concentration in flames). In contrast, CO2 acted as an inhibitor, reducing these parameters. Therefore, 20% PODE3/isooctane mixture with CO2 diluent showed the shortest flame height, least flame luminous intensity, and lowest flame temperature, indicating enhanced combustion efficiency, with an almost negligible mean KL factor. Moreover, He dilution produced soot particles with a chain-like morphology, less dense structure, longer fringe length, lower tortuosity, and less disordered, which can be attributed to its higher thermal conductivity and lower heat capacity than N2. Conversely, soot particles generated from pure isooctane with CO2 dilution resulted in smaller fringe length, higher tortuosity, and lower graphitization level compared to N2 and He diluents, even with 20% PODE3 addition. The combined effect of CO2 with 20% PODE3 led to the smallest fringe length, largest tortuosity, and least graphitization with more amorphous soot particles. These findings were supported by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses.