Effects of 2, 5–dimethylfuran/ethanol addition on soot formation in n-heptane/iso-octane/air coflow diffusion flames

Abstract

The effects of 2, 5-dimethylfuran (DMF)/ethanol addition on the atomized combustion soot formation characteristics of n-heptane/iso-octane coflow diffusion flames are systemically studied. The changes of soot particles and deposition morphology at different heights in the flame of reference fuel with 15% ethanol (E15) and 15% DMF (DMF15) are analyzed. Results show that the addition of oxygenated fuels can increase the flame heights and decrease the brightness. With the addition of 15% ethanol and DMF, the flame heights are increased by 11.29% and 19.35% respectively, and the brightness of the edge region of the middle section of DMF15 flame is enhanced. At the nozzle, the average temperature of E15 flame increases by 80 K. The high temperature region of E15 flame enlarged and the average temperature difference of different heights is higher than 50 K. As for DMF15, the high temperature region of flame is constant, and the average temperature difference between various heights is smaller than 35 K. For three fuels, the size of the sediment particles formed at the low height above burner (HAB = 20 mm) is large, and the obvious pore structure can be seen at HAB = 60 mm. The addition of oxygenated fuels makes the pore structure distribution regular and the porosity decreases, and DMF15 (porosity = 0.2670) has larger pore structure than E15 (porosity = 0.1557). The oxygenated fuel can inhibit the formation of soot precursors and the agglomeration of soot particles, but the increase of particle size in the middle region of flame is promoted because of the increase of flame height and residence time of soot particles. As a result, the soot particle size of oxygenated fuel is decreased greatly in the oxidation stage, and the soot particle size of DMF15 (40.4%) is decreased more than that of E15 (37.1%). The effect of ethanol and DMF addition on the activation energy of low position soot is not obvious, which is about 130 kJ/mol. In the middle position of the flame, the activation energy of E15 is the highest (159.32 kJ/mol), and the activation energy of DMF15 is the smallest (148 kJ/mol). At the high position, E15 (212.07 kJ/mol) and DMF15 (226.73 kJ/mol) are less than that of DE0 (235.03 kJ/mol) respectively, indicating that the activation energy of the mature soot reduces, which is beneficial to the oxidative removal. Ethanol can dilute the fuel and increase the oxygen concentration in the mixture, while DMF can delay the ignition and prolong the residence time of soot and increase the content of disordered carbon in mature soot particles.