Optical diagnosis study of fuel volatility on combustion characteristics of spray flame and wall-impinging flame

Finding suitable alternative fuels and reducing carbon emissions are important development directions for future engines. Many carbon-neutral fuels, such as alcohol fuels are widely used, and they are more volatile than traditional diesel fuel. Therefore, exploring the development characteristics of spray flames and near-wall flames of different volatile fuels has important theoretical significance and application value for the optimization of fuel characteristics and the development of efficient and clean combustion technology. In this paper, the flame structure, flame development characteristics and soot evolution of three fuels, including n-hexane, 74%v n-heptane mixed 26%v isooctane (PRF26) and 53%v n-dodecane mixed 47%v isooctane (D53) were tested, that the cetane number of three fuels is essentially same. High speed self-luminous imaging and RGB two-color method were used to investigated the effect of fuel volatility on combustion. Results can be seen as following. Firstly, during the early stages of n-hexane combustion, a blue premixed flame can be found, and there is a pale-yellow flame in the later stage. As the boiling point increases, the area of the blue flame in the first stage gradually decreases, the area of the yellow flame in the later stage gradually increases, and the spatially integrated natural luminosity (SINL) of flame gradually increases. In addition, as the boiling point increases, the initial spontaneous combustion ignition point is away from the nozzle, the ignition delay time reduces and flame area becomes larger. Furthermore, the flame of high boiling fuels result in higher soot emissions. Secondly, in the event of flame wall-impingement, the near-wall flame of the three fuels has higher flame SINL and more soot formation in comparison to the free spray flame. The flame development of the three fuels is similar. However, there are some differences in the flame brightness distribution. The bright flame areas in n-hexane and PRF26 are concentrated near the wall, while the bright flame area in the D53 flame is concentrated on the wall and the flame head area rolled up with a larger distribution area. The flame near the wall is impacted by the wall temperature that the lower wall temperature leads to prolonged fuel evaporation and mixing time, which increases the ignition delay time. In conclusion, fuel volatility has less influence on flame structure and development, but a significant impact on ignition delay time and emissions. Regardless of whether wall impingement occurs, higher volatile fuels can extend the ignition delay time, increase the flame area and decrease soot emissions from combustion.