Experimental study on the relationship between the molecular structure and droplet combustion/flame characteristics of various alkane hydrocarbon fuels

Abstract

A detailed experimental study was conducted on the combustion characteristics and flame shape of single droplets of 19 alkane hydrocarbon fuels at room temperature and atmospheric pressure under normal gravity. The diameter of the suspended droplets and the flame shape were photographed using a high-speed camera, and the resulting continuous images were analyzed. During combustion, all the fuel droplets maintained a spherical shape and all exhibited D²-law curve characteristics. The ignition delay time of the droplets tended to increase in the order of N-alkanes, cycloalkanes with one cyclohexyl, bicycloalkanes with two cyclohexyls, and isoalkanes. The burning rate constant K_d was largest for isoalkanes, which have low boiling points, high combustion heat and high vapor pressure, and decreased in order of N-alkanes, cycloalkanes and bicycloalkanes. In addition, detailed analysis of flame height H_f was carried out for all fuels, and it was found that the square value of H_f changed linearly with time during the second half (70%) of the entire period of stable combustion. The flame height H_f at time t can be expressed using a first-order approximation formula like the D² −law, using the flame height change rate K_f obtained from the analysis of the experimental results. This flame height change rate K_f increased in the order of N-alkanes, cycloalkanes, bicycloalkanes, and isoalkanes, and a different trend was obtained from the order of the burning rate constants K_d. Furthermore, a positive correlation was obtained between the initial droplet diameter and the flame height change rate K_f.