Experiment and Modeling on Thermal Cracking of n-Dodecane at Supercritical Pressure

摘要

A comprehensive understanding of the thermal cracking behavior of hydrocarbon fuels is important for thermal protection applications and investigations into the combustion of thermally cracked fuels. In the present study, n-dodecane is selected as a surrogate for aviation kerosene and it is subjected to a series of thermal cracking experiments at supercritical pressure. According to variations in chemical heat sink, fuel-conversion rate, and gas-production rate, the thermal cracking of n-dodecane is divided into three regions: primary, secondary, and severe. In the primary cracking region, the fuel-conversion rate is lower than 13%, and the liquid products contain only chain alkanes and alkenes. Owing to the mass fraction of main products being proportional to the fuel-conversion rate, a one-step global reaction kinetics is constructed. The secondary cracking region is characterized by rapidly increasing chemical heat sink, fuel-conversion rates, and gas-production rates with increasing fuel temperature, and the appearance of monocyclic aromatic hydrocarbons (MAHs) and cycloalkenes. A kinetic model containing three reactions is proposed for this region. This also considers the thermal decomposition of chain alkanes and alkenes, which result in the formation of MAHs and cycloalkenes. Severe cracking is observed for fuel-conversion rates above 71% where a rapid increase in the concentration of monocyclic and polycyclic aromatic hydrocarbons (PAHs) occurs. The increasing rate of chemical heat sink slows in this region which is characterized by the formation of MAHs, PAHs, and coke. A three-dimensional numerical model is built for the primary and secondary cracking regions, taking the effects of the flow, heat transfer, and thermal cracking of n-dodecane into consideration. Predicted values for the outlet temperature, fuel-conversion rate, and distribution of the main species in all tested cases agree well with the experimental results, validating the numerical model and kinetics for the primary and secondary thermal cracking of n-dodecane.