An experimental and modeling study of n-hexadecane autoignition under low-to-intermediate temperatures

Abstract

N-hexadecane is a potential candidate of diesel surrogate fuels and is also the largest linear alkane (n-alkanes) with known chemical kinetic models. The objective of this study is to investigate the autoignition characteristics of n-hexadecane in the low-to-intermediate temperature region and to validate the existing kinetic models. In this study, the ignition delay times (IDTs) of n-hexadecane were measured using a heated rapid compression machine (RCM) at two pressures of 7 and 10 bar, and over equivalence ratios ranging from 0.5 to 1.3. Two-stage ignition characteristic and the negative temperature coefficient (NTC) behavior of total ignition delay time were experimentally captured. This study paid special attention to the influence of pressure, equivalence ratio, and oxygen content on the IDTs of n-hexadecane. It is observed that both the total IDTs and the first-stage IDTs decrease with the rise of those parameters. It is worth noting that the first-stage IDT is found to show a greater dependence on temperature but a weaker dependence on other parameters compared to the total IDT. The observed IDT dependence in the low-temperature region (LTR) were quantitatively described by ignition delay time correlations. The newly measured IDTs were then validated against two kinetic models (LLNL and CRECK). Simulation results show that both models underpredict the first-stage IDT but generally capture the temperature dependence. The CRECK model well predicts the total IDTs of n-hexadecane while the LLNL model significantly underpredicts the total IDTs at most investigated conditions. To the best of our knowledge, this study is the first investigation on n-hexadecane autoignition under low-to-intermediate temperatures, which deepens the understanding of large n-alkane oxidation and contributes to the improvement of the existing kinetic models.