No abstract

The autoignition of n-heptane mixtures was studied over a wide range of conditions using a rapid compression machine (RCM). The experiments were performed using several n-heptane mixtures at average compressed gas pressures of 6.55 and 13.35 bar, compressed gas temperatures ranging from 605 to 757 K, equivalence ratio of one, and two inert dilution ratios of 79, and 84%. Two detailed kinetic mechanisms (LLNL and NUI Galway kinetic models) were used to simulate the measured data. Three-dimensional (3-D) and zero-dimensional (0-D) computational fluid dynamics models were used to simulate the autoignition at the studied conditions. The mechanism models performed well using 84% diluted mixture at both pressures, while 79% dilution performed well only at lower pressure. The NUI Galway mechanism model predicts a longer ignition delay with respect to the measured data at most of the studied conditions. The modeled ignition delays are longer using 3-D model with respect to the 0-D model, which is due to the simulation of the flow pattern inside the combustion chamber. However, the difference between the two models becomes smaller at higher gas temperatures. The results show that the 3-D model is necessary to simulate the n-heptane mixture ignition delay and validate the kinetic model at the RCM low-temperature conditions. In addition, some of the measured ignition delays were compared with the shock tube data, and they are slightly shorter than the shock tube data.

An outwardly expanding spherical flame speed has been studied using a constant volume chamber, a spherical bomb, and a shock tube. However, the literature reports very few laminar flame speed measurements at high gas temperatures relevant to the combustion system conditions. Therefore, this research primarily focuses on measuring the laminar flame speeds at high gas temperatures using a new experimental technique called rapid compression machine flame (RCM-Flame). The laminar flame speeds were measured using a stoichiometric mixture of methane, oxygen, and argon at an average compressed gas pressure of 3.04 bar and a range of gas temperatures from 740 to 765 K. The effects of the RCM chamber size, ignition energy, and temperature inhomogeneity on the flame propagation process were investigated by performing both one-dimensional (1D) and three-dimensional (3D) numerical modeling. The AramcoMech 3.0-detailed kinetic model was reduced under the studied conditions by considering ignition delay times and flame speeds to perform 3D numerical modeling. The linear extrapolation method of expanding a spherical flame showed that laminar flame speeds obtained through the experiments agree well with simulated data generated using 1D and 3D numerical modeling.

Laminar flame speed and autoignition are two fundamental characteristics of a hydrocarbon’s combustion. These two characteristics are extensively studied due to their importance in designing and developing combustion systems. However, a regime in which these two characteristics simultaneously affect the combustion process is not well understood. Thus, the primary focus of this investigation is to understand the autoignition-assisted flame regime prior to the first-stage heat release. The premixed mixture of n-heptane in oxygen–nitrogen–diluent at an equivalence ratio of 1.0, a compressed gas pressure of 6.85 bar, and compressed gas temperatures of 621 K (when using argon) and 616 K (when using helium) were studied in this work. At these initial conditions, the mixture ignition delays are 32.31 ms (using argon) and 53.92 ms (using helium). The flame-propagating images were recorded using a high-speed camera at different spark ignition times to measure the flame locations. The Rapid Compression MachineFlame (RCM-Flame) apparatus was used to perform experiments, and one-dimensional modeling and three-dimensional numerical modeling were performed. The simulations provided data to understand the effect of stretch on the measured spherical burning velocity. The measured and simulated autoignition-assisted laminar flame speeds were in excellent agreement, the linear method can be used to remove the stretch from the spherical burning velocity, and the autoignition-assisted flame speeds show a negligible dependency on the spark ignition times when the first-stage heat release is negligible.