Numerical and Experimental Investigation of n-Heptane Autoignition in the Ignition Quality Tester (IQT)

Abstract

Development of advanced compression ignition and low-temperature combustion engines is increasingly dependent on chemical kinetic ignition models. However, rigorous experimental validation of kinetic models has been limited under engine-like conditions. For example, shock tubes and rapid compression machines are usually restricted to premixed gas-phase studies, precluding the study of heterogeneous combustion and the use of low-volatility surrogates for commercial diesel fuels. The Ignition Quality Tester (IQT) is a constant-volume spray combustion system designed to measure ignition delay of low-volatility fuels, having the potential to validate ignition models. However, a better understanding of the IQT's fuel spray and combustion processes is necessary to enable chemical kinetic studies. As a first step, n-heptane was studied because numerous reduced chemical mechanisms are available in the literature as it is a common diesel fuel surrogate, as well as a calibration fuel for the IQT. A modified version of the KIVA-3V software was utilized to develop a three-dimensional computational fluid dynamics (CFD) model that accurately and efficiently reproduces n-heptane ignition behavior and temporally resolves temperature and equivalence ratio regions inside the IQT. Measured fuel spray characteristics (e.g., spray-tip velocity, spray cone-angle, and flow oscillation) for n-heptane were programmed into the CFD model. Sensitivity analyses of fuel droplet size and velocity showed that their effects on ignition delay were small compared to the large chemical effects of increased chain branching in the isomers 2-methylhexane and 2,4dimethylpentane. CFD model predictions of ignition delay using reduced/skeletal chemical mechanisms for n-heptane (60-, 42-, and 33-species, and one-step chemistry) were compared, again indicating that chemical kinetics control the ignition process.

Performance of lignin derived compounds as octane boosters

“…Bogin et al have modified the IQT controls and operating parameters for use in more fundamental ignition delay and kinetic mechanism validation studies [63][64][65]. The authors observed that for a sufficiently long auto-ignition delay time (ID) (e.g., >40 ms), the mixture conditions are nearly homogeneous in nature.…”

Investigation of Physical and Chemical Delay Periods of Different Fuels in the Ignition Quality Tester

“…This indicates that the oscillations in the pressure trace are related to the injection process. This rela- tionship was investigated in the National Renewable Energy Laboratory (NREL) [29,32] by monitoring the N.L and the corresponding changes in high speed images taken for the fuel spray during the injection process. NREL concluded that the oscillations were caused by the changes in the rate of fuel delivery during the injection process.…”

“…It is worth noting that a detailed analysis of the pressure traces for the 32 tests, used to determine the DCN in this investigation, indicated test-to-test variability in these oscillations. Such variability can be explained by the test-to-test variability in the needle oscillations reported by NREL [29,32].…”

Experimental and Computational Study of n-Heptane Autoignition in a Direct-Injection Constant-Volume Combustion Chamber

“…The culmination of the large body of experimental work was the development of a detailed kinetic mechanism for n-heptane, primarily through efforts by researchers at Lawrence Livermore National Laboratory [22,23]. Recent results have been reported on autoignition of nheptane in the IQT [5,6], which is particularly relevant to this study due to the similarity between the IQT and CID devices. The chemical kinetic mechanism of n-heptane predicts a clear NTC (negative temperature coefficient) region in which, counterintuitively, ignition delay increases with increasing temperature.…”

“…The experimental platform is similar to the well-known ignition quality tester (IQT). Recent work on autoignition of n-heptane and several of its isomers suggests that systems like the IQT and CID can, under the right circumstances, be used to validate chemical-kinetic mechanisms [5,6]. A key factor in such efforts is the ignition delay itself, which must be sufficiently long to allow for significant fuel-air mixing and thus a relatively homogeneous mixture at the time of ignition.…”