On the Chemical Kinetics of Ethanol Oxidation: Shock Tube, Rapid Compression Machine and Detailed Modeling Study

Abstract

Auto-ignition characteristics of ethanol were experimentally investigated using two Shock Tube (ST) facilities and a Rapid Compression Machine (RCM). Ignition delay times for stoichiometric ethanol-air mixtures were measured for temperatures between 775–1300 K in a High Pressure Shock Tube (HPST) at pressures close to 80 bar by probing pressure time histories and CH* emission. In some experiments the HPST was additionally employed for schlieren imaging to visualize ignition behavior by probing density gradients during ignition for ethanol-air mixtures. The ignition delay experiments in HPST were complemented by RCM measurements for extending the temperature regime to the Low Temperature Combustion (LTC) regime, down to 705 K, providing kinetic model validation data over a very wide temperature and pressure range. The current results also extend the earlier shock tube measurements performed in the same laboratory for pressures around 40 bar for temperatures down to 800 K [Heufer et al., Shock Waves 20 (2010) 307]. Furthermore, a Rectangular Shock Tube (RST) was solely used for additional schlieren imaging experiments to acquire information on ignition modes in stoichiometric ethanol-air mixtures around 10 bar. An improved chemical kinetic model was developed based on the Li et al. mechanism [Li et al., “Ethanol Model v1.0”, Princeton University, 2009] which was updated with evaluated rate parameters from the literature and validated through results obtained from the aforementioned facilities. The model predictions were compared to previously published low-pressure, premixed flat flame molecular beam mass spectrometry speciation data [Kasper et al., Combust. Flame 150 (2007) 220; Wang et al., J. Phys. Chem. A 112 (2008) 9255] where reasonable agreement is obtained considering the uncertainties in experiments and model. However, the model provides excellent agreement for the auto-ignition results obtained in the RCM and the high temperature shock induced ignition delays. Significant disparities with the model predictions are obtained for the shock tube results at temperatures below 1000 K as it transitions from the intermediate to the low temperature regime. The reasons for these deviations are assigned to strong fuel specific “pre-ignition” effects observed in ethanol auto-ignition, in contrast to other investigated fuels, which was satisfactorily explained through schlieren experimental results. To our knowledge this work is first of its kind that combines results from complementary experimental methods from three different facilities providing a holistic description on the auto-ignition behavior of ethanol. Furthermore, this paper reports ignition delay measurements for ethanol in air, at the highest pressures applicable to practical combustors.

Autoignition of ethanol in a rapid compression machine

“…In contrast to the study of Lee et al [10], Cancino et al [9] did not report any pre-ignition pressure rise and adjusted the kinetic mechanism to mimic the plateau in the ignition delay profile at low temperatures. Specifically, a significantly higher value for the rate constant of the ethanol + HȮ 2 reaction was adopted [10].…”

“…By using Schlieren imaging in a shock tube, Lee et al [10] also noted deflagrative behavior in ethanol autoignition. Pressure measurements as well as emission signals showed strong preignition behavior with pressure increasing by more than 100% in some cases before autoignition.…”

“…Several investigations have focused on studying the chemical kinetics of ethanol combustion using laminar flames [1][2][3][4], shock tubes [5][6][7][8][9][10], flow reactors [11][12][13], jet-stirred reactors [14,15] and a RCM [10]. Ethanol autoignition has been studied in shock tubes mostly at high temperatures and at pressures of less than 5 bar [5][6][7][8][9][10].…”

“…Few studies have been conducted at elevated pressures and at temperatures lower than 1000 K using shock tubes [8][9][10] and RCM [10]. At low temperatures and elevated pressures, HȮ 2 radical chemistry can play a dominant role.…”

“…Lee et al [10] determined ignition delay times for stoichiometric ethanol mixtures at 775-1000 K and 80 bar using a shock tube and complemented these with RCM measurements at 35 bar. Autoignition at low temperatures can be influenced by phenomena that are facility specific.…”