Comparative study of the counterflow forced ignition of the butanol isomers at atmospheric and elevated pressures

Abstract

In support of the development of robust combustion models, the present study describes experimental and computational results on the non-premixed counterflow ignition of all four butanol isomers against heated air for pressures of 1-4 atm, pressure-weighted strain rates of 200-400 s-1 , and fuel molar fractions in nitrogen-diluted mixtures of 0.05-0.25. Comparison of the parametric effects of varied pressure, strain rate, and fuel loading amongst the isomers facilitates a comprehensive evaluation of the effect of varied structural isomerism on transportaffected ignition. The experimental results are simulated using isomer-specific skeletal mechanisms developed from two comprehensive butanol models available in the literature, and are used to validate and assess the performance of these models. Comparison of the experimental and computational results reveal that while both models largely capture the trends in ignition temperature as functions of pressure-weighted strain rate, fuel loading, and pressure, for all isomers both models over-predict the experimental data to an appreciable extent. In addition, neither model captures the experimentally-observed ignition temperature rankings, with both models predicting a large spread amongst n-/iso-/sec-butanol which does not appear in the experimental results. Sensitivity and path analyses reveal that the butene isomers play a significant role in determining the ignition temperatures of the butanol isomers in both models, with the relative branching ratios likely accounting for the ignition temperature rankings observed using each model. It is observed that the reactivity of the butene isomers varies appreciably between the two butanol models, which may account for some of the variability in predictions between the two models. Furthermore, effects of transport properties and their uncertainties on ignition temperature predictions are discussed.

Reduced Chemistry for Butanol Isomers at Engine-Relevant Conditions

“…Brady et al 8 observed the same problem in their study on forced ignition of the butanol isomers. Perhaps more worryingly, when the same reaction is used in CHEMKIN-PRO 59 , the software proceeds with the calculation without showing any warning or error messages.…”

“…Several of these studies compared combustion properties among the four butanol isomers and found differences in ignition propensity [5][6][7][8] and flame propagation [13][14][15] , demonstrating that the four isomers exhibit differing reactivities as a result of their different molecular structures, which Fig. 1 depicts.…”

Development and validation of a n‐butanol reduced chemical kinetic mechanism under engine relevant conditions

“…Despite these reported results, the aforementioned n‐butanol reduced mechanisms 37–43 found in the literature were either large in mechanism size or validated only under limited engine conditions. However, among others, Feng et al.’s 37 reduced mechanism is considered to be the most well validated mechanism and also having a reasonable size for CFD simulations.…”

“…Nonetheless, the predicted ID times by the reduced mechanism 37 at equivalence ratios of 0.5 and 2.0 were not benchmarked against the detailed mechanism 34 . Meanwhile, Brady et al 39,40 . and Hui et al 41 .…”

“…separately developed reduced mechanisms for butanol isomers including n‐butanol. Brady et al.’s 39,40 reduced mechanism was validated against the experimental measurements in a counterflow burner, whereas Hui et al.’s 41 reduced mechanism was validated under ST ID times, laminar flame speed and 0D HCCI engine. Nonetheless, their reduced mechanisms were considered large in size for practical CFD simulations.…”