Counterflow Ignition of n-Butanol at Atmospheric and Elevated Pressures

Authors: K. B. Brady, X. Hui, C. J. Sung, K. E. Niemeyer

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Critical to the development of predictive combustion models is a robust understanding of the coupled effects of chemical kinetics and convective–diffusive transport at both atmospheric and elevated pressures. The present study describes a new variable-pressure non-premixed counterflow ignition experiment designed to address the need for well-characterized reference data to validate such models under conditions sensitive to both chemical and transport processes. A comprehensive characterization of system behavior is provided to demonstrate boundary condition and ignition quality as well as adherence to the assumption of quasi-one-dimensionality, and suggest limitations and best practices for counterflow ignition experiments. This effort reveals that the counterflow ignition experiment requires special attention to ignition location in order to ensure that the assumption of quasi-one-dimensionality is valid, particularly at elevated pressures. This experimental tool is then applied to the investigation of n-butanol for pressures of 1–4 atm, pressure-weighted strain rates of 200–400 s−1, and fuel mole fractions of 0.05–0.25. Results are simulated using two n-butanol models available in the literature and used to validate and assess model performance. Comparison of experimental and numerical ignition results for n-butanol demonstrates that while existing models largely capture the trends observed with varying pressure, strain rate, and fuel loading, the models universally over-predict experimental ignition temperatures. While several transport coefficients are found to exhibit order-of-magnitude or greater sensitivities relative to reaction rates, variation of transport parameters is not able to account for the large deviations observed between experimental and numerical results. Further comparison of ignition kernel structure and fuel breakdown pathways between two literature models suggests that an under-prediction in the radical pool growth with respect to temperature variation may be responsible for both the deviation from the experimental results and the discrepancy in ignition temperature results observed between models.

Citation: K. B. Brady, X. Hui, C. J. Sung, K. E. Niemeyer, “Counterflow Ignition of n-Butanol at Atmospheric and Elevated Pressures,” Combustion and Flame 162 (10), 3596-3611 (2015).