Ignition delay measurements are frequently carried out in Rapid Compression Machines (RCMs). When data is compared from different RCM facilities, the ignition delay times are often inconsistent for any particular fuel at a specific compressed condition. The literature has attributed the discrepancy to experimental uncertainties and/or facility effects; however, this issue has yet to be examined more thoroughly. Due to the limited studies on understanding the effect of “facility-dependent” factors on low-temperature reactions and the subsequent ignition of the fuel, one of the main goals of this work is to determine the root cause of the discrepancy. Further analyzing the impact of these factors would facilitate the comparison of ignition delay results across RCM facilities. Additionally, validating chemical kinetic mechanisms against experimental data with these effects in mind would help yield more accurate and consistent ignition delay predictions. Different RCM facilities employ various combinations of compression ratio, initial temperature, initial pressure, diluent gas composition, etc., to achieve the same compressed conditions. Therefore, this work aims to determine the effects of different compression ratios, mixture preparation methods, and diluent gas composition on the measured ignition delay.First, experiments and 0-D CHEMKIN simulations were carried out for stoichiometric mixtures of ethanol and air. To study the effect of diluent gas composition, two different mixtures, one consisting of nitrogen as the diluent gas and other consisting of argon were used. Additionally, this study also examined the effect of compression ratio and mixture preparation methods. Furthermore, 3-D CFD simulations were carried out to investigate the consistent top-to-bottom flame propagation behavior observed in the optical experiments of ethanol auto-ignition. Once a strong fundamental understanding of the factors causing discrepancies in ignition delay measurements of a simple fuel like ethanol was achieved, the study then moved on to a more complex fuel, iso-octane, which exhibits two-stage ignition delay and has a pronounced negative temperature coefficient (NTC) region. The iso-octane studies were focused on the effect of compression ratio; therefore, experiments and 0-D CHEMKIN simulations were carried out for rich mixtures (φ = 1.3) of iso-octane and air, using five different compression ratios. Using numerical analysis, the sensitivity of ignition delay to changes in compression ratio at different equivalence ratios and using different diluent gases was also studied. The results show that the method used to obtain the compressed condition, particularly the use of different compression ratios and diluent gas compositions, can strongly influence the ignition delay times, especially under two-stage ignition conditions. Changes in compression ratio and/or diluent gas composition leads to changes in initial conditions and post-compression heat loss. It was observed that a reduction in post-compression heat losses led to shorter ignition delay times. Furthermore, changing the compression ratio also changes the value of t50, defined as the duration of the last 50% pressure rise during compression. For conditions at which the ignition delay time is short (<15 ms), a longer t50 initiates reactivity prior to the end of the compression stroke, thus reducing the ignition delay time. This range of simulation and experimental conditions helps provide a diverse array of auto-ignition results, involving single-stage and two-stage auto-ignition fuels.
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