Impact of non-ideal behavior on ignition delay and chemical kinetics in high-pressure shock tube reactors

Here, we study real gas effects on high-pressure combustion by comparing simulated and experimentally-measured shock tube ignition delay measurements for n-dodecane/O2/N2 mixtures. Experiments and simulations occur at conditions relevant to diesel engines: 40-80 atm, 774-1163 K, equivalence ratios of 1.0 and 2.0, and O2 concentrations of 13-21%. At these conditions the fuel, oxidizer and intermediate species may exist in a supercritical state during combustion, requiring a real gas equation of state to incorporate non-ideal effects on thermodynamics, chemical kinetics, and the resulting ignition characteristics. A constant-volume, adiabatic reactor model is developed to simulate the reflected shock tube experiments, and simulations compare results for real and ideal gas equations of state, with different expressions for reacting species' activity concentrations. This paper focuses particularly on the cubic Redlich-Kwong equation of state and thermodynamically consistent chemical kinetic rate calculations based on it. Results demonstrate that the equation of state can have considerable influence on ignition delay times with increasing pressure, particularly in the negative temperature coefficient region. Additionally, the results establish important practices for incorporating real gas effects, namely that (i) the compressibilities of key species (i.e. those participating in rate-limiting reactions) are the appropriate way to screen for real gas effects, rather than the average mixture compressibility; and (ii) incorporating a real gas equation of state without also incorporating thermodynamically consistent chemical kinetics significantly under-predicts the magnitude of real gas effects.