Nonideal State Equations to Evaluate the Laminar Flame Speed and Ignition Delay Times at Subcritical, Transcritical, and Supercritical Conditions of Ethanol

Several studies have been conducted to identify an efficient method for reducing particulate emissions from vehicle exhaust gases, which are significant contributors to air pollution in large urban areas. One promising approach involves using supercritical combustion, injecting fuel directly at its critical temperature and pressure. Supercritical fluids possess a lower viscosity and surface tension than liquids and higher diffusion rates, facilitating a more uniform mixture distribution, enhancing thermal efficiency, and reducing particulate emissions. This study focuses on investigating ethanol supercritical combustion as a viable biofuel option. It proposes a simplified kinetic mechanism comprising 53 species and 385 reactions derived through sensitivity analysis and directed relation graph error propagation. To validate this mechanism, simulations were conducted using Cantera with a cubic Peng-Robinson (PR), Redlich-Kwong (RK), and an ideal (I) equation of state (EoS) for 1D laminar flame speed (LFS) and 0D constant-volume autoignition delay time (IDT) simulations for anhydrous ethanol. The IDT results agreed with experimental data across a temperature range of 700-1250 K at 10, 30, 50, 75, and 80 atm, showing good agreement with LFS experiments conducted at 298-949 K, 1-10 atm, and (ϕ) of 0.6-1.8. A normalized computational time ratio was calculated for each EoS relative to the ideal gas, revealing computational costs almost seven times higher for R-K and nearly nine times higher for P-R EoS compared to the ideal gas EoS. The study also examined the limitations of the ideal gas equation of state (EoS) in capturing real gas effects, particularly under ultrahigh-pressure conditions (greater than 100 atm), which revealed significant disparities in simulations at 500 atm. The results indicate that while the ideal gas EoS suffices for ethanol under atmospheric and transcritical conditions, a real gas EoS is crucial for accurate simulations under ultrahigh-pressure conditions.