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

## TL;DR

This study evaluates ethanol combustion under various pressure conditions using different equations of state to improve simulation accuracy and reduce emissions.

## Contribution

A simplified ethanol combustion mechanism and comparison of real and ideal gas equations of state for accurate high-pressure simulations.

## Key findings

- The ideal gas EoS is sufficient for ethanol simulations under atmospheric and transcritical conditions.
- Real gas EoS like Peng–Robinson and Redlich–Kwong are essential for accurate simulations at ultrahigh pressures (greater than 100 atm).
- Computational costs for real gas EoS are significantly higher than for the ideal gas EoS.

## Abstract

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.