Dilution, Thermal, and Chemical Effects of Carbon Dioxide on the Exergy Destruction in n-Heptane and Iso-octane Autoignition Processes: A Numerical Study

Abstract

A numerical analysis based on the second-law thermodynamics was conducted for the n-heptane and iso-octane autoignition processes in an adiabatic constant-volume system with CO2 addition. Detailed chemical mechanisms of n-heptane and iso-octane were used in the autoignition processes simulation at the initial temperatures located in the low-temperature region and the high-temperature region. The dilution, thermal, and chemical effects of the CO2 addition were numerically isolated to evaluate each individual effect on the exergy destruction in different reaction stages of the autoignition processes, namely, the fuel-series reaction stage, the fuel-fragment reaction stage, the H2O2 loop reaction stage, and the H2-O2 reaction stage. It was observed that the exergy loss in the fuel-series reaction stage of iso-octane autoignition was lower than that of n-heptane, but the individual effect of CO2 played the same role in the exergy destruction for both fuels. The dilution effect of CO2 reduced the exergy loss in the H2O2 loop reaction stage, and the reduction magnitude increased with the increased initial temperature or CO2 concentration. The thermal effect of CO2 slightly increased the exergy loss at both initial temperatures due to the increase in the overall heat capacity and the decrease in the bulk temperature. The chemical effect of CO2 reduced the exergy loss in the H2-O2 reaction stage at both initial temperatures. The exergy loss due to the incomplete combustion became apparent at the high initial temperature condition and was increased by the dilution and chemical effects of CO2 while decreased by the thermal effect of CO2.

Effects of mechanism reduction on the exergy losses analysis in n-heptane autoignition processes

“…Pathways to reduce the total exergy losses were explored by evaluating the influences of control parameters, such as oxygen concentration, equivalence ratio, initial temperature and pressure. Zhang et al 8 classified the overall chemical reactions in the fuel kinetic mechanisms into four stages and evaluated the dilution, thermal and chemical effects of carbon dioxide addition on the exergy losses in each reaction stage. Moreover, Zhang et al 9 studied the exergy losses in the autoignition process of dimethyl ether (DME) and alcohols blends.…”