Exergy losses in auto-ignition processes of DME and alcohol blends

Abstract

The exergy losses in the auto-ignition processes of binary blends of dimethyl ether (DME) and methanol (MeOH)/ethanol (EtOH) were numerically studied in an adiabatic constant-volume system. Three detailed kinetic mechanisms for the DME/alcohol blends were employed in the auto-ignition simulation at changed temperature, pressure, equivalence ratio and oxygen concentration conditions. Four reaction stages, namely the fuel-series reaction stage, the fuel-fragment reaction stage, the H2O2 loop reaction stage and the H2-O2 reaction stage were defined for the auto-ignition processes based on the reaction pathway analysis, and the exergy destructions in each stage were evaluated for different fuels. It was observed that the exergy loss characteristics calculated by three mechanisms were consistent. Meanwhile, increased initial pressure and reduced oxygen concentration could lower down the overall exergy losses. In addition, MeOH/EtOH addition does not show significant effects on the total exergy losses but the MeOH addition was more effective than EtOH in extending the area of the minimum exergy losses. Furthermore, the interaction effects between the reaction kinetics of alcohols and DME on the exergy loss characteristics were analyzed at the low and high initial temperatures, respectively. The exergy loss rates of different fuels featured triple-peak traces as the initial temperature was located at the low temperature region, while double-peak traces were shown at the high initial temperature condition. At the low initial temperature conditions, MeOH/EtOH addition to DME suppressed the low-temperature reactions, causing the reduced magnitude of the first peak in the exergy loss rate curves. In addition, the magnitude of the second peak increased with MeOH addition while decreased with EtOH addition. The magnitude of the third peak showed insignificant change with MeOH/EtOH addition. At both low and high initial temperatures, the addition of MeOH to DME increases the exergy losses by the H2O2 loop reactions while decreases the exergy losses by the fuel-fragment reactions. In contrast, the addition of EtOH decreases the exergy losses by the H2O2 loop reactions while increases the exergy losses by the fuel-fragment reactions.