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

Abstract

Exergy analysis in the n-heptane autoignition process was numerically conducted in an adiabatic constant-volume system with the detailed, skeletal and reduced n-heptane chemical mechanisms. The results revealed that the top five contributors to the system entropy change are N2, CO2, H2O, O2 and n-heptane at all cases. The relative errors of the exergy losses by chemical reactions calculated by the three mechanisms were less than 1% for all the initial conditions, indicating that mechanism reduction does not significantly influence the calculation results of the exergy losses by chemical reactions. However, the calculation results of the exergy losses by incomplete combustion products were less convinced by the skeletal and reduced mechanisms, with the relative error up to 3.45%. However, the overall satisfactory agreement of the total exergy losses calculated by different mechanisms suggested that the skeletal and reduced mechanisms might be suitable for the exergy efficiency calculation in engine combustion simulation. Furthermore, reactions in the mechanisms were classified to eight classes according to the carbon chain length. It was observed that the skeletal and reduced mechanisms overestimated the exergy losses by the C0–C1 reactions but underestimated the exergy losses by the C2 reactions. The calculated exergy losses of the C7 reactions were relatively consistent when the detailed, skeletal and reduced mechanisms were used.