Experimental and Kinetic Modeling Study of Autoignition Characteristics of n-Heptane/Ethanol by Constant Volume Bomb and Detail Reaction Mechanism

摘要

The purpose of this work is to investigate the autoignition characteristics of n-heptane/ethanol including the autoignition temperature (AIT), ignition delay time (tau(ig)) and the low/intermediate/high-temperature ignition. The AIT is measured by the combustion bomb under wide range conditions (phi = 0.2-2.0, P-init, = 0.772-3.861 MPa) while the influencing factors of tau(ig) and the dominant reactions of the high/intermediate/low-temperature ignition are investigated by detail chemical kinetic model. The results indicate that first, the maximum AITs difference is 46, 56, 62, 84, and 124 degrees C when the ethanol blending ratio is 0%, 25%, 50%, 75%, and 100% while the minimum AITs usually locate at the region of phi = 1.0-2.0, P-init = 3.282-3.861 MPa. Second, the AITs become more vulnerable to the pressure and equivalence ratio at 3.809-3.861 MPa and the maximum AIT difference increases from 6 to 44 degrees C as the ethanol blending ratio increasing from 0% to 100%. Third, the sensitive factors for tau(ig) rank as initial mixture temperature ethanol blending ratio = equivalence ratio = initial mixture pressure. Fourth, the upper temperature limit of the NTC region can be determined by the mole fraction-balanced coefficient (MBC) equal to 2 which is the ratio of the maximum mole fraction of OH radicals to the maximum mole fraction of H2O2 molecule. The NTC region can be quantified by combining the criteria of MBC = 2 and the ceiling temperature. Fifth, at low-temperature regime, the H atom abstraction from n-heptane is the rate limiting step; at the NTC region, the reaction type of QOOH = olefin + HO2 displaces QOOH + O-2= O(2)QOOH as the dominant reaction; the unimolecular fuel decomposition acts as the major initiation reactions and fuel consumption reactions at the high-temperature regime.