Ignition Quality Tester Guided Improvements to Reaction Mechanisms for n-Alkanes: n-Heptane to n-Hexadecane

摘要

While most published detailed reaction mechanisms for n-alkanes have been validated against shock-tube data that use pre-vaporized fuels, they have not been tested extensively using engine conditions. This is partly due to the complications of the effects of both spray and evaporation on ignition and on the gas-phase kinetics. In this study, CFD simulations of Ignition Quality Tests (IQT™) are used as a tool to validate the detailed reaction mechanisms, supplementing other validation tests that use more fundamental shock-tube data. The Ignition Quality Tester is a new ASTM standard for measuring the Cetane Number (CN) of fuels. Shock-tube data in the literature are limited for heavy n-alkanes of interest for engine fuels, which make CN data valuable for mechanism validation. The IQT employs a stationary combustion chamber that involves spray evaporation and mixing followed by combustion. These processes also play an important role under engine-like conditions, but they are not tested by fundamental experiments. To capture the fuel chemistry accurately in these simulations, we have used the FORTF: CFD Simulation Package to model the IQT, which allows us to employ detailed reaction mechanisms with hundreds of chemical species and thousands of reaction steps. This work focuses on long-chain alkanes from n-heptane (C7) to «-hexadecane (C_(16)). n-Alkanes are present in diesel and gasoline fuels, and are key components to their associated reference fuels and model surrogates. Characteristic of high Cetane Number (CN) fuel components, they both ignite earlier than most other classes of components and control the onset of ignition in a wide variety of engine configurations. Accurate reaction mechanisms for n-alkanes are therefore necessary to capture the ignition location in engine simulations. Ignition times for n-alkanes from C_7 to C_(16) in the IQT simulations using the initial mechanism and based on previously published work indicated a contradictory trend in ignition times, with n-hexadecane exhibiting slower ignition than n-heptane. Based on the IQT simulations and other fundamental data, we have updated the detailed mechanism for long-chain n-alkanes by updating the rate rules pertaining particularly to the low-temperature kinetics that is relevant for IQT conditions. The updated reaction mechanism accurately predicts the correct trend in the ignition time in IQT simulations as expected by the corresponding CN of n-alkanes. Although the predicted ignition times in IQT are semi-quantitative, the current work presents an important first step towards using IQT as a tool for validation of reaction mechanisms under more engine-relevant conditions than the fundamental experiments, which is important for establishing confidence in kinetics models for simulating advanced engines using complex fuels.