Kinetic mechanisms for hydrocarbon ignition

摘要

The origins of detailed and reduced kinetic models for the ignition and oxidation of alkanes and aromatic hydrocarbons have been traced in this study and their development and application in the prediction of some aspects of hydrocarbon fuels have been explored. The scope and limitations for the application of certain formal model structures has been illustrated. The major objective in development and use of kinetic models has been concerned with autoignition and homogeneous combustion as a promising combustion mode for reducing emissions in reciprocating engines. Clearly, there is a need for models that can be used in conjunction with fluid dynamic codes and for this purpose brevity is pre-requisite. Comprehensive kinetic schemes can play a very important part in the validation process, but it would seem that more has yet to be achieved in validating comprehensive models as a satisfactory benchmark for hydrocarbon combustion in the low temperature region. A major objective of this work has been to develop kinetic mechanisms for model fuels in order to simulate the complex physico-chemical interactions in practical combustion systems. The mechanisms, after they have been assembled, have been validated against a wide range of combustion regimes. The latter include laminar premixed and diffusion flames as well plug flow reactors and shock tubes. In Chapter 3 a mechanism for n-heptane is presented and successfully validated against experimental data. The agreement between calculations and measurements is very well. Thereafter, a mechanism for n-decane, which has to be the aliphatic compound for surrogate fuels in representing real diesel and kerosene properties, has been extensively validated. In the mechanism only a small number of chemical species and reactions has been retained without losing in accuracy. This is of great advantage for using detailed chemistry in the flame let model for describing the chemistry-turbulence interactions when simulating autoignition, combustion or pollutant formation in internal combustion engines. The mechanism’s ability to reproduce the main experimental observations on intermediate species has been demonstrated by examining successively the main steps of the mechanism for the combustion of n-decane. The agreement between calculated and experimental mole fraction profiles is good for most species. Computed laminar burning velocities, ignition delay times and oxidation in jet-stirred reactors at moderate pressure show good agreement with experimental data. Toluene and 1,2,4 Trimethylbenzene have also successfully been modelled. Thereafter an extensively investigation of the autoignition in strained flow fields for all the above fuels has taken place. In Chapter 4 different methods for reducing chemical mechanisms are briefly described. An algorithm based on the CSP ( Computational Singular Perturbation ) method is used to derive a reduced mechanism for n-heptane. Finally in chapter 5 a new combustion mode, the so called Homogeneous Charge Combustion Ignition (HCCI) has been presented. The feasibility of this concept has been analysed based on simulations performed, using simple models. The potential in reducing emissions especially Soot and NOx has been demonstrated.