Fundamental investigation of localised forced ignition in turbulent homogeneous and stratified mixtures

Abstract

Localised forced ignition (e.g. spark or laser ignition) of combustible mixtures has a number of important applications ranging from Gasoline Direct Injection (GDI) engines, Lean Premixed Prevaporised (LPP) combustors to high-altitude relight in aero-gas turbines. An improved understanding of localised ignition of combustible mixtures is essential for fire/explosion safety and also for designing a reliable ignition system in Internal Combustion (IC) engines and gas turbines so that self-sustained combustion subsequent to ignition can be achieved without compromising on emissions and energy-efficiency. The aforementioned importance of forced ignition serves the motivation behind the current analysis. In this thesis, localised forced ignition of turbulent homogeneous and stratified mixtures has been analysed in detail using three-dimensional compressible simple chemistry Direct Numerical Simulations (DNS) for range of different parameters (e.g. rootmean- square (rms) values of turbulent velocity (u′) and equivalence ratio fluctuations, and the length scales of turbulent fluid motion and mixture inhomogeneity). The influences of energy deposition characteristics on localised forced ignition of turbulent homogeneous mixtures have been analysed for different turbulent velocity fluctuations in order to identify the favourable conditions in terms of the characteristic width and duration of ignition energy deposition ignition for the purpose of achieving selfsustained combustion. Moreover, the ignitability of turbulent homogeneous mixtures and the corresponding value of Minimum Ignition Energy (MIE) in the case of localised forced ignition have been numerically analysed for different values of Karlovitz number Ka. It has been found that for the given values of equivalence ratio and u′, an increase in the energy deposition width leads to an increase the magnitude of minimum energy requirements for successful ignition and self-sustained combustion. Additionally, the influences of initial mixture distributions for both globally stoichiometric and fuel-lean stratified mixture on localised forced ignition of stratified mixtures have been analysed for different rms values of equivalence ratio, turbulent velocity fluctuations, and the Taylor micro-scale of equivalence ratio variation for initial presumed Bi-modal and Gaussian distribution of equivalence ratio. It has been demonstrated that the initial equivalence ratio distribution has significant effects on the early stages of burning of stratified mixtures following successful localised forced ignition. The results show that the rate of heat transfer from hot gas kernel increases with increasing turbulent intensity, which acts to reduce the extent of burning, and in some extreme cases may lead to flame extinction. The results demonstrated that favourable conditions in terms of initial mixture distribution, equivalence ratio variation, length scale of mixture inhomogeneity, and rms turbulent velocity fluctuation are required for self-sustained combustion on following successful ignition of stratified mixtures. Furthermore, the effects of fuel Lewis number LeF (ranging from 0.8 to 1.2) on ignitability of both homogeneous and stratified mixtures and early stage of burning following localised forced ignition have been analysed based on DNS data. It has been found that the ignition energy which leads to successful ignition and subsequent self-sustained flame propagation increases with increasing LeF for both stratified and homogeneous mixtures. Detailed physical explanations have been provided for the influences of the various parameters which significant affect the performance of localised forced ignition. Qualitative similarities between computational findings and experimental observations have been indicated, where possible, and limited number DNS simulations of localised forced ignition in n-heptane-air mixture with detailed chemistry have been carried out to confirm qualitative similarities between simulation results obtained using simple and detailed chemical mechanisms.