Abstract
The influences of fuel Lewis number L e F on localised forced ignition of globally stoichiometric stratified mixtures have been analysed using three-dimensional compressible Direct Numerical Simulations (DNS) for cases with L e F ranging from 0.8 to 1.2. The globally stoichiometric stratified mixtures with different values of root-mean-square (rms) equivalence ratio fluctuation (i.e. ϕ ′= 0.2, 0.4 and 0.6) and the Taylor micro-scale l ϕ of equivalence ratio ϕ variation (i.e. l ϕ /l f = 2.1, 5.5 and 8.3 with l f being the Zel’dovich flame thickness of the stoichiometric laminar premixed flame) have been considered for different initial rms values of turbulent velocity u ′. A pseudo-spectral method is used to initialise the equivalence ratio variation following a presumed bi-modal distribution for prescribed values of ϕ ′ and l ϕ /l f for global mean equivalence ratio 〈ϕ〉=1.0. The localised ignition is accounted for by a source term in the energy transport equation that deposits energy for a stipulated time interval. It has been observed that the maximum values of temperature and the fuel reaction rate magnitude increase with decreasing L e F during the period of external energy deposition. The initial values of L e F , u ′/S b(ϕ=1), ϕ ′ and l ϕ /l f have been found to have significant effects on the extent of burning of the stratified mixtures following localised ignition. For a given value of u ′/S b(ϕ=1), the extent of burning decreases with increasing L e F . An increase in u ′ leads to a monotonic reduction in the burned gas mass for all values of L e F in all stratified mixture cases but an opposite trend is observed for the L e F =0.8 homogeneous mixture. It has been found that an increase in ϕ ′ has adverse effects on the burned gas mass, whereas the effects of l ϕ /l f on the extent of burning are non-monotonic and dependent on ϕ ′ and L e F . Detailed physical explanations have been provided for the observed L e F , u ′/S b(ϕ=1), ϕ ′ and l ϕ /l f dependences.
Highlights
Localised forced ignition of inhomogeneous mixtures plays a pivotal role in smooth functioning of Direct Injection (DI) engines and high altitude relight in aero-gas turbines and improved understanding is needed to identify the conditions which lead to successful ignition and self-sustained combustion
For a given value of lφ/ lf, an increase in φ leads to a reduction of burned gas mass for the stratified mixture cases, whereas the influence of lφ/ lf on the extent of burning has been found to be non-monotonic and dependent on φ and LeF
The above findings demonstrate that the favourable conditions in terms of lφ/ lf, φ and u /Sb(φ=1) for successful ignition and self-sustained combustion in stratified mixtures are dependent on fuel Lewis number LeF
Summary
Localised forced ignition of inhomogeneous mixtures plays a pivotal role in smooth functioning of Direct Injection (DI) engines and high altitude relight in aero-gas turbines and improved understanding is needed to identify the conditions which lead to successful ignition and self-sustained combustion. Three-dimensional Direct Numerical Simulation (DNS) studies [8,9,10,11,12,13] demonstrated that an increase in turbulent velocity fluctuation for a given value of integral length scale of turbulence is detrimental to the success of localised ignition of inhomogeneous gaseous and droplet-laden mixtures. Patel and Chakraborty [18] used three-dimensional simple chemistry DNS simulations to demonstrate that the rms value of equivalence ratio fluctuation φ and the Taylor micro-scale lφ of equivalence ratio φ variation, in addition to the rms values of turbulent velocity u , have significant effects on the extent of burning and the possibility of attaining self-sustained combustion following successful ignition
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