Abstract
ABSTRACTThe influences of initial mixture distribution on localized forced ignition of globally stoichiometric stratified mixtures have been analyzed using three-dimensional compressible direct numerical simulations. The globally stoichiometric mixtures (i.e., ) for different root-mean-square (rms) values of equivalence ratio (i.e., = 0.2, 0.4, and 0.6) and the Taylor micro-scale of equivalence ratio variation (i.e., 2.1, 5.5, and 8.3 with being the Zel’dovich flame thickness of stoichiometric mixture) have been analyzed for different initial rms values of turbulent velocity . The equivalence ratio variation is initialized following both Gaussian and bi-modal distributions for a given set of values of and in order to analyze the effects of mixture distribution. The localized forced ignition is accounted for by considering a source term in the energy conservation equation that deposits energy for a stipulated time interval. It has been demonstrated that the initial equivalence ratio distribution has significant effects on the extent of burning of stratified mixtures following successful localized forced ignition. It has been found that an increase in ) has adverse effects on the burned gas mass, whereas the effects of on the extent of burning are non-monotonic and dependent on for initial bi-modal mixture distribution. The initial Gaussian mixture distribution exhibits an increase in burned gas mass with decreasing , but these cases are more prone to flame extinction for high values of than the corresponding bi-modal distribution cases. Detailed physical explanations have been provided for the observed mixture distribution, , , and dependences on the extent of burning following localized forced ignition of stratified mixtures.
Highlights
Premixed combustion offers an option of controlling flame temperature and reducing pollutant (e.g., NOx) emission, but, in practice, perfect mixing is often difficult to achieve and combustion in many engineering applications takes place in turbulent stratified mixtures
The probability of finding non-premixed pockets is relatively greater in the case of initial bi-modal distribution than in the initial Gaussian distribution cases because the cases with initial Gaussian mixture distribution show greater probabilities of finding φ % hφi 1⁄4 1:0 than the corresponding initial bi-modal distribution cases
For a given value of lφ=lf, an increase in φ0 leads to a reduction of burned gas mass for both Gaussian and bi-modal distributions, whereas the influence of lφ=lf on the extent of burning has been found to be nonmonotonic and dependent on φ0 for initial bi-modal mixture distributions, whereas the cases with initial Gaussian mixture distribution show an increase in burned gas mass with decreasing values of lφ=lf for all initial values of φ0 considered here
Summary
Premixed combustion offers an option of controlling flame temperature and reducing pollutant (e.g., NOx) emission, but, in practice, perfect mixing is often difficult to achieve and combustion in many engineering applications takes place in turbulent stratified mixtures. A number of previous analyses concentrated on flame propagation in stratified mixtures based on experimental (Anselmo-Filho et al, 2009; Balusamy et al, 2014; Grune et al, 2013; Kang and Kyritsis, 2005; Mulla and Chakravarthy, 2014; Renou et al, 2004; Samson, 2002; Sweeney et al, 2013; Zhou et al, 1998, 2013) and direct numerical simulations (DNS) (Cruz et al, 2000; Haworth et al, 2000; Hélie and Trouvé, 1998; Jiménez et al, 2002; Malkeson and Chakraborty, 2010; Patel and Chakraborty, 2014; Pera et al, 2013; Swaminathan et al, 2007) data These studies demonstrated that the flame propagation statistics are strongly affected by the local gradient of equivalence ratio. A similar qualitative conclusion was drawn by Swaminathan et al (2007) based on a preliminary analysis centered on simple chemistry 3D DNS simulations. Patel and Chakraborty (2014) have recently demonstrated, based on DNS simulations, that the root-mean-square (rms) value of equivalence ratio and the length scale of equivalence ratio fluctuations have profound influences on the extent of burning following successful forced ignition of stratified mixtures where the equivalence ratio fluctuation was initialized by a presumed bi-modal distribution
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