Numerical investigation of localised forced ignition of pulverised coal particle-laden mixtures: A Direct Numerical Simulation (DNS) analysis

Abstract

Localised forced ignition of mono-disperse pulverised coal particle-laden mixtures has been analysed based on three-dimensional Direct Numerical Simulations for the carrier phase with simplified chemistry for the combustion of volatile gases. The coal particles are treated as point sources and tracked in a Lagrangian manner. The coupling between Eulerian gaseous and Lagrangian particulate phases has been achieved by appropriate source terms in the mass, momentum, energy and species conservation equations. A detailed parametric analysis has been carried out to analyse the effects of particle equivalence ratio Φp (which is defined based on the total available primary volatile fuel in the particulate phase), root-mean-square of turbulent velocity u′ and particle diameter dp on the early stages of combustion. Both non-premixed and premixed modes of combustion have been observed in the reaction zone for the flames resulting from localised ignition. An increase in Φp is found to be detrimental for sustaining combustion, whereas a reduction in particle size may adversely affect the extent of burning. It has been found that an increase in u′ increases the rate of mixing of devolatilised fuel with the surrounding air, which, though beneficial for sustaining combustion, increases the heat transfer rate from the hot gas kernel, thus leading to flame extinction for high values of u′. Detailed physical explanations have been provided to explain the observed effects of Φp, root-mean-square turbulent velocity fluctuation u′ and particle diameter dp on combustion of coal particle-laden mixtures following successful localised forced ignition.