A numerical investigation of flame lift-off in diesel jets

Flame lift-off heights are modeled in diesel jets by using diffusion flamelet extinction as a criterion for identifying the lift-off. It is shown that the axial distance in the jet where the stoichiometric scalar dissipation rate matches computed extinction scalar dissipation rate correlates with the lift-off height. The influence of injection pressures (40–138 MPa), chamber densities (14.8–58.5 kg/m³), chamber temperatures (1000–1300 K) and O₂ molar concentrations (10–21%) are studied. N-heptane is chosen as a surrogate for diesel fuel. Two chemical kinetic mechanisms, a 37-species, 56-step mechanism and a 159-species, 1540-step mechanism, are employed. Consistent with experimental findings, the computed results indicate that the flame lift-off height decreases with increase in chamber temperature, chamber density and oxygen concentration and increases when the injection velocity is increased. It is observed that across the range of chamber conditions considered, the computed extinction scalar dissipation rates correlate well with the measured lift-off heights. When chamber temperatures and O₂ concentrations are varied, the results are found to be sensitive to the choice of the chemical kinetic mechanism.Rishikesh Venugopal and John Abraha

Direct numerical simulation of a liquid sheet in a compressible gas stream in axisymmetric and planar configurations

A thin liquid sheet present in the shear layer of a compressible gas jet is investigated using an Eulerian approach with mixed-fluid treatment for the governing equations describing the gas-liquid two-phase flow system, where the gas is treated as fully compressible and the liquid as incompressible. The effects of different topological configurations, surface tension, gas pressure and liquid sheet thickness on the flow development of the gas-liquid two-phase flow system have been examined by direct solution of the compressible Navier-Stokes equations using highly accurate numerical schemes. The interface dynamics are captured using volume of fluid and continuum surface force models. The simulations show that the dispersion of the liquid sheet is dominated by vortical structures formed at the jet shear layer due to the Kelvin-Helmholtz instability. The axisymmetric case is less vortical than its planar counterpart that exhibits formation of larger vortical structures and larger liquid dispersion. It has been identified that the vorticity development and the liquid dispersion in a planar configuration are increased at the absence of surface tension, which when present, tends to oppose the development of the Kelvin-Helmholtz instability. An opposite trend was observed for an axisymmetric configuration where surface tension tends to promote the development of vorticity. An increase in vorticity development and liquid dispersion was observed for increased liquid sheet thickness, while a decreasing trend was observed for higher gas pressure. Therefore surface tension, liquid sheet thickness and gas pressure factors all affect the flow vorticity which consequently affects the dispersion of the liquid