Direct numerical simulation of lifted flames in diesel engine conditions

Increasing energy demand and stringent pollution norms necessitate the design of engines with higher efficiency and lower emission levels. A fundamental understanding of combustion in diesel engines can enable the design of such engines. Of special interest is the lift-off height, the distance between the injector orifice and the start of the high-temperature reaction zone. The lift-off height controls the amount of mixing between fuel and oxidiser before combustion and hence combustion efficiency and pollutant levels. A comprehensive understanding of the flame stabilisation mechanism, which determines the lift-off height, is thus critical. Arriving at this understanding is not trivial due to the high-temperature and high-pressure conditions present in diesel engines which limit the capabilities of experiments. Consequently, important aspects such as the flame structure and its stabilisation mechanism are not well understood. With the aim to elucidate the flame structure and stabilisation mechanism in diesel engine conditions, direct numerical simulations are performed in this thesis. The ambient conditions are matched to the Engine Combustion Network’s Spray A flame. A canonical configuration of two-dimensional laminar lifted flame is considered first. The response of the flames to inlet velocity and scalar dissipation rate is studied. The flames transition from attached, to lifted propagation stabilised to lifted ignition stabilised upon increasing the inlet velocity. A complex multibrachial flame structure with up to five branches is observed. The propagation stabilised and ignition stabilised flames exhibit characteristically different structure, transport budget and displacement speeds. These observations are then employed to identify the stabilisation mechanism of a three-dimensional spatially developing turbulent round jet flame. The turbulent flame structure, transport budget and the displacement speed at the flame base closely resemble the characteristics of the two-dimensional propagating laminar flames indicating that the turbulent flame is stabilised by flame propagation. The DNS data are then utilised to a priori assess the chemistry tabulation combustion models, important design tools used in industry. Results show that these models give good performance in a priori comparison with the DNS data when the dimension and the choice of the control variables are appropriately considered