Direct Numerical Simulation and Modelling Study of the Structures and Propagation of Partially Premixed Turbulent Flames

Sammanfattning: Turbulent partially premixed flames are found in common combustion applications, such as the lifted jet flames in diesel engines. There is less theory surrounding the structure and propagation of turbulent partially premixed flames than for classical premixed flames and diffusion flames, especially in regards to modelling. This thesis presents direct numerical simulations (DNS) and analysis of the structures and propagation of turbulent partially premixed flames. The aim is to collect detailed information on turbulent partially premixed flames and to provide insight for the development of models for turbulent partially premixed flames. The fuels considered are methane and hydrogen, with unity Lewis number and Lewis number less than one, respectively. A detailed chemical kinetic mechanism and mixture averaged transport properties were used in the DNS. The computations were carried out at room temperature and atmospheric pressure in a rectangular domain of size of 20 mm, 10 mm and 10 mm in the streamwise and cross flow directions respectively. Isotropic turbulence was imposed at the inflow boundary and the inflow velocity was selected such that the flame was stabilized in the domain during the time span of the simulation. The two simulated flames have similar turbulence conditions. The Karlovitz number of the methane/air flame is about 50 at the triple-point of the flame, and for the hydrogen/air it is about 5. The DNS was carried out using a high order finite difference code. Both the turbulent methane/air and hydrogen/air partially premixed flames consist of a main premixed flame front trailed by two thin lean premixed flames at the edges and a diffusion flame following the stoichiometry line. The lean premixed flames can be sustained at much lower equivalence ratio than the corresponding planar unstretched laminar premixed flames. The leading front has a w-shaped structure, with weak reactions distributed on the burned side of the premixed flame. Turbulent eddies wrinkle the flames in both cases and broaden the preheat zone. The hydrogen flame is wrinkled at smaller scales than the methane flame, despite having very similar turbulence. The lean trail and the diffusion flame both have a heat release rate roughly two orders of magnitude lower than that of the premixed front. The propagation of turbulent partially premixed flames are shown to be highly sensitive to local equivalence ratio, curvature and strain rate of the flame, and local turbulent motion. The triple flame enhances the displacement speed of stoichiometric mixture of hydrogen due to the interaction between the diffusion flame and premixed flames. The effect of turbulent eddies is shown to highly disturb the flame structures and local displacement speed, which poses a great challenge to the development of cost-effective CFD models for turbulent partially premixed flames under high Karlovitz numbers.