Abstract
In this thesis, ignition of wedge-type chemically reacting boundary-layer flows is systematically investigated by the method of large activation asymptotics. Physical models of different kinds are adopted including non-catalytic and catalytic wall, premixed and non-premixed combustible gas, and forced and mixed convection. Explicit criteria ofignition corresponding to the condition for lower vertical tangency on an S-shaped ignition-extinction response curve are derived. Based on these explicit ignition criteria, the relevant system parameters such as flow conditions, thermophysical properties, and overallkineticmechanism of reactants affecting the ignition are investigated in detail. The region of ignition for the wedge angle greater than * exists near the wedge tip rather than far away from it for the wedge angle less than *. For ignition by non-catalytic hot wall, the region of ignition increases with the Prandtl number (Pr) and the temperature of wall, but decreases with the wedge angle.In the strongly catalytic limit (diffusionally controlled limit), the region of ignition for an adiabatic wall is greater than that for cooling wall. The effect of Schmidt number (Sc) is important for the ignition in this limit. For an adiabatic wall, the region ofignition decreases monotonically as the Schmidt number increases, but for external cooling an optimal Schmidt number exists for which the region of ignition is greatest. Ignition of non-premixed boundary-layer flows is governed by two mechanisms,i.e., the kinetically controlled and diffusionally controlledignition processes. The transitionfrom the kinetically controlled to the diffusionally controlled ignition mechanism occurs when the value of the injected mass flux reaches a critical value, at which the ignition temperature at wall for the stagnation-point flow and the ignition distance for the wedge angle smaller than * are minimum, but theregionof ignition for the wedge angle greater than * is maximum. The kinetically controlled ignition region increases with the Pr number and the temperature of the porous wall, but decreases with the wedge angle. For a fixed injection of mass flux, the ignition becomes more difficult as the Sc number of the oxidizer increases but easier as the Sc number of the fuel increases. For small wedge angles under mixed convection, the results of ignition distances for the upper and lower boundary-layer flows are distinct qualitatively and quantitatively. The qualitative behaviors of ignition distances on the upper side are quite similar to those for pure forced convection. The magnitude of the ignition distance for mixed convection on the upper side is greater than that for pure forced convection. For a great mixed convection ratio atignition, the ignition distance on the lower side decreases initially with the wedge angle and then increases with it after the wedge angle reaches an optimal value. The optimal wedge angle decreases with the wall temperature.For the stagnation-point flow, ignition phenomena are highlyhly sensitive to the buoyancy force. The ignition behaviors for the wedge angle greater than * are qualitatively consistent with those under pure forced convection due to sufficiently great flow accelerations.