温度和压力对处于贫油预混预蒸发燃烧阶段火焰的影响

The purpose of this experimental work is to study lean premixed n-heptane/air flames in order to identify the factors governing flame structure and flame stabilization. The experimental burner is composed of a cylindrical combustion chamber placed in a pressurized and cooled casing. Downstream, a converging nozzle with variable section allows to rise the pressure up to 8 bar inside the combustion chamber. Combustion air can be preheated up to 600°C. The n-heptane/air injector is a radial swirler injector which induces an estimated value of 1.74 for the swirl number. The premix tube is a 70mm length and 10 mm diameter cylinder. The droplets are detected by laser tomographic imaging and their size is measured by light diffraction. The reacting zone is visualized by CH* chemiluminescence imaging and the structure of inner recirculation zone by P-LIF-OH imaging. The analysis of laser tomographic images points out the pre-heating effect on the vaporization of liquid fuel. At T = 25°C and P = 1 bar, droplets are not totally evaporated in spite of their small initial diameter, (50 μm without combustion). Totally prevaporized regime is achieved for temperature values above 100°C. Measurements of CH* chemiluminescence for a fixed mean flow velocity V = 70 m/s, and for T = 25°C, 100°C and 200°C show that the flame remains weakly anchored at the nozzle exit. The flame stability evolves with the combustion air temperature and is linked with the position of the Inner Recirculation Zone (IRZ). On the other hand the pressure is a stabilizing factor for constant conditions of velocity and equivalence ratio, because the position of the IRZ does not fluctuates. At constant injection velocity, aerodynamic structure is modified by pressure increasing. At a given critical pressure value, the flame becomes unstable until it reaches a stable flash-back state.