介观尺度下活性炭微粒的光镊捕捉、点火和扩散燃烧特性研究

Trap, ignition, and diffusion combustion characteristics of active carbon micro-particles at a meso-scale studied by optical tweezers

为探索介观尺度下固体燃料微粒的燃烧现象,本文提出采用光镊工具对活性炭微粒进行捕捉、悬浮、定位,再通过激光点燃,研究其着火及扩散燃烧特性.介观尺度燃烧室中,光镊捕捉7.0μm活性炭微粒的最低捕捉功率为3.2 mW,捕捉速率范围为103.7—70.0μm/s;活性炭微粒在静止气流中的最低点火功率为3.2 mW,颗粒的等效粒径、周长、面积和圆形度对最低点火功率影响甚微,点火延迟时间约48 ms,提高点火功率,点火延迟时间缩短,最小点火延迟时间小于6 ms;活性炭在着火后先发生无焰燃烧,紧接着发生有焰燃烧,无焰燃烧的扩散燃烧速率满足粒径平方直线规律,其燃烧速率范围为15.0—8.0μm/s;有焰燃烧的火焰面积和强度随燃烧时间发生闪烁,其闪烁频率约29.1 Hz.对于粒径为3.0μm的活性炭微粒,从加热到完全燃烧殆尽所需时间约0.648 s.结果表明:对于聚焦后的高能激光束点燃活性炭微粒的着火属于联合着火模式,在挥发份析出之前,活性炭非均相着火而发生无焰燃烧,挥发份析出后被点燃发生均相着火,火焰面始终保持圆形.

To study combustion characteristics of solid fuels at the meso-scale, this paper presents a study on trap, ignition, and diffusion combustion characteristics of active carbon micro-particles at a meso-scale by optical tweezers. In the meso- scale combustor, minimum trap power for active carbon micro-particles with a diameter of 7.0 μm is 3.2 mW, and the trap velocity is in the range of 103.7--70.0μm/s. The active carbon micro-particles in static air flow can be ignited when the laser power is 3.2 mW. The effective diameter, perimeter, area and roundness of the particles have little effect on the minimum power for ignition. The ignition delay time is -48 ms for active carbon micro-particles with a diameter of 3.0 μm, and it will decrease till below 6 ms with increasing laser power. After ignited, the active carbon micro-particle shows flameless combustion first. The diffusion combustion velocity agrees with the diameter square linear-relationship, and the velocity is of 15.0--8.0 μm/s. Then the active carbon micro-particle continues to carry out combustion reactions with bright flames repetitiously, and the flash frequency is 29.1 Hz. For the active carbon micro-particle with a diameter of 3.0 pro, it can burn out thoroughly in an overall time -0.648 s （including the heating and combustion processes）. Results demonstrate that ignition of the active carbon micro-particle heated by high power density laser belongs to the combined ignition mode. Before volatile matter precipitates, the active carbon micro-particle is ignited heterogeneously and carries out a flameless combustion. However, after the volatile components are precipitated, it is ignited homogeneously, and the ombustion flame always shows a spheried shape.