不同湍流强度下煤粉颗粒群着火及燃烧特性的光学诊断研究

利用Hencken型平面火焰燃烧器搭建携带流反应系统，研究了不同湍流强度下煤粉颗粒群的着火及燃烧特性。煤粉被一次风送入温度、氧含量（本文所称"氧含量"是指氧的摩尔分数，mole fraction）可调节的高温烟气中形成稳定的射流火焰，利用OH平面激光诱导荧光技术（OH-Planar Laser-Induced Fluorescence，OH-PLIF）观测煤粉射流火焰着火、群燃等阶段的瞬态结构，基于对火焰图像的处理探究煤粉颗粒群的着火及燃烧特性。OH-PLIF的测量结果表明，在煤粉射流火焰的上游，射流外围区域的煤粉首先发生脱挥发分并着火，外围已燃的煤粉释放出大量热量并不断向射流内部传递，促进了射流内部区域煤粉颗粒群挥发分的析出。在高速一次风的卷吸及扰动作用下，析出的挥发分与氧之间不断扩散、混合，燃烧的OH锋面逐渐向射流中心区域延伸并连接成片，出现挥发分群燃火焰。实验结果表明:层流状态下，煤粉射流火焰窄而明亮；随着一次风湍流强度的增强，射流中煤粉颗粒的扩散运动变得剧烈，火焰形态发生变化，着火距离显著缩短。本文定量地研究了不同湍流强度下背景烟气温度（1200~1700 K）、烟气氧含量（10%~30%）以及一次风氧含量（5%~45%）对煤粉颗粒群着火延迟的影响规律。随着背景烟气温度、送风氧含量的升高，着火延迟时间逐渐缩短，但存在阈值现象，一旦背景烟气温度或送风氧含量超出某一阈值，其对煤粉颗粒群着火延迟的影响变弱，控制煤粉颗粒群着火行为的主导因素随之发生改变。

Experimental study on ignition and combustion of pulverized coal particles clouds under laminar and turbulent conditions

The ignition and combustion behaviors of pulverized coal particles clouds in a jet with different levels of turbulence and primary O₂ mole fractions were evaluated in a lab-scale optical entrained flow reactor. Bituminous particles were injected into the hot flue gas environment produced by a Hencken burner, with environment temperatures varying from 1200 K to 1700 K and secondary O₂ mole fractions from 0.1 to 0.3. Digital photography and OH-Planar Laser-Induced Fluorescence (OH-PLIF) techniques were employed to record the coal jet flame behaviors and capture the transient structure of the flame. The coal jet flame was narrow and smooth under laminar conditions, and become wider as the turbulent intensity of the primary flow increased. The OH-PLIF data reveals that upstream of the turbulent coal jet flame, reactions occur only at the periphery of the clusters of the pulverized coal particles where the high-temperature environment provides sufficient heat and oxygen. Downstream of the turbulent coal jet flame, reactions could occur also within the clusters of the pulverized coal particles, because of the continuous coal devolatilization process along the streams, and the mixing process between the volatiles and the oxygen entrained by the secondary flow. The ignition delay time is reduced and the combustion intensity is enhanced with the increase of the environment temperature and O₂ mole fraction in the primary or secondary flow. The frontal OH edge expands inward to the centerline of the clusters of the pulverized coal particles with the increase of the primary O₂ mole fraction. In addition, the effects of the environmental temperature and O₂ mole fraction on the ignition of pulverized coal particles clouds exhibit a threshold phenomenon. Once the environment temperature or the O₂ mole fraction exceeds a certain threshold, its influence on the ignition delay of the coal jet becomes weaker and the leading factors controlling the ignition behavior of the pulverized coal particle clouds change accordingly.