Lifting and Splitting of Nonpremixed Methane/Air Flames Due to Reactant Preheating

In order to assess the impact of initial reactant temperature on the occurrence of local extinction (LE) and the subsequent lifting process of non-premixed attached flames with increasing fuel injection velocity, hydroxyl radical planar laser-induced fluorescence (OH-PLIF) and high-speed CH*-chemiluminescence visualizations were conducted in a methane/air jet-flame, with preheating up to 1000 K. LE occurrence probability increases when approaching lifting, and the preheating level (Tox,ref) affects the probability density function (PDF) shape of LE axial origin. At low Tox,ref, partial lifting events occur near the burner lip, eventually leading the flame to lift directly from the very flame base. At higher Tox,ref, partial lifting events no longer occur, and LE is mostly witnessed in the flame breakpoint zone (axially from 1 to 3 jet diameters), resulting in a breakpoint lifting process. For very high Tox,ref (1000 K), local extinctions become widespread in the breakpoint zone so that a stable split flame is achieved prior to the lifted regime.

为评估初始反应物温度对局部熄火（local extinction, LE）的发生以及燃料喷射速度提升时非预混附着火焰后续抬升过程的影响，本研究在预加热温度最高可达1000 K的甲烷-空气射流火焰中，开展了羟基平面激光诱导荧光（hydroxyl radical planar laser-induced fluorescence, OH-PLIF）与高速CH*化学发光可视化实验。局部熄火发生概率在接近火焰抬升时升高，且预加热水平（Tox,ref）会影响局部熄火轴向起源位置的概率密度函数（probability density function, PDF）形态。在较低的Tox,ref工况下，部分抬升事件会出现在燃烧器喷口附近，最终使火焰直接从火焰根部完成抬升；在较高的Tox,ref工况下，不再出现部分抬升事件，局部熄火多发生于轴向范围为1~3倍射流直径的火焰断点区域，进而形成断点抬升过程。当Tox,ref极高（1000 K）时，局部熄火会在断点区域广泛出现，从而在火焰进入抬升工况前形成稳定的分裂火焰。