The unsteady effects of buoyancy-induced instabilities on jet diffusion flames are investigated experimentally under normal gravity conditions. Methane and propane are used as test fuels that are lighter and heavier than ambient air, respectively. A similar Froude (Fr) and Reynolds (Re) number relationship is realized in both hydrocarbon fuels with different tube diameters ranging from 6 to 24.2 mm. The Schlieren visualization technique and high-speed imaging synchronized with chemiluminescence signal measurement are used to identify changes in global flame shape and dominant frequency. Buoyancy-induced instabilities generate two forms of diffusion flames with varying frequencies in space. Both laminar and turbulent jet flames exhibit natural and subharmonic frequencies, as well as a shift between them. The methane-propane Re–Fr relationship confirms the instability mode transition. In addition, Strouhal (St) and Froude number relations are obtained as \(St \propto \ Fr^{-0.50}\), with a slope difference between natural and subharmonic modes in both fuels.

在正常重力条件下通过实验研究了浮力引起的不稳定性对射流扩散火焰的不稳定影响。甲烷和丙烷分别用作比环境空气更轻和更重的测试燃料。类似的 Froude ( Fr ) 和 Reynolds ( Re) 数量关系在两种碳氢燃料中实现，管径从 6 到 24.2 毫米不等。纹影可视化技术和与化学发光信号测量同步的高速成像用于识别全局火焰形状和主频率的变化。浮力引起的不稳定性会在空间中产生两种不同频率的扩散火焰。层流和湍流喷射火焰都表现出自然和次谐波频率，以及它们之间的偏移。甲烷-丙烷Re - Fr关系证实了不稳定模式的转变。此外，Strouhal ( St ) 和 Froude 数关系为\(St \propto \ Fr^{-0.50}\)，两种燃料的自然模式和次谐波模式之间存在斜率差异。