A closer look at determining flame speeds with imaging diagnostics

A comparison of flame speed measurements of highly luminescent reacting powders utilizing various filtering and illumination techniques is presented. Reactive energetic composites are often highly luminescent and quantifying reaction propagation can be difficult because of sensor saturation. To explore the influence of image saturation on flame speed measurements, experiments were designed using micron powders aluminum (Al) and molybdenum trioxide (MoO₃) and further studied with nano-powders of Al + MoO₃. Powder mixtures were loaded into burn tubes at various densities to produce speeds over a 2-3 order of magnitude spectrum. The flame speeds were measured in three ways: (1) using a high speed camera; (2) a high speed camera with a series of neutral density filters attached to the camera lens that provide limited light transmission to the camera sensor; and, (3) using a 511nm notch filter on the camera lens and Copper Vapor Laser (CVL) to illuminate the reaction and filter the majority (i.e., >99.9%) of reaction illumination. The results show that the flame speeds measured through the filtering techniques were not affected by the various levels of filtration. This conclusion is evidenced by the greatest percent difference in average flame speeds in the micron powder was 15% and in the nano powder 4.2%. Also with extreme filtering techniques such as a single bandwidth filter and single wavelength illumination, a more detailed view of thermite powder reaction can be observed. This advanced filtration technique can be used to analyze and characterize combustion phenomena.