An intriguing oscillating combustion phenomenon

Strobes are pyrotechnic compositions that emit bright flashes of white or colored light at regular time intervals. The strobe effect has applications in various fields, most notably in the fireworks industry and in the military area (signaling – missile decoys – crowd control). However, the chemical and physical mechanisms involved in this curious combustion phenomenon remain unknown. This study investigates parameters that influence the strobe effect (frequency, sharpness of flashes). Variations were applied to the fuel particle size and weight fraction of the ternary composition ammonium perchlorate, magnalium (fuel) and barium sulfate. Both parameters are related to the heat generation and transfer within the composition. The light emission was recorded with photodiodes and a high speed camera (5000 fps). The experimental results served as input for a model to explain the strobe behavior based on layer-by-layer combustion. Moreover, it was established that the flash frequency is dependent on a delicate balance between heat generation in one layer and heat transfer to a next layer. The model gives new insights into the behavior of pyrotechnic strobes and enables a better control of the flash frequency by variation of the size and content of the metal fuel particles.