Senko-hanabi under various ambient conditions

A traditional Japanese sparkler, senko-hanabi, is consists of paper string wrapping 0.1 g of black powder. The sparkler requires heat production by combustion of contained charcoal with ambient oxygen, showing off beautiful sparks luminous by heat radiation. In the present study, the appearance of the sparks is investigated under various ambient conditions, in which the total pressure is from vacuum to 5.0 bar and the oxygen concentration rate is set up to 40 %. The sparkler is ignited by Joule heating inside a closed vessel. Profound effects are confirmed by the ambient conditions on the sparks. As the total pressure rises, sparks spread actively, because the boundary layer surrounding the fireball, created at the bottom end of the paper string, becomes thinner, and the amount of oxygen supply increases. The oxygen concentration rate directly contributes to the amount of oxygen supply. The criterion given by molar flux of oxygen supply to the fireball is identified as 10(-1) mol . s(-1) . m(-2), rate-controlled by molecular diffusion through the boundary layer, for embodying fragile beauty over the centuries.