As an introduction of the special issue for the combustion science in fireworks, features of Japanese fireworks, competitions of fireworks display, names/gyokumei of individual firework, and science of fireworks are explained. The science of fireworks include physical, health, and environmental issues related to fireworks, such as searching of substitute for sensitive oxidizing substance, preventing mass explosion, searching of substitute for toxic or harmful ingredients, and preventing smoke emission.

A traditional Japanese sparkler, called Senko-hanabi, has been popular in Japan since the Edo-period. The simple composition of only 0.1 g black powder wrapped in a twisted paper generates a residue suspended at the bottom end of the paper string, emitting light streaks similar to branched pine needles, with ever smaller ramifications. The surprising events involve an exothermic reaction with the oxygen of air, chemical reactions of metastable compounds in the melt, gas bubble nucleation and bursting, liquid ligaments and droplets formation, all occurring in the sequential fashion. These complex processes at play in fireworks have remained elusive over the centuries. In this article, we introduce the research history on the science of Senko-hanabi starting from the 19th century, originated in Europe and then took place in Japan. We show the recent progress of the detailed cascade occurring at the spark ramifications as successive droplet fragmentation, thus answering a century old question. However, there still remain essential matters, which should be clarified in the future for the full understanding of the fragile beauty.

A simple model for the smoke formation in black powder combustion is developed. Smoke formation is modeled as nucleation from gas phase molecules. The precursor molecules for this nucleation process for black powder with 75 wt% of KNO₃ are identified as potassium salts such, K₂CO₃ and K₂SO₄. This determination is based on the partial-equilibrium calculations in which chemical species in the condensed phase are excluded. Standard classical nucleation theory (CNT) is adopted to estimate the radius and formation rate of the critical nuclei of smoke particles. The main components of smoke from black powder are K₂CO₃ or K₂SO₄ particles, depending on the sulfur content. The predicted nucleation rates of the particles are very fast. The time variation of the averaged particle radius and volume fraction of smoke is also evaluated by solving the population valance equation (Smoluchowski equation). The volume fraction of smoke produced by black powder combustion is predicted to be of the order of 10^-4. This study also investigates how ammonium perchlorate (NH₄ClO₄, AP) added to the black powder affects smoke formation, using CNT and the Smoluchowski equation. CNT predicts that the critical radius of K₂CO₃ and K₂SO₄ particles can be considerably increased by the addition of AP to black powder. This intervention could thus reduce smoke-particle formation. Although CNT predicts that no KCl particles will form because of the high vapor pressure of KCl, the Smoluchowski equation indicates that KCl particles will be produced with a large amount of added AP. Solutions of the Smoluchowski equation also indicate that the average particle diameter and volume fraction of smoke decrease if the amount of added AP is increased.

Principles and safety of fireworks as well as the database that the authors have been opened to the public are overviewed. First, chemical substances used to make fireworks are explained. Especially, oxidizers, which are solid powders including oxygen, are focused on. The reasons why the oxidants that are currently used have been selected to make fireworks are scientifically explained. Moreover, issues related to combustible substances are explained in detail. Next, the safety of fireworks is outlined. AIST has been addressing this issue for a long time. Various types of laboratory-scale sensitivity tests, evaluation tests for the hazard of spontaneous ignition, and safe production methods are explained. Finally, in “Physical Hazard Database of Chemicals,” a database developed by the authors and opened to the public, the parts related to fireworks are introduced.

Japanese fireworks began in the Edo period. And the technology has advanced with the support by the people over time. Currently, Japanese fireworks are highly respected worldwide. I think the reason is that Japanese are creating the clear bright color and sound technology using the characteristics of aluminum powder (Al powder) well. On the other hand, severe accidents such as “mass explosion” which affect almost the entire explosives virtually instantaneously during manufacture and storage of fireworks articles including Al powder had continued until recently. It was similar in overseas. For this reason, investigation and research have been conducted at various levels, such as international, national and industrial levels, and regulations had been also tightened. International regulation on transportation of dangerous goods including fireworks should follow the regulation called "UN Recommendations". By these investigation and researches, the regulations of fireworks were changed dramatically in 2005, and then continued to change. Additionally, ISO standards of fireworks have been developed from 2012 and published in 2017. The transport classification of fireworks should be decided by the flash composition test of UN Recommendation. This flash composition test is composed of two kinds tests, a time pressure test with 0.5g sample and DDT test with 25g one. It became clear by Japanese studies that the time pressure test overestimates due to the scale effect. It seems that the combustion behavior of Al powder at high temperature and high pressure tightly relates to mass explosion, but the actual behavior is unknown now time.

Combustion simulation is a promising tool in many ways such as understanding the mechanism of ignition and extinction, engine design, emission prediction and control, and fuel development. Regarding the chemical kinetics mechanism for the oxidation of hydrocarbon fuels, there are many investigations on the elementary reaction process, and now we have detailed chemical kinetics models which quantitatively predict the ignition timing and laminar flame velocity for the representative components and their mixtures included in the natural gas, gasoline, diesel fuels, kerosene, and jet fuels. When we combine these large chemical kinetics models into fluid dynamic simulations, the computational cost inhibits the practical use. Thus, the need for the reduction of detailed chemical kinetics model is increasing. This article describes the concept of surrogate fuel which mimics the combustion properties of real fuels with few or several representative hydrocarbon components. Then, we see the mechanism reduction method briefly. Finally, we see the construction of simplified model for gasoline surrogate fuel which predicts the autoignition timing and laminar flame velocity under the temperature and pressure related to the internal combustion engines.

In the present work, an underwater shock wave was applied to an explosive bubble, which then generated a spherical shock wave after shrinking process. A special care was taken for arrangement of the bubble and a pressure transducer so as to measure pressure behind the spherical shock wave originating from single bubble expansion. The bubble was made of a stoichiometric ethylene-oxygen mixture and its initial equivalent radius ranged from 1.0 mm to 2.2 mm. An incident underwater shock wave (ISW) was driven by gaseous detonation which propagated towards water surface, giving a peak pressure behind ISW (P_i) from 6 MPa to 23 MPa. Shadowgraph images of the bubble show that it starts to shrink after passage of ISW and then emits a light indicating combustion during the shrinking phase. When the bubble turns to expansion, it generates a shock wave (BSW) propagating spherically into the surrounding water. The experimental results reveal that the non-dimensional maximum pressure behind BSW (P_peak/P_i) is almost inversely proportional to the non-dimensional measurement distance from the bubble based on the initial radius of the bubble. The energy conversion efficiency from the bubble energy to the shock energy is dependent on the momentum acquired by water around the bubble, which is estimated based on the concept of the Kelvin impulse. The measured shrinking time of the bubble is found to be in good agreement with the Rayleigh collapse time.

Active radical species are generated on thermally-excited titanium dioxide (TiO₂) under air conditions. Oxidation and decomposition of unburned compounds, such as CO and acetaldehyde, were investigated using this activation of TiO₂. The materials used were TiO₂ bead and TiO₂/SiO₂ composite bead. The supporting ratio of TiO₂ on the composite bead used was 14% or 20%. Oxidation of CO to CO₂ was 70% at 500 ℃ using TiO₂ bead, though that was about 20% using TiO₂/SiO₂ bead. Decomposition of acetaldehyde was over 90% at 300 ℃ using TiO₂ bead or TiO₂/SiO₂ bead. The existence of TiO₂ particles on the surface and in the nano-pore of silica would allow the effective decomposition of acetaldehyde. It was assumed that the thermally-excitation of TiO₂ was caused by the formation of lattice defect, that is oxygen defect, in TiO₂ under high temperature followed by the active radical generation.