[Objective] Unlike fire accidents caused by direct combustion, which are easy to detect and control, numerous accident investigations have shown that ignition caused by high-temperature molten metal droplets is a key link in the spread and escalation of fire events. Recently, a number of studies in China and other countries have investigated the ignition mechanism of fire spreading owing to high-temperature molten metal droplets. However, most of these studies have investigated the ignition mechanism alone, and only a few studies have explored the dynamic characteristics of molten metal droplets impacting the wall surface. Consequently, in this experimental study, the dynamic of molten aluminum particles with different particle diameters impacting noncombustible stainless-steel plates and combustible expanded polystyrene (EPS) foam were explored. [Methods] To address the problems of adhesion of molten metal particles to the heating vessel and the low upper limit of the heating temperature in traditional research, this study adopted a non-contact method to heat and control the metal particles and recorded the spreading and retraction of molten metal droplets and the pyrolysis and combustion phenomena of the substrate material in the collision process using a high-speed camera. The morphology of the molten metal can be used to visualize droplet motion and assess the heating conditions of the impacted substrate and surrounding objects. The droplet spreading diameter is one of the most important parameters for studying the dynamic characteristics of molten metal droplets impacting the wall surface; however, after impact, molten metal droplets do not have standard circular surfaces, and their contours cannot be directly obtained. Using Image-Pro Plus software to draw the outer contour of the thin layer of metallic aluminum particles, the number of pixels in the coil is calculated and multiplied by the scale; furthermore, the actual spreading area of the thin layer of molten metal S0 is calculated, and the equivalent spreading length D is subsequently obtained. In the experiment on molten droplets impacting on a stainless-steel metal surface, the abovementioned factors were analyzed by varying the particle diameter of the molten aluminum droplets, the distance of impingement, and the particle temperature to analyze the effects of the above factors on the spreading pattern, spreading distance, and motion changes of the droplets. In addition to considering the effects of pyrolysis of the stainless-steel wall, the effects of pyrolysis of EPS foam on the droplet spreading process should also be analyzed in an experiment on the impact of molten metal droplets on the EPS foam surface. [Results] The results of the experiments of molten droplets impacting stainless-steel metal surfaces and EPS foam surfaces showed that: (1) Aluminum droplets oxidize on the surface of the substrate particles during the impact process; this effect was more pronounced for particles with larger diameters, and larger particles took a longer time to reach the maximum spreading diameter. (2) The spreading characteristics of particles with different initial temperatures were roughly the same, and the maximum spreading length achieved by the particles was approximately 25 mm. (3) The collision of aluminum particles with the EPS foam reached a maximum spreading at t=20.0 ms during the aluminuny particles motion, followed by a decrease in the spreading length of the thin layer of metal particles. [Conclusions] Larger initial diameters and impact distances imply that the peak of the spreading diameter is also larger. The initial temperature has little effect on the spreading motion of molten droplets in the study range. When the base plate material is EPS foam, the spreading diameter generally increases and subsequently decreases with time, and there is no stabilization phase.
Key words
molten droplets /
high-temperature metal /
collision /
motion characteristics
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