Microgravity combustion of polyethylene droplet in drop tower
Introduction
The microgravity environment allows for an analytical description of the droplet combustion process by eliminating the buoyancy and natural convection [1]. The classical theory of one-dimensional (1-D) droplet combustion was first formulated by Spalding and Godsave in the early 1950s [2,3]. Since then, numerous microgravity experiments and numerical simulations have been performed subsequently to understand the combustion of liquid fuel droplets, as well as to verify and update the classical d-square law [2,3]. A wide range of droplet sizes up to 5 mm and fuel types, e.g., heptane, octane, diesel, methanol, have been tested, which have been reviewed in detail (e.g. [4], [5], [6], [7], [8], [9]).
In addition to liquid-fuel droplets, the combustion of plastic spherical fuels in a solid phase was found roughly following the classical d-square law even if burning in normal gravity [10,11]. Some unique burning behaviors of plastic fuels, like bubbling and bursting, were found in burning three different plastic materials, polymethylmethacrylate (PMMA), polypropylene (PP), and polystyrene (PS), with diameters from 2 to 6.35 mm in low-gravity aircraft experiments [12,13]. Recently, a series of drop tower experiments on the PMMA sphere with a diameter of 10–40 mm were conducted to investigate the curvature effect on the flame extinction of solid fuels in microgravity [14]. Nevertheless, in all these experiments, spherical plastic fuels either were thermosets or did not fully melt into the liquid, so that the condensed-phase heat transfer will play an important role in the burning phenomenon [15].
Different from thermosetting polymers (e.g., cast PMMA), thermoplastic polymers, such as the polyethylene (PE), will first melt into liquid before ignition. Thus, the burning of molten thermoplastics is close to liquid fuel combustion, although the pyrolysis of melts is fundamentally different from the evaporation of liquid [16]. On Earth, the molten and burning thermoplastics tend to develop the flooring [17] and dripping [18,19] as driven by gravity. In microgravity, the burning thermoplastic materials tend to shrink into a ball under the surface tension force [20,21], which behaves like the classical droplet combustion. Today, thermoplastic materials are widely used for wire insulations and electrical devices aboard the spacecraft [22]. Thus, it is important to examine the combustion of thermoplastic droplets and its fire risk in microgravity spacecraft.
So far, very limited microgravity combustion experiments are available for the thermoplastic droplet. Most thermoplastics are in the solid phase at room temperature and have high melting and pyrolysis points. Thus, the time required for melting the entire PE particle into a liquid droplet and forming a stable flame takes several seconds, which is comparable to the short microgravity period of the drop tower. For example, the ignition delay time of PE thin-film almost took the entire microgravity time [23]. Although plastic fuels can be ignited and generate dripping before the microgravity time, the droplet will detach under gravity in a random fashion. Thus, it is challenging to heat, melt uniformly, and ignite the thermoplastic droplet, and synchronize the droplet detachment time with the free-fall time.
In this work, the microgravity combustion of molten PE droplet was studied in a 3.6-s drop tower with a gravity level of 10−3–10−4 g. The pre-ignited PE droplets were continuously generated and detached from burning wires. The burning rates of PE droplets with and without forced flow were measured, which were verified against the classical d-square law and compared with liquid-fuel droplets. The unique phenomena of the PE droplet combustion, such as rebound, sliding, bubbling, and bursting (or ejecting), were discussed, which could shed light on fire hazards in spacecraft environments.
Section snippets
Experimental methods
The experiments were conducted in the 3.6-s drop tower in the National Microgravity Laboratory of China (NMLC), which offers a microgravity level of about 10−3~10−4 g for the single-capsule test [24]. The effective space inside the sealed capsule was about 1 m3, so that it includes about 300 g oxygen under the oxygen concentration of 21% and the pressure of 1 atm (Fig. 1(a)). Thus, for a short burning duration, the influence of oxygen depletion by combustion could be neglected. Inside the
Comet flame of moving droplet
Figure 2(a) shows the trajectory and the burning process of the PE droplet in the microgravity drop period. Because an initial downward velocity was required for the droplet to detach from the parent fuel, the PE droplet had an initial velocity. Thus, the Stage-I burning of the moving droplet showed a comet-shaped flame. The observed yellow flame might be the soot shell or the inner layer of the flame sheet, where the outer blue flame layer was not visualized by the current high-speed imaging
Conclusions
In this study, the combustion of the molten thermoplastic droplet has been tested under the microgravity drop tower. A unique experimental setup was designed to successfully produce the pre-ignited PE droplets with a diameter of 2~3 mm and a small initial velocity within less than 40 cm/s. Once the drop started, two stages of droplet combustion were observed,
- (I)
a comet-shape flame for a low-velocity droplet with strong bubbling and ejecting processes and the burning-rate constant (K) of 2.6 ± 0.3
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is supported by National Natural Science Foundation of China under the Grant No. U1738117 and 51876183, the CAS Strategic Priority Research Program on Space Science (Nos. XDA04020410 and XDA04020202-10).
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