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Flame Spread over Polyethylene Film: Effects of Gravity and Fuel Inclination

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Abstract

The flammable thermoplastics are widely used in our daily life and manned space travels in microgravity, posing a potential fire risk. This work studies the flame spread along with the thin polyethylene (PE) film (15–100 µm) in the microgravity drop tower and normal gravity. Microgravity flame spread faster than vertically downward flame spread in normal gravity due to the weak buoyancy flow and greater flame preheating length. For the ultra-thin film, the influence of melting on the opposed flame spread is negligible. The upward flame spread rate reaches a maximum constant (45 ± 10 mm/s for 20 μm film) when the inclination angle is larger than 30°, due to the dripping removal of molten fuel. The upward flame spread rate changes under the competition between the enhanced flame heating by buoyancy and the dripping removal of the molten fuel. The vertically upward spreading flame cannot be maintained due to the dripping removal of the molten fuel, and a critical extinction condition was determined and analyzed. This work provides valuable data on flame dynamics in plastic films and can help develop more sophisticated material flammability tests for fire safety in space travel.

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Abbreviations

Bi :

Biot number (-)

c :

Specific heat (kJ/kg/K)

D :

Diameter (m)

g :

Gravitational acceleration (m/s2)

h :

Convection heat transfer coefficient (W/m2/K)

l :

Length (m)

L :

Characteristic length (m)

L v :

Latent heat of vaporization (J/kg)

M :

Mass (g)

\(\dot{m}\) :

Mass loss rate (g/s)

Nu :

Nusselt number (-)

Pr :

Prandtl number (-)

\({\dot{q}}^{^{\prime\prime}}\)  :

Heat flux (kW/m2)

R :

Radius (m)

Ra :

Rayleigh number

t :

Time (s)

T :

Temperature (K)

V :

Spread rate (m/s

α :

Thermal diffusivity (m2/s)

β :

Thermal expansion coefficient (K−1)

δ :

Length (m)

θ :

Angle (°)

λ :

Heat conductivity (W/m/K)

μ :

Viscosity (kg/m/s)

ν :

Kinematic viscosity (m2/s)

ρ :

Density (kg/m3)

σ :

Surface tension (N/m)

τ :

Thickness (m)

∞:

Ambient

b :

Burning

cr :

Critical

dr :

Drip

f :

Flame

g :

Gas

ig :

Ignition

m :

Melt

s :

Solid

tot :

Total

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Funding

This work is supported by the National Key R&D Program of China (Grant No. 2021YFA0716203), the Opening Fund of State Key Laboratory of Fire Science (SKLFS) under Grant No. HZ2021-KF12, and the National Natural Science Foundation of China under Grant No. U1738117.

Author information

Authors and Affiliations

  1. Key Laboratory of Microgravity, Institute of Mechanics, Chinese Academy of Sciences, Beijing, 100190, China

    Feng Zhu & Shuangfeng Wang

  2. Research Centre for Fire Safety Engineering, Department of Building and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China

    Xinyan Huang

  3. State Key Laboratory of High-Temperature Gas Dynamics, Chinese Academy of Sciences, Beijing, 100190, China

    Shuangfeng Wang

  4. School of Engineering Science, University of Chinese Academy of Sciences, Beijing, 100049, China

    Shuangfeng Wang

Authors
  1. Feng Zhu
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  2. Xinyan Huang
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  3. Shuangfeng Wang
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Corresponding authors

Correspondence to Xinyan Huang or Shuangfeng Wang.

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Appendix

Appendix

The thermogravimetric analysis (TGA) analysis of PE sample was conducted with a PerkinElmer STA 6000 Simultaneous Thermal Analyser. The initial mass of PE sample was 3–5 mg, and samples were heated at the constant rate of 10 K/min and under both nitrogen and air environments. Experiments were repeated twice for each case, and good repeatability is shown. Figure 11 shows the mass-loss rate curves of the PE insulation sample. In general, the pyrolysis temperature of PE is about 410 °C, and the ignition temperature is slightly higher.

Fig. 11
figure 11

DTG curves of HDPE at the heating rate of 10 K/min

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The position of flame leading edge as a function of time under microgravity experiments and normal gravity environment for the vertically downward spreading flame. The coefficients of determination (R2) for each fitting curve are listed in the Fig. 12.

Fig. 12
figure 12

The time evolution of the flame leading edge position for the film with a thickness of 20 μm for (a) downward flame spread and microgravity flame spread (b) upward flame spread

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The Bi number is defined as \(Bi=h\tau /{\lambda }_{s}\), where h is the heat-transfer coefficient, τ is the thickness of the film, which is used as the characteristic length, λs is the thermal conductivity of the film. We first determine the flow condition and the heat transfer coefficient.

$$R{a}_{L}=\frac{g\beta \left({T}_{ig}-{T}_{\infty }\right){L}^{3}}{\alpha \nu }=1\times 1{0}^{6}\ll 1{0}^{9}$$

Thus, the flow is laminar, and we can estimate the convection coefficient as

$$\bar{N}{u }_{L}=0.68+\frac{0.670R{a}_{L}^{1/4}}{{\left[1+{\left(0.492/\mathrm{Pr}\right)}^{9/16}\right]}^{4/9}}$$
$$h=\frac{\bar{N}{u }_{L}{\lambda }_{g}}{L}=7.4\; \text{W/m}^{2}{\text{K}}$$

Then, the Bi number can be calculated for PE film of \(\tau =40\) μm as

$$Bi=\frac{h\tau }{{\lambda }_{s}}=6\times 1{0}^{-4}$$

where properties of gas are evaluated at the film temperature \({T}_{f}=\left({T}_{ig}+{T}_{f}\right)/2=950 K\), where Tig is the ignition temperature of the PE, and Tf is the flame temperature. The properties of air are shown in Table 1. The length of the sample is used as the characteristic length, that is L = 0.15 m.

Table 1 Properties of air at 450 K (Incropera et al. 2013)
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Zhu, F., Huang, X. & Wang, S. Flame Spread over Polyethylene Film: Effects of Gravity and Fuel Inclination. Microgravity Sci. Technol. 34, 26 (2022). https://doi.org/10.1007/s12217-022-09945-4

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