Flame Spread Over Ultra-thin Solids: Effect of Area Density and Concurrent-Opposed Spread Reversal Phenomenon

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There are no existing experimental studies of flame spread rate trends for ultra-thin solid samples. Previous theory has predicted that for concurrent flame in kinetic regime, the flame spread rate decreases as the sample thickness decreases and there is a critical thickness below which burning is not possible. To test this hypothesis, a series of microgravity experiments of concurrent-flow flame spread over samples of ultra-low area densities are conducted using NASA Glenn Research Center’s Zero Gravity Research Facility (the 5.18 s drop tower). The tested samples are cellulose-based materials of various area densities, ranging from 0.2 mg/cm2 to 13 mg/cm2, as low as one order of magnitude less than those ever tested before. Each sample is 30 cm long by 5 cm wide and is burned in a low-speed concurrent air flow (5 to 30 cm/s). The results show that the concurrent flame spread rate is proportional to the flow velocity relative to the flame and is inversely proportional to the sample area density. A theoretical formulation, provided in this work, suggests that the flame length has a linear relationship with the relative flow speed and has no direct dependency on the sample area density. The experimental data supports this conclusion. From the images recorded in the experiments, a unique flame base tubular structure directed upstream away from the burnout zone is observed for thin samples. This structure is suspected to be due to flame stretching and localized blowoff caused by the oxidative pyrolysis Stefan flows at the sample burnout. This can be an indication that the chemical time becomes comparable to the flow time of the Stefan flow and the tested samples are approaching the kinetically-limited thickness. For the thinnest tested sample (0.2 mg/cm2), flames with concurrent and opposed dual natures are observed when the air flow rate is low (< 20 cm/s). At the lowest tested flow rate (5 cm/s), the flame spread rate exceeds the air flow rate and the flame transits to an opposed flame in the concurrent flow. The dual nature and flame transition are presented and discussed. This study provides detailed examination through high-resolution images of the transition between the concurrent to opposed flame spread modes.

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  1. 1.

    Any use of tradenames in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.


D :

Species diffusivity

\( k_{g} \) :

Gas-phase thermal conductivity

L :

Latent heat of a solid fuel

\( L_{f} \) :

Flame length

\( L_{p} \) :

Pyrolysis length

\( \dot{m}' \) :

Total burning rate of a sample per unit width \( ( = \rho \tau V_{f} ) \)

\( \overline{{\dot{m}''}} \) :

Average burning rate of a sample \( \left( { = \frac{{\rho \tau V_{f} }}{{L_{f} }}} \right) \)

\( \overline{{\dot{q}_{c} ''}} \) :

Average conductive heat flux

Re :

Reynolds number

Re x :

Local Reynolds number

t :

Time after drop

\( T_{f} \) :

Flame temperature

\( T_{p} \) :

Pyrolysis temperature

\( V_{f} \) :

Flame spread rate

\( V_{rel} \) :

Relative flow velocity (concurrent, dual nature, and concurrent-reversed: \( V_{rel} = \left| {V_{\infty } - V_{f} } \right| \); opposed: \( V_{rel} = V_{\infty } + V_{f} \))

\( V_{\infty } \) :

Forced flow velocity

x :

Distance away from the upstream leading edge of a sample

\( y_{f} \) :

Cross-stream location (away from sample surface) of a flame

\( \alpha \) :

Gas-phase thermal diffusivity

\( \rho \tau \) :

Sample area density

\( \nu \) :

Kinematic viscosity


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This work is supported by National Science Foundation under Award #1740478 (Division of Chemical, Bioengineering, Environmental, and Transport Systems) and NASA Glenn Research Center under Award #NNX16AL61A. We would also like to thank the crew members of the Zero Gravity Research Facility, Eric Neumann, Luke Ogorzaly, Mingo Rolince, Moses Brown, and Vittorio Valletta, for their tremendous help during the experiment operation.

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Correspondence to Ya-Ting Liao.

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Vetturini, A., Cui, W., Liao, Y. et al. Flame Spread Over Ultra-thin Solids: Effect of Area Density and Concurrent-Opposed Spread Reversal Phenomenon. Fire Technol 56, 91–111 (2020).

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  • Flame spread
  • Material flammability
  • Quenching limit
  • Concurrent-opposed reversal
  • Ultra-thin solid fuel