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Numerical Modeling of Flame Shedding and Extinction behind a Falling Thermoplastic Drip

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Abstract

The dripping of molten thermoplastics is a widely observed phenomenon in cable and façade fire, where the large drips can often carry a blue chain flame during the free fall to ignite other flammable materials and escalate the fire hazard. This work simulated the flame evolution behind a falling thermoplastic drip with the DNS model and finite-rate flame chemistry. The accelerated free-fall of drip was modeled by fixing the position of drip, increasing the upward airflow, and setting a fuel jet on the top of the drip. Modeling reproduces the dripping flame and reveals the flame shedding to be a combination of a lifted flame and a vortex street, where the lifted flame caused by the gravity acceleration of drip is identified as the critical factor that governs the shedding formation. As the diameter of drip decreases, the falling drip becomes difficult in forming a stable shedding structure in the wake region, so that the dripping extinction occurs due to the dilution and cooling of airflow, agreeing well with the experimental observation. This work reveals the underlying mechanism of stabilizing the dripping flame and helps evaluate the fire risk and hazard of dripping phenomena.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 52006185) and Hong Kong Polytechnic University (BE04). The authors thank Peiyi Sun and Yanhui Liu (HK PolyU) for assisting the data and imaging processes.

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Correspondence to Xinyan Huang.

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Supplementary Information

Video S1_Formation process of a burning polymer drip

Video S2_Structure of dripping flame at 60 fps

Video S3_Structure of flame shedding at 960 fps

Video S4_A non-reactive base of Karman vortex street

Video S5_A reactive base of diffusion flame

Video S6_Flame shedding with the fuel injection velocity of 0.5 m/s

Video S7_Full evoluction of dripping structure

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Appendix

Appendix

Given that \(\eta = L_{k} /D\), a cylinder bluff body flow (Sallet 1964) follows:

$$S_{t}^{2} \eta^{3} + 0.529S_{t} \eta^{2} - 1.528\eta + 1.592\left( {C_{D} - \frac{4}{{\sqrt {Re} }}} \right) = 0$$
(A1)

where \(L_{k}\) is the distance between two successive vortices of the vortex street, D is the drip diameter, St is the Strouhal number and CD is the drag coefficient. For the current regime, \(S_{t} = 0.21\) and \(C_{D} = 1.05\) (Sallet 1964). As D changes from 1 to 5 mm, the Reynolds number \(Re = V_{at} \cdot D/v\), where \(V_{at}\) denotes the maximum airflow velocity and  \(v=\)15.89 × 10–6 N·s/m2 is the kinetic viscosity of air at 300 K, varies from 252 to 1260. Thus, the fuel injection velocities \(V_{f} \approx a\Delta t = \sqrt {2gL_{k} }\) are 0.28, 0.4, 0.49, 0.56 and 0.63 m/s, respectively.

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Xiong, C., Huang, X. Numerical Modeling of Flame Shedding and Extinction behind a Falling Thermoplastic Drip. Flow Turbulence Combust 107, 745–758 (2021). https://doi.org/10.1007/s10494-021-00250-5

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  • DOI: https://doi.org/10.1007/s10494-021-00250-5

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