Simulated effects of main properties and heat flux on first dripping behaviors of polymers impinged by small flames

Abstract

Dripping plays a significant role in understanding the UL-94 vertical test. In this study, the dripping of polymers impinged by small flames was simulated using the volume of fluid method and the enthalpy-porosity technique. The UL-94 flame was approximated using a 2026K gas flow with a velocity of 0.9 m/s, meeting with ASTM-D5207. The small-size drop and the large-size drop were characterized by aspect ratios of no more than 1.1 and greater than 3.1, respectively. Decreasing surface tension (σ), viscosity (μ), thermal conductivity (k), or increasing the heat flux tends to result in small-size drops. When σ ≤ 0.04 N/m, the first drop is small-size, and the equivalent radius of the first drop follows a power-law relationship with σ. As σ varies from 0.03 N/m to 0.07 N/m, the critical value of μ decreases from 0.30 Pa s to 0.02 Pa s. If σ = 0.07 N/m and k ≤ 0.1W/m·K, the first drop tends to be small-size. When σ is 0.03 N/m, the size of the first drop is independent of k, and the critical heat flux is approximately 48 kW/m2. The first dripping time increases with μ, and the large-size dripping time is linear with the product of viscosity and characteristic length divided by drop mass.