CFD模拟表面张力对垂直燃烧测试中热塑性聚合物小尺寸滴落行为的影响

CFD Simulations of the Influence of Surface Tension on the Small-size Dripping Behaviors of Thermoplastics in the Vertical Burning Test

熔体滴落是热塑性聚合物的火灾特性之一,燃烧熔滴会引发二次灾害并扩大火势。采用流体体积(VOF)法耦合焓-孔隙度法处理多相流和相变,模拟了垂直燃烧测试中聚合物的熔融相变过程和小尺寸滴落行为。模拟结果表明:相变过程中样条表层存在较大温度梯度,熔体运动会改变下端样条的厚度并影响聚合物表层温度分布,熔滴掉落时熔体相体积分数的增长速率达到峰值。当表面张力较小时会因气流曳力而出现飘浮熔滴,垂直燃烧测试条件下飘浮熔滴的临界直径为0.64 mm。掉落熔滴的半径和滴落时间均为表面张力的幂函数,掉落熔滴的质量或尺寸受表面张力控制,无量纲熔滴体积与Bond数倒数呈线性关系。模拟结果与理论分析基本吻合。

Dripping is one of the characteristic fire performance of thermoplastics.Burning drops acts as the ignition source to spread fire and increase the fire growth rate.In this study,numerical simulations are performed in order to investigate the melting and dripping process of thermoplastic polymer in the UL-94 vertical burning test.The Volume of Fluid(VOF)method and the Enthalpy-Porosity method are applied to model the multiphase flow and the melting process respectively.Simulation results show that the temperature gradient at the surface layer is high.The movement of melt changes the thickness and temperature distribution of the bottom end of specimen.The growth rate of the liquid volume fraction reaches a peak at the detachment of drop.If the surface tension is low,the small drop will float due to the buoyancy of hot gas flow.The critical diameter of floating drop in the vertical burning test is 0.64 mm.Both the radius of drop and the dripping time are power law functions of the surface tension.The drop mass or size is controlled by the surface tension and the dimensionless drop volume is linear with the reciprocal of Bond number.The simulation results agree well with the theoretical analysis.