Simulations On Dripping Process Of Polymer Melt Under Vertical Burning Test Conditions

Thermoplastic polymeric materials are widely used in electric,construction and decoration industries due to their low price and excellent performance.They have great application prospects and advantages.However,due to its flammability and the characteristics of softening,melting and flowing in the fire,burning droplets lead to the pool fire and secondary disaster which brings severe challenges to the fire prevention.High temperature and heat flux conditions significantly restrict experimental researches.Therefore,the numerical simulation method is used to explore the dynamic process of melting and dripping of thermoplastic polymer under vertical burning test conditions.The Volume of Fluid(VOF)method coupled with the Enthalpy-Porosity method is employed to solve the unified equations in the whole field and to effectively track the interface between air and polymer and the melting front of the sample.The multi-phase flow model and the solidification/melting model numerically simulate the melting process of the polymer specimen under vertical burning test conditions.The vertical combustion experimental space is simplified to a two-dimensional centrally symmetric computational domain.To simulate the heat flux condition required by the vertical burning test standard,the ignition flame is simulated by the air velocity inlet boundary with air temperature of 2026 K and velocity of 0.9m/s.Other boundaries are pressure boundaries.The simulation results show that the material distribution interface and the dynamic process of the melting interface during the melting process are well predicted.When the surface tension,the velocity and the thermal conductivity are respectively 0.03N/m,0.1kg/(m·s),and 0.32W/(m·K),the sample first undergoes the phase transformation at the bottom and lateral surfaces of the specimen and then generates a liquid droplet.There is a large temperature gradient in the polymer surface layer.The growth rate of the melting volume fraction is 4%/s.When the first droplet detaches from the specimen,the growth rate of the melting volume fraction reaches a peak.When the surface tension is small,the floating droplets appear due to the airflow drag force.Theoretical analyses show that the critical diameter of the floating droplets under the vertical burning test conditions is 0.64 mm.The dripping of the polymer melt can be analyzed according to the force balance.The gravity causes the dripping.The surface tension and the viscous force impede the melt dripping.The type of dripping behavior changes with the surface tension coefficient,viscosity,and thermal conductivity of the polymer.When the small-size dripping occurs,the dimensionless volume of the droplet is linear with the reciprocal of the Bond number.The surface tension controls the size of the droplet,and the viscosity is only related to the dripping time.For the large-size dripping which breaks at a cross section of the specimen,the size of the droplet increases with the viscosity.The dripping time is dependent on their respective time scale of the two dripping types.When the surface tension is large enough,the increase of the thermal conductivity is favorable for forming the large-size dripping.