Molten Thermoplastic Dripping Behavior Induced By Flame Propagation Over Energized Polyethylene-insulated Wires

With social development,people's life are being electrified and intelligent.The accidents of electrical fire happen frequently as with electronics widely used,resulting in a serious threat to the peoples' life and property safety.Wire is one of the most important sources of electrical fires.And the polyethylene(PE)insulated wires are widely used in the crowded conditions because of their non halogen and low smoke characteristics.It is of great significance to do the research on the PE-insulated wires.Due to the Joule heating effect of wire metal core under energized,the tightly adhered insulation is heated.The molten insulation accumulates blow the core and flows with flame propagation along the wire,and gradually increases the volume.When the volume exceeds its surface tension,a drop would occur.The dripping behavior could effectively expand the scope of combustion,resulting in the rapid development of fire and very serious consequences.This paper is focused on the study of molten dripping behavior induced by flame propagation over energized polyethylene-insulated wires.Combined with the theory analysis and the experiment research,the mechanism of formation and evolution and the critical conditions for the dripping behavior are investigated.This work could enhance the understandings of wire fire,and be the support of wire fire safety.First of all,on the basis of wire combustion characteristics,a theoretical model is proposed to study the rising process and the equilibrium temperature for the energized wire.Mass transfer number is used to calculate the burning rate of the molten thermoplastic,and the accumulation rate of melting insulation is obtained with flame propagation.From surface tension,the droplet volume is derived and then the theoretical expression of wire dripping frequency is achieved.Second,three types of polyethylene insulated wires,with the same thickness and different core diameters,are used to do the research under normal and lower pressure atmosphere.As the results shown,the occurrence of molten dripping behavior is a procedure from quantitative development to a qualitative change.Here,the quantitative development refers to the accelerated accumulation of molten thermoplastic as with the rising currents,and the qualitative change is the formation of droplets.Under a large current,the thermoplastic insulation is in a state of mild pyrolysis,leading to a reduced energy required for melting and increasing the mass loss rate.Part of the insulation loss is consumed in combustion,and the other is coming into molten.With the increase of current,the limited increase of burning rate could not catch with the accumulating rate,resulting in the increase of accumulation.When the total accumulation is beyond the limit of droplet surface tension,dripping phenomenon is inevitable.Then,the molten dripping process is analyzed.The droplet flame is in a cone-shape and retain a relatively stable state in the falling procedure.After a short distance,the droplet still has a high temperature with the ability to maintain combustion and even ignite other combustible materials nearby.Based on the analysis of drop distance,the velocity and acceleration is achieved.The molten dripping behavior is presented to be periodic.The dripping time and frequency are obtained theoretically and experimentally,and they are in good agreement.For the wires with minimum core section area,the dripping frequency and the square of current are in a positive linear correlation.And the dripping time decreases exponentially with the increasing of wire temperature.In addition,dripping behavior has an obvious effect on the wire flame propagation,mainly for the flame height.Before and after a drop,the flame height is changing rapidly from the maximum to the minimum.But the flame width is only a slight vibration during the drop and the flame propagation velocity is limited affected.Finally,by the experimental study under lower pressure,it could be concluded that the molten thermolysis rate decreases by the decline of flame radiation,which is the main reason for the increasing dripping frequency.Drop occurs more easily under lower pressure,resulting in large amount of heat exhaustion from wire flame propagation,and the flame could disappear for a short time or even completely extinguish.In addition,the critical current for the dripping behavior is smaller as with the lower pressure.But the changing curve is nonlinear.In the stage of 1.0-0.8 atm,the critical current changes a little,and in the stage of lower than 0.8 atm,the critical current decreases rapidly.Moreover,the dripping frequency increases with the decrease of the ambient pressure.Especially as with the current increasing,the changing curves of dripping frequency and lower pressure develop from linear to parabolic.The molten loss under lower pressure has a stronger correlation with the wire equilibrium temperature.With flame propagation,the higher the wire temperature,the shorter the time required for the accumulation of the molten thermoplastic before a drop and the greater the dripping frequency.