Influence Of Wire Spacing And Current On Flame Spread Characteristics Of PE Wires With Different Core Diameters

In recent years,with the development of society,wires and cables are widely used in power plants,substations,nuclear power plants and other places because of their excellent performance.At the same time,due to line overload,short circuit,leakage,illegal operation,etc.,fire accidents occur frequently.According to relevant accident statistics,fire caused by cable fire is the most important cause of electrical fire.The outer insulation layer of these cables is mostly flammable polymer.Once the fire is on,the flame will spread along the direction of the cable.It has the characteristics of fast spreading,large amount of toxic and harmful smoke,and high heat release rate.Life and property have caused great harm.Therefore,it is of practical significance to study the fire spread and combustion mechanism of wires and cables,which can provide theoretical basis for fire assessment and fire protection design.In this paper,the wire with copper（Cu）core material and polyethylene（PE）insulation layer was selected as the experimental research object.The influence of wire metal core diameter on wire flame spread characteristics was experimentally studied.The interaction between the flames during the flame spread of a single wire was compared with that of the single-wire combustion,revealing the influence of the wire spacing on the flame interaction.At the same time,a double-wire flame spread experiment was carried out under the current,and the influence of the current size on the flame interaction was analyzed to reveal the coupling effect of the current size and the wire spacing on the flame interaction.First,seven kinds of wires with different copper core diameters were selected to study the influence of copper core diameter on flame spread.The results show that with the increase of copper core diameter,the flame width and flame area first increase and then decrease.For intermediate size copper core（6 mm,7 mm,8 mm and 10 mm diameter）wires,the relationship between flame area A and copper core diameter d_cis established as A～d_c.Moreover,by analyzing the change of the temperature field during the flame spreading process,it is found that the temperature gradient of the wire with a diameter of 3 mm to 10 mm is basically the same when the flame spreads,indicating that the flame height will affect the radiant heat.When the diameter is 12 mm,the thermal conductivity is small,indicating that the core material Cu has a cooling effect on the flame.Therefore,as the core diameter increases,the dominant role of the Cu core changes from a heat source to a heat sink.Then,on this basis,the double-wire flame spread is studied,and the effect of spacing on the wire-fire interaction is analyzed.The interaction between the flames in the process of double-wire flame spreading is analyzed by the merging probability.It is found that the size of the wire spacing affects the air entrainment between the flames,and the air entrainment affects the flame merger,and has a certain influence on the flame.The stretching action also accelerates the heat transfer to the preheating zone.In addition,when the spacing is too small,the insulating layers between the wires will stick,which not only inhibits the normal spread of the flame,but also may cause a“re-burning”phenomenon,which increases the fire hazard.Next,energize a single wire,change the magnitude of the current,and study the effect of the current on the spread of the flame on the wire.The results show that:under the condition of small current,the current has a promoting effect on the flame spread on the wire,and this promoting effect will increase with the continuous increase of the current.When the current is too large（I≥40A）,the current will inhibit the spread of the flame,resulting in a decrease in the size of the flame and the speed of the flame spread.At this stage,because the current is too large,a large amount of Joule heat is generated,which increases the temperature of the preheating zone,produces a large number of drippings,and takes away the heat and fuel required for the spread of the flame,which eventually leads to the inhibition of the spread.In addition,under the same current,the larger the diameter of the copper core,the greater the Joule heat generated.Different core diameters are affected by the current differently,and the wires with larger core diameters are more affected by large currents.Finally,combined with the first three experiments,the coupling effects of core diameter,current and wire spacing on the flame spread of double wires are studied.When the distance between the wires increases,the flame spread rate of the double wires will gradually decrease,and eventually approach the flame spread rate of a single wire.At the same time,the increase of the current will increase the flame spread effect within a certain range,which will slow down the the separation of the flames caused by the increased spacing changes the critical spacing for the flame to transition from the“continuous merging”stage to the“intermittent merging”stage.Also,according to the temperature riseΔT_addchange in the additional entrainment region,the heat transfer mechanism of the double-wire flame spread is revealed.The reduction of the spacing will lead to stronger flame interaction,which,coupled with the Joule heat generated by the current,will increase the flame spread and have a higher fire hazard.