| As the important transmission media,wires and cables have been widely used in residential buildings,industrial plants,nuclear power plants,aerospace,etc..The insulating materials of wire and cable are mostly polymers,with or without flame-retardant treated,which are prone to decomposing and catching fire under the influence of overload,short circuit,external heating,etc..Once wire and cable are ignited,the flame will not only spread rapidly along the laying direction,but also cause the other nearby combustibles to catch fire.This can expand the fire scale,and release a lot of toxic smoke/gases as well as increase the fire hazard.In China,fire statistics indicate that more than half of electrical fires attribute to the ignition of the faulty wire and cable.Therefore,it is of great theoretical value and practical significance to study fire behaviors of wire and cable.Because of their unique structure,they are different from the general solid combustibles in terms of combustion and flame spread.And the corresponding fire behavior is susceptible to environmental pressure,electric current,external radiation flux,thermal aging degree,environmental temperature,etc..In addition,two kinds of insulating materials,polyethylene(PE)and polyvinyl chloride(PVC),commonly used in the current wire and cable industry,are selected to insulate the wire and cable studied in this work.In summary,this paper focuses on the pyrolysis characteristics of typical insulating materials,combuation and flame spread behaviors of wires and cables.The experimental observation and theoretical analysis are adopted to carry out the relevant researches on wire and cable fire behaviors under different working conditions,mainly covering the following three aspects:Firstly,the pyrolysis characteristics of PE and PVC insulations were studied.In nitrogen atmosphere,PE insulation has only one pyrolysis stage,while PVC insulation has three pyrolysis stages dominated by first two stages.However,in air atmosphere,pyrolysis process becomes more complex due to the existence of oxidation reaction.The mass loss(TG)and mass loss rate(DTG)curves shift to the higher temperature direction with the increase of heating rate and thermal aging degree.Three model-free methods,Kissinger-Akahira-Sunose(KAS)method,Flynn-Wall-Ozaw(FWO)method and Friedman method,were employed to estimate the activation energy at different conversion rates.Moreover,based on thermogravimetric data at multiple heating rates,the pyrolysis reaction mechanisms were determined by two model-fitting methods including Criado method and Generalized Master-plots method.The results show that the activation energy of aged PE insulation is higher than that of unaged one,but the reaction mechanism of both conforms to two-dimensional geometric model R2.The activation energy of aged PVC insulation in the first and second pyrolysis stages is lower than and higher than that of the unaged one,and the reaction mechanisms are nucleation growth models A2 and A3 respectively.While the reaction mechanisms of the unaged PVC insulation are in accordance with nucleation growth models A4 and A3 respectively.In addition,SCE(Shuffled Complex Evolution)global optimization algorithm was applied to optimize the kinetic parameters and realize the Thermogravimetric simulation of PE insulation.The ANN(Artificial Neural Network)model developed based on MATLAB code was used to simulate the pyrolysis process of PVC insulation.The predicted data are in good agreement with the data of the thermogravimetry.Secondly,the effects of external radiant heat flux and thermal aging on the combustion characteristics of commercial cables were analyzed.A series of combustion experiments were carried out with cone calorimeter and flame propagation apparatus.The ignition characteristics,heat release characteristics,gas consumption and generation characteristics,etc.,were discussed.The results show that ignition characteristics of cables mainly depend on the properties of the sheath.The ignition time decreases with the increase of radiation intensity and increases with the increase of thermal aging degree.It is nearly proportional to the-2 power of the radiation intensity,which conforms to the classical ignition model of thermally thick materials.There is a power function relationship between ignition time and thermal aging degree,and the exponent is related to the cable type.As the radiation intensity increasing or thermal aging degree decreasing,the heat release rate and mass loss rate increase.And the maximum peak heat release rate and maximum mass loss rate have a good linear relationship with the radiation intensity,but the linear slope is affected by thermal aging and cable type.The variations of O2 consumption and CO2/CO generation with radiation intensity and thermal aging degree are roughly consistent with the law of heat release rate.In addition,the macroscopic morphology of the combustion residue was also compared and analyzed.Thirdly,the flame spreading behavior of PE wire and PVC cable under the pressure and thermal environment was studied.In order to study the influence of the pressure,the flame spread experiments of PE wires,in a hypobaric chamber(50-100 kPa)and in three cities(64,76,and 100 kPa),were carried out.It is found that with the increase of pressure,the flame height increases while the flame width decreases.The flame spread rate also increases with the pressure increasing.By combining the defined thermal characteristic length and the heat transfer analysis,a simplified correlation between the flame spread rate,pressure,and wire configuration was proposed to predict flame spread along PE wires.Then,the flame spread experiments of PE wires under the different pressures(64,76,and 100 kPa)and currents(0-16 A)were conducted.The relationship between flame spread rate and current,pressure and wire size was revealed.The results show that the flame spread rate increases with the pressure and current.It is directly proportional to the square of the current and has a power function relationship with the pressure.However,it is negatively correlated with the insulation thickness and the core diameter.In order to study the melt dripping behavior,the flame spread experiments were carried out under varied pressures(40-100 kPa)and currents(0-16 A),It is observed that the dripping frequency increases with the decrease of pressure or the increase of current.Three dripping zones,including non-dripping zone,transition zone and frequent dripping zone,are defined.A simplified prediction model of the dripping frequency under the coupling of pressure and current was proposed.Finally,the experiments were performed to characterize the influence of ambient temperature(20-70?)and radiation intensity(4.5-8.5 kW/m2)on the flame spread over PE wires and PVC cables.The results show that the flame spread rate increases with the increase of the ambient temperature and radiation intensity.The flame spread of cables is accompanied by special fire behaviors such as "stagnant flame","jumping flame",which can affect the fire spread rate.The-1 power of the flame spread rate of wires is proportional to the ambient temperature,while-1/2 power of the flame spread rate of cables is proportional to the radiation intensity.Besides,the double cables with a distance of 10 mm promote the increase of the flame spread rate due to the enhanced heat feedback and combustion driven by the flame interaction effect. |
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