Study On Effects Of Pressure And Oxygen Concentration On Flame Spread And Combustion Characteristics Of PE-coated Electric Wire

Special environments exist widely in the fields of production and life,such as the high-pressure environment during the period of air tightness test in the containment of nuclear power plants,and the high-pressure and hyperoxic environment in the hyperbaric oxygen chamber.The combustion characteristics and flame spread behaviors are urgent to be studied in these special conditions of abnormal pressure and oxygen concentration.Therefore,in this study,polyethylene-insulated electric wire which is common combustible is selected as the research subject.The characteristics of pyrolysis,combustion and flame spread are investigated with the effects of pressure,oxygen doncentration and radiant heat flux experimentally and theoretically.The main research contents and conclusions are as follows.Effects of heating rate,environmental atmosphere and radiation intensity on pyrolysis properties of insulation material are studied.TG-FTIR technique is used to study the pyrolysis characteristics with different heating rates and different ambient atmosphere.Fire propagation apparatus is used to study the effect of radiation intensity.(1)The results show that the TG and DTG curves of insulation material moves to the high temperature region with the increase of heat rate.The activation energy is obtained by different pyrolysis kinetics methods and the value is about 253.8kJ/mol.(2)For Nitrogen atmosphere,there is only one peak in DTG curve,and the pyrolysis products mainly consists of CO2,H2O,HCl,C=O,C=C and-OH.When the ambient atmosphere is air,there are two peaks in DTG curve and the pyrolysis products mainly are CO2 and a small amount of H2O and HCI.(3)With the increase of radiation intensity,the beginning time of thermal decomposition is reduced as well as the complete degree of the decomposition.In addition,the average heat release rate is linear with the radiant heat flux and the heat of evaporation is 2.48kJ/g calculated by the linear relation.The combustion characteristics of the polyethylene-coated copper core wire is analyzed under different oxygen concentrations and ambient radiation intensities.The limiting oxygen concentration(LOC)and the ignition time decrease with the increase of the radiation intensity.The ignition time is proportional to the-1 power of the radiant intensity which is consistent with the theoretical model.The mass loss rate increases with the increase of oxygen concentration and radiation intensity.The average mass loss rate can be used to calculate the heat blockage coefficient and the calculated values increase with oxygen concentration.The coupling effects of oxygen concentration and radiation intensity on the heat release rate and CO2 production are similar.When the radiation intensity is low,the heat release rate increases with oxygen concentration.As the radiation intensity increases,the heat release rate firstly increases and then decreases with the maximus value in air atmosphere.The heat release rate presents increasing trend with increasing radiation intensity under air atmosphere and it increases first and then decreases under non-air atmosphere with the maximus values at radiation intensity of 35kW/m2.The effective combustion heat increases firstly and then decreases with oxygen concentration in each radiation intensity and the value is biggest in air atmosphere.Under the oxygen concentration of 18%and the air atmosphere,the effective heat of combustion increases with the increase of the radiation intensity.Under high oxygen concentration,it increases first and then decreases,reaching the maximum at a radiation intensity of 35kW/m2.The smoke production rate increases with the increase of two kinds of factors.High radiation intensity promote the rate of CO yield and it is higher for the non-air atmosphere than air-atmosphere.That is to say,in environment of low oxygen,high oxygen and high radiation,more toxic gases are released and the fire hazard is bigger.The effect of pressure and inclination angle on the spread of wire fire was studied.When the pressure ranges from 40kPa to 100kPa,blue area appears at the bottom of the flame and the area increases as the pressure decreases.However,when the tilt angle increases,blue flame area does not appear as the pressure drops.The blue area no longer appears when the pressure is increased over 100kPa.And the flame color changes from light yellow to orange when the pressure increases.The flame height increases with the pressure first,and then decreases.The wire material reaches a maximum at a pressure of 200 kPa.While the flame width of electric wire decreases with increasing pressures.For the spread of the wire in inclined state,the flame width decreases as the pressure increases.While the length of the pyrolysis zone in the flame zone decreases first and then increases as pressure increases with the maximum value under normal pressure condition.Whether horizontal or inclined flame spreads,the flame spread rate increases with the increase of pressure.The flame spread models of wire fire under horizontal and inclined conditions under different pressure conditions are established respectively.The horizontal flame spread model indicates that the flame plays a leading role in the heat transfer mechanism of the flame front zone.For the flame spread under the coupling factors of the tilt angle and pressure,the model shows that the convective heat transfer of the flame to the flame front plays a dominant role in the heat transfer mechanism.This study provided basic data of fire characteristics for wire fires under special conditions of high pressure,low pressure,high oxygen,and low oxygen,and established a theoretical basis for fire prevention and safety design under special conditions.