| Thermal insulation materials are widely used in the fields of buildings,chemical industry,transportation and ocean engineering.Because of their special combustion characteristics in fire,having become one of the research hotspots in fire science.In many fire insulation structures,such as building facades,cryogenic liquid containers pipe,cryogenic liquid storage tank(LNG,LPG,etc.)and offshore integration of cryogenic liquid storage unit(VLGC,FLNG,etc.).In recent years,there have been occasional fires of thermal insulation materials in storage tanks of liquefied gas(LNG,LPG,etc.)of onshore and offshore terminal.Taking transportation in shipping,port and storage of liquefied gas as an example,it is expected to expand the research on the fire safety performance of structures with thermal insulation materials.The commonly used insulation materials in LPG storage tanks are Expanded Polystyrene(EPS)and Polyurethane(PU).Once a fire occurs,it will affect and destroy the tank structure itself and the pressure balance in the tank,which results in serious safety consequences.The thermal-mechanical coupled characteristics of the steel-attached combustible material in the thermal insulation material fire of storage tank are discussed,which involve the heat transfer of the thermal insulation material,the heat transfer of combustible materials attached to structural steels and corresponding mechanical response.There have been researches on fire performance of thermal insulation materials and fire resistance of steel structures at home and abroad,but there is still a lack of research on thermal-structure-mechanical coupling of thermal insulation material-steel plate system.Therefore,this paper starts with two fire scenarios and heat transfer process involved in steel bonded insulation material fire.Using Q345 steel commonly used in LNG storage tank structures as the attachment carrier of thermal insulation materials,an experimental platform for thermodynamic coupling characteristics of steel-bonded combustible materials was designed and built.The heating effect of insulation material ignition on steel structure and the thermal-mechanical response of the structure were studied.In the aspect of heat transfer by pyrolysis and ignition of steel attached thermal insulation materials,two typical insulation materials,EPS and PU,were selected and 12 groups of tests were carried out with three material sizes of 0.5×0.5×0.05 m,0.4×0.4×0.05 m and0.3×0.3×0.05 m,the experimental data of 6 groups of working conditions were obtained,then the time and space of the combustion heat transfer of insulation materials were carried out division and theoretical analysis.The results show that there are pyrolytic gasification layer on the surface of thermal insulation materials,equivalent temperature layer of thermal feedback,full combustion zone and plume zone during the combustion and heat transfer process of thermal insulation materials.Among them,the equivalent temperature layer of thermal feedback close to the gas phase above the material can be used to calculate the spatial and temporal distribution of heat flow feedback on the surface of pyrolytic materials.Based on this,a feedback model of combustion heat for steel-bonded insulation materials was established,in which the convective heat transfer coefficient,which determines the convective heat transfer on the pyrolysis surface,was obtained by using two-order center scheme formula when considering the convective term and combining with the heat transfer correlation developed in previous experiments.When considering the radiative component,the point source model is coupled with traditional cone flame model and plane flame model,and the effect of the equivalent temperature layer of heat feedback is considered comprehensively.The heat conduction term reflects the horizontal distribution gradient of heat feedback.The model calculation result shows that after the thermal insulation material enters the stable combustion stage,predicting results gradually approach experimental values,because the smoke completely covers the surface of the pyrolytic material.In addition,empirical formula and experimental observation data were used to study the heat and mass transfer rules of thermal insulation material combustion.Results show that EPS in fire can be divided into four stages: melting ignition,stable combustion,pool fire formation and extinguishing,while PU in fire can be divided into three stages: pyrolysis ignition,stable combustion(accompanied by double peak)and extinguishing.When the equivalent temperature layer of heat feedback enters the pool fire combustion stage of EPS and the stable combustion stage of PU,the temperature gradient in the layer decreases gradually,but the decreasing trend slows down gradually.The overall ignition time of EPS is greater than that of PU,and the heat release rate is the minimum of 117.7k W /s and the flame height is the lowest when the plate width is 0.39 m.Based on this special experimental phenomenon,from the perspective of heat transport,it is pointed out that the generation mechanism of critical combustion size is generated by the cross control of thermal radiation and convection entrainment of air.The mass loss rate of PU and EPS has a positive power relationship with flame height and material width.The 600 K isotherm of two-dimensional drawn by measured data can well describe the height of the flame after image processing.In the section of thermal-mechanical response study of steel structure,thermophysical property of thermal insulation materials were analyzed,and the phenomenon of combustion diffusion experiment and solid heat transfer behavior of thermal insulation materials were explained based on the classical model of thermoplastic pool fire of EPS.Then,the thermalmechanical response model of the steel structure was established in seven steps,as follows: 1)According to the established thermal feedback model,the feedback of heat flux distribution over the insulation material were obtained.2)The theoretical heating of steel plate is calculated by using heat feedback.3)Considering the thermal conductivity model,the actual temperature rising of the steel plate is calculated by measuring the temperature at the bottom of the steel plate.4)Analyzing the correlation between feedback of heat flux and actual temperature rising of steel plate.5)Determining the thermal attenuation function of heat feedback through insulation material and steel plate.6)The maximum temperature rising rate of the steel plate with three sizes of the attached material were calculated respectively.7)The maximum internal force of steel plate was calculated according to the rate of temperature rising.Based on this,changing patterns of internal force of steel plate were obtained.Calculation results of the model show that the thermal attenuation range is 0.133,0.182,0.283,respectively.With the increase of the burning area,the feedback of heat flow gradually increases,which are 43.98kw/m~2,68.86kw/m~2 and 83.56kw/m~2,respectively.The heating rate of steel plate increases,which are0.38484℃/s,0.54663℃/s and 0.5775℃/s,respectively.The maximum internal stress of steel plate gradually increases,which are 1.13 MPa,1.607 Mpa and 1.698 MPa,respectively,and finally reaches the maximum design stress at L=52.8m,to reach the ultimate yield strength at L=122m.In addition,for the increase of the burning area of EPS insulation material,the maximum design stress will reach when L=17.5m,and the ultimate yield strength will reach when L=41m,which achieves the separate calibration of the mechanical thermal response of the steel structure with the steel-bonded combustible material.According to results obtained in this paper,the thermal insulation material combustion is affected obviously by the geometric characteristics of the material,especially the width of materials.As the burning size of the thermal insulation material increases,the harm to the steel structure attached to it will increase accordingly.Then,with the increase of external heat flow,the diffusion rate of combustion of thermal insulation material will be faster,and the internal pressure generated by heat entering the storage tank for gasification of liquefied gas will be superimposed with the thermal response of the tank mechanics,which will increase the risks and hidden dangers in combustion control.The results obtained in this paper are of scientific value for the prediction of the behavior of the fire coupled structure with steel-bonded combustible materials. |
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