Experimental And Model Study On Pyrolysis And Ignition Temperature Of PMMA Under Wind Influence

The use of fire is accompanied by the development of human civilization.At the same time,fire disasters threaten public safety and social development,causing a large number of casualties and huge property losses to the society.Whether it is a building exterior fire or a forest fire,the effect of environmental wind on the fire cannot be ignored.Therefore,it is necessary to study the influence of environmental wind on the fire field.In fires,radiation heat transfer is a very common heat transfer method.At the same time,previous studies have shown that radiation is the main heat transfer mode for large fuel beds.Many studies have proved that external radiation conditions have a very important effect on the pyrolysis and ignition process of materials.Therefore,in order to better simulate the fire scene,this paper adopts the experimental conditions of external constant heat radiation.In this paper,typical non-carbonized polymer material PMMA is represented,and its pyrolysis and ignition laws under the action of ambient wind and constant heat flux are systematically studied.Through the measurement of various key parameters of the material’s internal,surface and gas phase environment,the pyrolysis and ignition characteristics of PMMA under different air velocity and external heat flux conditions are summarized.Then combined with theoretical analysis,the influence of different absorption assumptions and in-depth absorption coefficients on the accuracy of predicting key parameters of fire is explored.Among the many key parameters of pyrolysis ignition,temperature parameters have the following advantages:easy to measure,high engineering practical value;wide selection of measuring equipment,high accuracy;less affected by wind velocity;no influence Wind field due to small probe volume.Therefore,it was selected as the key parameter of this research.Research on the internal temperature of the material found that under constant external heat flux and wind conditions,the air flow velocity only affects the internal temperature of a sample with a certain thickness.The influence of air velocity on internal temperature increases first,and then decreases with the increase of external heat flux.This is because the external heat flux and the air flow have a competitive relationship(that is,the air flow takes away surface heat,while the external heat flux increases the surface heat).When the influence of the external heat flux increases,the proportion of the total heat taken by the air flow will decrease;The in-depth absorption assumption is more suitable to explain the internal temperature changes of transparent non-carbonized materials under windy conditions.Research on material surface temperature,critical temperature and mass loss found that:critical surface temperature,critical mass loss rate,and ignition delay time are positively correlated with airflow velocity;the increase in external heat flux will greatly offset the influence of airflow velocity and ignition.The competition mechanism between the external radiant heat flux and the wind environmental velocity needs further study.The in-depth absorption assumption is more suitable for characterizing PMMA ignition under forced air flow.The newly proposed analytical approximation method works good and predicted PMMA ignition time well in the wind under the conditions(for a certain range of radiant heat flux and air flow velocity),and this method can be applied to engineering practice.Research on the gas phase temperature of the material and the ignition of different ignition sources found that the external radiant heat flux mainly controls the pyrolysis rate of PMMA,while the air flow controls the transportation and mixing process of the pyrolysis gas.Under low heat flux(11.53kW/m2),the production of pyrolysis volatiles is slow,even a very low air velocity can still make it evenly distributed in the space.But when the wind velocity increases,it will over-dilute the pyrolysis concentration,which will cause the ignition to become random.After increasing the heat flux,due to the rapid generation of pyrolysis volatiles,the lower flow rate air flow cannot transport the pyrolysis volatiles in time,which will cause them to accumulate at a certain downstream position.The accumulation position is most likely to reach the lean flammable limit,so it is easier ignite.At this time,increasing the air flow velocitu can make the pyrolysis gas fill the space,but the pyrolysis volatiles are generated near the center of the sample quickly.The instantaneous pyrolysis gas concentration is higher than the entire space,and the center position becomes the entire field.The most easily ignited position in the middle.