Concurrent Flame Spreading And Mutation Behaviors Over Typical Thin Fabric Fuel Surfaces

The flame spread over solid fuel surfaces is the main performance of fire growth,which basically determines the initial fire growth.Due to the same direction of flame propagation and entrainment air flow,the rate of concurrent flame spreading is much more rapid than that of countercurrent configuration.The concurrent flame spreading often results in a large-area and uncontrollable fire in a short time.Therefore,it is of great significance to study concurrent flame spreading over solid fuel surfaces for building fire protection.At present,there are still not thorough and comprehensive investigations on the concurrent flame spread over solid fuel surfaces with effects of different factors,especially the heat transfer in flame spreading process.In this work,the heat transfer mechanism and relevant fire spread model for concurrent flame spreading over solid fuel surfaces are explored and established.In this work,the thin cotton fabric and flax fabric fuels are chosen as the typical combustible solid.A series of comparative experiments were conducted to investigate the effects of inclination angle,sample width,limited distance,etc.on the concurrent flame spread over thin solid surfaces.The key propagation parameters were obtained and analyzed,including pyrolysis length,flame length,flame thickness,flame spread rate.The model of heat transfer with effects of different factors was theoretically established and internal mechanisms of mutation behaviors(the flame acceleration and one-sided flame self-induced blow off)in flame spreading were explained in detail.The present work investigated the two-sided concurrent flame spread over inclined surfaces by performing experiments on cotton fabric sheets with inclination angles varying from 0° to 90°.The results show that the pyrolysis length,preheating length and spread rate increase with the inclination angle.The above mentioned flame spread parameters show a sharp increase when the inclination angle increases from 10° to 15°,which indicates that the flame spreading changes from the steady region to acceleration region.Moreover,the empirical model of concurrent flame spread rate and inclination angle is established.The heat flux received by the virgin surface is qualitatively analyzed using upper flame tilt angle and the standoff distance between flame and fuel surfaces.The basic cause of flame acceleration is revealed and the critical criterion for this mutation behavior is established.The effects of sample width and porosity on the concurrent flame spread over thin flax fabric fuels were experimentally investigated and the varying trends of flame length,pyrolysis length and spread rate were analyzed.The empirical models of dimensionless flame length,dimensionless pyrolysis length,flame spread rate and sample width are established.The mutation behavior of one-sided flame self-induced blow off can be observed in flame spreading when the sample width is less than a critical value,whose internal mechanism can be interpreted based on the buoyant selfinduced air velocity.Moreover,the relationship between Damkole number and flame length is proposed.The heat transfer models for two-sided flame acceleration and one-sided steady flame spreading are proposed.The sharp changes of flame spread rate after the self-induced blow off occurs can be explained using energy conservation,while the influence mechanism of width effects on concurrent flame spreading is analyzed according to the theory of a lateral diffusion of pyrolysis gas throughout the width.The effects of limited distance and sample width on the concurrent flame spread over flax fabric fuels were experimentally studied.It can be seen that the maximum flame length,steady flame length,maximum spread rate and steady spread rate all first increase and then decrease with limited distance ranging from 1.0 cm to 3.5 cm.The one-sided flame self-induced blow off does not occur randomly on the unlimited or limited side.When the negative effect of limiting oxygen supply is dominant,the one-sided flame self-induced blow off occurs on the unlimited side,whereas when the flame spreading is controlled by the positive effects of chimney effect and heated wall radiation,the mutation behavior occurs on the other side.The proposed heat transfer models for two-sided and one-sided flame spreading are proposed can illustrate that the heat flux received by the preheating zone first increases and then decreases,which can be verified by the surface temperature distribution in the preheating region.The flame spreading is controlled by the competition between the negative and positive effects of limited distance.