Effects Of Convection And Sample Width On Solid Ignition And Concurrent Flame Spread

The studies of solid ignition and flame spread phenomenon are very significant for fire safety.The flame spread can usually be divided into two major categories:concurrent-flow flame spread and opposed-flow flame spread.In the former case,the flame spread process is quite fast as the gas flows move in the same direction as that of the propagation.However,the study of concurrent-flow flame spread is much less developed than that of opposed-flow flame spread.And the controlling mechanism of concurrent-flow flame spread needs further explored.Moreover,solids of finite width are usually used in the study of solid fire,which must be considered when applying the results obtained from small-scale fires to real fire scenarios.Therefore,the effects of gas convection and sample width on the ignition and concurrent-flow flame spread over solid fuels(Paper and PMMA)are conducted in both natural and forced convection modes.In a natural convection flow,the flow is induced by buoyancy and the flow field varies with the gas phase environment.At first,the effects of ambient pressure,oxygen mole fraction and ignition power on piloted ignition over thin papers were studied.Four typical scenarios were observed as the ignition conditions were varied.The ignition delays of upward and downward flames on the frontside(pilot heated)and backside(not pilot heated)surfaces were firstly examined.It was found that as the pressure,oxygen concentration or ignition power was increased,the ignition delays were decreased.After the ignition was achieved,the subsequent upward flame spread was one-sided under certain conditions,while the downward flame spread was always two-sided.Differences between one-sided and two-sided flame spreads were investigated.It was found that the one-sided flame propagated much more slowly than the two-sided flame,and the one-sided flame had a narrower flammability range.The effects of ambient pressure,oxygen mole fraction and width effect on the two-sided upward flame spread over thermally thin paper fuels were also investigated.A flame break-off phenomenon was observed,which occurred more likely at high oxygen and high pressure conditions.This phenomenon was considered to be attributed to the multi-step pyrolysis.Pressure modeling of upward flame spread obtained a power law relationship between the flame spread rate and the length-averaged Grashof number with an exponent of 0.257,which was very close to the theoretical value of 1/4.The good accuracy obtained here indicates that for upward flame spread over narrow and thin solids,the radiation and gas phase chemical kinetics are less important compared to the heat convection from the flame to the fuel surface in the spread process.This conclusion holds true for near extinction conditions.In contrast,for the opposed-flow flame spread,the flame spread is primarily controlled by the gas phase chemical kinetics for near extinction conditions.The effects of sample width on upward flame spread over thermally thin fuels were as follows.The flame spread rate as well as flame length increased with the sample width as a power law.The smaller sample width narrowed the flammability limits of solid fuels.In a forced flow,the effects of wind velocity and sample width on concurrent-flow flame spread over thermally thick PMMA solids were investigated.It was found that as the sample width was smaller than 6cm,the flame spread rate and flame length increased with the sample width;as the sample width was larger than 6cm,the flame spread was almost width-independent.As the flame propagated,width effects on the concurrent flame were initially primarily controlled by the lateral fuel diffusion and heat loss,and then primarily controlled by the air entrainment.Turning point of the controlling mechanism occurred when the thermal boundary layer thickness was comparable to the sample width.By using the method of calculating the B number from the standoff distance,the B-numbers as functions of wind velocity,sample width,space and time were obtained.As the sample width was smaller than 6 cm,the B number increased with the sample width,while as the sample width was larger than 6 cm,the B number was almost width-independent.Wind velocities in this study(2 and 3 m/s)had little effect on the B-number.An average B-number was used to predict the flame spread rate,which showed a good agreement between the theoretical prediction and the experimental results.The controlling mechanisms of flame ignition,concurrent-flow flame spread and width effects were revealed in this study.The results provide theoretical basis for fire safety under gas convection conditions,especially for situations of an environment with non-standard pressure and oxygen mole fractions,such as aircraft and spacecraft.