| In recent years,fire accidents are reported frequently in the country,which seriously threaten the safety for life and property of the people.In long and narrow spaces,when the firing position is closer to the ceiling or the fire power is large,a rising flame will impinge on the ceiling,it will be deflected and spread out along the ceiling radially.During this process,the relatively small cone-shaped vertical flame will turn into a disk of horizontal flame,which dramatically enhances the risk of fire propagation.Though widely implemented,the existing fire compartments have certain disadvantages.Water mist has been a common method among other techniques for fire protection because of its economical merits and environmental issues.As a practical consideration,a method of suppression ceiling flame with water mist curtain in long and narrow spaces is proposed.First,the enveloping surface of existing 7N water mist head is not preferable as water mist curtain,due to it was originally designed for extinguish the existing fire under the water mist head.This results the air gap exists among the adjacent enveloping surfaces.To obtain the relative dense and porosity water mist curtain with a horizontal certain momentum,a modified water mist curtain head is designed.A series of full scale experiments were conducted to verify the effectiveness of suppression ceiling flame with the same flow rate water mist curtain generated by 7N and modified water mist curtain head.The results indicate that two types of water mist curtain have a certain suppressing effect on the ceiling flame.When the water mist curtain is actuated,ceiling temperature and the fire smoke flow velocity after water mist curtain decrease obviously.The results also show that water mist curtain having a high thermal insulation effect.In addition,it’s confirmed that the effects in the above became more remarkable when modified water mist curtain is applied.Note that the open of water mist curtain will result in smoke settling at the water mist curtain coverage area.The length of settled smoke decreases as the fire power increases and the length of settled smoke caused by modified water mist curtain is shorter than the 7N water mist curtain.However,smoke stratification still presentafter smoke settled area.Second,FDS software was used to simulate the development of ceiling jet flame under three conditions: no water mist curtain,7N and modified water mist curtain.The changes of ceiling flame length,temperature fields,concentrations of related gases and flow field are studied.Meanwhile,the mechanism of suppression ceiling flame with water mist curtain is discussed in detail.The results indicate during application of water mist curtain,partial droplet is evaporated by absorbing heat from ceiling flame,reducing the temperature of the combustion zone with a larger latent heat of vaporization.In this process,a large amount of water vapor is produced by the evaporation of water mist.Therefore,the oxygen and combustible vapor downstream of the water mist curtain is diluted and the combustion speed is suitably suppressed.At the same time,due to the sensible heat absorption of water vapor,the ceiling temperature at the protection segment is greatly reduced.Smoke movement under different water mist curtain parameters was also discussed.Based on the full scale experimental setup,the effect of suppression ceiling flame with water mist curtain under different water mist pressures were studied.The difference in the shape of ceiling flame was observed between the 7N water mist curtain series and modified water mist curtain series.For modified water mist curtain series,the ceiling flame is blocked as the water mist pressure is smaller and with the increasing of water mist pressure,the length of ceiling flame gradually decreased.For7 N water mist curtain series,the shape of ceiling flame is affected by water mist.When the pressure is low,with increasing of water mist pressure,the flame surface at the water mist curtain coverage area gradual uplift,when the pressure increases to some extent,the flame front bend downward as it emerges from the position of water mist curtain head.For higher pressures,flame front fragmentation occurs.The ceiling temperature and the fire smoke flow velocity after water mist curtain decreases obviously.The experimental result showed that modified water mist curtain is better than the 7N water mist curtain on the suppression ceiling flame.The experimental results were similar with the simulation results. |
