Experimental Study On The Effects Of Low Ambient Pressure Conditions At High Altitude On The Fire Suppression Performance Of Water Mist

China is a long-history country and has considerable numbers of cultural heritages,especially in the west plateau area,there are many historical buildings of temples,such as the Potala Palace which is a representative historical building of Tibetan Buddhism and had been inscribed on the World Heritage List.However,most of the historical buildings are wood structures or wood-stone structures,whose fire resistance level is very low,and there are lots of flammable materials in them,so their fire safety status is very serious.Study on the effects of low ambient pressure conditions at high altitude on the fire suppression performance of water mist is very important for developing effective and suitable fire extinguishing technologies for historical buildings at plateau areas.Water mist has been received considerable attention for fire suppression in recent years due to many advantages,such as high fire extinguishing effectiveness,less water consumption,no pollution to environment, safety to protect objects etc.Large numbers of studies and applications of water mist in normal altitude areas have been carried out.Especially,owing to the above advantages,studies on fire protection of historical buildings with water mist were also performed recently.Fire suppression effectiveness of water mist will change with altitude and ambient pressure conditions.However,few studies on fire suppression effectiveness and mechanism in low ambient pressure conditions have been done. Systemic experimental research on fire suppression effectiveness of watermist under lower ambient pressure conditions were carded out in this paper.In order to conduct the experiments conveniently,a small-scale experimental platform with adjustable inner ambient pressure was built.Meanwhile,a water mist fire suppression experimental platform was built in Joint Laboratory of SKLFS(State Key Lab.of Fire Science) and Tibet,where research of the interaction between water mist and flame was performed in a relatively large scale.Good experimental conditions were provided for the study on fire suppression effectiveness and mechanism of water mist.Investigations on the effects of ambient pressure on flow coefficient of water mist nozzles were carried out by measuring the operation pressure and flow rate of water mist nozzles at different ambient pressures.By means of taking pictures of spray field lightened by a laser light sheet at different ambient pressures and measuring the volume of water mist collected in different regions below the nozzle,relations between droplet distribution characteristics and ambient pressures were got.The results showed that when work pressure of the nozzle was constant,the distribution of mist droplets congregate towards spray cone surface with reducing ambient pressure for both pressure-whirl hollow cone nozzle and pressure-whirl solid cone nozzle. Consequently,spray intensity around spray cone surface increases while that in the center of spray field decreases.Pump set is a key part of the pump supplying water mist fire suppression system. The performance of pump set has direct effects on the water supply for the nozzle and the spray intensity,et al.and finally affects the fire suppression effectiveness of water mist.Influence of ambient pressure on the flow characteristic of piston pump and pump set efficiency was also studied by comparative experimental study between Hefei and Lhasa.In addition,combustion characteristics of n-heptane and gasoline pool fire,such as fuel's mass burning rate,flame shape,flame temperature,CO concentration in flue gas etc.in different ambient pressure conditions were researched on low ambient pressure simulative experimental platform.In order to reflect the relation between burning characteristics and ambient pressure veritably,experiments of combustion characteristics of various representative combustibles,such as petrol,diesel oil,ghee and wood crib,were carried out in Hefei and Lhasa comparatively.The results revealed the influence law of low ambient pressure on combustion characteristics and provided reference for the investigation of influence mechanism of low ambient pressure conditions on fire suppression performance.Finally,comparative experiments were conducted to gain the fire extinguishing effectiveness of representative combustibles with both high and low pressure water mist fire suppression system.In the experiments,by designing several test conditions composed of different nozzles,operating pressures and fuels,fire extinguishing effectiveness with water mist under different attitude conditions was studied,and the relations between ambient pressure conditions and fire suppression performance of water mist system were analyzed as well.The results indicated that water mist system has better in overall fire suppression performance in Lhasa than in Hefei.By means of theoretical analysis of spray characteristics and mechanism of water mist interacting on fire under different ambient pressures,in combination with the results of fire suppression experiments,the changes of the three key fire suppression mechanisms of water mist,including flame cooling,oxygen depletion and burning surface cooling, under different ambient pressure conditions were obtained.The influence law of low ambient pressure on fire suppression mechanisms of water mist was revealed.