Effects Of High Altitude On Pool Fire Behavior And Radiative Heat Feedback To Fuel Surface

As one of the most devastating disaster,fire hazards pose threats to life and property.It is,therefore,the prime object of safety systems to detect,remove or reduce the risk of fire threatened by those potential hazards.Fuel burning characteristics and fire development law is the rudimentary knowledge needed in fire safety prevention design and control engineering,thus it has been extensively studied in normal environment.The largest and highest plateau in the world is the Tibetan Plateau,sometimes metaphorically described as the "roof of the world".With its long history,distinctive regional characteristics and profound cultural traditions,Tibet has created a unique style of architecture art rich in scientific and aesthetic value.Thus,the fire safety should be highly respected in those ancient building considering the relic protection.However,the fire characteristics at high altitude would be affected through heat feedback from flame to fuel,among which the radiation feedback term dominates in real fires.Hence,the in-depth understanding of high altitude effect on flame heat feedback and other related parameters will be of great scientifically significance and engineering application value for fire prevention at high altitude.To reveal the heat feedback effect accompanied with air pressure effect in an intuitive manner,experiments of small scale pool fire with different external irradiance were conducted in an altitude chamber firstly and field tests also used to for validation.It was found that the mass loss rate,or burning intensity increases with the elevated radiative heat flux,and the duration of the combustion decreases with that,regardless of the fixed pressure.For configurations with sufficient imposed irradiance,the burning was directly went into the boiling stage after a short rapid rising stage and then switched into the decline stage,during which the quasi-steady stage is hard to be distinguished.Though the thin air for reduced pressure may attenuate the combustion,for n-heptane pool fire tests,the differences in mass loss rate between two pressures appears to be vague for irradiances larger than 2 kW/m2 for both chamber tests and field tests.As the irradiance reaches to 10 kW/m2,the burning intensity at lower pressure is even higher due to the lower boiling temperature.For Jet-A pool fires,the ratio of average MLR in Lhasa vs in Hefei is growing monotonously with the rise of additional radiation heat flux,but the disparity is still distinct.It could be inferred that the threshold value of the transition from the pressure effect dominating pattern to imposed irradiance dominating pattern for Jet-A is higher than the upper limit of the tested range(10 kW/m2).Besides,the temperature under low pressure condition,is smaller at lower vertical height,and subsequently exceeds the temperature at normal pressure.The transition point lies between the measurement points of 30?45 cm.The variation of radiative heat flux is lower at reduced pressure because depressed the soot formation or soot volume fraction.With the aid of the same configured calorimeters as dimensions of 40%of that in ISO 9705 in Hefei(24 m,100.8 kPa)and Lhasa(3650 m,64 kPa),the influence of high altitude on heat release rate and combustion efficiency were investigated.Two group of liquid pool fires for moderate sizes(D=0.15,0.25 m)with fuel level maintain system were tested at two sites,respectively.Typical fuels with different sooting levels,i.e.N-heptane and Jet-A were selected.The ambient air pressure effect were introduced by modifying the standard calculation method of heat release rate in ISO 9705.Experimental results indicated that the dimensionless burning intensity in the quasi-steady stage for both fuel could be accorded to pressure modeling with acceptable accuracy.And it could be correlated to radiation modeling well for Jet-A while that of n-heptane failed and this may be explained by the flame convection feedback could not be neglected for moderate sooty fuel of moderate sizes.The combustion efficiency at high altitude is slightly higher than that at atmospheric pressure,and it will gradually increase with the decreasing pool dimension regardless of the ambient pressure.To investigate the effect of low air pressure on the flame radiative feedback to fuel surface of pool fires,a sequence of n-heptane pool fires with stable liquid surface was performed by employing three round burners with diameters of 0.15 m,0.25 m and 0.35 m in both Lhasa and Hefei.Incident heat flux at fuel surface and other burning characteristics related to radiation feedback including flame shape and temperature distribution were compared at two sites.Though flame shape is similar at the two ambient pressures and shows significant difference in three pool sales,the flame envelope for 0.15 m appears to be cylinder while that for 0.35 m tends to be cone,and the mean flame height is larger in reduced pressure atmosphere for the same pool dimension.The averaged flame temperature is relatively higher in Lhasa due to reduction of air entrainment and thermal radiation loss,and the axial temperature rises could be scaled in the form of z(P/Q)2/5.Experimental findings also reveals that low pressure reduces the radiation feedback evidently for different scale pool fires and the averaged value could be correlated against Tf5P2Lm.The radiation feedback fraction Xa defined as the ratio of the radiative heat absorbed by fuel surface to that needed for evaporation are both around 30%for 0.15 m pool fires,while it is 55%in Lhasa for 0.35 m pool fire,lower than that 76%in Hefei.