| Fire research under low pressure condition is an important aspect in fire science. One of the purposes is to solve the issue of fire protection of historic buildings in high altitude area. A representational place is the Qinghai-Tibet plateau in China, named "Roof of the World" with an average altitude about4000m, which owns thousands of buddhism buildings including the world famous Potala Palace. These buildings include not only huge cultural value, but also important political significance. It would bring inestimable loss if fire occurs.Due to the special characteristic of high altitude buildings (dense and low fire resistance rating) and the modern economy development (the increasing use of fire, electric and gas), the fire accidents in plateau area increased every year. It should be noticed that the fire in high altitude has already became a problem which can not be ignored.Fire detection is the key technology of fire protection. It’s based on the combustion parameters, and thermal or physical phenomenon of fire development process. Because of the rise of altitude, atmospheric pressure and air density there are much lower than the normal plain area, such as Lhasa city in Tibet, the air pressure is only about2/3atm. Experiments had shown that the change of atmospheric pressure has a significant influence on burning behaviors, fire dynamics is affected obviously by low air pressure and oxygen density, these phenomena would bring new problems to fire protection engineering in Plateau areas. So the aim of this paper is to study the variation of combustion parameters under low pressure condition, which is instructive and fundamental.Liquid pool fire was selected as the main object considering the representational and good repeatability. According to the theory of fire dynamic and combustion, pool fire burning behavior such as burning rate, temperature, radiation heat flux, flame height, flame puffing frequency depending on air pressure was investigated by theoretical explanation. The detailed work is as follow:In theory research, firstly two scale modeling on pressure variation (pressure modeling and radiation fire modeling) are introduced. Then a staged prediction model about the influence of pressure on pool fire burning rate is developed considering the different heat feedback control mechanisms (conduction, convection and radiation). Radiation fraction is found to be almost independent of pressure based on radiation fire modeling. Combining the fluid theory, flame image parameters including flame height and puffing frequency were analyzed using non-dimensional method. Finally, the theoretical prediction model of low air pressure on pool fire burning characteristics was established.In experimental research, two series of full scale comparison experiments were designed and conducted in different altitudes (or air pressure conditions). First, n-heptane and ethanol pool fire experiments are carried out for good repeatability and their different soot production abilities at different altitude area in Hefei (99.8kPa) and Lhasa (66.5kPa) respectively to campare with the theoretical analysis, different scale fires with pan equivalent diameter range from4.5cm to37cm were used in order to investigate the influence of pressure on different size pool fires. The results agreed well with the theory model. Second, rectangular n-heptane and ethanol pool fires with aspect ratio n=l/w varying from1to8were also studied at Hefei and Lhasa. Phenomenological models of the low pressure and aspect ratio coupling effects on burning rates, flame height, puffing frequency were established and validated by the experimental results. |
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