Experimental And Numerical Study On The Effects Of Low Air Pressure On The Cardboard Box Fires

Fire is one of the most frequent and devastating disasters in human society and the research of its development laws under low-air pressure is a relatively new subject in the field of fire research in China. On one hand, the Tibetan Plateau is the sacred place of Tibetan Buddhism with many ancient buildings. However, those ancient buildings are mostly wooden structure, and there are large amounts of combustible materials and ignition sources, which will easily lead to a fire. Moreover, the rapid increase of the consumption amount of fire, electricity, gas in Tibet also caused a rising trend of fire accidents. On the other hand, tremendous loss would be caused if fire occurs when an aircraft is in the state of cruising, generally low pressure inside. Therefore, it is necessary to study the combustion characteristics and development laws of fires under low pressure and low oxygen concentration.Low pressure and low oxygen concentration at high altitude will directly affect the chemical reaction rate, heat transport process in the fire, which will lead to the variation of ignition of combustibles, fire spread rate and smoke spread, etc. Many Researches have indicated that the burning rate of small pool fires and wood crib fire will decrease under low air pressure, and the temperature field and radiation fraction might be also changed. However, there are little researches on the large solid fire in low pressure, and little directly measured data of heat release rate. Therefore, a large-scale heat release rate calorimeter platform was built at Lhasa based on ISO9705standard to measure the heat release rate directly. The cardboard boxes were chosen as the fire load in this research, which was usually treated as typical fuel in fire research, and the fire experiments were performed in the large-scale heat release rate calorimeter platform respectively at Hefei and Lhasa.To study the effect of pressure on the fire with different size and layout,12configurations of cardboard boxes layouts have been tested, with characteristic length range from0.5to1.5m. And test for each configuration was repeated at least3times to ensure repeatability. The heat release rate was measured directly, and the curves were characterized by using a triangle approximation for the convenience of fire modelers’ use. By comparing the measured heat release rate, burning rate, flame temperature, and flame radiation fraction between Hefei and Lhasa, it was found that:(1) the combustion is more incomplete under low air pressure, which was outstandingly shown in two aspects:incomplete pyrolysis of solid combustible and incomplete combustion of pyrolysis gas;(2) the effect of low air pressure on the fire development mainly manifested in the increase of characteristic combustion time, which means the curves of dimensionless burning rate vs. dimensionless time would overlap together for different ambient pressure;(3) the convection plays a dominant role in energy transfer process within the scale of fire in our research. The peak value of dimensionless heat release rate or mass loss rate was proportional to the1/3power of Grashof number, which was consistent with the pressure model of fire. The time difference of temperature rising between different locations gave indirect evidence to the dependence of fire spread rate on the P2/3, which is also consisted with the pressure model of fire;(4) the fire plume temperature distribution at Lhasa is not entirely conform to the classical plume model. The temperature was higher near the fire than the classical plume model, but lower far from the fire. It is necessary to perform more accurate experiments under low air pressure to verify and correct the classical plume model;(5) the radiative heat flux measured at the same location was equal at Hefei and Lhasa, which means that the radiation fraction at Lhasa was larger than Hefei with the same size of fire load. The flame temperature at Lhasa was about100℃lower than that of Hefei, which also proved this point.On the basis of theoretical analysis and experimental results, some exploratory research was made in this paper about the numerical simulation method of fire under low air pressure. In this research, the simulation method based on the heat release rate per unit area was used to predict the heat release rate of the cardboard-box fires. For low air pressure, the peak value of heat release rate per unit area was reduced in proportion to P2/3, and the growth time was also prolonged with the same factor. The simulation result shows that this method could predict the development process of fire under normal and low pressure environment. Because the collapse of cardboard box was difficult to achieve through the simulation method, the predicted heat release rate of larger-size fire would be larger than the measured value. The flame temperature also can’t be predicted accurately.