Study On Burning Characteristic And Heat Feedback Of Unsteady Combustion Process Of Small Scale Pool Fires

A typical fuel leakage fire is pool fire with a certain fuel thickness, as well as boilover burning of fuel spilled on water. Burning rate, which depends on the flame heat feedback received by fuel surface, is one of the most basic characteristic, parameters of the combustion process. Profound understanding of the unsteady combustion process and heat transfer mechanism of pool fire through studying its burning rate characteristic and heat feedback law will provide theoretical basis for controlling pool-fires.N-heptane and aviation fuels are both important risk sources in fuel leakage fires. This dissertation, firstly, investigated unsteady burning characteristics and temperature distributions of small cale pool fires. Subsequently, thin-layer boilover characteristics of pool fires with water sublayer were experimentally studied. Finally, heat transfer analysis of pool fire was carried out by using heat balance equation, and the calculation models of heat feedback and burning rate were established. The detail research contents are as follows:A series of experiments on small scale pool fires and pool fires with water sublayer were carried out in Multi-function Test Hall in the State Key Laboratory of Fire Science. Four circular stainless steel containers with diameter of 0.10m,0.14m, 0.20m and 0.30m were used in these studies. The initial temperature of fuel ranged from ambient temperature to boiling point. The initial thickness of fuel layer was 6.4mm,12.8mm and 19.6mm respectively. And the thickness of the water sublayer was 12.8mm fixed. The electronic balance was employed to measure the mass loss rate of pools. The temperature distributions of fuel layer and container wall were measured by several K-type micro-thermocouples.The mass information of small scale pool fires was analyzed to research the variation of instantaneous burning rate. Results showed that the combustion process of small scale pool fires can be identified into four typical phases based on the experimental data and phenomenological analysis, which involved pre-burning, quasi-steady state burning, boiling burning and decaying phase. It was found out through boiling theory that boiling burning will occur when the wall temperature is not less than the fuel boiling point. Thus the lower the boiling point the more prone to the phenomenon of boiling combustion, and the boiling intensity increases with initial fuel thickness and temperature.The effects of pool diameter, initial fuel thickness, initial fuel temperature and water sublayer on the burning rate of pool fires were studied. Results showed that the stable burning rate, boiling burning rate and average burning rate increased with pool diameters, and it was satisfied with a certain relationship between burning rate and diameters. The duration of quasi-steady burning phase and boiling burning rate increased with initial fuel thickness, but stable burning rate was almost unchanged. The quasi-steady burning phase was dramatically shortened, but the boiling burning rate increased significantly with the initial fuel temperature. The burning rate of heptane pool fire with water sublayer was slightly lower than that of without water sublayer and that of RP-5 pool fire burning rate greatly increased due to boilover when burning spilled on water.The characteristics and formation conditions of thin layer boilover were studied by performing a series of experiments on fuel burning with water sublayer. Results of these studies proved that thin layer boilover was due to the nucleation boiling of the water at the fuel/water interface, which will emerge under the temperature between 110 to 130℃. Consequently, thin layer boilover occurred in RP-5 pool fires which had much higher boiling point than water rather than in heptane pool fires. The pre-boilover burning rate and start time increased as the pool diameters. Thicker fuel layer resulted in a larger burned mass ratio and stronger boilover intensity.Flame heat feedback and mass burning rate in unsteady combustion process of small scale pool fires were investigated through combination of experimental measurements and theoretical calculations. Results showed that the emission power of the flame determined from the heat balance equations of the combustion system was approximately equal to the mean radiative flux measured in the literature. The instantaneous and stable mass burning rates calculated based on heat feedback analysis were in accordance with the experimental results. The rates of heat loss from container wall to surrounding air were considerably large, and cannot be neglected in comparison to the direct heat input from the flame into the fuel.