| Self-ignition is an important fire phenomenon. Under natural conditions, combustible materials may release heat due to internal self-heating reactions, meanwhile lose heat by the convection and radiation heat exchange with the ambient environment. If heat release exceeds heat loss, the excess heat will gradually accumulate inside the system. Once the internal heat excess exceeds the critical value, self-ignition will occur. The exploring of the self-ignition phenomenon, as well the characteristic of temperature rise, the influencing factors and the critical condition, are very important for the determination of the storage condition of combustible material, the formulation of safety standards, and also the effective prevention of accidents.Many researchers explore the self-ignition phenomenon by means of establishing theoretical models and performing experiments. Based on the energy balance equation, three widely used methods are developed, i.e., the basket heating method, the CPT method and the HR method. These methods allow for the determination of the critical conditions of self-ignition and the exploring of kinetic parameters. The recently proposed multipoint temperature method theoretically overcomes the application limitations of the previous three methods, however, the reliabilities of experimental mechanisms and results still need to be examined. The present work investigates the operating process and measuring results of multipoint temperature method by combination of experimental and theoretical analyses.The present work uses the one-dimensional conduction system in literature to conduct the tobacco powder self-ignition experiments. The reliability of multipoint method is verified by analyzing the experimental results. The major results are summarized as follows:(1) It is verified that the established experimental system has reliable property of one-dimensional heat conduction;(2) The specific heat of samples is measured using the DSC method;(3) Results obtained by two different layouts of thermocouples are compared;(4) The temperature characteristics during the self-ignition process are analyzed;(5) The kinetic parameters are calculated;(6) The effect of humidity on the self-ignition phenomena is explored. To make sure that the experimental results will not be affected by the physical parameters, the samples are simultaneously prepared and hermetically saved. Thus the samples will have almost the same percentage of moisture and quality in each test. It is analyzed that, the use of the multipoint method in exploring self-ignition is simple in operation and has reliably results, and the experimental repeatability is reasonably well. The present work also analyzes the temperature characteristics during the self-ignition process and the major findings are summarized as follows:(1) The thermocouples are fixed to conform the coincident positions of measuring points in whole tests, which is successful in avoiding the ruins to the measuring point positions and the probe relative to the way which inserting the thermocouples after the filling of samples. It is found that this method has high experimental repeatability.(2) The established experimental system has a characteristic of one-dimensional heat conduction.(3) By use of the symmetrical thermocouple layout, the obtained temperature characteristics accord with theories. Namely, the temperatures are symmetrical on the center point of system and self-ignition starts at the center point. Conversely, the results under the asymmetrical thermocouple layout are inconsistent with theory.(4) The inherent fault of the system in the self-ignition tests will lead the set ambient temperature to be about4℃higher than the ambient environment temperature, this has been modified in this work.(5) It is found that the maximum rate of temperature rise at the center point increases with higher ambient temperature, and the occurrence time is in advance.(6) With different ambient temperatures, the results of temperature rise of system can be classified into three categories. Firstly, with the occurrence of self-ignition, temperature at the center point is considered to be infinite. Secondly, self-ignition cannot occur, temperature at the center point is specifically higher than the ambient temperature. Finally, with low ambient temperature, temperature at the center point equals to the ambient temperature or is a little bit lower.(7) The calculation results of kinetic parameters of samples are reasonable.(8) With higher humidity environment, the critical self-ignition temperature decreases and the temperature process speeds up. In addition, the specific heat of sample is measured by the DSC method, which provides the basis for the calculation of kinetic parameters. |
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