This paper intends to present our investigation and experimentation results of our study on auto-ignition characteristics of wooden material at a variable heat flux so as to disclose the auto-ignition mechanism. For our research purpose, we have also observed and tested the auto-ignition behaviors of wood under the conditions of a variable linear increase of the heat-flux with the device for fire performance at the early stage (DEPES) . In our study, the output power of the radiation plate in the DFPES were adjusted to 30% , 40% , 50% , 60%, 70% , 80% and 100% so as to form a sequence of variable heat flux with an increasing rate from 0.065 kW/(m~2·s) to 0.968 kW/(m~2·s) . When we did our experiments, we were trying to use four kinds of wood, I. E. Paulownia, toon, elm and acacia, with the ignition time, the incident heat flux and interior temperature being measured carefully. The heat flux to be introduced to the sample surface was measured by a non-water-cooled heat-flux gauge and the interior temperature was checked by three thermocouples which were inserted in the sample before the test. Test results of ours show that all of the samples can be ignited if the heat flux increasing rate is greater than 0.199 kW/(m~2·s). But if the heat flux increasing rate is less than 0.199 kW/(m~2·s), some of the samples used would fail to get ignited. Thus, the experimental facts we have found prove that the critical heat flux increase rate for wood ignition is between 0.065 kW/(m~2·s) and0.103 kW/(m~2·s). Based on the heat conduction model, a numerical model has been developed to calculate the variation of the wood temperatures. Since the wood ignition time can be measured in the tests, the surface ignition temperature can be worked out with the help of the numerical model, with the value of the autoignition temperature being about 500 °C for wood. And, finally, a comprehensive autoignition criteria, including the surface temperature and the critical heat flux increasing rate can be shown in accordance with the testing and calculation results.
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维普
