The Research On Heat Transfer In Combustible Porous Media

This dissertation deals with the spontaneous combustion and ignition behavior of porous media stack. Combustible porous media with inner heat source is ubiquitous in our daily life. Research on such porous media's inner temperature distribution, finding out the spontaneous combustion disciplinarian, avoiding self-ignite during the deposit and transportation and supervising the media's security have very important meanings. This article bases on heat self-ignite theory, using coal and ammunition as investigate objects, systemically analyzed the heat exchange process in the combustible porous media. And it numerically simulates such representative media's inner temperature distribution and the influence factors. Using experiment's results validate the model's validity used in the simulation at the end of this paper.Because of the microstructure's complexity, the research on microcosmic transfer process with chemistry reaction in combustible porous media just limited in qualitative analysis at present. This paper introduces heat self-ignite theory and dynamics disciplinarian of chemistry reaction process at first, analyzes the influence of chemistry dynamics parameter (such as activation energy and frequency gene and reaction heat, etc) to temperature ascend.The self-ignite phenomenon of porous media can be predigested into an unsteady heat transfer process with inner heat source. In this paper, we obtained the theory results under Cartesian coordinate, pole coordinate and sphere coordinate with invariable inner heat source. However, the accuracy results always contain infinite series and they can't express the temperature trend clearly.In practice the inner heat source changes along with the temperature. In this paper using coal parameter to establish Cartesian coordinate heat transfer model with changeable inner heat source and acquired inner temperature change direction. The simulation results show that when inner heat source changing with temperature, the inner heat ascend velocity increases gradually and coal is in danger circumstance. Spherical coal stack's inner temperature distribution in high temperature circumstance (like summer) and in low temperature circumstance (like winter) are simulated in this paper.Then, ammunition stack's temperature distribution under two dimensional poles coordinate is simulated, inner heat source's change is analyzed, and environment temperature's influence is calculated in the paper, compared finite and infinite long columelliform stack's inner temperature distribution. The result shows that the highest temperature ascendance in infinite long column quicker than that in finite column. So using infinite long column to calculate is the worst emanate heat circumstance. Such results have the best safety guarantee. Different ammunition type have different chemistry dynamics parameters, and the temperature ascend process in ammunition is different. Activation energy reflects the needed input energy in chemistry reaction. The lower the activation energy is, the easier the chemistry to react. The bigger the frequency factor is, the bigger the collide odds between molecule is. Then the temperature will goes up quickly. Generally speaking, the most dangerous exothermic reaction system is such that has low activation energy and high frequency factor.Convection heat exchange coefficient, stack size and porosity's influence to the highest temperature in porous media are also discussed in this paper. The results show that the smaller the convection heat exchange coefficient, the bigger the stack size and the smaller the porosity is, the quicker the temperature ascends in porous media and the easier to combust.In real ammunition temperature ascend is very slow. So spontaneous combustion experiment study with full size will take very long time, and the experimental condition is hard to control. In this paper using a small size experiment drug stack to instead of real ammunition experimented under constant wall temperature. The measure results show that the higher the wall temperature is, the shorter the ignite time. The experimental measure results about the highest temperature change in ammunition almost consistent with numerical simulation results under the same condition.