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Numerical simulation of development of a fire in the longwall goaf |
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Numerical simulation of development of a fire in the longwall goaf Author: W. Dziurzyński
Issues related to the forecasting of the ventilation process developing in the mine ventilation after occurrence of an underground fire locatcd in a longwall goaf resulted in the development of a method to determine parameters specifying the unstable condition that may develop in the mine ventilation network. This forecasting method was based on a numerical simulation of the ventilation process for the network of mine excavations. The mathematical model of the phenomena under consideration is a complex system of non-linear partial differential equations that are mutually interlinked by physical parameters and boundary and initial conditions. The phenomena described by the mathematical model may be divided into three basic cateorics: I. Distribution of the mixture of air and gases and determination of the flow velocity of the mixture in excavations and goafin relation to various ventilation conditions. 2. Changes in concentration levels of individual components of the mixture, taking into account varying flow velocity and sources of in flow of combustion gases in time. 3. Time and spatial distribution of the temperature of the fire itself and that of the surrounding goaf area. In this paper a mathematical model of a fire in a goaf is presented, which includes the coal combustion process. The result of combustion is a fall in the oxygen content (2, 1), which determines the flow-stream of the generated heat (2.12) and the stream of gases gencrated by thc combustion (2.6). To specify the parameters in the equation that describes the coal combustion process, results of experimental research conducted in conditions of mine excavation were used (Dziurzyński, Tracz 1994). This mathematical model of the fire is presented in the form of cylindrical coordinates (3.8) permitting the for calculation of the fire temperature in the goaf, taking into consideration transport and conductivity of heat generated by the combustion process. Furthermore, absorption of heat by conductivity of the fire's surrounding environment (3.9) was included in the equation of the temperature distribution in the fire. For the mathematical model, boundary and initial conditions were determined for the model- equations (2.8), (3.8), (3.9). To obtain the solution, a numerical method was used based on the approximation of the non-overt (5.1) and overt (5.8), (5.11) differential models. Two examples of simulation of development of the fire under different-vcntilation conditions are investigated. The results of the simulation are presented graphically as time diagrams (Fig. 10) and in a form of spatial distribution diagrams (Figs. 2 to 9). On the basis of the model and the numerically ealeulated solutions, it was concluded that the combustion proeess in the fire occurs mainly at the contact of its face with the inflowing air. As a result of the low flow velocity in the goaf, amounting only to a few mm/s, oxygen is consumed rapidly by the combustion process. The mine fire moves towards the inflow offresh air, thus increasing in size both alongside and across the line of the flow-stream. The simulations performed indicate that there is a certain border value of flow velocity in the goaf, exceeding which results in a sudden development of the fire. This condition is characterised by an increase of the solid's radius, with an increased flow rate of the generated combustion gases. The calculated size of the mine fire is several metres and the shape of the solid that it forms resembles a falling drop of water.
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2009