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Boundary Settings. The gasification channel boundaries are defined as temperature boundaries, the temperature of which is 1400K. This model does not consider the heat effects of bottom strata, thus the bottom boundaries of the model are defined as thermal insulation. Suppose the model region is big enough, which means that the surrounding boundaries are not influenced by the gasification temperature field. Therefore, they are also temperature boundaries with ambient temperature. Left and right boundaries of the structural mechanics model are stated as fixed boundaries. Top, bottom and interior boundaries are considered as free boundaries. There is a 2. 47e7N/m2 uniform load applied on the top boundary, which is equivalent as the burial depth is 1000m. Likewise, there is also a uniform load valued as 2. 5e7 N/m2 acted on the bottom boundary. Results and Analysis This paper mainly studies the heat conductive process at the transverse section during UCG. Fig. 3 represents the temperature distribution on the center vertical direction of the gasification transverse section as the burning time is 6 days. It shows that heat conducts to overlying strata linearly at the beginning. The temperature rising speed decreases over time, which means the nonlinear character becomes more and more obvious as time passes. The heat radius is 3 meters when burning 6days. Fig. 3 Temperature distribution on center vertical direction of gasification Fig. 4 Temperature distribution on horizontal direction of the roof of coal seam Fig. 4 gives the horizontal temperature distribution of the coal seam roof. It shows that the conduction regulation is similar compared with vertical conduction. The influence range is only 2m, which is lesser than the vertical conduction. In conclusion, heat conduction presents a linear to nonlinear pattern as time passes. The reason for this may be the influence weights of convection term and diffusion term change as time elapses. When time is limited, the convection term, which is the first-order item, plays a leading role in heat transfer. However, with the extension of time, the contribution of the diffusion term, which is second-order, becomes bigger and bigger. Consequently, the curves showed nonlinearity. The chemical reactions of coal occur only when the temperature is higher than 300℃. There are just physical changes, such as water desorption or coal shrinkage, happened when temperature is lower than 100℃.
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The performance of sandstone does not change a lot before the temperature reaches 200℃, either [2, 3]. Accordingly, it is known that performances of overlying strata change only when the temperature is higher than 400K. Thus, stratum properties are only affected in a small area during UCG. The temperature field influences 1-2 meters in this model. Conclusions This simulation has built a two-dimensional coupled heat conduction and structural mechanics model to investigate the transverse section temperature distribution during UCG. Based on general heat transfer partial differential equation, the temperature change curves on horizontal and vertical directions have been given. Calculation and analysis indicate that only a 2 meter range of strata is affected by heat. Based on the study in this paper, current experimental studies of rock performance at elevated temperature could be utilized in UCG. This paper also puts forward a new method, multi-physics field coupling analysis, of numerical simulation. The UCG process could be reflected more accurately and vividly using this method. Future work may be carried out in the following aspects: 1. Optimize the transient analysis model by considering the variation of temperature field as time changes. 2. Simulate the temperature field distribution during UCG in three-dimensional model. 3. Couple the heat transfer, structural mechanics and chemical engineering modulus, to simulate a more authentic gasification process. Nomenclature k=Thermal Conductivity ρ=Density Cp=Heat capacity at constant pressure E=elastic modulus ν=Poisson's ratio References.
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