基于格子Boltzmann方法的饱和土壤渗流与传热数值研究

发布时间：2018-03-14 13:57

本文选题：多孔介质　切入点：格子Boltzmann方法　出处：《大连理工大学》2015年硕士论文　论文类型：学位论文

【摘要】：地热源热泵技术由于其良好的环境和经济效益,近年来发展迅速。在地埋管换热器与土壤换热过程中,地下水渗流影响不可忽视,且热渗耦合机制也较为复杂。在以往研究中,大多针对宏观方面开展相关的实验和数值模拟分析。本文尝试从微观角度出发,采用数值模拟计算的方法以揭示土壤内部结构演化及其热渗耦合规律。本文首先引入了不可压耦合双分布函数格子Boltzmann模型,介绍了其基本原理和边界处理格式,同时借助封闭方腔自然对流验证了选取模型和边界处理的可行性和准确性。其次本文通过随机四参数生成法(QSGS)重构得到了与真实土壤形态相近的二维饱和多孔介质模型,通过调整模型参数之间的关系可以表征出多孔介质更多细节信息。在此基础上,从孔隙尺度角度出发,在无渗流纯导热情况下对有效导热系数同多孔介质结构之间的关系作了探讨。最后对饱和土壤渗流与传热开展了相关数值模拟研究。在模拟中分别针对不同渗流压差、孔隙率、土壤固相颗粒尺寸分布、温度梯度方向对热渗耦合特性影响进行了详细的讨论,同时对土壤相关的水力学参数进行了计算分析。数值模拟结果表明：(1)无渗流影响时,土壤有效导热系数随着孔隙率增加而近似呈指数形式递减,且关于土壤固相颗粒导热系数呈幂函数关系；对于同一孔隙率,土壤固相颗粒尺寸越小,有效导热系数越大。(2)在有渗流时,土壤渗流速度与渗流压差呈线性递增关系,且随着速度增加,土壤内部传热机制逐渐由固液热传导占优转变为对流换热为主,整个土壤内部平均温度上升速率减缓。(3)随着土壤孔隙率的增加,土壤内部孔隙间连通性变好,渗流速度随之增大；由于孔隙水与周围固体骨架形成的液桥增加,接触热阻减小,加之渗流速度大幅度增加,渗流方向与导热方向一致,对流换热作用得到强化,导致土壤内部温度呈明显上升趋势。(4)在同一孔隙率和渗流压差下,土壤固相颗粒尺寸较大时,多孔介质内部会出现局部流速突增情况,形成典型的优先流效应。随着颗粒尺寸的减小,颗粒之间接触更加紧密,生成随机多孔介质有效导热系数增大,对流换热系数减小,会造成传热过程中热扩散阻力增加,土壤内温度变化逐渐趋于平缓,平均温度下降。(5)对于热源相同作用位置,流动与传热两者驱动势同向比反向时土壤内平均温度要高,且随着孔隙率增加,温度差距也随之加大。
[Abstract]:The geothermal source heat pump technology has developed rapidly in recent years because of its good environment and economic benefits. In the process of heat transfer between ground heat exchanger and soil, the influence of groundwater seepage can not be ignored, and the mechanism of heat and permeability coupling is more complicated. Most of the experiments and numerical simulation are carried out on the macro aspect. This paper tries to start from the micro point of view. In this paper, the incompressible coupled double distribution function lattice Boltzmann model is introduced, and its basic principle and boundary treatment scheme are introduced. At the same time, the feasibility and accuracy of the selection model and boundary treatment are verified by natural convection of closed square cavity. Secondly, a two-dimensional saturated porous media model similar to the real soil morphology is obtained by the stochastic four-parameter generation method (QSGS) reconstruction. By adjusting the relationship between the parameters of the model, more detailed information of porous media can be expressed. On this basis, from the point of view of pore scale, The relationship between effective thermal conductivity and porous media structure is discussed in the case of pure thermal conductivity without seepage. Finally, the numerical simulation of seepage and heat transfer in saturated soil is carried out. The effects of particle size distribution and temperature gradient direction on the thermo-osmotic coupling characteristics were discussed in detail, and the soil hydraulic parameters were calculated and analyzed. The effective thermal conductivity of soil decreases exponentially with the increase of porosity, and the thermal conductivity of soil solid particles is a power function, and for the same porosity, the smaller the size of soil solid particles is, The larger the effective thermal conductivity is, the more the seepage velocity increases linearly with the seepage pressure difference when there is seepage, and with the increase of the velocity, the heat transfer mechanism in the soil gradually changes from solid-liquid heat conduction to convection heat transfer. With the increase of soil porosity, the connectivity between pores in soil becomes better and the seepage velocity increases, and the contact thermal resistance decreases due to the increase of liquid bridge between pore water and surrounding solid skeleton. In addition, the seepage velocity is greatly increased, the direction of seepage flow is consistent with the direction of heat conduction, and the convection heat transfer is strengthened, which results in an obvious upward trend of soil internal temperature. (4) under the same porosity and seepage pressure difference, the particle size of soil solid phase is larger. With the decrease of particle size, the contact between particles becomes closer, the effective thermal conductivity of random porous media increases, and the convection heat transfer coefficient decreases. During heat transfer, the resistance of thermal diffusion increases, the variation of temperature in soil tends to be gentle, and the average temperature drops at the same position of heat source, and the driving potential of both flow and heat transfer is higher than that of the reverse, and the average temperature in soil is higher when the driving potential of flow and heat transfer is the same as that in reverse. With the increase of porosity, the temperature gap also increases.
【学位授予单位】：大连理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：S152

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