基于LBM的传输过程模拟及在多孔介质中的应用研究

发布时间：2019-06-21 11:16

【摘要】：格子Boltzmann方法(LBM)作为一种介观方法,凭借天然并行性、边界条件处理简单、程序易于实施等自身独特的优势,广泛应用于微尺度流动和传热、多孔介质、生物流体、磁流体、纳米流体、多相流、燃料电池等众多领域,为多种复杂现象的机理研究提供了新的研究思路。但是目前在层流受限冲击射流、随机多孔介质的流动和换热应用上,LBM存在不足和空白之处,有待于进一步的完善和拓展。因此,本文采用LBM对上述两方面做了有益的尝试,为深刻认识这些问题提供了新的启示,同时为后续研究工作打下了良好的基础。概括来说,本文的工作主要包括:(1)基于LBM方法模拟计算了正形和菱形排布的四圆柱绕流问题,揭示了Re数和间距比L/D对流动形态、阻力系数、升力系数和Nu数的影响规律。通过对四圆柱绕流的流动特性和受力特性的数值模拟研究,结果表明:不同Re数下流动模式发生转变时对应的临界L/D不同;流动模式发生转变时,平均阻力系数、平均升力系数发生突变,这些结论与已知的研究结果一致。基于此,文中对目前报道较少的四圆柱绕流换热问题进行了深入探讨,结果表明:圆柱表面Nu数随Re数增大快速增大;随着L/D的增大流动模式发生改变时,Nu数大幅度增加;对于同一流动模式,Nu数随着L/D的增加几乎不变。这部分研究成果为随机多孔介质流动和换热问题的研究提供了基础。(2)对冲击射流的研究,本文模拟计算了平面和曲面冲击射流,揭示了Re数、无量纲冲击距离H/W对层流受限冲击射流流动特性、温度分布特性的影响规律。不同于传统冲击射流模拟结果对选取的湍流模型具有巨大依赖性,本文采用LBM这种新兴的数值模拟方法,避免了经验参数的确定。数值仿真结果表明:随着Re数增大,流动从稳定结构转变为非稳定结构,H/W越大,流动结构发生转变对应的临界Re数越小。滞止点处Nu数随Re数增加而增加,随H/W增加而减小。在这两个参数中,Re数对换热特性有着更显著的影响。对射流冲击曲面换热问题的研究结果表明:圆柱表面Nu数随角度0变化较大,当θ从0°增加到90°时,Nu数逐渐减小,并且在90°达到最小值;随着0从90°增加到135°,Nu数逐渐增大。这部分工作验证了LBM能够对强旋流动进行准确地数值模拟。(3)对于随机多孔介质,重点研究了多孔结构随机性、粒径分布和颗粒形状对流动和换热特性的影响。(a)通过查阅大量文献发现,不同学者提出的多孔介质渗透率与孔隙率关系式存在着巨大的差异,充分证明了多孔介质复杂的内部结构的影响。本文采用基于过程的重构技术生成随机多孔介质,对孔隙内流场进行了深入的探讨。结果表明,构成多孔介质的颗粒粒径和数量不变,随机生成的不同多孔结构的无量纲渗透率K相差很大,内部多孔结构对流动有着不可忽视的影响。对于二维结构,提出了适用于直径符合高斯分布的圆柱形成的多孔介质的渗透率K计算式。在此基础,上,给出了用于Forchheimer方程的一个新的b'Den计算式。对于三维结构,重点考察了粒径分布规律和颗粒形状的影响。当不同粒径颗粒种类不大于5时,渗透率随颗粒种类增加而增大。球形颗粒的无量纲渗透率大于立方体颗粒。同时,本文将立方体的研究结果拓展到长方体长度(a)小于高度(b)的情况,随着a/b增大,渗透率K减小。(b)通过查阅大量文献发现,不同形式多孔介质换热系数关联式相差很大,甚至存在一些矛盾的结论。基于此,本文针对随机多孔介质内部的换热特性展开了研究。结果表明,构成多孔介质的颗粒粒径和数量不变,随机生成的不同多孔结构的Nu数相差很大,内部多孔结构对换热有着不可忽视的影响。对于二维结构,给出了考虑孔隙率影响的Nu数表达式。对于三维结构,重点考察了粒径分布规律和颗粒形状的影响。当不同粒径颗粒种类不大于5时,Nu数随颗粒种类增加而减小。对于形状相同粒径分布不同的颗粒,等直径颗粒组成的多孔介质孔隙率对Nu数的影响作用最显著;对于粒径种类相同形状不同的颗粒,立方体颗粒组成的多孔介质孔隙率对Nu数的影响作用远大于球形颗粒。总之,本文的研究工作对深入认识微尺度冲击射流、随机多孔介质的流动和换热特性具有指导意义。这些研究不但具有重要的工程应用价值,而且拓展了LBM在这些领域的研究范围。
[Abstract]:The lattice Boltzmann method (LBM), as a mesoscopic method, is widely used in micro-scale flow and heat transfer, porous media, biological fluid, magnetic fluid, nano-fluid, multi-phase flow, In many fields, such as fuel cell, this paper provides a new approach to the mechanism of many complex phenomena. However, in the application of the laminar flow limited impact jet, the flow of the random porous medium and the heat transfer application, the LBM has insufficient and empty space, which is to be further improved and expanded. Therefore, this paper makes a useful attempt to the above-mentioned two aspects by using the LBM, and provides a new inspiration for the deep understanding of these problems, and lays a good foundation for the follow-up research work. In general, the work of this paper mainly includes: (1) the problem of four-cylinder flow around the regular and diamond-shaped arrangement is simulated based on the LBM method, and the influence of the number of Re and the spacing ratio L/ D on the flow pattern, the drag coefficient, the lift coefficient and the Nu number is revealed. The numerical simulation of the flow characteristics and the force characteristics of the four-cylinder flow around the cylinder shows that the critical L/ D is different when the flow pattern is changed under different Re number, and the average drag coefficient and the average lift coefficient change when the flow pattern is changed. These conclusions are consistent with the known study results. The results show that the number of Nu in the cylindrical surface increases rapidly with the increase of the number of Re, and the number of Nu increases with the increase of L/ D, and for the same flow pattern, The number of Nu increases with the increase of L/ D. The results of this study provide the basis for the study of the flow and heat transfer of the porous media. (2) In order to study the impact jet, this paper simulates the impact jet of plane and surface, and reveals the influence of the Reynolds number, the dimensionless impact distance H/ W on the flow characteristics and the temperature distribution characteristics of the laminar flow limited impact jet. In this paper, a new numerical simulation method of LBM is used to avoid the determination of the empirical parameters. The numerical simulation results show that, with the increase of the number of Re, the transition from the stable structure to the non-stable structure, the larger the H/ W, the smaller the critical Re number corresponding to the transition of the flow structure. The number of Nu at the stagnation point increases with the increase of the number of Re, and decreases with the increase of H/ W. In these two parameters, the Re number has a more significant effect on the heat transfer characteristics. The results of the study on the heat transfer of the jet impingement surface show that the number of Nu of the cylindrical surface increases with the angle of 0, and the number of Nu decreases gradually when the angle increases from 0 掳 to 90 掳, and the minimum value is reached at 90 掳, and the number of Nu increases with the increase of 0 from 90 掳 to 135 掳. This part of the work verifies that the LBM can accurately simulate the strong spin flow. (3) For the random porous media, the influence of the randomness of the porous structure, the particle size distribution and the particle shape on the flow and heat transfer characteristics is studied. (a) It is found that the relationship between the permeability of the porous media and the porosity of the porous media has a great difference, and the influence of the complex internal structure of the porous media is well demonstrated. In this paper, a process-based reconstruction technique is used to generate a random porous medium, and the flow field in the pore is discussed in-depth. The results show that the particle size and quantity of the porous media are the same, the dimensionless permeability K of the randomly generated porous structure is very different, and the internal porous structure has a non-negligible effect on the flow. For the two-dimensional structure, a formula for permeability K of a porous medium, which is suitable for cylindrical formation with a Gaussian distribution, is proposed. On this basis, a new b 'Den calculation formula for the Forschheimer equation is given. The effect of particle size distribution and particle shape on the three-dimensional structure is studied. When the particle size of the different particle sizes is not more than 5, the permeability increases with the increase of the particle type. The non-dimensional permeability of the spherical particles is greater than the cubic particles. At the same time, the research result of the cube is extended to the case where the length (a) of the cuboid is less than the height (b), and the permeability K decreases as a/ b increases. (b) According to a large number of literature, the correlation formula of the heat transfer coefficient of different forms of porous media is different, and even some contradictory conclusions are present. Based on this, this paper studies the heat transfer characteristics of the interior of the porous media. The results show that the size and quantity of the particles that make up the porous medium are the same, and the number of the Nu of the randomly generated porous structure is very different, and the internal porous structure has a non-negligible effect on the heat transfer. For the two-dimensional structure, the Nu number expression considering the influence of porosity is given. The effect of particle size distribution and particle shape on the three-dimensional structure is studied. When the particle size of the different particle size is not more than 5, the number of Nu decreases with the increase of the particle size. The effect of the porosity on the number of Nu is most significant for the porous media with the same particle size distribution, and the effect of the porosity on the number of the Nu is much larger than that of the spherical particles for the particles with the same particle size. In conclusion, the research work in this paper is of great significance to the in-depth understanding of the micro-scale impact jet and the flow and heat transfer characteristics of the random porous media. The research not only has important engineering application value, but also extends the research scope of the LBM in these fields.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TK124

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