基于格子-Boltzmann方法的固液相变界面特性研究

发布时间：2018-03-15 05:02

本文选题：糊状区　切入点：固液相变　出处：《山东建筑大学》2017年硕士论文　论文类型：学位论文

【摘要】：多孔介质内固液相变问题广泛存在于自然界和众多工业领域。固液相变材料作为一种被广泛应用的储能材料,在实际应用中往往不是单一组分的物质,相变发生时的温度并非在某一恒定值,而是在一个区间范围内,此时存在一个固相和液相共存的区域——糊状区。糊状区的存在及其特性对相变过程有着重要的影响。研究固液相变的流动和热质交换规律,探讨固液相变界面(糊状区)的发展、迁移及热质传递特性规律有着重要的意义。本文基于对糊状区的物理特性分析,提出一种描述糊状区流动特征的“多相流-多孔介质”两区域模型,即将糊状区中靠近固相表面的低液相率区域采用BrinkmannForchheimer-Darcy多孔介质渗流模型;靠近液相的高液相率区域采用多相流模型。为获得考虑多孔骨架条件下,骨架对固液相变的整场和局部流动换热特性以及对糊状区影响的规律,本文基于焓法分别建立了多孔骨架内固液相变的表征元(REV)尺度和孔隙尺度的数学模型。在数学模型的求解方面,采用格子-Boltzmann方法(LBM),通过选择适当的离散速度模型、平衡态分布函数及源项,分别建立了研究固液相变糊状区的REV尺度和孔隙尺度下的LBM模型。并通过一系列的数值实验验证了模型或方法应用的有效性及正确性。本文以方腔内固液相变过程为研究对象,首先模拟研究了无多孔骨架的固液相变过程,重点关注糊状区的特性,分析了无量纲参数(普朗特数Pr、瑞利数Ra,斯蒂芬数Ste)对糊状区发展、迁移及其内部的流动和传热的影响规律。研究结果表明:1)糊状区的厚度与相变温度半径T_R直接相关,T_R越小,糊状区厚度越薄;2)糊状区常数C及高、低液相率区的分界点trg的选取与相变材料的物性及糊状区内部构造有着密切的关系;3)相变材料的无量纲物性参数(Prandtl数Pr、Rayleigh数Ra、Stefan数Ste)对糊状区和相变过程有着明显的影响;低Pr数条件下,相变前期会产生多涡结构,会影响糊状区的形状和迁移;Ra数的大小影响着液相区和糊状区的自然对流强度,同时影响着糊状区的形状及内部的换热流动,Ra数的增大,融化速率随之增大,糊状区减薄,对应热壁面的换热能力增强;Ste影响融化速率的快慢,Ste越大,糊状区厚度增大,其下部区域的速度随之增大,在整个相变过程中系统的蓄热量随Ste数增大而减小。以上述为基础,本文进一步模拟分析了考虑多孔骨架时骨架对固液相变过程的影响。基于REV尺度分析了骨架对相变过程中糊状区及整场流动换热的影响,研究表明:1)骨架的达西数Da的增大或孔隙率e的减小,相应液相区和糊状区的流动效应越明显,糊状区的厚度越薄,其形状则越弯曲,孔隙率e增大一方面降低了系统的换热能力,另一方面相变材料体积的增加,可以增大系统的蓄热量;2)骨架的添加并不一定对相变过程起促进作用,其他条件相同时,骨架的导热能力越强,糊状区的边界弯曲度越小,准稳态阶段未融化的体积越大。基于孔隙尺度下研究了骨架对相变过程中流动换热的局部细节的影响规律,研究显示:1)在相变过程中,由于糊状区边界和骨架的影响,在局部会产生涡流;2)孔隙尺度下骨架孔隙率e对相变过程及糊状区的影响与REV尺度所得结论一致,且骨架的高导热性对系统整体的传热特性有一定的促进作用,随着骨架的导热性能的增强,但其促进效果逐渐减弱。
[Abstract]:Solid-liquid phase change in porous media widely exist in nature and many industrial fields. The solid-liquid phase change materials has been widely used as a storage material, in the practical application is often not a single component material, the temperature is not the phase change occurs at a constant value, but in a range, at this time: the mushy zone region of a solid phase and liquid phase coexistence. The existence and characteristics of the mushy zone has an important influence on the phase transition process. Research on solid-liquid phase change flow and heat exchange rules of solid-liquid interface (mushy zone) development, plays an important role in migration and the heat and mass transfer characteristics analysis. In this paper, based on the physical characteristics of the mushy zone, presents a description of flow characteristics of the mushy zone "of multiphase flow in porous media -" two region model, the mushy zone in low liquid fraction near the solid surface by Br InkmannForchheimer-Darcy porous medium seepage model; multiphase flow model with high liquid fraction near the liquid phase. For considering the porous skeleton under the condition of the whole skeleton on solid-liquid phase change and the local flow and heat transfer characteristics and the influence on the law of the mushy zone, based on the enthalpy method were established for characterization of solid-liquid phase change element within the porous framework (REV) mathematical model of scale and at the pore scale. In the mathematical model, the lattice -Boltzmann method (LBM), by selecting the appropriate discrete velocity model, the equilibrium distribution function and the source term, we established the LBM model of the REV scale and the pore scale study of solid-liquid phase change of mushy zone under the effective. And the correctness of the application model or method is verified through a series of numerical experiments. The solid-liquid phase change process in cavity as the research object, studies the non porous solid simulation first The liquid phase change process, focus on the characteristics of the mushy zone, analysis of the dimensionless parameters (Prandtl number Pr, Rayleigh number Ra, the Stephen number Ste) on the development of mushy zone, influence of flow and heat transfer and internal. The results show that: 1) the thickness and phase transition temperature of T_R is directly related to the radius of the mushy zone T_R, the smaller the mushy zone thickness; 2) mushy zone constant C and high and low liquid fraction zone selection TRG cut-off point of phase change materials in the mushy zone and the internal structure are closely related; 3) the dimensionless physical parameters of phase change materials (Prandtl Pr number, Rayleigh number Ra, Stefan Ste) has a significant impact on the mushy zone and the phase change process; low Pr conditions, early phase will produce eddy structure, will affect the shape and migration of the mushy zone; the number of Ra affects the intensity of natural convection in liquid region and mushy zone, also affect the mushy zone the shape and the The internal flow and heat transfer, the increase of Ra number, the melting rate increases, mushy zone thickness, corresponding heat wall heat capacity enhancement; Ste effect of the melting speed of the Ste, the greater the mushy zone thickness increases, the lower area of speed increases, heat storage system in the whole process with phase transformation the Ste number increases. On the basis, this paper further analyze the influence of skeleton on solid-liquid phase change process with porous skeleton. REV scale analysis of the impact of the skeleton of the mushy zone and the transformation process of flow and heat transfer based on the research shows: 1) the skeleton of the Darcy number Da increases or porosity e the effect of the corresponding flow decreases, liquid phase region and the mushy zone is more obvious, the mushy zone thickness is thin, its shape is bent, the porosity of e increased while reducing the heat transfer capacity of the system, the phase change material volume increased on the other hand, can increase Large quantity of heat storage system; 2) framework does not necessarily add on the phase change process and play an important role in other conditions, thermal conductivity of skeleton is stronger, the mushy zone boundary curvature smaller, quasi steady stage not melt the larger volume. Based on the pore scale studied, skeleton of details heat flow during the phase change process of the research shows: 1) in the transformation process in the mushy zone due to the influence of edge and skeleton, eddy current generated in the local council; 2) under the influence of pore size porosity of e phase transformation process and mushy zone and REV scale consistent, high thermal conductivity and the skeleton are the effect on the heat transfer characteristics of the whole system, with the enhancement of thermal conductivity of the skeleton, but its effect is gradually weakened.

【学位授予单位】：山东建筑大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TB34

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