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Heat Transfer in Wall-Bounded Gas-Solids Flows

机译：有边界的气固两相流中的热传递

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摘要

Despite the remarkable advancements made by modern science, multi-scale problems still pose a significant challenge to the fields of engineering and mathematics. The transport phenomena occurring within a gas-solids flow is a quintessential multi-scale problem. Specifically, the physics occurring at the sub-particle scale is strongly coupled to the macroscopic behavior of many flowing particles. In addition to a fundamental significance, the mathematical description of a particle-laden flow is crucial to the efficient design and operation of many industrial systems. Products from multiphase unit operations encompass (but are not limited to): petroleum, pharmaceuticals, polymers, limestone, and energy. In recent years, solid particles have played a key role in the area of renewable energy. Namely, efficient designs for concentrated solar power plants (CSPs) have been proposed that utilize solid particles as the heat transfer fluid. Particle-based CSPs employ a near black body (NBB) receiver (i.e., granular heat exchanger) to transfer thermal energy from concentrated sunlight to a gas-solids mixture. Generally, concentrated sunlight is irradiated upon the NBB domain walls and the solid particles undergo heat transfer with the hot walls as they flow through the receiver. To assess the feasibility of these new CSP designs, the accurate prediction of wall-to-particle heat transfer is of primary significance. Due to the novelty of solid particle CSPs, the first steps in quantifying wall-to-particle heat transfer have been concerned with convective and conductive mechanisms only (no radiation). As a result, the present work will be concerned with the convective and conductive transport of thermal energy within a wall-bounded gas-solids flow.;The work here begins with an application-based study of the heat transfer within a NBB receiver. The discrete element method (DEM) is employed to simulate a granular heat exchanger whose domain walls are exposed to a constant total heat flux (irradiated sunlight). Current state-of-the-art methods for simulating wall-to-particle heat transfer were implemented within the DEM framework. Specifically, closures for the direct conduction mechanism occurring between a particle and wall in contact and indirect conduction mechanism occurring between a particle and wall separated by a thin layer of fluid were added to DEM. Furthermore, a new, total heat flux boundary condition was developed to properly describe the NBB geometry. Previous boundary conditions required that a heat flux be specified to each phase and leads to different wall temperatures for each phase. Physically speaking, the total heat flux at the NBB domain wall is what may be approximated (irradiated sunlight) while the partition of the total heat flux amongst the gas and solids phase will vary in space and time according to the local hydrodynamics. A numerical framework for implementing the latter interpretation for a constant, total heat flux boundary condition is derived and verified against DEM simulation of four granular heat exchanger designs.;Simulation of the NBB receiver showed that a majority of the heat transfer to the particles is due to the indirect conduction mechanism. A comparison of the thermal resistances associated with direct and indirect conduction is formulated and it is noted that indirect conduction will dominate for a wide variety of systems. In light of the significance of indirect conduction, the sensitivity of indirect conduction theory to its two theoretical inputs (fluid lens thickness and surface roughness) is assessed for dynamic, multi-particle systems. Analytical techniques commonly employed by kinetic theory are utilized to average indirect conduction theory and quantify a macroscopic heat transfer coefficient. Inputs to indirect conduction theory (fluid lens thickness and surface roughness) are perturbed individually to quantify their effect upon the macroscopic heat transfer coefficient. The analytical sensitivity analysis is found to agree with DEM simulations and shows that indirect conduction is most sensitive to the fluid lens thickness. However, no rigorous means have been established for setting the fluid lens thickness, and thus greater exploration is warranted to test the validity of indirect conduction theory.;In order to rigorously assess indirect conduction theory, the high fidelity direct numerical simulation (DNS) framework must be utilized. A hybrid lattice Boltzmann - random walk code was provided by Professor Xiaolong Yin at the Colorado School of Mines. However, challenges associated with inter-phase heat transfer needed to be corrected before the DNS code could be utilized to simulate heat transfer in gas-solids flows - e.g., discontinuity in the thermal diffusivity field due to the presence of fluid and particle. The work here details the modifications made to the DNS code and the case studies utilized to verify the new algorithm.
机译：尽管现代科学取得了令人瞩目的进步，但多尺度问题仍然对工程和数学领域构成了重大挑战。发生在气固两相流中的传输现象是一个典型的多尺度问题。具体而言，在亚粒子尺度上发生的物理学与许多流动粒子的宏观行为密切相关。除了基本意义外，载有粒子的流的数学描述对于许多工业系统的有效设计和操作也至关重要。多相单元操作产生的产品包括（但不限于）：石油，制药，聚合物，石灰石和能源。近年来，固体颗粒在可再生能源领域中发挥了关键作用。即，已经提出了利用固体颗粒作为传热流体的集中式太阳能发电厂（CSP）的有效设计。基于粒子的CSP使用近乎黑体（NBB）的接收器（即颗粒热交换器）将热能从集中的太阳光转移到气固混合物中。通常，聚集的阳光照射在NBB畴壁上，并且当固体颗粒流过接收器时，固体颗粒会与热壁进行热传递。为了评估这些新的CSP设计的可行性，准确预测壁与颗粒之间的传热至关重要。由于固体颗粒CSP的新颖性，量化壁到颗粒传热的第一步仅涉及对流和传导机制（无辐射）。因此，当前的工作将涉及壁垒式气固两相流中热能的对流和传导传输。此处的工作首先是基于应用的NBB接收器内传热研究。离散元方法（DEM）用于模拟粒状热交换器，其畴壁暴露于恒定的总热通量（照射的阳光）下。在DEM框架内实现了模拟壁到粒子传热的最新技术。具体地，将用于发生在接触的粒子和壁之间的直接传导机制的封闭物和发生在由流体薄层隔开的粒子和壁之间的间接传导机制的封闭物添加到DEM。此外，开发了一种新的总热通量边界条件，以正确描述NBB的几何形状。先前的边界条件要求为每个相指定热通量，并导致每个相的壁温不同。从物理上讲，NBB畴壁上的总热通量是近似值（照射的太阳光），而气相和固相中总热通量的分配将根据局部流体动力学在空间和时间上变化。得出了用于对恒定的总热通量边界条件进行后一种解释的数值框架，并通过对四种颗粒换热器设计的DEM模拟进行了验证。间接传导机制。给出了与直接和间接传导相关的热阻的比较，并注意到间接传导将在多种系统中占主导地位。鉴于间接传导的重要性，我们针对动态多粒子系统评估了间接传导理论对其两个理论输入（流体透镜厚度和表面粗糙度）的敏感性。动力学理论通常采用的分析技术被用于平均间接传导理论，并量化宏观的传热系数。间接传导理论（流体透镜的厚度和表面粗糙度）的输入会被单独扰动，以量化它们对宏观传热系数的影响。发现分析灵敏度分析与DEM模拟一致，表明间接传导对液镜厚度最敏感。但是，尚未建立用于设定流体晶状体厚度的严格方法，因此有必要进一步探索以检验间接传导理论的有效性。；为了严格评估间接传导理论，高保真直接数值模拟（DNS）框架必须加以利用。科罗拉多矿业学院的尹小龙教授提供了混合格子Boltzmann-随机步行码。但是，与相间传热相关的挑战需要纠正，然后才能使用DNS代码模拟气固流动中的传热-例如，由于流体和颗粒的存在而导致的热扩散率领域的不连续性。此处的工作详细介绍了对DNS代码进行的修改以及用于验证新算法的案例研究。

著录项

  • 作者

    Lattanzi, Aaron Michael.;

  • 作者单位

    University of Colorado at Boulder.;

  • 授予单位 University of Colorado at Boulder.;
  • 学科 Chemical engineering.;Fluid mechanics.;Applied mathematics.
  • 学位 Ph.D.
  • 年度 2018
  • 页码 222 p.
  • 总页数 222
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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