Presidential Young Investigators Award: Convective Transport Complex Flows

流固两相流动中的热质传递包括许多工程学科，从颗粒材料的处理到使用填充床和流态化床的化学合成。尽管石化、流程和能源行业广泛使用了这些技术，但管理散装液-固传输率的模型主要基于经验数据。为了简化分析，流固体系通常被视为宏观上均质的；然而，在微观或孔隙水平上，介质由周围流体中单个颗粒的随机分布组成。孔隙水平的输运取决于床层结构、颗粒碰撞的频率、颗粒与流体之间的热量、质量和动量交换以及与边界表面的相互作用。尽管包含这些局部或微观影响的准确描述似乎难以解决，但目前的努力侧重于理解局部机制，目的是进一步发展具有重要工业意义的宏观关系。计划中的分析和实验研究将调查颗粒流动的整体和边界区域中的传输。在主体区域中，整体流动看起来是均匀的，但在局部，单个粒子进行横向旋转和平移。这种局部运动增加了横流的热量和质量扩散，并产生了更高的传输率。在边界表面附近，粒子不是均匀分布的，而是在不同的点处接触表面，导致粒子在近壁排列。这种分布取决于壁面条件、颗粒类型和流速，并影响速度分布、分子扩散系数和局部颗粒对流。与这项PYI资助一起进行的工作将强调流固传输，包括接触主导的颗粒流中的热量和质量传输。这一领域在能源、加工和食品工业中有许多工业应用，也结合了较简单的固定床问题和较复杂的移动颗粒流的各个方面。人们对颗粒流的这些方面知之甚少，但它们包含了控制热量、质量和动量传输的机制。这些领域的持续研究将推动颗粒状物质流动的建模，并可能导致其他流固研究领域，如流态化床和悬浮流。

Heat and mass transfer in fluid-solid flows encompasses many engineering disciplines from handling and processing of granular material to chemical synthesis using packed and fluidized beds. Though petrochemical, process, and energy industries have extensively used these technologies, models governing the bulk fluid-solid transport rates are based largely on empirical data. To simplify the analysis the fluid-solid systems are often viewed as macroscopically homogeneous; however, on the microscopic or pore level, the medium consists of a random distribution of individual particles in a surrounding fluid. The pore-level transport depends on the bed structure, frequency of the particle collisions, heat, mass and momentum exchange between the particles and fluid, and interactions with bounding surfaces. Though exact descriptions incorporating these local or microscopic effects appear inextricable, the current efforts focus on understanding the local mechanisms with the intent of furthering development of the industrially-important macroscopic relations. The planned analytical and experimental research will investigate transport in the bulk and in the boundary regions of the particle flows. In the bulk region, the overall flow appears uniform, but locally the individual particles rotate and translate laterally. This local motion increases the cross-stream heat and mass diffusion and yields higher transport rates. Near a bounding surface, the particles are not uniformly distributed but contact the surface at distinct points causing a near-wall alignment of particles. The distribution depends on the surface conditions of the wall, the type of particle, and the flow rate and affects the velocity profile, molecular diffusivity and local particle convection. The work performed with this PYI grant will emphasize fluid-solid transport involving heat and mass transfer in contact-dominated granular flows. This field has many industrial applications in energy, process, and food industries, and also combines aspects of the simpler fixed-bed problems with the more complex moving- particle flows. These aspects of granular flows are poorly understood and yet they comprise the mechanisms that govern the heat, mass and momentum transport. Continued research in these areas will advance the modeling of granular material flows and may lead to other fluid-solid research areas such as fluidized beds and suspension flows.