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Development of Closure for Heat Exchangers Based on Volume Averaging Theory.

机译：基于体积平均理论的换热器封闭装置的开发。

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摘要

The design of heat exchangers, which has always been important, has become increasingly critical in today's world and has also become closely coupled with optimization. The commonly used optimization methods, such as design of experiment (DOE), genetic algorithm (GA), etc., require a large population of design points/individuals to be evaluated. To shorten the optimization process, the evaluation of these large number of design points/individuals should be fast, which has precluded the use of time-consuming evaluation methods, such as direct numerical simulations (DNS) due to the significant computational costs of performing the flow and heat transfer in such heterogeneous (and porous), hierarchical devices with conjugate effects included, nor experimental evaluation that requires a large number of different heat exchangers be fabricated which is not only time-consuming but also expensive. This makes population-based optimization quite difficult if combined with system level CFD or experimental evaluation.;To enable these optimization methods, a fast-running solver is necessary to perform an evaluation of the many needed design points/individuals. Breakthroughs in the modeling of transport phenomena in heterogeneous media with Volume Averaging Theory (VAT) have allowed engineers to fully simulate flow and heat transfer in thermal devices in seconds, in comparison with hours or days it takes to do so with system level CFD simulation over one single design of a heat exchanger.;VAT is a hierarchical modeling method that includes mathematical description on two different levels. The lower level is flow and heat transfer at the pore scale for individual subscale elementary volume, which is described by the point-wise Navier-Stokes and thermal energy equations. The upper level is for the whole heat transfer device, described by the VAT-based mass, momentum, and thermal energy transport equations. The two levels are rigorously connected by mathematical scaling procedures, yielding additional integral and differential terms which need to be evaluated.;Proper evaluation of these extra terms is called the closure problem of the VAT based model, which has been the primary measure of advancement and for measuring success in research on transport in porous media. A method to obtain closures of the VAT based governing equations by CFD evaluation is developed. The procedure of the closure evaluation consists of eight main steps: 1) select the representative elementary volume correctly, which is simply the periodic unit cell in the case of periodic media; 2) define a proper characteristic length scale; 3) select a proper numerical method according to the flow condition; 4) discretize the computational domain carefully and conduct validation and verification of the adopted numerical method; 5) determine the number of REVs needed to obtain reasonable fully developed local values; 6) conduct numerical simulation over the selected REVs; 7) extract the macroscopic hydrodynamic and thermal characteristics from the microscopic results by evaluating the closures over the REV; 8) collect the evaluated results for friction factor and heat transfer coefficient and develop the corresponding correlations. Since the closure evaluation is conducted by carrying out direct numerical experiments at the pore scale of the representative elementary volume (REV) which usually has a quite small computational domain, therefore the computation load is much lower than simulation at the device scale. With the closure correlations, the upper-level governing equation set is relatively simple and allows a nonlocal description of transport phenomena in heterogeneous thermal devices, with the morphology directly incorporated into the field equations and conjugate effects fully treated. The simplicity of the VAT based governing equations makes it possible for them to be solved discretely and a rapid evaluation of the design points to be performed.;With a fast-running solver being developed, the population based optimization methods can be exploited to guide the design to its optimal configuration. To demonstrate how the VAT based solver can be combined with commonly used optimization methods, a fin-and-tube heat exchanger is optimized using genetic algorithm and a heat sink with scale-shaped surface roughness is optimized using design of experiment. It should be noted that the development of the optimization algorithms is not the focus of the present work. The algorithms adopted in the present study are quite basic ones, which were borrowed from other researchers.
机译：热交换器的设计一直很重要，在当今世界变得越来越重要，并且与优化紧密结合。常用的优化方法，例如实验设计（DOE），遗传算法（GA）等，需要评估大量设计点/个人。为了缩短优化过程，对这些大量设计点/个人的评估应快速进行，由于执行计算所需的大量计算成本，因此无法使用耗时的评估方法，例如直接数值模拟（DNS）。在这种具有共轭效应的异质（和多孔）分层装置中进行流动和传热，也不需要进行实验评估，因为实验评估要求制造大量不同的热交换器，这不仅耗时而且昂贵。如果与系统级CFD或实验评估相结合，这将使基于总体的优化变得非常困难。；要启用这些优化方法，必须使用快速运行的求解器来评估许多所需的设计点/个人。使用体积平均理论（VAT）在异质介质中传输现象建模方面的突破使工程师能够在几秒钟内完全模拟热设备中的流动和传热，而在整个系统级的CFD模拟中则需要花费数小时或数天的时间。热交换器的单一设计。增值税是一种分层建模方法，其中包括两个不同级别的数学描述。较低的级别是各个子级别基本体积在孔尺度下的流动和传热，这由点式Navier-Stokes和热能方程式描述。上层代表整个传热设备，由基于增值税的质量，动量和热能传输方程式描述。这两个级别通过数学换算程序严格连接，产生了需要评估的其他积分和微分项。对这些额外项的正确评估称为基于VAT的模型的关闭问题，这已成为改进和评估的主要方法。用于测量多孔介质运输研究的成功。开发了一种通过CFD评估获得基于VAT的控制方程式闭包的方法。封闭性评估的程序包括八个主要步骤：1）正确选择代表性的基本体积，在周期性介质的情况下，它只是周期性的晶胞； 2）定义适当的特征长度尺度； 3）根据流动情况选择合适的数值方法； 4）仔细离散计算域，并对采用的数值方法进行验证和验证； 5）确定获得合理的充分发展的当地价值所需的REV数量； 6）对所选REV进行数值模拟； 7）通过评估REV上的封闭，从微观结果中提取宏观的水动力和热学特征； 8）收集摩擦系数和传热系数的评估结果，并建立相应的相关性。由于封闭性评估是通过在代表性基本体积（REV）的孔尺度上进行直接数值实验来进行的，该孔通常具有很小的计算域，因此，计算量比在设备规模上的模拟要低得多。通过闭合相关性，上级控制方程组相对简单，并且可以对非均质热设备中的传输现象进行非局部描述，其形态直接纳入场方程中，并充分处理了共轭效应。基于VAT的控制方程式的简单性使其可以离散地求解，并且可以快速评估设计点。通过开发快速运行的求解器，可以利用基于总体的优化方法来指导求解。设计到最佳配置。为了演示如何将基于VAT的求解器与常用的优化方法结合使用，使用遗传算法对翅片管式换热器进行优化，并通过实验设计对具有鳞片状表面粗糙度的散热器进行优化。应当指出，优化算法的开发不是当前工作的重点。本研究采用的算法是非常基础的算法，是从其他研究人员那里借来的。

著录项

  • 作者

    Zhou, Feng.;

  • 作者单位

    University of California, Los Angeles.;

  • 授予单位 University of California, Los Angeles.;
  • 学科 Engineering Mechanical.
  • 学位 Ph.D.
  • 年度 2014
  • 页码 261 p.
  • 总页数 261
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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