A Fundamental Study on the Effect of Surface Roughness Structures on Fluid Flow and Heat Transfer at the Microscale Level

提案标题：微观尺度下表面粗糙结构对流体流动和换热影响的基础研究主要研究员：Kandlikar，Satih G.机构：罗彻斯特理工学院提案编号：CBET-0829038摘要本项目旨在改变我们对微观和宏观尺度不可压缩层流中粗糙度的传统理解，并为控制具有设计粗糙度结构的微尺度器件中的传输过程提供新的见解。这项工作包括建立一个理论模型，该模型通过数值模拟以及压降和换热实验进行验证，其中的结构粗糙度包含在10 mm宽、间隙从0.2 mm到2 mm的矩形通道的壁上。测试的粗糙度高度覆盖了0-30%的相对粗糙度范围(定义为粗糙度高度与渠道水力直径之比)。我们对粗糙度的传统理解主要源于几十年前尼库拉泽和穆迪等研究人员在大口径管道研究的基础上所做的工作。在相对粗糙度小于5%的范围内，流体流动和压降不受壁面粗糙度的影响。这种差异在很大程度上是由尼库拉泽？S实验中的巨大不确定性造成的。近五年来在严格控制的条件下进行的微尺度流体流动实验表明，层流流动确实受到壁面粗糙度的影响。关于粗糙度对换热的影响，文献中提供的数据很少，对粗糙度对这一重要传输现象的影响缺乏很好的了解。由于水力直径较小，微通道中经常会遇到层流和过渡流，这一点尤为重要。目前，人们还不能很好地了解这种渠道中粗糙度引起的流动结构的变化和不稳定性。以前由其他人和RIT进行的研究表明，随着相对粗糙度的增加，压降和换热增加。粗糙度的存在导致表面积与体积比的增加，这在微尺度上已经大于常规尺寸的沟槽。低相对粗糙度的影响以前已经在RIT使用润滑理论进行了建模。对于给定的粗糙度剖面，该模型可以预测由此对压降的影响。为了准确地模拟较大的相对粗糙度的影响，需要对壁面进行进一步的修改。前人的工作表明，粗糙微通道中存在较早的从层流到湍流的转变，这些影响也将被用来研究传热特性。理论工作将与RIT热分析和微流体实验室的数值模拟和先进的实验技术(使用水)相补充。这项工作将提供不同类型粗糙表面(均匀粗糙度、交叉线和均匀间隔肋条)的基本表征，并基本理解表面粗糙度如何影响层流和过渡区中微通道和微通道中的流体流动、换热和层流/湍流转变。计划开展的具有可控粗糙度单元的低雷诺数流动的研究将为微尺度流体流动装置提供基本的设计工具。例如，在电子冷却应用中，虽然微通道中的换热系数很高，但需要更高的值，水和气流的压降损失更小(航空航天领域对空气冷却的新重视)。特别设计的粗糙度元件为提高性能提供了有效的方法。这样的？设计师？表面还将在其他应用中加强各自的传输过程，例如生物和化学分析系统、微全系统分析、微推进系统、空间探索中的机载微尺度设备、质子交换膜燃料电池的冷却通道和微型混合器。作为这项工作的结果，将从各种参数的精心控制的微型实验中获得一组详尽的数据。在教育方面，这项工作将为这一新兴领域的本科生提供令人兴奋的研究机会。它将通过与微系统工程项目中的PI合作的博士生来巩固RIT的微流体研究基础设施。这项工作的结果将在多学科会议上介绍，以交流思想。该项目将出现在E3博览会上，这是自1991年以来由PI为中学生举办的一年一度的活动。国际学生联合会将继续其对多元化的坚定承诺，今年迄今已雇用了两名少数族裔学生和五名女学生(一名听力残疾学生)。

Proposal Title: A Fundamental Study on the Effect of Surface Roughness Structures on Fluid Flow and Heat Transfer at the Microscale LevelPrincipal Investigator: Kandlikar, Satish G.Institution: Rochester Institute of TechProposal No: CBET-0829038AbstractThis project aims at transforming our conventional understanding of roughness in incompressible laminar flow at both microscale and macroscale, and providing new insight in controlling transport processes in microscale devices with designer roughness structures. The work includes developing a theoretical model that is validated through numerical simulation as well as pressure drop and heat transfer experiments with structured roughness incorporated on the walls of rectangular channels that are 10-mm wide with gaps ranging from 0.2 mm to 2 mm. The roughness heights tested cover a relative roughness range (defined as the roughness height to the channel hydraulic diameter) from 0 to 30 percent.Our conventional understanding of roughness was derived mainly from the work of researchers such as Nikuradse and Moody several decades ago on the basis of their studies on large diameter tubes. Fluid flow and pressure drop were believed to be unaffected by the wall surface roughness in the relative roughness range below 5 percent. The large uncertainties in Nikuradse?s experiments are largely responsible for this discrepancy. Microscale fluid flow experiments conducted in the last five years under carefully controlled conditions showed that laminar flow is indeed affected by the wall roughness. Regarding the roughness effects on heat transfer, there is very little data available in the literature and a good understanding is lacking on the roughness effects on this important transport phenomenon.Due to the small hydraulic diameter, laminar and transition flows are often encountered in microchannels and are of particular importance. The changes in the flow structure and instabilities caused by roughness in such channels are not well understood at the present time. Previous studies performed by others as well as at RIT indicate an increase in pressure drop and heat transfer with an increase in relative roughness. Presence of roughness leads to an increase in the surface area to volume ratio, which is already larger at the microscale than in conventional-sized channels. The effects of low relative roughness have been previously modeled at RIT using lubrication theory. For a given roughness profile, this model can predict the resulting effect on pressure drop. Further wall modifications will be necessary in order to accurately model the effects of larger relative roughness. Previous work indicates an earlier transition from laminar to turbulent flow in rough microchannels, and these effects also will be investigated for heat transfer characteristics. The theoretical work will be complemented with numerical simulations and advanced experimental techniques (using water) in the Thermal Analysis and Microfluidics Laboratory at RIT.This work will provide basic characterization of different types of rough surfaces (uniform roughness, cross-hatched, and uniformly spaced ribs), and a fundamental understanding of how surface roughness impacts the fluid flow, heat transfer and laminar/turbulent transition in microchannels and minichannels in the laminar and transition regions. The planned work on low Reynolds number flows with controlled roughness elements will provide a fundamental design tool for microscale fluid flow devices. For example, in electronics cooling applications, although the heat transfer coefficients in microchannels are high, even higher values are desired with lower pressure drop penalties for water as well as air flow (new emphasis on air-cooling in aerospace field). Specially designed roughness elements offer an effective way to improve the performance. Such ?designer? surfaces will also enhance respective transport processes in other applications, e.g. biological and chemical assay systems, micro-total system analysis, micro propulsion systems, on-board microscale devices in space exploration, cooling passages for PEM fuel cells, and micromixers. As a result of this work, an exhaustive set of data will be available from carefully controlled microscale experiments over a wide range of parameters. On the educational front, the work will provide exciting undergraduate research opportunities in this emerging field. It will consolidate microfluidics research infrastructure at RIT through Ph.D. students working with the PI in the Microsystems Engineering program. The results of this work will be presented in multidisciplinary meetings for cross-fertilization of ideas. The project will be featured at the E3 Fair, an annual event heralded by the PI for middle school students since 1991. The PI will continue his strong commitment to diversity, with employment of two minority and five female students (one with hearing disability) so far this year.