The Critical Heat Flux Condition in Micro-Channels

两相传热比单相传热具有显著的优势，但临界热流密度（CHF）条件是微通道沸腾传热设计中必须考虑的一个关键因素，它决定了微通道运行的热上限。 关于微通道中的CHF条件的文献相当稀少，并且对于常规尺寸的通道的现有CHF相关性对微通道的适用性是未知的;因此需要更好的预测微通道的CHF条件的能力。 该项目的目标是对单个和多个平行微通道中的CHF条件进行更好的定量和定性理解，以评估当前CHF相关性对微通道的适用性，开发一种技术以最小化或消除多个微通道中的流动不稳定性和流动分布不均，并开发一个现象学模型的CHF条件，考虑到在固体中的传导的混淆效果。 为了实现这些目标，进行实验和分析研究。 在实验研究中，圆管和矩形通道加热的所有方面进行了测试，在一系列的通道直径，平行的通道，流量，压力和功率水平的数量。 测试部分配置包括具有不同管壁材料和壁厚的单个圆管，加工成大型铜块的多个通道，以及在硅晶片上制造的微流体系统，所有这些都配备了快速响应的温度和压力传感器阵列。 使用两种流体，制冷剂R123和水。 由于流动不稳定性可能是显着的沸腾在多个平行通道，随时间变化的数据被获得和通道的几何形状被修改的微流体和铜块系统通过添加上游流动限制，以减少或消除不稳定性;这些相同的通道修改用于最小化或消除流量分布不均。 高速，微观流动可视化研究进行补充定量测量。 在分析研究中，一个唯象模型，包括局部流体和固体条件和动态接触角的影响，通过扩展先前开发的CHF模型池沸腾流动沸腾在微通道。 由于壁导热系数和热容量（密度比热产品）的影响，同时评估局部壁温条件。 该模型进行了测试，对单管CHF数据。 CHF条件在单个和多个，平行通道之间的差异，清楚地阐明通过实验中的微流体和铜块系统。 将上游的个人流量限制器的微流控系统和个人可拆卸的插入物中的铜块系统的新方案被用来研究它们对改善CHF在并行通道操作的效果。更广泛的影响一个更好的方法来预测CHF将允许工程师追求的想法，现在不能实施，因为在热操作限制，如高功率电子，计算机，激光等的不确定性的研究结果是在国家会议上提出，发表在适当的期刊，并在访问和研讨会期间与行业和国家实验室共享。 为了提高这项研究的价值，国际短期课程的微通道传热结合ASME罗切斯特传热章。 此外，该项目还通过一个授予博士学位的机构（伦斯勒理工学院）与一个不授予博士学位的机构（罗切斯特理工学院）建立伙伴关系，增加了教育机会。 少数民族研究生是从历史悠久的黑人学院和大学招收的。 由于早期接触研究已被证明是一个显着的刺激本科生继续研究生学位，有前途的本科生被确定参加该项目。

Two-phase heat transfer has significant advantages over single-phase heat transfer, but a crucially important factor that must be taken into account in the design of micro-channel boiling heat transfer is the critical heat flux (CHF) condition, which sets the upper thermal limit on the micro-channel operation. Literature on the CHF condition in micro-channels is quite sparse and the applicability to micro-channels of existing CHF correlations for conventionally sized channels is unknown; therefore better capability to predict the CHF condition for micro-channels is needed. The objectives of this project are to develop a better quantitative and qualitative understanding of the CHF condition in single and multiple, parallel micro-channels, to assess the applicability of current CHF correlations to micro-channels, to develop a technique to minimize or eliminate flow instabilities and flow maldistribution in multiple micro-channels, and to develop a phenomenological model of the CHF condition that takes into account the confounding effects of conduction in the solid. To accomplish these objectives, both experimental and analytical investigations are performed. In the experimental study, circular tubes and rectangular channels heated on all sides are tested over a range of channel diameters, number of channels in parallel, flow rates, pressures, and power levels. The test section configurations include single circular tubes with varying tube-wall materials and wall thicknesses, multiple channels machined into large copper blocks, and microfluidic systems fabricated on silicon wafers, all instrumented with fast-responding arrays of temperature and pressure sensors. Two fluids, refrigerant R123 and water, are used. Because flow instabilities can be significant with boiling in multiple parallel channels, time-varying data are obtained and the channel geometry is modified in both on the microfluidic and copper block systems by the addition of upstream flow restrictions to reduce or eliminate instabilities; these same channel modifications are used to minimize or eliminate flow maldistribution. High-speed, microscopic flow-visualization studies are undertaken to complement the quantitative measurements. In the analytical study, a phenomenological model that includes the effects of local fluid and solid conditions and dynamic contact angles is produced by extending the previously developed CHF model for pool boiling to flow boiling in micro-channels. Effects due to wall thermal conductivity and thermal capacity (density-specific heat product) are incorporated while evaluating local wall temperature conditions. The model is tested against the single-tube CHF data. Differences between the CHF condition in single and multiple, parallel channels are clearly elucidated through experiments in the microfluidic and copper-block systems. Novel schemes of incorporating upstream individual flow restrictors in the microfluidic system and individual removable inserts in the copper-block system are used to study their effects on improving CHF in parallel-channel operation. Broader impactA better method for prediction of CHF will permit engineers to pursue ideas that cannot be implemented now because of uncertainties in thermal operating limits, such as higher-power electronics, computers, lasers, etc. The research findings are presented at national conferences, published in appropriate journals, and shared with industry and national laboratories during visits and seminars. To enhance the value of this research, an international short course on micro-channel heat transfer is held in conjunction with the ASME Rochester Heat Transfer Chapter. Also, the project enhances educational opportunities through the partnering of a PhD-granting institution (Rensselaer Polytechnic Institute) with one that does not grant PhD degrees (Rochester Institute of Technology). Minority graduate students are recruited from historically black colleges and universities. Because early exposure to research has been proven to be a significant spur for undergraduates to continue for a graduate degree, promising undergraduates are identified to participate in the project.