The Critical Heat Flux Condition in Micro-Channels

两相换热相对于单相换热具有明显的优势，但在微通道沸腾换热设计中必须考虑的一个至关重要的因素是临界热流密度（CHF）条件，它设定了微通道运行的热上限。关于微通道中CHF条件的文献很少，对于常规尺寸的通道，现有CHF相关对微通道的适用性尚不清楚；因此，需要更好的预测微通道CHF条件的能力。该项目的目标是更好地定量和定性地了解单个和多个平行微通道中的CHF条件，评估当前CHF相关性对微通道的适用性，开发一种技术来减少或消除多个微通道中的流动不稳定和流动不均匀。并建立考虑固体中传导的混杂效应的CHF条件的现象学模型。为了实现这些目标，进行了实验和分析调查。在实验研究中，在不同的通道直径、平行通道数量、流速、压力和功率水平范围内，对四面加热的圆形管和矩形通道进行了测试。测试部分配置包括具有不同管壁材料和壁厚的单圆管，加工成大型铜块的多个通道，以及在硅片上制造的微流体系统，所有设备都配有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。由于在多个平行通道中沸腾时，流动不稳定性会很明显，因此可以获得时变数据，并且通过增加上游流动限制来修改微流体和铜块系统中的通道几何形状，以减少或消除不稳定性；这些相同的通道修改用于减少或消除流动不均匀。为了补充定量测量，进行了高速、微观流动可视化研究。在分析研究中，将先前建立的池沸腾CHF模型扩展到微通道内的流动沸腾，建立了包含局部流固条件和动态接触角影响的现象学模型。由于墙体导热系数和热容量（密度比热积）的影响被纳入评估当地的墙体温度条件。用单管CHF数据对模型进行了验证。通过微流控和铜块系统的实验，清楚地阐明了单通道和多通道、并联通道中CHF条件的差异。采用在微流控系统中加入上游单个节流器和在铜块系统中加入单个可移动插片的新方案，研究了它们对提高并行通道运行CHF的影响。更广泛的影响一种更好的预测CHF的方法将使工程师能够追求目前由于热操作限制的不确定性而无法实现的想法，例如更高功率的电子设备、计算机、激光等。研究结果在国家会议上提出，在适当的期刊上发表，并在访问和研讨会期间与工业和国家实验室分享。为了提高这项研究的价值，与ASME罗切斯特传热分会一起举办了一个关于微通道传热的国际短期课程。此外，该项目通过博士学位授予机构（伦斯勒理工学院）与不授予博士学位的机构（罗切斯特理工学院）的合作，增加了教育机会。少数族裔研究生是从传统的黑人学院和大学招收的。由于早期接触研究已被证明是本科生继续攻读研究生学位的重要动力，因此确定有前途的本科生参与该项目。

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.