Collaborative Research: Supercritical Fluids and Heat Transfer - Delineation of Anomalous Region, Ultra-long Distance Gas Transport without Recompression, and Thermal Management

超临界（SC）条件下的流体，即不再存在明显的液相和气相，存在于自然界和技术系统中，这些系统可以利用在临界温度和压力附近发生的极端变化。SC流体的传热增加、流体摩擦减少和高溶解度在天然气管道输送、作为碳捕获和储存过程一部分的二氧化碳输送、热电厂和核电站的工作流体、太阳能和地热能转换系统以及电子设备和数据中心的增强冷却方面具有当前和潜在的应用。尽管SC流体的独特性质具有优势，但由于对异常行为的理解不足，其广泛应用受到限制，异常行为的特征是临界区域热物理性质的大规模变化，导致热和流动振荡以及其他有害现象。此外，特定于每种物质的临界温度和压力范围可能不适合特定的技术需求。这项研究将解决一组重要的SC流体的异常行为知识空白，这将为高容量、节能、环保的流体流动和热管理系统的新技术打开大门。例如，SC条件下的天然气（SNG）可以通过陆上、地下和海底管道运输，运输距离超过2000公里。煤制天然气运输节能，可以减少途中再压缩站的数量或完全取消它们，从而实现目前不可能实现的新的跨洋路线，既符合美国的国家利益，也为世界其他地方提供能源安全。与液化天然气（LNG）相比，SNG运输成本更低，对环境的影响更小，而且更安全。在研究天然气运输过程中产生的知识将有助于确定二氧化碳从海岸线运输到海底进行封存时的热力学状态。同样，开发运输SC氧、氮和其他重要工业/医疗气体的方法也将受益于这项工作。这项研究的更广泛的影响包括SC传输现象的教育机会，以及利用广泛的当前和潜在的SC技术来激发对热力学的兴趣，向代表性不足的群体进行宣传。先前的研究表明，临界点附近的异常流体输运行为始于三相点以上的亚临界状态，并继续深入到SC状态。在这个研究项目中，将建立一个基于吉布斯自由能的热力学模型来描述异常态的温度-压力边界和表征高阶相变。它将适用于一系列自然/工业上重要的SC流体，包括水、二氧化碳、甲烷、氩气和氮气。该分析将有助于确定超临界流体（包括超临界天然气（SNG））在不进行再压缩的情况下长距离运输的安全条件。通过开发SC热输运的一维计算模型，考虑输运特性变化和可压缩性、环境热条件、焦耳-汤姆逊现象和管道的热阻，可以量化SNG输运的全部潜力。将开发一个三维模型来检查入口和出口区域的流动和热行为以及周围环境温度变化的影响。热力学模型还将用于设计定制的SC流体混合物，以实现有效的热管理。建议的研究包括计划制造一个SC流体流动和热分析的实验装置，以产生与SC流体特性相关的数据，检查模型验证的参数效应，并探索增强传热的方法。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Fluids under supercritical (SC) conditions, where distinct liquid and gas phases no longer exist, are found in nature and technological systems that can take advantage of the extreme changes that take place near the critical temperature and pressure. The increases in heat transfer, reductions in fluid friction, and high solubility of SC fluids have current and potential applications in pipeline transport of natural gas, delivery of carbon dioxide as part of carbon capture and storage processes, working fluids for thermal and nuclear power plants, solar and geothermal energy conversion systems, and enhanced cooling of electronic devices and data centers. Despite the advantages offered by the unique properties of SC fluids, their wide-spread use has been curtailed because of inadequate understanding of anomalous behaviors, characterized by large-scale variations in thermophysical properties in the critical region, resulting in thermal and flow oscillations and other detrimental phenomena. Additionally, the critical temperature and pressure range, specific to each substance, may not fit a particular technological need. This research will address the anomalous behavior knowledge gaps of an important set of SC fluids, which will open the door to new technologies for high-capacity, energy-efficient, and environmentally responsible fluid flow and thermal management systems. For example, natural gas under SC conditions (SNG) can be transported via overland, underground, and undersea pipelines for distances greater than 2,000 km. SNG transport is power-efficient and can reduce the number of enroute recompression stations or eliminate them altogether, enabling new trans-oceanic routes that are currently impossible, serving the US national interest as well as providing energy security elsewhere in the world. In comparison with liquified natural gas (LNG), SNG transport can be less expensive, have reduced environmental impact, and be more secure and safe. Knowledge generated in the study of SNG transport will be useful in determining the thermodynamic states of carbon dioxide when it is transported from shorelines to the ocean floor for sequestration. Likewise, development of methods to transport SC oxygen, nitrogen, and other important industrial/medical gases will benefit from this work. The broader impacts of this research include educational opportunities in SC transport phenomena and outreach to underrepresented groups using the wide range of current and potential SC technologies to motivate interest in thermodynamics.Previous research has shown that anomalous fluid transport behavior near the critical point starts in the subcritical state above the triple point and continues deep into the SC state. In this research program, a thermodynamic model based on Gibbs free energy will be developed to delineate the temperature-pressure boundaries of the anomalous states and characterize the higher-order phase transitions. It will be applied to a set of naturally/industrially important SC fluids including water, carbon dioxide, methane, argon, and nitrogen. This analysis will lead to the identification of safe conditions at which SC fluids, including supercritical natural gas (SNG), can be transported over long distances without recompression. The full potential for SNG transport will be quantified by developing a one-dimensional computational model for SC thermal transport, accounting for transport property variations and compressibility, environmental thermal conditions, Joule-Thomson phenomena, and thermal resistance of the pipeline. A three-dimensional model will be developed to examine the flow and thermal behavior in the inlet and outlet regions as well as the impact of temperature variations in the surrounding environment. The thermodynamic models also will be employed to design customized mixtures of SC fluids for effective thermal management. The proposed research includes plans to fabricate an experimental apparatus for SC fluid flow and thermal analysis to generate data relevant to SC fluid properties, examine parametric effects for model validation, and explore methods to enhance heat transfer.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.