Turbulent-convection experiments at extreme conditions using cryogenic nitrogen

自然对流发生在流体被加热并开始上升的时候。提出的研究的重点是调查自然对流在极端条件下的参数，这是重要的过程。在几个重要的自然和工程流程中都有应用。大规模的自然系统，如地球的大气层——它的海洋和地幔——以及恒星（如太阳）的内部，都在不同程度上受到自然对流的影响。用来理解这种普遍存在的热传递机制的简化物理模型是瑞利-巴姆纳德（RB）对流，这是一种由具有绝热侧壁的实验单元的顶板和底板之间的温差驱动的流体流动。尽管这门学科历史悠久，近年来在理论、数值和实验领域取得了进展，但仍有许多问题尚未解决。一个基本问题涉及高湍流对流中的传热标度，特别是关于过渡到增强传热的渐近状态。其他问题与这些极端流动参数下的流动结构有关，特别是与不规则多边形结构的存在有关，这种结构类似于在对流开始上方观察到的那些结构。该项目旨在通过在乔治亚理工学院的一个设施中使用接近其共存曲线的液态或气态氮，从1atm的77K到其临界点（约126K和34 bar/500psi），探索极端参数下的RB对流。提出的研究将在不存在的条件下提供流动结构的可视化。在这些极端条件下，现有的实验结果似乎是相互矛盾的，而拟议的工作有望解决这个问题。除了研究生和本科生的教育活动外，还计划继续与Guerilla Science合作，这是一家位于纽约和伦敦的专业外展组织，pi与该组织合作开发并运行流体动力学启发的竞赛。其目标是吸引年轻学生学习工程学。

PI: Ranjan, DeveshProposal Number: 1604468Natural convection occurs when fluid is heated and starts to rise. The focus of the proposed research is to investigate natural convection at extreme conditions in terms of the parameters that are important to the process. Applications are found in several important natural and engineering flows. Large scale natural systems such as the Earth's atmosphere - its oceans as well as the Earth's mantle - and the interior of stars such as the Sun, are all affected to various degrees by natural convection.The simplified physical model used to understand this ubiquitous heat transport mechanism is Rayleigh-Bénard (RB) convection, which is a fluid flow driven by a temperature difference between the top and bottom plates of an experimental cell with adiabatic sidewalls. Despite the long history of the subject and the recent progress in theoretical, numerical and experimental domains, many questions remain unresolved. A fundamental question concerns the heat transfer scaling in highly turbulent convective flows and in particular regarding the transition to the asymptotic regime of enhanced heat transfer. Other questions are related to the flow structures at these extreme flow parameters, and in particular about the existence of irregular polygonal structures that resemble those observed just above the onset of convection. This project aims to explore RB convection at extreme parameters by using liquid or gaseous nitrogen close to its coexistence curve, from 77K at 1 atm to its critical point (about 126K and 34 bar/500psi) in a facility at Georgia Institute of Technology. The proposed research will provide visualization of the flow structures at conditions for which none exist. Available experimental results at these extreme conditions appear to be in conflict, and the proposed work promises to settle the issue. In addition to graduate and undergraduate educational activities, there is a plan to continue collaborating with Guerilla Science, a professional outreach organization based in NYC and London, with which the PIs developed and run a fluid dynamics-inspired competition. The goal is to attract young students to engineering.