CAREER: Near-Critical Heat Transfer

在高温高压下，流体可以存在于一个临界点以上，在这个临界点上，它们可以在“类液体”和“类气体”性质之间转换，而不会发生相变（如气泡或液滴的形成）。近年来，超临界流体作为高效发动机、先进航空航天应用和电子冷却领域的工作流体获得了极大的兴趣。临界点附近的超临界流体具有独特的热物理性质，通常可以增强传热，但在某些操作条件下，这些性质也会导致破坏性的流动不稳定性和传热退化。这些破坏性的近临界退化和振荡的原因和控制尚不清楚。该项目将使用一种新颖的实验方法来理解控制近临界流体传热特性的物理现象，以预测和控制其潜在的破坏性影响。除了研究目标，这项工作将与一个独特的教育计划的发展联系起来，致力于培养具有全球能力的工程师，准备将超临界技术推向市场。具体来说，这项研究将使用光纤传感器来测量超临界流体的瞬态对流换热，并使用这些数据来开发定制的非均匀热交换器基板的共轭换热瞬态模型。这项工作将为超临界流体非定常对流换热系数中浮力、流动加速度和热物性变化之间的关系提供基础见解，并为超临界流体瞬态过程中高低频流动振荡的演变提供新信息。在稳态条件下建立的理论和模型将使用更能代表实际超临界系统运行的实验进行评估。数据将用于指导非均匀基板中共轭效应的建模，为通过新制造技术制造组件奠定基础。最后，本文对光纤仪器在分布式高精度传热实验中的应用进行了论证，对热输运界具有一定的参考价值。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。

At high temperatures and pressures, fluids can exist above a critical point where they can transition between “liquid-like” and “gas-like” properties without undergoing a phase change (such as the formation of bubbles or droplets). Recently, supercritical fluids have gained significant interest as working fluids in highly efficient engines, advanced aerospace applications, and electronics cooling. Supercritical fluids near the critical point have unique thermophysical properties that often provide enhanced heat transfer, but these properties can also lead to damaging flow instabilities and degraded heat transfer during certain operating conditions. The cause and control of these damaging near-critical degradations and oscillations are not well understood. This project will use a novel experimental approach to understand the physical phenomena that govern the heat transfer properties of near-critical fluids in order to predict and control their potentially damaging effects. In addition to the research objectives, this work will be linked with the development of a unique educational program dedicated to developing globally competent engineers prepared to bring supercritical technology to the market.Specifically, this research will use fiber optic sensors to measure transient convective heat transfer of supercritical fluids, and use the data to develop transient models of conjugate heat transfer in tailored, non-homogenous, heat exchanger substrates. This proposed work will provide fundamental insights on the relationship between buoyancy, flow acceleration, and varying thermophysical properties on the unsteady convective heat transfer coefficient of supercritical fluids, and new information on the evolution of high- and low-frequency flow oscillations during transients of supercritical fluids. Established theory and models developed for steady state conditions will be evaluated using experiments more representative of actual supercritical system operation. Data will be used to guide the modeling of conjugate effects in non-homogeneous substrates, laying the groundwork for components fabricated through new manufacturing techniques. Finally, the demonstration of the fiber optic instruments for distributed high accuracy heat transfer experiments is of value to the broader thermal transport community.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.