New Engineering Concepts from Phase Transitions: A Leidenfrost Engine

莱登弗罗斯特效应最初是以约翰·戈特洛布·莱登弗罗斯特(Johann Gotlob LedenFrost，1715-1794)的名字命名的，他在1756年出版的《关于公共水的性质的论文集》中仔细描述了他是如何使用抛光铁匙的，他如何在发光的煤上加热，并注意到一滴水掉进发光的勺子里，当他接触到较冷的铁时，就不会像水习惯的那样粘在汤匙上。(Quéré，Annu.流体机械牧师。45,2013)。这对研究液氮的物理学家来说很熟悉，液滴可以在地板上自由滚动，工程师们也很熟悉，因为他们研究的是从高温固体到液体的微弱传热。莱登弗罗斯特效应是一层水在接触到比液体沸点高得多的固体表面时瞬间转化为蒸汽的现象。蒸汽层消除了液滴在固体表面上通常观察到的液-固接触，提供了一种隔热屏障和几乎无摩擦的运动。经过几个世纪的好奇和低强度的研究，莱登弗罗斯特效应突然出现，成为一个快速增长的研究领域，最初是作为完美(超级)疏水表面的模型。随着人们认识到这样的表面可以提供显著的减阻，表面可以被微结构以产生直线运动，并且可以修改表面材料/结构来降低转变温度，人们的兴趣已经增长。此外，莱登弗罗斯特效应的范围已扩大到包括升华产生的蒸汽层，因此可以使用类似的概念来理解固体-蒸汽相变以及液体-蒸气相变。在很大程度上，最近的重点仍然是科学理解，而不是工程应用。2015年，我们在《自然通讯》(第6卷，2015)上发表了一个概念验证--莱登弗罗斯特发动机--这既是一个实现旋转的机械发动机，也是有史以来第一个基于升华的热机的演示。这是基于热涡轮机一样的底物允许产生和引导蒸汽的想法，从而实现了干冰圆盘、水滴和固体圆盘的旋转运动-通过表面张力耦合到旋转的水滴。一旦实现了旋转，我们就证明了可以产生小电压。这项提议探索了基于具有涡轮形状表面图案的衬底的莱登弗罗斯特发动机的新概念，并将使用两种类型的相变：i)薄膜沸腾(液-气相变)，ii)二氧化碳升华(固-气相变)。我们的目标是研究i)一系列表面纹理设计，以创建有效的莱登弗罗斯特涡轮机表面，ii)使用液体和固体作为“燃料”和“工作物质”，以及iii)间歇和连续模式操作的设计。我们的目标是研究这两种小规模的设计，用于表面积与体积比高且摩擦占主导地位的情况，以及在较大规模的情况下，重力是一个问题，因此蒸汽悬浮的能量成本很高。因此，我们将把小规模的受控悬浮配置(CLC)设计和较大规模的固定轴承配置(FBC)设计整合到发动机原型中。通过这样做，我们希望这个项目将建立基于薄膜沸腾和升华的热机的明确设计原则，从而将最近的科学进步转化为工程可能性。

The Leidenfrost effect was first named after Johann Gotlob Leidenfrost (1715-1794), who carefully described in his Treatise on the Properties of Common Water, published in 1756, how he used polished iron spoons "heated over glowing coals" and noticed that a drop of water falling into the glowing spoon "does not adhere to the spoon, as water is accustomed to do, when touching colder iron." (Quéré, Annu. Rev. Fluid Mech. 45, 2013). It is familiar to Physicists working with liquid nitrogen whose droplets can roll freely across a floor and to Engineers working on poor heat transfer from hot solids into liquids. The Leidenfrost effect is the instantaneous conversion of a layer of water to vapour upon contact with a solid surface that is substantially hotter than the liquid's boiling point. The vapour layer removes the liquid-solid contact usually observed for a droplet resting on a solid surface and imparts both a thermally insulating barrier and a virtually frictionless motion.After centuries of curiosity and low intensity study, the Leidenfrost effect has burst into life becoming a rapidly growing field of research, initially, as a model of a perfectly (super) hydrophobic surface. Interest has grown as it has been realized that such surfaces may offer significant drag reduction, that surfaces may be micro-structured to create linear motion and that it is possible to modify surface materials/texture to reduce the transition temperatures. In addition, the scope of the Leidenfrost effect has been widened to include vapour layers created by sublimation so that solid-vapour phase transitions, as well as liquid-vapour phase transitions can be understood using similar ideas. Largely, this recent focus has remained on scientific understanding rather than engineering applications.In 2015 we published a proof-of-concept in Nature Communications (vol. 6, 2015) - a Leidenfrost Engine - which was both a mechanical engine achieving rotation and the first ever demonstration of a sublimation-based heat engine. This was based on the idea of hot turbine-like substrates allowing vapour to be created and directed, such that rotational motion of discs of dry ice, droplets of water and solid discs-coupled by surface tension to rotating droplets of water was achieved. Once rotation had been achieved, we demonstrated that a small voltage could be generated.This proposal explores the new concept of a Leidenfrost Engine based on substrates with turbine shaped surface patterns, and will use two types of phase changes: i) thin film boiling (liquid-vapour phase transition), and ii) carbon dioxide sublimation (solid-vapour phase transition). We aim to investigate i) a range of surface texture designs to create effective Leidenfrost turbine surfaces, ii) the use of liquids and solids as "fuels" and "working substances" and iii) designs for batch and continuous mode operation. We aim to investigate both small-scale designs, for use where there is high surface area to volume ratio and friction is a dominant concern, and at larger scales, where gravity is a concern and thus levitation by the vapour is energetically costly. We will therefore integrate controlled-levitation configuration (CLC) designs at small scales and fixed-bearing configuration (FBC) designs at larger scales into engine prototypes. By doing so, we expect this project to establish clear design principles for heat engines based on thin-film boiling and sublimation, thereby translating recent scientific advances into engineering possibilities.