New Engineering Concepts from Phase Transitions: A Leidenfrost Engine

莱顿弗罗斯特效应最早是以约翰·戈特罗布·莱顿弗罗斯特（Johann Gotlob Leidenfrost，1715-1794）的名字命名的，他在1756年出版的《普通水的性质》中详细描述了他如何使用抛光的铁勺子“在灼热的煤炭上加热”，并注意到一滴水落入灼热的勺子“不会像水接触较冷的铁时习惯的那样粘附在勺子上。（Quéré，Annu. Rev. Fluid Mech.45，2013）。这对于研究液氮的物理学家来说是很熟悉的，因为液氮的液滴可以在地板上自由滚动，对于研究从热固体到液体的不良传热的工程师来说也是很熟悉的。莱顿弗罗斯特效应是一层水在与固体表面接触时瞬间转化为蒸汽，固体表面的温度远远高于液体的沸点。蒸汽层消除了通常在固体表面上观察到的液-固接触，并赋予热绝缘屏障和几乎无摩擦的运动。经过几个世纪的好奇心和低强度研究，莱顿弗罗斯特效应已经成为一个快速发展的研究领域，最初，作为完美（超）疏水表面的模型。随着人们认识到这种表面可以提供显著的减阻，表面可以被微结构化以产生线性运动，并且可以修改表面材料/纹理以降低转变温度，兴趣已经增长。此外，莱顿弗罗斯特效应的范围已经扩大到包括由升华产生的蒸气层，从而可以使用类似的想法来理解固-汽相变以及液-汽相变。2015年，我们在《自然通讯》（Nature Communications）杂志上发表了一个概念验证--莱顿弗罗斯特发动机（Leidenfrost Engine），它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力发动机。这是基于这样的想法，即热的涡轮状基底允许蒸汽被产生和引导，使得干冰盘、水滴和固体盘的旋转运动通过表面张力耦合到旋转的水滴。一旦旋转已经实现，我们证明，一个小的电压可以产生。这个建议探讨了莱顿弗罗斯特发动机的新概念的基础上与涡轮机形状的表面图案的基板，并将使用两种类型的相变：i）薄膜沸腾（液体-蒸汽相变），和ii）二氧化碳升华（固体-蒸汽相变）。我们的目标是调查i）一系列表面纹理设计，以创建有效的莱顿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”和iii）设计的批量和连续模式操作。我们的目标是调查两个小规模的设计，用于有高的表面积与体积比和摩擦是一个主要的问题，并在较大的规模，重力是一个问题，因此悬浮的蒸汽是精力充沛的成本。因此，我们将在小规模的控制悬浮配置（CLC）的设计和固定轴承配置（FBC）的设计，在更大的规模集成到发动机原型。通过这样做，我们希望该项目能够为基于薄膜沸腾和升华的热机建立明确的设计原则，从而将最新的科学进展转化为工程可能性。

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.