Micro Leidenfrost Heat Engine

摩擦一直是产生机械能的一个重要因素。在微观尺度上，高比表面积与体积比会导致显著的摩擦磨损和相关的能量损失。最近，将热能转化为机械功的无摩擦发动机的概念得到了验证。发动机在涡轮机激励的衬底上使用莱登弗罗斯特液滴工作，在衬底上，定向蒸汽垫产生无摩擦的旋转。(Wells，2015)莱登弗罗斯特液滴是一种薄膜沸腾的情况，热的固体表面和液滴之间的直接接触是通过一层蒸发物来防止的。这层蒸发物减缓了热固体表面的热量传递，从而减缓了液滴的沸腾。当莱登弗罗斯特液滴在非对称衬底上沸腾时，观察到了自推进。这种不对称性被认为是为了纠正莱登弗罗斯特液滴漂浮在其上的蒸发物的流动。与之相关的粘性阻力导致悬浮部件移动，控制这些衬底的设计可以产生定向推进。在宏观尺度上，已经表明，通过不断补充液体，能量转换可以持续，证明了一个封闭的热力学循环。(Agrawal，2019)在微尺度上，基于对数曲线设计的基板几何形状显示出显著的扭矩。不幸的是，扭矩会因蒸汽气泡的形成而减少。由于高蒸汽压力降低了液体覆盖率，在液体体积中形成了蒸汽气泡，对旋转稳定性产生了不利影响。具有高凹槽深度的衬底减少了气泡的形成并增加了功率输出。(Agrawal，2019)Anthony Buchoux博士后来证明，通过改变衬底的几何形状，蒸汽气泡的形成可以用来驱动热机。实验评估表明，气泡驱动热机的性能可以超过纯莱登弗罗斯特热机。作为研究的一部分，我希望为爱丁堡大学与诺森比亚大学合作开发的莱登弗罗斯特热机的开发做出贡献。千米级发动机正在使用快速原型技术(数控铣削和3D打印)和净室工艺(无掩膜制造和晶片加工)制造。我希望使用一种集成了微型加热器的发动机，它可以在两种配置下运行：(I)纯莱登弗罗斯特；其中被输送的蒸汽以表面特征驱动发动机；(Ii)气泡驱动；莱登弗罗斯特效应提供无摩擦轴承，但发动机是由局部核沸腾推动的。我希望了解优化发动机输出功率的关键参数，并希望为当前的设置开发一种连续进给机构。此外，我还想开发莱登弗罗斯特热机，探索利用多孔性吸胀(纳米孔膜)。在希望有足够稳定的设置之后，我希望研究莱登弗罗斯特热机的效率，即流量和功率输入，以持续产生机械能。我知道流量、进料频率和流体入口温度都会影响发动机的性能，我想调查这些因素的重要性--它们是如何影响发动机的--以及如何才能使这些因素提高性能。

Friction has always been an important factor in the production of mechanical energy. At the micro-scale, high surface-area to volume ratios cause significant frictional wear and associated energy losses. Recently, the concept of a frictionless engine which converts thermal energy to mechanical work has been proven. The engine works using Leidenfrost droplets in turbine-inspired substrates where a cushion of directed vapour causes frictionless rotation. (Wells, 2015)A Leidenfrost droplet is a case of thin-film boiling, where direct contact between a hot solid surface and the liquid droplet is prevented by a layer of evaporate. This layer of evaporate slows the transfer of heat from the hot solid surface, consequently slowing the boiling of the droplet. Self-propulsion has been observed when Leidenfrost droplets boil on asymmetric substrates. The asymmetry is understood to rectify the flow of the evaporate on which the Leidenfrost droplet levitates. The viscous drag associated causes the levitating component to move and controlling the design of these substrates can result in directed propulsion.At the macro-scale, it has been shown that by continuously replenishing the liquid, energy conversion may be sustained, demonstrating a closed thermodynamic cycle. (Agrawal, 2019) At the micro-scale, substrate geometries based on a logarithmic curve design have shown significant torque. Unfortunately, torque is reduced by the formation of vapour bubbles. Vapour bubbles form in the liquid volume due to the high vapour pressure where the liquid coverage is reduced, and the rotation stability is negatively affected. Substrates with high groove depth reduce bubble formation and increase power output. (Agrawal, 2019) Dr. Anthony Buchoux has since shown that by altering the substrate geometry, the formation of vapour bubbles can be used to drive the heat engine. Experimental assessment shows that bubble driven heat engines could outperform pure Leidenfrost heat engines.As part of the research I wish to contribute to the development of the Leidenfrost engine, developed here at the University of Edinburgh in collaboration with Northumbria University.Millimetre scale engines are being manufactured using rapid prototyping technologies (CNC milling and 3D printing) and clean room processes (maskless manufacturing and wafer processing). I hope to work with an engine which integrates a micro-heater and can run in two configurations: (i) pure Leidenfrost; where the vapour being channelled drives the engine with surface features, and (ii) bubble driven; the Leidenfrost effect provides a frictionless bearing but the engine is propelled by localised nucleate boiling. I wish to understand the key parameters for the optimisation of the power output of the engine and wish to develop a continuously fed mechanism for the current setup. Furthermore, I would like to develop Leidenfrost heat engines which explore the use of porous imbibition (nanoporous membranes).Following hopefully sufficiently stable set-ups - I wish to investigate the efficiency of Leidenfrost heat engine w.r.t flowrate and power input for the sustained production of mechanical energy. I understand that the flow rate, drop feeding frequency and fluid inlet temperature all influence the behaviour of the engine and I would like to investigate the significance of each of these factors - how they influence the engine - and what can be done to have these factors improve performance.