Micro Leidenfrost Heat Engine

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.