Beyond Leidenfrost levitation: A thin-film boiling engine for controlled power generation
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Overcoming friction between moving components is important for reducing energy losses and component wear. Hydrodynamic lubrication via thin-film boiling provides an opportunity for reduced friction energy and mass transport. A common example of such lubrication is the Leidenfrost effect, where a liquid droplet levitates on a cushion of its own vapor on a surface heated to temperatures above the liquid's boiling point. An asymmetry in this vapor flow, self-propels the droplet on the surface due to viscous drag, converting thermal energy to mechanical motion, like a heat engine. Although levitation significantly reduces friction, the induced self-propulsion depends on substrate geometry and material properties, which limits dynamic propulsion control. Therefore, the ability to control the power output is a significant challenge in realizing operational mm and sub-mm scale virtually frictionless engines. Here, we present a thin-film boiling engine where we control the power output mechanically. The rotor, which comprises of a working liquid coupled to a non-volatile solid, is manually positioned over a heated turbine-inspired stator in a thin-film boiling state. We show that by controlling the position of the rotor over the substrate the power output from the rotation can be controlled above and below the Leidenfrost temperature (~250 °C). We explain these experimental observations using a hydrodynamic analytical model. Additionally, we achieve propulsion outputs almost 4 times higher than levitation-based propulsion systems. The ability to control the rotation characteristics of such virtually frictionless engines allows potential applications in extreme environments such as at microscales or for space and planetary exploration.Keywords:
Leidenfrost effect
Heat Engine
Abstract Friction is a major inhibitor in almost every mechanical system. Enlightened by the Leidenfrost effect – a droplet can be levitated by its own vapor layer on a sufficiently hot surface – we demonstrate for the first time that a small cart can also be levitated by Leidenfrost vapor. The levitated cart can carry certain amount of load and move frictionlessly over the hot surface. The maximum load that the cart can carry is experimentally tested over a range of surface temperatures. We show that the levitated cart can be propelled not only by gravitational force over a slanted flat surface, but also self-propelled over a ratchet shaped horizontal surface. In the end, we experimentally tested water consumption rate for sustaining the levitated cart and compared the results to theoretical calculations. If perfected, this frictionless Leidenfrost cart could be used in numerous engineering applications where relative motion exists between surfaces.
Leidenfrost effect
Cart
Gravitational force
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Cooling of stainless steel surfaces with flat fan nozzles was studied experimentally. Several configurations of jets and pressures were tested. Tests were done with variable coolant (water) temperatures (20 °C, 40 °C, 60 °C and 80 °C). The influence of coolant temperature on the heat transfer coefficient was investigated. An increase in coolant temperature caused a significant decrease of the Leidenfrost temperature (temperature at which the character of boiling is changed - the film boiling is changed into nucleate boiling). Changing the water temperature from 20 °C to 80 °C caused a change of the Leidenfrost temperature of about 140 °C. Furthermore it was observed that in a high temperature region (above Leidenfrost temperature) the heat transfer coefficient has the highest value for the lowest water temperature and for the high coolant temperature (80 °C) the cooling intensity is the lowest.
Leidenfrost effect
Boiling point
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Leidenfrost effect
Boiling point
Bubble point
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Droplet impact on high-temperature wall can be seen in a spray cooling to heated steel. Impacting droplets can levitate when the wall has sufficiently high temperature. This levitating phenomenon of a droplet is called the Leidenfrost effect. Many researches have been conducted to investigate the conditions of occurrence of the Leidenfrost effect. It is reported that a wettability of the wall and an impact velocity of droplets have affected the occurrence of the Leidenfrost effect. However, the detailed mechanism of the Leidenfrost effect is still poor understood. In this study, we used the molecular dynamics (MD) simulation to investigate the mechanism of the levitating phenomenon of impacting nanodroplet. We found that evaporation from in the vicinity of the three-phase contact line may affect the levitation of nanodroplet.
Leidenfrost effect
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E. Zielinski discovered experimentally a comparatively easy method to reproduce film boiling. According to his method, if a wire which is making nucleate boiling with heating current adjusted to a rather low heat flux is pulled out of water for a short time, the surface of the wire dries up and its temperature rises because of the contact of the wire and air, even when heat flux is kept constant. If the wire is dipped in water later on, for the second time, it heats red-hot in a few seconds, followed by the appearance of film boiling. Film boiling is to be reproduced with comparative easiness by breaking temporarily the contact of heating surface with water. The authors who paid attention to Zielinski's method followed it with the result in which nucleate boiling and film boiling are made to coexist stably on the same heating surface by heating them with a given heat flux. This is considered to be a kind of transition boiling, and the present paper deals with the boiling phenomena at the coexistence of nucleate and film regions in saturated water, subcooled water and sodium oleate solution as well as their relation with boiling characteristic curve. In this region the coefficient of heat transfer decreases with the increase of temperature difference, while heat flux increases with the increase of temperature difference.
Leidenfrost effect
Subcooling
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We report that a volatile liquid deposited on a hot substrate with a gradient of temperature does not only levitate (Leidenfrost effect), but also spontaneously accelerates to the cold. This thermophobic effect is also observed with sublimating solids, and we attribute it to the ability of temperature differences to tilt (slightly) the base of the "object", which induces a horizontal component to the levitating force. This scenario is tested by varying the drop size (with which the acceleration increases) and the substrate temperature (with which the acceleration decreases), showing that the effect can be used to control, guide and possibly trap the elusive Leidenfrost drops.
Leidenfrost effect
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Leidenfrost effect
Liquid nitrogen
Film temperature
Boiling point
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The present study provides a detailed theoretical investigation of the thermo-fluid-dynamics of the inverse Leidenfrost levitation phenomenon of a microscale droplet/solid on a liquid pool, and also the conditions essential for solid/liquid spherical objects to levitate. The theoretical model is developed for the floating characteristic of liquid/solid objects based on the thermo-fluid-dynamics of the vapor film during the inverse Leidenfrost effect. A very small thickness of the vapor layer, approximately of the order of micrometers, formed between the object and liquid pool during levitation, and its variation with the angular position and time history is considered in contrast to previous works. The actual magnitude of the overlapping contact angle is estimated and also incorporated in the present study. The effects of various influencing parameters, like nondimensionalized sphere radius, contact angle, and density ratio, on the levitation possibility and dynamics, are analyzed. The model is validated against experimental observations of the inverse Leidenfrost phenomenon for water drop levitating on a nitrogen liquid pool, and the effects of droplet parameters on total levitation time and dynamics are noted to provide accurate predictions. The approach presented is noted to provide a more accurate estimate of inverse Leidenfrost levitation compared to previous reports.
Leidenfrost effect
Microscale chemistry
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The latest version of the newly developed liquid-vapor phase-change lattice Boltzmann method, with a conjugate thermal boundary condition imposed at the solid-fluid interface, is applied to simulate numerically pool boiling from smooth, infinitely long, upward-facing, horizontal heated surfaces under controlled wall temperature conditions. A parametric study is carried out to investigate effects of wettability as well as heater and fluid physical properties on pool boiling curves, from onset of nucleation to critical heat flux (CHF) through transition boiling to stable film boiling. It is found that although a heater’s wettability has no effect on film boiling, it has important effects on nucleate boiling and transition boiling. Decreasing heater wettability shifts the nucleate and transition boiling curves to the left, decreases the maximum heat flux, decreases the minimum heat flux (MHF), and lowers the Leidenfrost temperature. With the increase of the heater’s thermal conductivity, both the MHF and the Leidenfrost temperature decrease, but this has no effect on nucleate boiling, CHF, or film boiling. On the other hand, increasing the vapor’s thermal conductivity has no effect on nucleate boiling, but it increases the MHF and decreases the Leidenfrost temperature in transition boiling as well as in film boiling.
Leidenfrost effect
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