Technical Brief

A Leidenfrost Thermostat

[+] Author and Article Information
Alexander Cole, Benjamin Jury

Department of Physics,
University of Bath,
Bath BA2 7AY, UK

Kei Takashina

Department of Physics,
University of Bath,
Bath BA2 7AY, UK
e-mail: k.takashina@bath.ac.uk

The fit also returns a value for room temperature TA = −4 °C from the offset, which is at least 20 °C colder than the air temperature in the laboratory. We feel that this is an acceptable discrepancy in view of the crudeness of the model.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received May 22, 2014; final manuscript received November 20, 2014; published online December 17, 2014. Assoc. Editor: W. Q. Tao.

J. Heat Transfer 137(3), 034502 (Mar 01, 2015) (5 pages) Paper No: HT-14-1334; doi: 10.1115/1.4029238 History: Received May 22, 2014; Revised November 20, 2014; Online December 17, 2014

A simple thermostat based on self-propelled Leidenfrost droplets is proposed and demonstrated. The proof-of-principle device sits on a heated hotplate, which provides the heat, but under dripping water which cools it. Using temperature dependent directionality of droplets on a substructured sawteeth surface, droplets are either discarded or fed into a region with high Leidenfrost temperature and enhanced heat-loss. The system can therefore adjust how much of the droplets’ cooling power it uses depending on its own temperature, and this feedback enables it to maintain a constant set temperature and act as a thermostat.

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Grahic Jump Location
Fig. 1

The ratcheted sawteeth surface: (a) A side-on optical micrograph image of the sawteeth surface. (b and c) Scanning electron micrographs of the surface. (micrographs in (a), (b), and (c) have also been used in Fig. 1(b), Figs. 3(e) and 3(f), respectively, in Ref. [8]—the sawteeth in the present study were machined using identical methods, including the machining tool.) (d) and (e) Positions where the droplets fall off the ratcheted bar. (d) A schematic of the zones. X marks the spot where droplets are dropped. (e) A graph showing the number of droplets falling at each zone, after 20 droplets were counted at each temperature Tb. Traces, taken at 5 °C intervals, are offset by equal increments of 10 counts. Asterisks in (d) and (e) mark the corner forming the juncture between zones 9 and 10, which sets Tset. The same corners are marked in Figs. 2(a) and 2(b).

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Fig. 2

The Leidenfrost thermostat and the minimal model: (a) photograph of the base assembly. An arrow highlights the position of the braided metallic mesh, fixed in a trench to raise the Leidenfrost temperature. (b) Photograph from a different angle of the assembly with the brass bar on top. Yellow asterisks mark the corner (in (a) and (b)) forming the juncture between zones 9 and 10 which sets Tset. (c) A schematic cross-sectional diagram of the thermostat. (d) A schematic diagram illustrating the flow of heat in the minimal model. (e) A graph to illustrate how Pbw changes with temperature.

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Fig. 3

Cooling, feedback, and nucleate boiling: Evolution of bar temperature Tb with time. Different symbols correspond to data taken with different hot plate temperature Tp. (a)–(d) Data at different water supply rates. When Tb > Tset, the cooling rate is greater as droplets evaporate in their entirety. Below Tset, the droplets are discarded, until the surface cools below the Leidenfrost point at which point the Tb falls rapidly gain ((c) and (d)).

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Fig. 4

Stability: (a) Tb against Tp under dynamic equilibrium (t = 210 s) at different water supply rates. (b) A contour plot of the same date with water supply rate and Tp as axes. The solid pink lines mark boundaries of the stable region described in the body text.




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