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Research Papers: Evaporation, Boiling, and Condensation

Effects of Droplet Diameter and Fluid Properties on the Leidenfrost Temperature of Polished and Micro/Nanostructured Surfaces

[+] Author and Article Information
Anton Hassebrook, Corey Kruse, George Gogos

Mechanical and Materials Engineering,
University of Nebraska-Lincoln,
Lincoln, NE 68588

Chris Wilson, Troy Anderson, Craig Zuhlke, Dennis Alexander

Electrical Engineering,
University of Nebraska-Lincoln,
Lincoln, NE 68588

Sidy Ndao

Mechanical and Materials Engineering,
University of Nebraska-Lincoln,
Lincoln, NE 68588
e-mail: sndao2@unl.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received July 15, 2014; final manuscript received October 19, 2015; published online January 27, 2016. Assoc. Editor: Amitabh Narain.

J. Heat Transfer 138(5), 051501 (Jan 27, 2016) (7 pages) Paper No: HT-14-1471; doi: 10.1115/1.4032291 History: Received July 15, 2014; Revised October 19, 2015

An experimental investigation of the effects of droplet diameters and fluid properties on the Leidenfrost temperature of polished and nano/microstructured surfaces has been carried out. Leidenfrost experiments were conducted on a stainless steel 304 polished surface and a stainless steel surface which was processed by a femtosecond laser to form above surface growth (ASG) nano/microstructures. Surface preparation resulted in a root mean square roughness (Rrms) of 4.8 μm and 0.04 μm on the laser processed and polished surfaces, respectively. To determine the Leidenfrost temperatures, the droplet lifetime method was employed using deionized (DI) water and HFE 7300DL. A precision dropper was used to vary the size of DI water droplets from 1.5 to 4 mm. The Leidenfrost temperature was shown to display increases as high as 100 °C on the processed surface over the range of droplet sizes, as opposed to a 40 °C increase on the polished surface. Average increases of the Leidenfrost temperature between polished and processed samples were as high as 200 °C. The experiment was repeated with HFE 7300DL; however, with no noticeable changes of the Leidenfrost temperatures with droplet size whether on the polished or the processed surface. The difference in the Leidenfrost behavior between DI water and HFE 7300DL and among the various droplet sizes can be attributed to the nature of the force balance and flow hydrodynamics at a temperature slightly below the Leidenfrost point (LFP).

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Figures

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Fig. 5

LFP results using DI water droplets: (a) Droplet lifetime curves of water droplets on polished and FLSP stainless steel test surfaces. (b) Enlarged view of lifetime curve for 1.5 mm droplets on ASG-Mounds. (c) Leidenfrost temperature as a function of droplet diameter shows substantially different rates of change for the mirror-polished and ASG-Mounds samples.

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Fig. 4

SEM images (top) and 3D topology scans (bottom) of (a) ASG-Mounds and (b) mirror polished test samples. SEM images taken at 600 × magnification—scale bars are 100 μm. It should be noted that the colors do not correspond on the topology scans. For the ASG-Mounds red represents a height of 30 μm, while the same color represents a height of 0.7 μm on the polished sample.

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Fig. 3

LFP experimental setup. A precision microdropper was used in conjunction with a temperature controller to evaporate droplets. Droplets were recorded on a high-definition camera and droplet lifetime was extracted from the video.

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Fig. 2

(a) Square flat top beam profile of Femtosecond laser pulses. (b) Laser raster pattern. X and Y were approximately 1.5 in. while d was 15 μm.

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Fig. 6

Schematic representation of a droplet on a hot surface transitioning to film boiling. (1) Nucleate boiling, (2) coalescence of vapor pockets typically occurs during transition boiling, (3) unstable vapor film just below the Leidenfrost Temperature, and (4) film boiling.

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Fig. 7

(a) Droplet lifetime curves of HFE 7300DL on polished and FLSP stainless steel test surfaces. (b) Leidenfrost temperature as a function of droplet diameter shows that the LFP is the same for both the mirror-polished and ASG-Mounds samples. Only minimal shifts in the LFP were observed over the range of droplet sizes.

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Fig. 8

Left: Leidenfrost temperature as a function of droplet diameter for both test fluids on the mirror polished sample. Right: Contact angle images for both fluids on the mirror polished surface.

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