Investigation of Nucleate Pool Boiling of Saturated Pure Liquids and Ethanol-Water Mixtures on Smooth and Laser-Textured Surfaces

ABSTRACT

Nucleate pool boiling experiments were performed on plain and five laser-textured stainless-steel foils using saturated pure water, 100% ethanol, 0.4% and 4.2% mole fraction ethanol – water mixtures. All laser-textured samples contained untreated, smooth 0.5 mm wide regions and intermediate textured surfaces, that differ in the width of the laser patterned regions (from 0.5 mm to 2.5 mm). For smooth surfaces, we measured significant decreases in average heat transfer coefficients (HTC) and increases in bubble activation temperatures in comparison with the laser-textured surfaces for all the tested working fluids. Significant enhancement in HTC (280%) on a textured heating surface with 2.5-mm-wide laser pattern was recorded using pure water. For pure ethanol, the highest enhancement of 268% was achieved on a heating surface with 1.5-mm-wide laser pattern. The highest enhancement of HTC for the tested binary mixtures was obtained using 2.0-mm wide-laser-textured regions (HTC improved by 235% and 279% for the 0.4% and 4.2% mixtures, respectively). Our results indicate that laser texturing can significantly improve boiling performance when the intervals of the laser-textured patterns are close to the capillary lengths of the tested fluids.

KEYWORDS:

• Pool boiling
• water
• ethanol
• binary mixtures
• laser surface engineering
• laser texturing
• capillary length

Highlights

• Study presents boiling of water and ethanol-water mixtures on laser-textured foils.

• Pulsed fiber laser was used to create active and non-active boiling regions.

• High-speed IR thermography was used to measure transient temperature fields.

• Laser-made microcavities provide active nucleation sites on all tested fluids.

• Boiling patterns close to the fluids’ capillary length show the best performance.

Acknowledgments

The authors acknowledge the financial support from the state budget by the Slovenian Research Agency (research core funding No. P2-0223 and P2-0392 and project No. J2-1741). The authors would also like to thank SPI Lasers Ltd. for their support of the research project Surface functionalization by nanosecond fiber laser texturing (nsFLaT) and Dr. Matej Hočevar from the Institute of metals and technology (Slovenia) for providing SEM images and 3D surface measurements.

Declaration of interest

The authors declare no competing financial interest.

Supplementary Material

Supplemental data for this article can be accessed publisher's website.

Nomenclature

M	=	molar mass (g/mol)
P	=	wavelength (m)
$\dot{q}$	=	heat flux (W/m2)
T	=	temperature (K or °C)
p	=	probability density (1/K)
g	=	acceleration of gravity (m2/s)
fps	=	frames per second (1/s)

Greek symbols

α	=	HTC; heat transfer coefficient (W/m2K))
${\lambda }_c$	=	capillary length (m)
χ	=	mole fraction ethanol-water mixtures; (/or %)
Δ	=	delta
ρ	=	density (kg/m3)
γ	=	liquid surface tension (N/m)

Subscript

w	=	wall
l	=	liquid
v	=	vapor

Additional information

Funding

This work was supported by the Javna Agencija za Raziskovalno Dejavnost RS [J2-1741,P2-0223,P2-0392].