Research Papers

The Hoodoo: A New Surface Structure for Enhanced Boiling Heat Transfer

[+] Author and Article Information
James F. Klausner

Mechanical and Aerospace Engineering,
University of Florida,
Gainesville, FL 32611

Edward Mckenna

Materials Science and Engineering,
University of Florida,
Gainesville, FL 32611

Manuscript received February 16, 2012; final manuscript received April 24, 2012; published online February 22, 2013. Assoc. Editor: Jovica R. Riznic.

J. Thermal Sci. Eng. Appl 5(1), 011003 (Feb 22, 2013) (11 pages) Paper No: TSEA-12-1028; doi: 10.1115/1.4007439 History: Received February 16, 2012; Revised April 24, 2012

The hoodoo is introduced as a beneficial surface structure for enhancing boiling heat transfer. A full parametric study was conducted to determine which attributes of the hoodoo structure promote boiling heat transfer enhancement. Hoodoo size and spacing were observed to have the most profound effect on boiling heat transfer, nucleation site activation, and critical heat flux (CHF). The CHF enhancement factor, defined as the ratio of CHF on the structured surface to that of a smooth surface, varies from 1.05 to 1.67 for FC-72 and hexane working fluids. Droplet spreading studies confirm the hemiwicking properties of the hoodoo surface, and it is postulated to be the primary mechanism for CHF enhancement. Measured wicking front speeds varied from 12 to 40 mm/s and were observed to obey a power-law dependence on time with an exponent of approximately 0.5. Plausible thermohydraulic mechanisms for CHF enhancement on the hoodoo surfaces are discussed.

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Fig. 1

Tilted SEM image of the 10 μm hoodoo surface

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Fig. 2

Schematic depiction of the fabrication process for surfaces composed of discrete reentrant surface structures. The process technique comprised deposition of SiO2, removal of the SiO2 in areas not intended to be the caps of structures, and finally etching down, around, and underneath the remaining SiO2 caps with a plasma that etches silicon but not SiO2.

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Fig. 3

SEM images taken normal to the 10, 20, 30, and 40 μm hoodoo surfaces with 3 μm gap spacing ((a) 10 μm, (b) 20 μm, (c) 30 μm, and (d) 40 μm)

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Fig. 4

SEM images taken of the 20 μm hoodoo surface with different hoodoo spacing ((a) 6 μm, (b) 12 μm, (c) 24 μm, and (d) 48 μm)

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Fig. 5

Geometric length scales defined for a hoodoo unit cell (d, hoodoo size; g, hoodoo spacing; u, hoodoo undercut; Dp, hoodoo post diameter; D, unit cell size)

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Fig. 6

Pool boiling chamber (1, substrate heater assembly; 2, stainless steel plate; 3, Viton O-ring; 4, primary bulk fluid heater; 5, Pyrex glass cylinder; 6, fill inlet; 7, inlet and outlet ports for condenser and secondary bulk fluid heater; 8, outlet to pressure transducer; 9, condenser coil; 10, secondary bulk fluid heater; 11, bulk fluid thermocouple; not shown are the outlet to the pressure relief valve and lower drain valve outlet)

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Fig. 7

FC-72 pool boiling curves for different hoodoo sizes

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Fig. 8

Hexane pool boiling curves for different hoodoo sizes

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Fig. 9

FC-72 and hexane CHF enhancement factor for different hoodoo sizes

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Fig. 10

FC-72 pool boiling curves for different hoodoo gap spacing with 20 μm hoodoo size

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Fig. 11

FC-72 pool boiling curves for different hoodoo morphologies. Cross sections of the different morphologies are shown in the inset.

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Fig. 12

Measured CHF enhancement for factor FC-72 with different hoodoo morphologies

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Fig. 13

FC-72 pool boiling curves for the 20 μm hoodoo with 3 μm spacing and undercut with and without a sputtered copper coating

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Fig. 14

Hemiwicking image time series with hexane on the 30 μm hoodoo surface (hemiwicking front and droplet edge are denoted by dotted and solid lines, respectively, for 16 ms tile)

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Fig. 15

A comparison of the measured wicking front speeds on the 10 and 30 μm hoodoo surfaces with FC-72 and hexane droplets. Front speeds are determined via relative linear displacement from the substrate edge.

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Fig. 16.

Liquid vortices penetrating the vapor blanket during film boiling of FC-72 on a hoodoo surface




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