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Research Papers: Evaporation, Boiling, and Condensation

Pool Boiling Heat Transfer of Water on Finned Surfaces at Near Vacuum Pressures

[+] Author and Article Information
Mark Aaron Chan, Christopher R. Yap

Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

Kim Choon Ng1

Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260mpengkc@nus.edu.sg

1

Corresponding author.

J. Heat Transfer 132(3), 031501 (Dec 29, 2009) (6 pages) doi:10.1115/1.4000054 History: Received March 23, 2009; Revised August 17, 2009; Published December 29, 2009; Online December 29, 2009

This research paper presents a study of boiling heat transfer from longitudinal rectangular-finned surfaces immersed in saturated water at low vapor pressures. Finned surfaces with assorted fin spacing, fin thicknesses, and fin heights on a copper based surface have been investigated. All the finned surfaces were found to increase both boiling heat transfer coefficients and critical heat fluxes. An optimal fin thickness was found for a configuration, and heat transfer coefficients have been obtained at the pressures. Factors affecting the boiling characteristics have been identified and the optimal enhancement requires a balance of the active nucleation sites, bubble flow resistance, natural convection, thin film evaporation, liquid superheating, heat transfer area, bubble coalescence, and liquid reflux resistance. High speed visualization of vapor plug and vapor film generation on the boiling surfaces has revealed significant insights into the boiling mechanisms at low saturation pressures.

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Figures

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Figure 1

Detailed schematic of the boiling/heater chamber

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Figure 2

Boiling curves of pool boiling on a plain copper surface at subatmospheric pressures of 2 kPa, 4 kPa, and 9 kPa. Experimental results with predictions from Cooper’s correlation (13).

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Figure 3

Boiling curves for finned surfaces having different fin thicknesses with constant fin space (g)=0.5 mm and fin height (h)=15 mm at pressures of 2 kPa and 9 kPa

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Figure 4

Heat transfer coefficient versus heat flux (of footprint) for finned surfaces having different fin thicknesses with constant fin spacing (g)=0.5 mm and fin height (h)=15 mm at pressures of 2 kPa and 9 kPa

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Figure 5

Boiling curves for finned surfaces having different fin spacings with constant fin thickness (t)=1.0 mm and fin height (h)=15 mm at 2 kPa pressure. The plain surface data is measured with the same apparatus.

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Figure 6

Boiling curves for finned surfaces having different fin spacings with constant fin thickness (t)=1.0 mm and fin height (h)=15 mm at 9 kPa pressure

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Figure 7

Boiling curves for finned surfaces having different heights with constant fin spacing (g)=0.5 mm and fin thickness (t)=0.5 mm at pressures of 2 kPa and 9 kPa. The CHF is reached for the low-finned surface.

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Figure 8

Boiling curves for rectangular and square pin-finned surfaces with fin spacing (g)=0.5 mm, fin thickness (t)=1 mm, and fin height (h)=15 mm at 2 kPa and 9 kPa pressures

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Figure 9

(a) Water boiling on a finned surface (fin height=15 mm, fin spacing=0.5 mm, and fin thickness=1.0 mm) with heat flux of 5 W/cm2 at 2 kPa pressure. The darkened slot at 40 ms shows the presence of a vapor plug within the fin space as no light is able to go through. At 50 ms, the same fin space is now filled with liquid as shown by the lighter contrast. (b) The graphical depiction of bubble growth and departure sequences in the narrow fin spacing.

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Figure 10

(a) Water boiling on a finned surface (fin height=15 mm, fin spacing=0.5 mm, and fin thickness=1.0 mm) with a heat flux of 20 W/cm2 at 2 kPa pressure and (b) the graphical depiction of growth, coalescence, and departure sequences of adjacent bubbles

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