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Research Papers: Evaporation, Boiling, and Condensation

Subcooled Boiling Flow Heat Transfer From Plain and Enhanced Surfaces in Automotive Applications

[+] Author and Article Information
Franz Ramstorfer

 The Virtual Vehicle Research Company, Inffeldgasse 21A, 8010 Graz, Austriafranz.ramstorfer.ext@siemens.com

Helfried Steiner

Institute of Fluid Mechanics and Heat Transfer, Graz University of Technology, Inffeldgasse 25F, 8010 Graz, Austriasteiner@fluidmech.tu-graz.ac.at

Günter Brenn

Institute of Fluid Mechanics and Heat Transfer, Graz University of Technology, Inffeldgasse 25F, 8010 Graz, Austriabrenn@fluidmech.tu-graz.ac.at

Claudius Kormann

 BASF AG, G-EVO∕MI-J550, 67056 Ludwigshafen, Germanyclaudius.kormann@basf.com

Franz Rammer

 MAN Nutzfahrzeuge Österreich AG, Schönauerstraße 5, 4400 Steyr, Austriafranz.rammer@at.man-mn.com

J. Heat Transfer 130(1), 011501 (Jan 25, 2008) (9 pages) doi:10.1115/1.2780178 History: Received July 20, 2006; Revised May 09, 2007; Published January 25, 2008

The requirement for the highest possible heat transfer rates in compact, efficient cooling systems can often only be met by providing for a transition to subcooled boiling flow in strongly heated wall regions. The significantly higher heat transfer rates achievable with boiling can help keep the temperatures of the structure on an acceptable level. It has been shown in many experimental studies that special surface finish or porous coatings on the heated surfaces can intensify the nucleate boiling process markedly. Most of those experiments were carried out with water or refrigerants. The present work investigates the potential of this method to enhance the subcooled boiling heat transfer in automotive cooling systems using a mixture of ethylene-glycol and de-ionized water as the coolant. Subcooled boiling flow experiments were carried out in a vertical test channel considering two different types of coated surfaces and one uncoated surface as a reference. The experimental results of the present work clearly demonstrate that the concept of enhancing boiling by modifying the microstructure of the heated surface can be successfully applied to automotive cooling systems. The observed increase in the heat transfer rates differ markedly for the two considered porous coatings, though. Based on the experimental data, a heat transfer model for subcooled boiling flow using a power-additive superposition approach is proposed. The model assumes the total wall heat flux as a nonlinear combination of a convective and a nucleate boiling contribution, both obtained from well-established semiempirical correlations. The wall heat fluxes predicted by the proposed model are in very good agreement with the experimental data for all considered flow conditions and surface types.

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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

The generation of vapor bubbles: (a) nucleation site on the smooth surface, (b) bubble nucleation from individual microchannels in the porous layer, and (c) formation of a continuous vapor film in the porous layer (adopted from Ref. 4)

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Figure 2

Flow boiling test loop schematic

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Figure 4

Photomicrograph of the surface S0. The position of the frame of the micrograph is schematically marked by the dashed square in the sketch of the cross section of the heated probe below.

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Figure 5

Photomicrograph of the surface S1. The position of the frame of the micrograph is schematically marked by the dashed square in the sketch of the cross section of the heated probe below.

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Figure 6

Photomicrographs of the surface S2 at two different resolutions: (a) base material with superficial coating and (b) detailed view of the coating. The position of the frame of the micrographs is schematically marked by the dashed square in the sketch of the cross section of the heated probe below.

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Figure 7

Experimentally measured flow boiling curves using a mixture 40vol% of ethylene-glycol and 60vol% of distilled water as the working fluid at p=3.2bars, Tb=100°C, and three different velocities of the bulk flow: circles (○), surface S0; diamonds (◇), surface S1; squares (◻), surface S2

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Figure 8

Correction factor ξ representing the enhancement of the convective heat transfer due to the macroscopic surface roughness of S1 for different bulk velocities ub

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Figure 9

Predicted flow boiling curves for the plain surface S0 at different velocities of the bulk flow. The symbols denote the measurements: full line (—), model prediction; dash-dotted line (-∙-∙-), natural convection; dashed line (---), pool boiling curve.

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Figure 10

Predicted flow boiling curves for the porous surface S1 at different velocities of the bulk flow. The symbols denote the measurements: full line (—), model prediction; dash-dotted line (-∙-∙-), natural convection; dashed line (---), pool boiling curve.

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Figure 11

Predicted flow boiling curves for the porous surface S2 at different velocities of the bulk flow. The symbols denote the measurements: full line (—), model prediction; dash-dotted line (-∙-∙-), natural convection; dashed line (---), pool boiling curve.

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