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Research Papers: Forced Convection

Flow Visualization and Local Measurement of Forced Convection Heat Transfer in a Microtube

[+] Author and Article Information
Boris Schilder

Department of Mechanical Engineering, Technische Universitaet Darmstadt, Petersenstrasse 30, 64287 Darmstadt, Germany

Simon Yu Ching Man

School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

Nobuhide Kasagi

Department of Mechanical Engineering, University of Tokyo, Tokyo 113-8656, Japan

Steffen Hardt

Center of Smart Interfaces, Technische Universitaet Darmstadt, Darmstadt D-64287, Germany

Peter Stephan1

Department of Mechanical Engineering, Technische Universitaet Darmstadt, Petersenstrasse 30, 64287 Darmstadt, Germanyp.stephan@ttd.tu-darmstadt.de

1

Corresponding author.

J. Heat Transfer 132(3), 031702 (Jan 04, 2010) (9 pages) doi:10.1115/1.4000046 History: Received October 20, 2008; Revised July 31, 2009; Published January 04, 2010; Online January 04, 2010

The pressure drop and the convective heat transfer characteristics of ethanol and water in a circular tube with a diameter of 600μm with and without phase change have been studied experimentally. The test section consists of a glass tube coated with a transparent indium tin oxide heater film. For single-phase liquid flow (including superheated liquid) it was found that the measured Nusselt numbers and friction factors are in good agreement with the theoretical values expected from Poiseuille flow. Subsequently, the boiling heat transfer of ethanol was studied. It was found that boiling with bubble growth in both upstream and downstream directions leaving behind a thin evaporating liquid film on the tube wall is the dominant phase change process. Wavy patterns on the film surface indicate shear forces between vapor and liquid phase during slug flow. Temporary dryout phenomena occur even at a low mean vapor quality due to film rupture as a result of film instabilities. Local Nusselt numbers are calculated for the two-phase flow at different heat fluxes and Reynolds numbers. Compared with single-phase flow the heat transfer is enhanced by a factor of 3–8.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic view of the experimental setup

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Figure 2

Change of electric resistance of an unprotected ITO film

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Figure 3

Friction factor compared with theory for laminar flow

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Figure 4

Outer wall, inner wall, and bulk temperature

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Figure 5

Nusselt number variation in flow direction for single-phase heat transfer

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Figure 8

Evolution of flow pattern during a boiling cycle, part 2: bubble nucleation upstream of the camera’s field of view

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Figure 9

Pressure drop, transition from single-phase flow to boiling, ṁ=237 kg/m2 s, and q̇=79,000 W/m2

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Figure 10

Nusselt numbers for boiling, ṁ=158 kg/m2 s

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Figure 11

Nusselt numbers for boiling, ṁ=237 kg/m2 s

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Figure 6

Nusselt number as a function of dimensionless length and experimental results compared with correlation of Grigull and Tratz for single-phase heat transfer

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Figure 7

Evolution of flow pattern during a boiling cycle, part 1: bubble nucleation in the camera’s field of view

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Figure 12

Nusselt numbers for boiling, ṁ=317 kg/m2 s

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Figure 13

Average Nusselt numbers for boiling at different mass flux densities compared with data of Yen (24,8) and Lin (25)

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