Research Papers: Experimental Techniques

Effect of Longitudinal Minigrooves on Flow Stability and Wave Characteristics of Falling Liquid Films

[+] Author and Article Information
Klaus Helbig, Ralph Nasarek, Tatiana Gambaryan-Roisman, Peter Stephan

Chair of Technical Thermodynamics, Technische Universität Darmstadt, Petersenstrasse 30, 64287 Darmstadt, Germany

J. Heat Transfer 131(1), 011601 (Oct 14, 2008) (8 pages) doi:10.1115/1.2993539 History: Received October 07, 2007; Revised May 05, 2008; Published October 14, 2008

Falling liquid films are used in many industrial apparatuses. In many cases the film flow along a wall with topography is considered advantageous for intensification of the heat and mass transport. One of the promising types of the wall topography for the heat transfer intensification is comprised of minigrooves aligned along the main flow direction. The wall topography affects the development of wavy patterns on the liquid-gas interface. Linear stability analysis of the falling film flow based on the long-wave theory predicts that longitudinal grooves lead to the decrease in the disturbance growth rate and therefore stabilize the film. The linear stability analysis also predicts that the frequency of the fastest growing disturbance mode and the wave propagation velocity decrease on a wall with longitudinal minigrooves in comparison with a smooth wall. In the present work the effect of the longitudinal minigrooves on the falling film flow is studied experimentally. We use the shadow method and the confocal chromatic sensoring technique to study the wavy structure of falling films on smooth walls and on walls with longitudinal minigrooves. The measured film thickness profiles are used to quantify the effect of the wall topography on wave characteristics. The experimental results confirm the theoretical predictions.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Experimental setup

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Figure 2

(a) Smooth and structured evaporator tubes; (b) structure geometry

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Figure 3

Arrangement for the simultaneous film thickness measurements by the CHR method and the shadow method

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Figure 4

Geometry of the minigroove

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Figure 5

Average film thickness (shadow method and numerical calculations)

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Figure 6

Wave propagation velocities on smooth and structured surfaces (l4=1 m)

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Figure 7

Interface arc length for the structured and smooth surfaces (l4=1 m)

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Figure 8

Evolution of film thickness at the smooth and structured surfaces (Re=30 and l5=1.30 m)

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Figure 9

Average wave frequencies for the smooth and structured surfaces (l5=1.30 m)

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Figure 10

Scaled disturbance growth rate versus the scaled disturbance wave number. (Solid lines) Structured wall. (Dashed lines) Smooth wall.

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Figure 11

Frequencies of the fastest growing waves: the ratio between the wave frequencies on the structured and smooth surfaces. The experimental data are taken from Fig. 9.



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