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Research Papers

Bubble-Induced Water Hammer and Cavitation in Microchannel Flow Boiling

[+] Author and Article Information
David W. Fogg

 Creare Inc, P.O. Box 71, Hanover, NH 03755dwf@creare.com

Kenneth E. Goodson

Department of Mechanical Engineering, Stanford University, Building 530, Room 224, Stanford, CA 94305

J. Heat Transfer 131(12), 121006 (Oct 15, 2009) (12 pages) doi:10.1115/1.3216381 History: Received February 13, 2008; Revised October 07, 2008; Published October 15, 2009

While microchannel flow boiling has received much research attention, past work has not considered the impact of acoustic waves generated by rapidly nucleating bubbles. The present work provides a theoretical framework for these pressure waves, which resembles classical “water hammer” theory and predicts a strong influence on bubble nucleation rates and effective convection coefficients. These pressure waves result directly from confinement in microchannel geometries, reflect from geometrical transitions, and superimpose to create large transients in the static liquid pressure. Feedback from the pressure waves inhibits bubble growth rates, reducing the effective heat transfer. Pressure depressions generated by the propagating pressure pulses can cause other bubbles to grow at lower than expected wall temperatures. The additional nucleation enhances heat transfer over short times but increased flow instability may inhibit heat transfer over longer periods. The limited quantitative measurements available in the literature indicate confined bubble growth rates in microchannels are significantly lower than those predicted by the classical Rayleigh–Plesset equation. The present model predicts confined bubble growth rates to within ± 20%. A nondimensional number indicative of the relative magnitude of the water hammer pressure to bubble pressure is proposed to characterize the transitions from conventional to microchannel flow boiling.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Growth history for a bubble nucleating from a cavity (dns=2.75 μm, Tb−Tw=20 K) in a 100 μm square microchannel

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Figure 2

Measured visible area of a pancake-shaped bubble growing in a 1000 μm by a 25 μm channel. Note the area grows linearly for this confined bubble and constrained in one dimension.

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Figure 3

Bubble growth rates measured in Ref. 29 are predicted within ±20% by the confinement pressure model

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Figure 4

The effect of reducing the channel cross-sectional area for bubbles nucleating at a wall superheat of 10 K, corresponding to a bubble overpressures of 0.39 bar. None of the bubble interfaces reach the asymptotic Rayleigh–Plesset velocity (Eq. 16) before confinement retards growth.

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Figure 5

The influence of wall superheat on a bubble growing in a square 250 μm channel

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Figure 6

The effect of heat transfer coefficient on bubble growth. The lines correspond to htp=50 kW/m2, 500 kW/m2, 5 MW/m2, and 50 MW/m2. The largest two lines are virtually on top of each other.

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Figure 7

Schematic of experimental manifold designs used in simulations to determine effect system design

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Figure 8

Evolution of liquid pressure pulses in microchannel systems caused by the nucleation and growth of a single bubble. Straight manifold design (left) and tapered manifold design (right). The bubble nucleates at at z=0 for a wall superheat of 20 K and experiences inertia-controlled growth at a constant pressure throughout the simulation. The dashed lines denote axial locations with changes in channel geometry.

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Figure 9

Time history of the bubble radial velocity in the straight manifold channel. The negative water hammer reflections effectively increase the bubble growth rate.

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Figure 10

Time evolution for the local liquid pressure at z=−11.4 mm. The bubbles nucleate at Tsup=20 K in both systems. The minimum Pl for the straight manifold is 0.909 bar and 0.866 bar for the tapered manifold.

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Figure 11

The evolution of the static liquid pressure in the tapered channel if a 2.44 μm diameter cavity is located at z=−11.4 mm

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Figure 12

Time history comparison of the liquid pressure at z=−6 mm for the tapered manifold channel with (solid line) and without (dashed line) additional nucleation from a 2.44 μm cavity at z=−11.4 mm

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Figure 13

Interaction of the second bubble in the tapered manifold channel with the growth of the first bubble

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Figure 14

The growth history for bubbles at z=−11.4 mm from sites nucleating at liquid pressures of 0.868 bar, 0.878 bar, and 0.936 bar for a Tw=393.15 K. These nucleation sites measure 2.392 μm, 2.415 μm and 2.557 μm in diameter, respectively.

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