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Research Papers: Evaporation, Boiling, and Condensation

Droplet and Bubble Dynamics in Saturated FC-72 Spray Cooling on a Smooth Surface

[+] Author and Article Information
Ruey-Hung Chen1

Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450chenrh@mail.ucf.edu

David S. Tan, Kuo-Chi Lin, Louis C. Chow, Alison R. Griffin, Daniel P. Rini

Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450

1

Corresponding author.

J. Heat Transfer 130(10), 101501 (Aug 07, 2008) (9 pages) doi:10.1115/1.2953237 History: Received June 29, 2007; Revised April 01, 2008; Published August 07, 2008

Droplet and bubble dynamics and nucleate heat transfer in saturated FC-72 spray cooling were studied using a simulation model. The spray cooling system simulated consists of three droplet fluxes impinging on a smooth heater, where secondary nuclei outnumber the surface nuclei. Using the experimentally observed bubble growth rate on a smooth diamond heater, submodels were assumed based on physical reasoning for the number of secondary nuclei entrained by the impinging droplets, bubble puncturing by the impinging droplets, bubble merging, and the spatial distribution of secondary nuclei. The predicted nucleate heat transfer was in agreement with experimental findings. Dynamic aspects of the droplets and bubbles, which had been difficult to observe experimentally, and their ability in enhancing nucleate heat transfer were then discussed based on the results of the simulation. These aspects include bubble merging, bubble puncturing by impinging droplets, secondary nucleation, bubble size distribution, and bubble diameter at puncture. Simply increasing the number of secondary nuclei is not as effective in enhancing nucleate heat transfer as when it is also combined with increased bubble puncturing frequency by the impinging droplets. For heat transfer enhancement, it is desirable to have as many small bubbles and as high a bubble density as possible.

FIGURES IN THIS ARTICLE
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Copyright © 2008 by American Society of Mechanical Engineers
Topics: Bubbles , Simulation
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Figures

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Figure 1

Schematic of spray cooling system

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Figure 2

Average bubble diameter versus time based on experimental observations. The curve fit is d(t)=Ct, with C=0.0100618m and t in seconds. Note that the experimental growth rates are nearly equal and the curve was therefore used for all three values of N in the simulation. The typical scatter of the experimental data is approximately 50μm.

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Figure 3

A snapshot (with on exposure time equal to 0.01ms) of bubbles and droplets arriving at the heater based on simulation result. Note that open circles represent bubbles having various diameters and solid dots represent droplets, which have a uniform size of 100μm in diameter. N=2.0×106∕cm2s; Csec=1; Rd=1; nfix=0.

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Figure 4

(a) For N=2.0×106∕cm2sto4.4×106∕cm2s and 8.2×106∕cm2s, with Δt=0.01ms, Csec=1, Rd=1, and nfix=0: surface bubble density (nb) as a function of time. (b) For N=2.0×106∕cm2sto4.4×106∕cm2s and 8.2×106∕cm2s, with Δt=0.01ms, Csec=1, Rd=1, and nfix=0: the number of bubbles punctured per cm2 per time step. Note that the simulation quickly reaches steady state after t⩾0.01s (i.e., after 1000 time steps).

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Figure 5

Nucleate heat transfer (qN″) versus bubble density (nb); Δt=0.01ms, Csec=1, 2, 5, 10, and 15, nfix=0, and Rd=1. Circles, squares and inverted triangles are for N=2.0×106∕cm2s, 4.4×106∕cm2s, and 8.2×106∕cm2s, respectively. Solid symbols are experimental results and open symbols represent simulated results. The typical scatter of the experimental heat transfer data is approximately ±8%.

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Figure 6

Fraction of bubbles with diameter d, P(d); Δt=0.01ms, Csec=1, nfix=0, and Rd=1

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Figure 7

Fraction of bubbles punctured at diameter d, P(db); Δt=0.01ms, Csec=1, nfix=0, and Rd=1

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