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Research Papers: Jets, Wakes, and Impingement Cooling

Numerical Simulations and Experimental Characterization of Heat Transfer From a Periodic Impingement of Droplets

[+] Author and Article Information
Mario F. Trujillo1

Department of Mechanical Engineering,  University of Wisconsin, Madison, WI 53706 e-mail: mtrujillo@wisc.edu

Jorge Alvarado

Department of Engineering Technology and Industrial Distribution,  Texas A&M University, College Station, TX 77843

Eelco Gehring

Graduate Research Assistant Department of Mechanical Engineering,  University of Wisconsin, Madison, WI 53706

Guillermo S. Soriano

Graduate Research Assistant Department of Engineering Technology and Industrial Distribution,  Texas A&M University, College Station, TX 77843

1

Corresponding author.

J. Heat Transfer 133(12), 122201 (Oct 05, 2011) (10 pages) doi:10.1115/1.4004348 History: Received December 23, 2010; Revised May 26, 2011; Published October 05, 2011; Online October 05, 2011

In this combined experimental and simulation investigation, a stream of HFE-7100 droplets striking a prewetted surface under constant heat flux was studied. An implicit free surface capturing technique based on the Volume-of-Fluid (VOF) approach was employed to simulate this process numerically. Experimentally, an infrared thermography technique was used to measure the temperature distribution of the surface consisting of a 100 nm ITO layer on a ZnSe substrate. The heat flux was varied to investigate the heat transfer behavior of periodic droplet impingement at the solid–liquid interface. In both experiments and simulations, the morphology of the impact zone was characterized by a quasi-stationary liquid impact crater. Comparison of the radial temperature profiles on the impinging surface between the experiments and numerical simulations yielded reasonable agreement. Due to the strong radial flow emanating from successive droplet impacts, the temperature distribution inside the crater region was found to be significantly reduced from its saturated value. In effect, the heat transfer mode in this region was governed by single phase convective and conductive heat transfer, and was mostly affected by the HFE-7100 mass flow rates or the number of droplets. At higher heat fluxes, the minimum temperature, and its gradient with respect to the radial coordinate, increased considerably. Numerical comparison between average and instantaneous temperature profiles within the droplet impact region showed the effect of thermal mixing produced by the liquid crowns formed during successive droplet impact events.

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Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Configuration used in this study showing a stream of droplets impinging a liquid film heated from below

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Figure 2

Experimental setup (a) consisting of: (1) frequency generator; (2) backlight illuminator; (3) droplet generator head; (4) high speed camera; (5) syringe pump; (6) heat exchanger; (7) chiller; (8) infrared camera; (9) power supply; (10) PC and data acquisition system; and (11) heater assembly. A zoomed view of the heater setup is shown in (b); it consists of: (1) holder; (2) optical grade epoxy; (3) ITO coating; (4) ZnSe substrate; (5) copper wire; and (6) electrical conductive epoxy.

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Figure 3

Schematic of temperature measurement device using ZnSe and ITO coating as heater

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Figure 4

Illustration of mesh refinement employing hanging-nodes

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Figure 5

A perspective view of the impact zone shown in (a) experiments and in (b) computations. The axisymmetry is readily apparent under both types of investigation. The vertical lines in the experimental image denote the crown location.

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Figure 6

Radial positions at which temperature profiles were examined for grid convergence

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Figure 7

Vertical temperature profiles at (a) r = 0.1 mm, (b) r = 0.2 mm, (c) and r = 0.3 mm as a function of grid resolution

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Figure 8

Grid comparison, top figure: Δ = 15 μm (coarse) and bottom figure Δ = 1.875 μm (fine)

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Figure 9

The radial displacement of the crater in the quasi-stationary period is relatively small as shown in (a). Its time history is given in (b), where after approximately 6 ms, the quasi-stationary period is established.

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Figure 10

Heat transfer coefficient from temporally-averaged VOF data

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Figure 11

Comparison between experimental and numerical values for the radial temperature profile below the ITO heater

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Figure 12

Experiments showing the relationship between the minimum temperature corresponding to various values of flowrate, Weber number, and frequency

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Figure 13

Contour map of the (a) average and (b) instantaneous temperature field within the impact zone

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