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TECHNICAL PAPERS: Micro/Nanoscale Heat Transfer

Spray Cooling of High Aspect Ratio Open Microchannels

[+] Author and Article Information
Johnathan S. Coursey, Kenneth T. Kiger

Department of Mechanical Engineering, University of Maryland, College Park, MD 20742

Jungho Kim1

Department of Mechanical Engineering, University of Maryland, College Park, MD 20742kimjh@umd.edu

1

Corresponding author.

J. Heat Transfer 129(8), 1052-1059 (Jan 18, 2007) (8 pages) doi:10.1115/1.2737476 History: Received July 21, 2006; Revised January 18, 2007

Abstract

Direct spraying of dielectric liquids has been shown to be an effective method of cooling high-power electronics. Recent studies have illustrated that even higher heat transfer can be obtained by adding extended structures, particularly straight fins, to the heated surface. In the current work, spray cooling of high-aspect-ratio open microchannels was explored, which substantially increases the total surface area and allows more residence time for the incoming liquid to be heated by the wall. Five such heat sinks were constructed, and their thermal performance was investigated. These heat sinks featured a projected area of $1.41×1.41cm2$, channel width of $360μm$, a fin width of $500μm$, and fin lengths of $0.25mm$, $0.50mm$, $1.0mm$, $3.0mm$, and $5.0mm$. The five enhanced surfaces and a flat surface with the same projected area were sprayed with a full cone nozzle using PF-5060 at $30°C$ and nozzle pressure differences from $1.36–4.08atm$$(69–121ml∕min)$. In all cases, the enhanced surfaces improved thermal performance compared to the flat surface. Longer fins were found to outperform shorter ones in the single-phase regime. Adding fins also resulted in the onset of two-phase effects (and higher-heat transfer) at lower wall temperatures than the flat surface. The two-phase regime was characterized by a balance between added area, changing flow flux, flow channeling, and added conduction resistance. Spray efficiency calculations indicated that a much larger percentage of the liquid sprayed onto the enhanced surface evaporated than with the flat surface. Fin lengths between $1mm$ and $3mm$ appeared to be optimum for heat fluxes as high as $124W∕cm2$ (based on projected area) and the range of conditions studied.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

Figure 1

Schematic diagram of the experimental apparatus

Figure 2

OFHC copper heating base and a detachable heating neck with four thermocouple holes, each positioned through a different face

Figure 3

Schematic of incident flow measurement system

Figure 4

Spray cooling curves for 1.36atm(20psig) nozzle pressure where solid lines indicate single-phase, two-phase, and drying-out regimes. Heat fluxes are based on the 2cm2 projected area.

Figure 5

Spray cooling curves for 2.72atm(40psig) nozzle pressure where solid lines indicate single-phase and two-phase regimes. Heat fluxes are based on the 2cm2 projected area.

Figure 6

Spray cooling curves for 4.08atm(60psig) nozzle pressure where solid lines indicate single-phase and two-phase regimes. Heat fluxes are based on the 2cm2 projected area.

Figure 7

Heat flux as a function of nozzle pressure difference for Twall=60°C (similar results are obtained at other wall temperatures). Heat fluxes are based on the 2cm2 projected area.

Figure 8

Area and heat flux enhancement as a function of fin length for Twall=60°C (similar results are obtained at other wall temperatures). Heat fluxes are based on the 2cm2 projected area.

Figure 9

One-dimensional model results as a function of fin length for Twall=60°C, ΔP=4.08atm(60psig) normalized by hflat=9784W∕m2K

Figure 10

Predicted temperature profile on 5mm fins using one-dimensional model for Twall=60°C, ΔP=4.08atm(60psig)

Figure 11

Spray efficiency as a function of fin length in the single-phase regime

Figure 12

Spray efficiency as a function of wall temperature for 1.36atm(20psig) nozzle pressure difference

Figure 13

Spray efficiency as a function of wall temperature for 2.72atm(40psig) nozzle pressure difference

Figure 14

Spray efficiency as a function of wall temperature for 4.08atm(60psig) nozzle pressure difference

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