Research Papers: Forced Convection

Heat Transfer Impact of Synthetic Jets for Air-Cooled Array of Fins

[+] Author and Article Information
Ri Li

School of Engineering,
University of British Columbia,
Kelowna, BC V1V 1V7, Canada
e-mail: sunny.li@ubc.ca

William D. Gerstler

Energy Systems Laboratory,
GE Global Research,
Niskayuna, NY 12309

Mehmet Arik

School of Engineering,
Ozyegin University,
Istanbul 34794, Turkey

Benjamin Vanderploeg

Electronics Design ECOE,
GE Aviation Systems,
Grand Rapids, MI 49512

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received October 22, 2014; final manuscript received September 17, 2015; published online October 13, 2015. Assoc. Editor: Ali Khounsary.

J. Heat Transfer 138(2), 021702 (Oct 13, 2015) (10 pages) Paper No: HT-14-1698; doi: 10.1115/1.4031647 History: Received October 22, 2014; Revised September 17, 2015

Free convection air cooling from a vertically placed heat sink is enhanced by upward concurrent pulsated air flow generated by mesoscale synthetic jets. The cooling enhancement is experimentally studied. An enhancement factor is introduced and defined as the ratio of convection heat transfer coefficients for jet-on (enhanced convection) to jet-off (natural convection) cooling conditions. To obtain the two coefficients, heat transfer by radiation is excluded. A high-resolution infrared (IR) camera is used to capture detailed local temperature distribution on the heat sink surface under both cooling conditions. Analysis is carried out to obtain local convection heat transfer coefficients based on measured local surface temperatures. The enhancement of convectional cooling by synthetic jets can be then quantified both locally and globally for the entire heat sink. Two categories of thermal tests are conducted. First, tests are conducted with a single jet to investigate the effects of jet placement and orifice size on cooling enhancement, while multiple jets are tested to understand how cooling performance changes with the number of jets. It is found that the cooling enhancement is considerably sensitive to jet placement. Jet flow directly blowing on fins provides more significant enhancement than blowing through the channel between fins. When using one jet, the enhancement ranges from 1.6 to 1.9 times. When multiple jets are used, the heat transfer enhancement increases from 3.3 times for using three jets to 4.8 times for using five jets. However, for practical thermal designs, increasing the number of jets increases the power consumption. Hence, a new parameter, “jet impact factor (JIF),” is defined to quantify the enhancement contribution per jet. JIF is found to change with the number of jets. For example, the four-jet configuration shows higher JIF due to higher contribution per jet than both three-jet and five-jet configurations.

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Fig. 1

A typical operation cycle of a synthetic jet consists of compression (a) and expansion (b). One of the synthetic jets used in the present work is shown (c).

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Fig. 2

Schematic of the aluminum finned heat sink. (a) Front and side views of the heat sink; (b) the dimensions of the fins: z = 12.70 mm, s = 13.34 mm, t = 1.65 mm, and b = 5.08 mm. The heater and insulation are not shown in (b).

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Fig. 3

Temperature measurement by IR camera (FLIR SC3000) with emissivity set to 0.97 in comparison with that by T-type thermocouple

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Fig. 4

The heat sink only cooled by free convection (jet-off): (a) contour of temperature Tf,1 (°C); (b) contour of effective heat transfer coefficient h1 (W/m2 K); and (c) contour of convection heat transfer coefficient hc,1 (W/m2 K)

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Fig. 5

Setup of single-jet tests: (a) schematic drawing and (b) experimental setup

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Fig. 10

(a) For all multiple-jet test configurations, each jet is placed x1=2 mm below the heat sink with on-fin orientation; (b) experimental setup for the test using three jets.

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Fig. 9

Overall enhancement of convection heat transfer E¯F.c of single-jet tests

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Fig. 8

The enhancement of convection heat transfer EF,c of the heat sink cooled by one synthetic jet (jet-on) with varied jet placements. Jets with two orifice sizes are used: (a) x3=8 mm and (b) x3=15 mm. The configuration highlighted with a dotted frame is selected for multiple-jet tests.

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Fig. 6

Temperature contours Tf,2 (°C) of the heat sink cooled by one synthetic jet (jet-on) with varied jet placements. Jets with two orifice sizes are used: (a) x3=8 mm and (b) x3=15 mm.

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Fig. 11

Thermal results of the heat sink cooled by multiple jets: (a) contours of temperature Tf,2 (°C); (b) contours of local convection heat transfer coefficient hc,2 (W/m2 K); and (c) contours of enhancement of convection heat transfer EF,c

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Fig. 12

Overall enhancement of convection heat transfer E¯F.c and overall enhancement of total heat transfer E¯F.t of multiple-jet tests

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Fig. 7

Contours of local convection heat transfer coefficient hc,2 (W/m2 K) calculated using the temperature data Tf,2 shown in Fig. 6 for the heat sink cooled by one synthetic jet: (a) x3 = 8 mm and (b) x3 = 15 mm

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Fig. 13

The JIF (enhancement contribution per jet) of multiple-jet tests. The value shown by the dotted line comes from the single-jet test shown in Fig. 8.




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