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Research Papers: Electronic Cooling

# Spray Cooling With Ammonia on Microstructured Surfaces: Performance Enhancement and Hysteresis Effect

[+] Author and Article Information
Huseyin Bostanci1

Rini Technologies, Inc. (RTI), 582 South Econ Circle, Oviedo, FL 32765huseyin.bostanci@rinitech.com

Daniel P. Rini

Rini Technologies, Inc. (RTI), 582 South Econ Circle, Oviedo, FL 32765dan@rinitech.com

John P. Kizito

Department of Mechanical and Chemical Engineering, North Carolina A&T State University, Greensboro, NC 27411jpkizito@ncat.edu

Louis C. Chow

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

1

Corresponding author.

J. Heat Transfer 131(7), 071401 (May 05, 2009) (9 pages) doi:10.1115/1.3089553 History: Received April 01, 2008; Revised December 15, 2008; Published May 05, 2009

## Abstract

Experiments were performed to investigate spray cooling on microstructured surfaces. Surface modification techniques were utilized to obtain microscale indentations and protrusions on the heater surfaces. A smooth surface was also tested to have baseline data for comparison. Tests were conducted in a closed loop system with ammonia using RTI’s vapor atomized spray nozzles. Thick film resistors, simulating heat source, were mounted onto $1×2 cm2$ heaters, and heat fluxes up to $500 W/cm2$ (well below critical heat flux limit) were removed. Two nozzles each spraying $1 cm2$ of the heater area used $96 ml/cm2 min$$(9.7 gal/in.2 h)$ liquid and $13.8 ml/cm2 s$$(11.3 ft3/in.2 h)$ vapor flow rate with only 48 kPa (7 psi) pressure drop. Comparison of cooling curves in the form of surface superheat $(ΔTsat=Tsurf−Tsat)$ versus heat flux in the heating-up and cooling-down modes (for increasing and decreasing heat flux conditions) demonstrated substantial performance enhancement for both microstructured surfaces over smooth surface. At $500 W/cm2$, the increases in the heat transfer coefficient for microstructured surfaces with protrusions and indentations were 112% and 49% over smooth surface, respectively. Moreover, results showed that smooth surface gives nearly identical cooling curves in the heating-up and cooling-down modes, while microstructured surfaces experience a hysteresis phenomenon depending on the surface roughness level and yields lower surface superheat in the cooling-down mode, compared with the heating-up mode, at a given heat flux. Microstructured surface with protrusions was further tested using two approaches to gain better understanding on hysteresis. Data indicated that microstructured surface helps retain the established three-phase contact lines, the regions where solid, liquid, and vapor phases meet, resulting in consistent cooling curve and hysteresis effect at varying heat flux conditions (as low as $25 W/cm2$ for the present work). Data also confirmed a direct connection between hysteresis and thermal history of the heater.

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Copyright © 2009 by American Society of Mechanical Engineers
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## Figures

Figure 1

Schematic of the experimental setup

Figure 2

RTI’s 1×2 vapor atomized nozzle array (a), 1×2 heater (b), and entire nozzle array-heater assembly (c)

Figure 8

Surface superheat as a function of time as heat flux changes in steps of 25–50 W/cm2 every 3 min for micro-p surface in heating-up and cooling-down modes

Figure 9

Surface superheat as a function of heat flux for micro-p surface in heating-up and cooling-down modes

Figure 10

Surface superheat as a function of heat flux for micro-p surface in heating-up and cooling-down modes (a close-up view of Fig. 9)

Figure 11

Surface superheat as a function of time as heat flux changes in steps of 100 W/cm2 every 3 min for micro-p surface in heating-up and cooling-down modes

Figure 12

Surface superheat as a function of heat flux for micro-p surface in heating-up and cooling-down modes

Figure 13

Surface superheat as a function of heat flux for micro-p surface in heating-up and cooling-down modes (a close-up view of Fig. 1)

Figure 14

Normalized heat transfer coefficients at constant heat fluxes as a function of maximum heat flux in heating-up mode indicating quantitative change in heat transfer due to hysteresis effect

Figure 3

Schematics and SEM images of micro-i (top) and micro-p (bottom) surfaces

Figure 4

Surface superheat as a function of time as heat flux changes in steps of 100 W/cm2 every 5 min from 0 to 500 then back to 0 W/cm2 for smooth and microstructured surfaces in heating-up and cooling-down modes

Figure 5

Surface superheat as a function of heat flux for smooth and microstructured surfaces in heating-up and cooling-down modes

Figure 6

Heat transfer coefficients as a function of heat flux for smooth and microstructured surfaces in heating-up and cooling-down modes

Figure 7

Enhancement factor as a function of heat flux for smooth and microstructured surfaces in heating-up and cooling-down modes

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