Heat Transfer Engineering, 33(3):175–204, 2012 Copyright C  Taylor and Francis Group, LLC ISSN: 0145-7632 print / 1521-0537 online DOI: 10.1080/01457632.2011.589305 Nucleate Boiling Enhancements on Porous Graphite and Microporous and Macro–Finned Copper Surfaces MOHAMED S. EL-GENK Institute for Space and Nuclear Power Studies, Chemical and Nuclear Engineering Department, and Mechanical Engineering Department, University of New Mexico, Albuquerque, New Mexico, USA Enhancements in nucleate boiling heat removal with dielectric liquids, by increasing either the bubbles nucleation sites density and/or the wetted surface area, are desirable for immersion cooling of high-power computer chips. This article presents the results of recent investigations of nucleate boiling enhancement of FC-72, HFE-7100, and PF-5060 dielectric liquids on porous graphite, copper microporous surfaces, and copper surfaces with square corner pins, 3 mm × 3 mm in cross-section and 2, 3, and 5 mm tall. All surfaces have a footprint measuring 10 × 10 mm. These investigations examined the effects of liquid subcooling up to 30 K and surface inclination, from upward-facing to downward-facing, on nucleate boiling heat transfer coefficient and critical heat flux. Natural convection of dielectric liquids for cooling chips while in the stand-by mode, at a surface average heat flux <20 kW/m 2 , is also investigated for the different surfaces. INTRODUCTION Major challenges in cooling high-power computer chips are the high thermal power dissipation and the formation of surface hot-spots, as a direct consequence of the ever increasing transis- tors’ density and processing speed. In addition, the junctions’ temperature needs to be kept sufficiently low to satisfy the manufacturer recommendations (<85–125 ◦ C, depending on the application). The local heat flux at the hot-spots could be >3 times the surface average, approaching 100 W/cm 2 . In addition to the high local heat flux, the thermal stresses caused by the hot- spots could compromise the chip performance, decrease service life and increase failure frequency (Figure 1). These concerns, in addition to the high heat dissipation of >100 W/cm 2 , need ef- fective cooling solutions of high-power computer chips (Figure 2). This figure compares the range of applicability of different cooling methods for removing the dissipated thermal power in electronics applications. For these applications, liquid cooling methods have been favored over forced air cooling. While cool- ing by impinging jets offers the highest capability for extremely This article is the text of the Donald Q. Kern Award Lecture given by the author at the 8th ASME-JSME Thermal Engineering Joint Conference (AJTEC2011), 17 March 2011, in Honolulu, HI. Address correspondence to Professor Mohamed S. El-Genk, Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131-0001, USA. E-mail: mgenk@unm.edu high heat flux, it has inherent practical limitations including the thermal stresses caused by the nonuniform surface cooling and temporal distribution. In addition, this cooling method is not suited for dealing with the high local heat flux at the hot-spots. Immersion cooling by nucleate boiling of dielectric liquids such as FC–72, PF–506, and HFE–7100 has been recognized to offer many advantages for cooling high–power chips. These in- clude moderate to high nucleate boiling heat transfer coefficient, low junctions temperature (<85 ◦ C), a relatively uniform surface temperature, and an effective mitigation of the hot–spot effect. In addition to being chemically compatible with most structure and heat spreader materials and being environmentally friendly, candidate dielectric liquids for immersion cooling of computer chips (Table 1) have relatively low saturation temperatures (54- 64 ◦ C) at atmospheric pressure that help maintain a sufficiently low junctions’ temperature without pressurization. Because of their very low surface tension (Table 1) and good wetting (static contact angle on most metal and silica surfaces <5 ◦ ), a concern with dielectric liquids is experiencing an excursion in surface temperature prior to initiating nucleate boiling. Such an excursion on plane copper (Cu) and silicon (Si) surfaces could reach or exceed 25 K [1–5], and thus is undesirable since it increases the junctions’ temperature. In addition to enhancing the nucleate boiling heat transfer of dielectric liquids on micro– and macrostructured, micro– and macroporous surfaces, the 175 Downloaded by [University of New Mexico] at 07:19 28 November 2011