Immersion cooling is making a resurgence for use in server applications driven by an increase in chip densities and the need for reduction in overall data center power usage. This paper focuses on pool boiling characteristics from an array of heaters which simulate electronic chips on a vertically-oriented printed circuit board. A pool boiling study was conducted on an array of four bare die using two different dielectric fluids; namely Novec 649 and HFE 7100, both with low global warming potential (GWP). Tests were conducted at two die spacings; 25mm and 10mm under two different pool conditions: saturated and 15°C subcooled representing a start-up transient. Data were collected for increasing and decreasing heat flux cycles. The flux dissipated was found to be 14.6 W/cm
 2
 and 14.2 W/cm
 2
 with Novec 649 and HFE 7100 respectively for the test board with 10mm die spacings under subcooled conditions. These values are recorded at self-imposed maximum surface temperatures that ensured operation well below the critical heat flux. These flux values are quite high especially since they are attained without the need to modify the surface or add heat sinks. In an effort to increase thermal performance, tests were also conducted on dies spaced 25mm apart augmented with two different enhanced heat sinks featuring microporous and microfinned surfaces. Enhanced heat sinks performed better than bare die and the flux dissipated was found to be 18.87 W/cm
 2
 and 18.31 W/cm
 2
 using Novec 649 and HFE 7100 respectively under subcooled conditions. Additionally, these values were achieved at surface temperatures 15°C lower than the surface temperatures recorded by the bare dies for dissipating the same amount of heat. High speed images were obtained to provide a better understanding of nucleation characteristics.

Effect of system and operational parameters on the performance of an immersion-cooled multichip module for high performance computing

The corrosion protection performance of commercial organic coatings over Stainless Steel (304 L) was investigated in highly acidic media. Three coatings were applied by air spraying and subsequently evaluated for their performance against chemical corrosion by concentrated H2SO4, HNO3, and HCl in terms of % mass loss. Open current potential (OCP) and potentiodynamic polarization (PD) measurements were performed. The water condensation properties of the coatings were monitored for their evaluation as potential protective coatings in condensing economizer applications. ZN691 coating exhibited better protection than ANORZINC and TROPIS versus time in contact with 40% of H2SO4, HNO3 and 37% HCl. Electrochemical results confirmed the superior performance of ZN691 over ANORZINC and TROPIS with corrosion rates at 0.017, 0.098 and 0.097 mpy respectively. Condensation experiments for ZN691 showed a water collection rate of 1.487 ml m−2 h−1 at 40 °C and 100% RH. According to the results, the Zn rich epoxy-based coatings are promising for corrosion protection of condensing economizer elements.

Evaluation of Zn- and Fe- rich organic coatings for corrosion protection and condensation performance on waste heat recovery surfaces

ii Acknowledgments iv List of Tables viii List of Figures ix Nomenclature xii CHAPTER 1: INTRODUCTION 1 1.1 NUMERICAL MODELING OF BOILING 1 CHAPTER 2: LITERATURE REVIEW 3 2.1 BOILING MODEL OF KURUL AND PODOWSKI (RPI BOILING MODEL) 3 2.2 NUMERICAL SIMULATIONS USING THE RPI BOILING MODEL 5 2.2.1 Numerical simulations of boiling in water 6 2.2.2 Numerical simulations of boiling in other fluids 9 2.3 NUMERICAL SIMULATIONS USING OTHER METHODS AND MODELS 15 2.4 CONCLUSIONS FROM LITERATURE 16 CHAPTER 3: SINGLE PHASE SIMULATIONS IN ANSYS ICEPAK 17 3.1 SINGLE PHASE MODEL 17 3.1.1 Five die model 18 3.1.1.1 Five die model description 18 3.1.1.2 Icepak solver settings for the five die model 19 3.1.1.2 Convergence criteria and mesh independence 19 3.1.1.4 Compact conduction models 20 3.1.2 Four die model 21

Numerical Simulations of Boiling in Dielectric Fluid Immersion Cooling Scenarios of High Power Electronics

The coalescence-induced jumping of tens of microns size droplets on super-hydrophobic surfaces has been observed in both experiments and simulations. However, whether the coalescence-induced jumping would occur for smaller, particularly nanoscale droplets, is an open question. Using molecular dynamics simulations, we demonstrate that in spite of the large internal viscous dissipation, coalescence of two nanoscale droplets on a super-hydrophobic surface can result in a jumping of the coalesced droplet from the surface with a speed of a few m/s. Similar to the coalescence-induced jumping of microscale droplets, we observe that the bridge between the coalescing nano-droplets expands and impacts the solid surface, which leads to an acceleration of the coalesced droplet by the pressure force from the solid surface. We observe that the jumping velocity decreases with the droplet size and its ratio to the inertial-capillary velocity is a constant of about 0.126, which is close to the minimum value of 0.111 predi...

\textbf{Coalescence-induced jumping of nanoscale droplets on super-hydrophobic surfaces}

The motion of droplets on a super-hydrophobic surface, whether by sliding or rolling, is a hot research topic. It affects the performance of super-hydrophobic materials in many industrial applications. In this study, a super-hydrophobic surface with a varied roughness is prepared by chemical-etching. The adhesive force of the advancing and receding contact angles for a droplet on a super-hydrophobic surface is characterized. The adhesive force increases with a decreased contact angle, and the minimum value is 0.0169 mN when the contact angle is 151.47°. At the same time, the motion of a droplet on the super-hydrophobic surface is investigated by using a high-speed camera and fluid software. The results show that the droplet rolls instead of sliding and the angular acceleration increases with an increased contact angle. The maximum value of the angular acceleration is 1,203.19 rad/s2 and this occurs when the contact angle is 151.47°. The relationship between the etching time, roughness, angular acceleration, and the adhesion force of the forward and backward contact angle are discussed.

/pdf/effect-of-surface-roughness-on-the-angular-acceleration-for-5adps7civd.pdf

Effect of surface roughness on the angular acceleration for a droplet on a super-hydrophobic surface

An alternative to air-cooling of high performance computing equipment is presented. Heat removal via pool boiling in FC-72 was tested. Tests were conducted on a multichip module using 1.8 cm × 1.8 cm test die with multiple thermal test cells with temperature sensing capability. Measurements with the bare silicon die in direct contact with the fluid are reported. Additional testing included the test die directly indium-attached to copper heat spreaders having surface treatments. A screen-printed sintered boiling-enhanced surface (4 cm × 4 cm) was evaluated. Tests were conducted on an array of five die. Parameters tested include heat flux levels, dielectric liquid pool conditions (saturated or subcooled), and effect of neighboring die. Information was gathered on surface temperatures for a range of heat flux values up to 12 W/cm2. The highest heat dissipated from a circuit board with five bare die was 195 W (39 W per die). Addition of the heat spreader allowed heat dissipation of up to 740 W (from a five-die array). High-speed imaging was also acquired to help examine detailed information on the boiling process. Numerical modeling indicated that placing multiple boards in close proximity to each other did not degrade performance until board spacing was reduced to 3 mm.© 2012 ASME

Cooling of High-Performance Server Modules Using Direct Immersion

Development of a Durable Vapor Phase Deposited Superhydrohobic Coating for Steam Cycle Power Generation Condenser Tubes

The condenser performance benefits afforded by dropwise condensation have long been unattainable in steam cycle power plant condensers due to the unavailability of durable and long lasting wetting inhibiting surface treatments. However, recent work in superhydrophobic coating technology shows promise that durable coatings appropriate for use on condenser tubes in steam cycle power generation systems may soon become a reality. This work presents a nano-scale, vapor phase deposited superhydrophobic coating with improved durability comprised of several layers of rough alumina nanoparticles and catalyzed silica with a finishing layer of perfluorinated silane. This coating was applied to solid, hemicylindrical test surfaces fabricated from several common condenser tube materials used in power generation system condensers: Titanium, Admiralty brass, Cupronickel, and Sea Cure stainless steel condenser tube materials as well as 304 stainless steel stock. The development evolution of the coating and its effect on condensation behavior on the above materials are presented. Results show that the performance enhancement, measured in rate of heat transfer spikes corresponding to condensate roll-off events, was best for the titanium surface which produced 64% more events than the next most active material when coated using the most durable surface treatment tested in this work. INTRODUCTION As electrical power production costs continually rise due to governmental regulations and increasing fuel costs, power producers must diligently seek capacity and efficiency increases in all generation system components and subsystems in order to keep up with ever increasing power consumption. Current condenser design procedures for steam power generation systems assume filmwise condensation instead of the preferable and more efficient dropwise mode of condensation because traditionally there has been no method of maintaining dropwise condensation in the harsh environment produced inside a power generation condenser [1]. The major hurdle to the widespread implementation of hydrophobic coatings on power generation condenser tubes to allow dropwise condensation is the poor durability of these coatings. Low durability coatings do not provide the longevity of coating Proceedings of the ASME 2017 Power Conference Joint With ICOPE-17 POWER2017-ICOPE-17 June 26-30, 2017, Charlotte, North Carolina, USA POWER-ICOPE2017-3080 1 Copyright © 2017 ASME effectiveness to make their use economically viable, and the difficulty in installing coated condenser tubes without damaging the coating further discourages their use. In order to realize the performance increases potentially afforded by dropwise over filmwise condensation sufficiently durable superhydrophobic coatings and improved installation procedures must be developed. The condensate mobility and heat transfer advantages offered by dropwise over filmwise condensation are well documented [2-5]. During dropwise condensation, droplet mobility greatly increases the rates of heat and mass transfer because as droplets move they expose portions of “fresh” condensing surface that are free of a liquid thermal barrier. Because of this, dropwise condensation can produce heat transfer coefficients up to ten times those produced during filmwise condensation [3]. This has motivated many efforts to study and produce superhydrophobic surfaces that excel at achieving dropwise condensation. Several methods have been explored but the one that has emerged as the most promising is microscopic surface texturing. This technique usually consists of microscale surface features covered with a nanoscale texturing. The nanoscale texture produces the superhydrophobic characteristics and the microscale features serve to protect the nano-texture from being completely removed by mechanical abrasion or filled with foreign material during handling or coming in contact with other objects [1]. One of the earlier works studying hierarchical micro/nano textured surfaces found that when new, these types of surface exhibited superhydrophobic characteristics that would quickly diminish during condensation but when a hydrophobic coating was applied over the two-tier surface texture the superhydrophobic properties were more stable and lasted longer, and a higher degree of droplet mobility was achieved [6]. Droplet mobility on superhydrophobic surfaces can be quite dramatic. Several studies have reported observing droplets remove themselves from the surface via a spontaneous jumping phenomenon that occurs as the result of a coalescence event [3,7-10]. The first such report was made by Boreyko et. al. [3] in which, jumping droplets were observed moving away from the condenser surface at speeds as high as 1 m/s. Subsequent studies have found methods of giving the jumping droplets a horizontal component of velocity [8] which can allow the jumping droplet to clear multiple other droplets from the condenser surface instead of only removing itself [9]. This is done by forcing the coalescence event to occur around a vertical obstacle, such as micromachined pillars, which gives the resulting jumping droplet a horizontal component of velocity which causes the moving droplet to collide with others on the surface taking them with it as it moves off of the surface [9]. While micro-machined pillars may not be a realistic surface treatment for condenser tubes in a power generation system without prerequisite advances in manufacturing and handling techniques and systems, the droplet mobility offered by these surfaces is impressive and shows the enormous potential improvements that may be made to condenser performance in the future. Currently there are several other methods of achieving dropwise condensation and even superhydrophobicity on tubular metallic substrates being studied. Manvel used several treatments to promote dropwise condensation on horizontal copper-nickel condenser tubes in [4]. He reported a 53% improvement in outer heat transfer coefficient when dropwise condensation was promoted by applying a fluorocarbon coating called Nedox, and a 45% improvement in outer heat transfer coefficient on similar dropwise condensing tubes sputtered with TFE compared to bare copper-nickel tubes [4]. Another study recorded a heat transfer coefficient improvement of an impressive 250% on gold plated aluminum tubes with a dropwise condensation promoting organic self-assembled monolayer (SAM) compared to a similar tube without the SAM [5]. Miljkvoic et. al. reported developing a promising technique for forming a superhydrophobic layer on copper tubes [10]. In this study a layer of nano-scale, sharp, knife like copper oxide structures was grown on the surface of copper tubes and then sealed with a vapor phase deposited silane coating. Dropwise condensation on these tubes produced heat transfer coefficients up 85% higher than on filmwise condensing bare copper tubes and under certain conditions small droplets were observed jumping from the tubes [10]. Although there are large heat transfer performance benefits to be gained by utilizing superhydrophobic coatings on condenser tubes in power generation plants and on many other phase change heat exchangers, this technology has yet to be widely implemented because all current methods of promoting dropwise condensation are relatively fragile [5]. The objective of this paper is to report the development and testing of an alumina nano-particle vapor phase deposited superhydrophobic coating for metal substrates adapted from a similar coating currently used to protect consumer electronics from water damage. This surface treatment shows promising durability performance potential while maintaining sufficient nonwetting characteristics and can be applied to condenser tubes in-situ through vapor deposition processes. TEST SURFACE FABRICATION Test surfaces were prepared using Southern Companyprovided one-inch-diameter condenser tube samples in the following materials: Titanium, Admiralty brass, SEA-CURE and Cupronickel. To produce solid test surfaces, capable of conducting heat, the following fabrication sequence was followed. Aluminum support mandrels were fabricated to match the inner diameter for each tube sample. These were pressed into the tube samples to support them radially and axially to allow the tubes to be clamped and machined like solid round rods. The tube and mandrel assemblies were then machined into one-inch long half cylinders, resulting in a 2 Copyright © 2017 ASME 25mm x 25mm base area matching the cooled surface footprint in the experimental facility (described below). The remaining mandrel material was then removed. Lead was cast directly into each half tube section. After cooling, the lead casting and tube section were separated and then permanently joined using silver based thermal interface material and epoxy. Additional flat and half round control surfaces were produced using 304 stainless steel stock machined to the desired dimensions. After fabrication was completed, surface treatments were applied in an RPX-540 Vapor Deposition System. The system deposits customized nano-composite structures using a hybrid VPD/ALD (vapor phase deposition/atomic layer deposition) process. Up to five precursors can be sequentially applied to create a variety of specialty films for each specific application. All the precursors are heated to create a reservoir of chemical vapor and their introductory timing, dose, and order is controlled by a LabVIEW-based software system with automatic process operation. A typical coating process is as follows. Incremental layers of Al2O3 nanoparticles are accumulated by repeating VPD cycles, followed by a 600-cycle pyridine-catalyzed silica protection film and a final perfluoronated silane finish layer for superhydrophobicity. SURFACE TREATMENT DEVELOPMENT Several coating methods have been studied in recent years. Technologies like CVD (Chemical Vapor Deposition) and ALD (Atomic Laye

Development of a Durable Vapor Phase Deposited Superhydrophobic Coating for Steam Cycle Power Generation Condenser Tubes

As phase change, specifically boiling becomes an ever more popular and advanced mechanism for cooling electronics, efforts must be made to find methods of increasing the thermal performance of condensers.

Christopher M. Duron

Papers

Cooling of High-Performance Server Modules Using Direct Immersion

Development of a Durable Vapor Phase Deposited Superhydrohobic Coating for Steam Cycle Power Generation Condenser Tubes

Development of a Durable Vapor Phase Deposited Superhydrophobic Coating for Steam Cycle Power Generation Condenser Tubes

Condensate mobility actuated by microsurface topography and wettability modifications