Aluminum based one and two dimensional micro fin array structures high throughput fabrication and heat transfer testing This content has been downloaded from IOPscience Please scroll down to see the f[.]
Home Search Collections Journals About Contact us My IOPscience Aluminum-based one- and two-dimensional micro fin array structures: high-throughput fabrication and heat transfer testing This content has been downloaded from IOPscience Please scroll down to see the full text 2017 J Micromech Microeng 27 025012 (http://iopscience.iop.org/0960-1317/27/2/025012) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 150.216.68.200 This content was downloaded on 24/02/2017 at 11:09 Please note that terms and conditions apply You may also be interested in: Cu microchannel devices: fabrication and analysis of flow and heat transfer characteristics Fanghua Mei, W A Phillips, B Lu et al Fabrication, assembly and heat transfer testing of low-profile copper-based MHEs Bin Lu, Ke Chen, W J Meng et al MEMS silicon-based micro-evaporator M Mihailovic, C M Rops, J Hao et al Effects of structural parameters on flow boiling performance of reentrant porous microchannels Daxiang Deng, Yong Tang, Haoran Shao et al Subcooled convective boiling in structured surface microchannels Shou-Shing Hsieh and Chih-Yi Lin Active control of flow and heat transfer in silicon microchannels Guohua Liu, Jinliang Xu, Yongping Yang et al Internal passivation of Al-based microchannel devices by electrochemical anodization Paul J Hymel, D S Guan, Yang Mu et al Pool boiling heat transfer on artificial micro-cavity surfaces in dielectric fluid FC-72 Chih Kuang Yu, Ding Chong Lu and Tsung Chieh Cheng Investigation of change in surface morphology of heated surfaces upon pool boiling of magnetic fluids under magnetic actuation Mostafa Shojaeian, Melike-Mercan Yildizhan, Ömer Cokun et al Journal of Micromechanics and Microengineering J Micromech Microeng 27 (2017) 025012 (9pp) doi:10.1088/1361-6439/aa53c9 Aluminum-based one- and two-dimensional micro fin array structures: high-throughput fabrication and heat transfer testing Philip A Primeaux1, Bin Zhang1, Xiaoman Zhang1, Jacob Miller1, W J Meng1, Pratik KC2 and Arden L Moore2 Mechanical and Industrial Engineering Department, Louisiana State University, Baton Rouge, LA 70803, USA Mechanical Engineering Department, Louisiana Tech University, Ruston, LA 71272, USA E-mail: wmeng1@lsu.edu Received 12 July 2016, revised December 2016 Accepted for publication 14 December 2016 Published January 2017 Abstract Microscale fin array structures were replicated onto surfaces of aluminum 1100 and aluminum 6061 alloy (Al1100/Al6061) sheet metals through room-temperature instrumented roll molding Aluminum-based micro fin arrays were replicated at room temperature, and the fabrication process is one with high throughput and low cost One-dimensional (1D) micro fin arrays were made through one-pass rolling, while two-dimensional (2D) micro fin arrays were made by sequential 90° cross rolling with the same roller sleeve For roll molding of 1D micro fins, fin heights greater than 600 µm were achieved and were shown to be proportional to the normal load force per feature width At a given normal load force, the fin height was further shown to scale inversely with the hardness of the sheet metal For sequential 90° cross rolling, morphologies of roll molded 2D micro fin arrays were examined, which provided clues to understand how plastic deformation occurred under cross rolling conditions A series of pool boiling experiments on low profile Al micro fin array structures were performed within Novec 7100, a widely used commercial dielectric coolant Results for both horizontal and vertical surface orientations show that roll molded Al micro fin arrays can increase heat flux at fixed surface temperature as compared to un-patterned Al sheet The present results further suggest that many factors beyond just increased surface area can influence heat transfer performance, including surface finish and the important multiphase transport mechanisms in and around the fin geometry These factors must also be considered when designing and optimizing micro fin array structures for heat transfer applications Keywords: microscale roll molding, aluminum micro fin arrays, pool boiling heat transfer (Some figures may appear in colour only in the online journal) 1. Introduction of heat transfer performance and mechanical integrity Microscale compression molding was used successfully to replicate one- and two-dimensional (1D/2D), microscale features directly onto surfaces of high thermal conductivity and high ductility metals, such as aluminum (Al) and copper (Cu) [2–4] Subsequent bonding of Al and Cu caps to open 1D microchannel arrays led to the formation of Al- and Cubased, enclosed, microchannel heat exchangers (MHEs) [5] Such Al and Cu MHEs can be made with low profile and high Metal-based microscale fin array structures are of interest for microscale heat transfer applications While the classic paper by Tuckerman and Pease showed the advantages of microscale structures with increased surface to volume ratio in singlephase convective heat transfer applications [1], silicon-based structures, such as those employed by Tuckerman and Pease in their original paper, are not optimal from the perspective 1361-6439/17/025012+9$33.00 © 2017 IOP Publishing Ltd Printed in the UK P A Primeaux et al J Micromech Microeng 27 (2017) 025012 Cut-away view of the roller sleeve with an array of circumferential microprotrusions Sheet metal Cylindrical roller Figure 1. A schematic of the roll molding process for replicating straight, 1D micro fin arrays on sheet metals cooling capacity in the single-phase, forced flow, convective heat transfer regime [6] Compression molding of metals at the microscale involves a mold insert containing a microscale surface pattern inverse to the final desired pattern on the metal work piece When the mold insert is placed onto the metal work piece with a compression load applied, patterns on the mold insert are transferred onto the work piece surface through plastic deformation [7] Because the key parameter controlling this compression molding process is the contact pressure [7], the compression force required to achieve replication of a given pattern scales linearly with the area of the pattern This fact, combined with the requirement on alignment accuracy between the mold insert and the molded metal substrate, makes scaling up of microscale compression molding to larger pattern footprints more difficult We have shown previously that roll molding can be used to generate 1D microchannel arrays with large depths on sheet metal surfaces in a parallel manner [8] In roll molding, metal sheets are passed through a device analogous to that used for flat rolling of sheet metals [9], in which one or both rollers can contain microscale surface protrusions Plastic deformation is induced in the metal sheet when it comes into contact with the patterned roller(s), thereby creating microscale patterns on one(both) surfaces of the metal sheet [8] Roll molding offers much increased fabrication throughput as compared to compression molding, and reduces the requirements on alignment The potential of using roll molding to generate 1D/2D surface patterns on metal substrates and the use of 1D/2D micro fin array structures for heat transfer applications provide motiv ation for the present study In addition to convective heat transfer in single-phase, forced liquid flow situations, enhancing two-phase heat transfer efficiency using metals with microscale 1D/2D surface patterns is also of interest for pool boiling environments [10–16], which have broad industrial processing applications including power plants, refrigeration systems, and food production Two-phase direct immersion cooling has become increasingly attractive as a means of achieving efficient thermal management of high density electronics systems and IT hardware [17–22] Besides facilitating lower operating temperatures or higher power/performance capabilities, an ideal two-phase immersion cooling surface enhancer should also be low profile so as not to require large printed circuit board spacing and negatively affect computing density Thus the types of low cost, high throughput micro fin array structures produced in this work may hold promise for facilitating enhanced heat transfer and achieving widespread industrial adoption Here we present preliminary results on the heat transfer performance of select Al micro fin array structures within pool boiling environments that mimic those encountered in direct immersion cooling of electronics, as a demonstration of potential and as a guide towards future work 2. Experimental setup and procedures 2.1. Roll molding of Al strips and characterization Roll molding of Al sheet metals was carried out on a customdesigned and built machine, analogous to a sheet metal roll [9] Rotation of the lower steel roller of the machine, with a nominal outer diameter (OD) of 108 mm (4.25 inches), was computer controlled with an angular speed range of 0–6 rpm and instrumented so the total input torque was measured The upper steel roller, with the same nominal OD, was attached to a hydraulically driven assembly, actuated to move in the vertical direction, and instrumented to measure the normal loading force and the upper roller displacement [8] Both steel rollers could accommodate roller sleeves containing microscale protrusions on their external surfaces In the present experiments, shown schematically in figure 1, the upper roller sleeve was made of hardened 52 100 steel and contained an array of circumferential micro-protrusions made by wire electrical discharge machining The protrusion cross sections were trapezoidal in shape, with a sidewall taper of ~7° [8] The total width of the roller sleeve was 25.4 mm (1 inch) The OD of the roller sleeve was 108.7 mm (4.28 inches), slightly larger than the steel roller OD Roll molding with such a roller sleeve produced 1D micro fin arrays with a similar sidewall taper [8] P A Primeaux et al J Micromech Microeng 27 (2017) 025012 Figure 2. Annotated illustrations of (a) the pool boiling experimental setup and (b) the sample stage Not to scale Al as well as 1D- and 2D- patterned samples fabricated by roll molding An annotated illustration of the experimental setup is given in figure The walls, bottom, and top of the cube-shaped test tank were stainless steel plates with interior surfaces possessing a #8 mirror finish to minimize bubble nucleation from surfaces other than the sample under study Three circular polycarbonate (Lexan) viewports were mounted over cutouts within the sidewalls with a gasket ring in between These viewports were utilized for visualization during the experiment The lid located at the top of the tank is mechanically clamped along with an interfacing edge gasket to minimize vapor escape A passive pressure valve fitted into the lid maintained the vessel at atmospheric conditions A pair of sealed feedthroughs allowed for cartridge heater power leads and Type K thermocouples (Omega) to pass through the setup’s lid and access the variable power source and data acquisition hardware One thermocouple was inserted within the heated stage in close proximity to the base of the mounted sample The remaining thermocouple was used to measure the temperature of the pool away from the sample stage Two coils of thin-walled copper tubing occupied interior space within the tank One coil resided within the pool and was used to maintain the temperature of the pool at the desired subcooled condition In subcooled pool boiling, the temperature of the pool away from the heat source is below the saturation temperature (Tsat, more commonly referred to as the boiling point) of the medium at the operating pressure The second copper coil was located above the free surface of the pool and served as a condenser for the coolant vapor produced during boiling After condensing, the coolant would fall back into the pool and thus a constant fluid level was maintained throughout the experiment Each coil was connected to its own water-based closed-loop flow system with dedicated liquid-to-air heat exchangers, pumps, and reservoirs The sample stage was mounted on a pair of adjustable mechanical supports made of low thermal conductivity Nylon The connection between the stage and the supports allowed the stage to rotate such that data could be obtained for arbitrary Commercial Al1100 (99% + Al) and Al6061 sheet metal strips were used as substrates The initial thickness of the Al strips was 6.35 mm (0.25 inch) The initial width of the Al strips was 31.8 mm (1.25 inches) Al strips were used in the asreceived condition, and after annealing at 300 °C and 500 °C for various durations Because the Al strip widths were larger than the pattern width on the roller sleeve of 25.4 mm, micro fin array patterns were imprinted onto strip surfaces with two untouched rims on the outside During roll molding, the lower steel roller without surface pattern and the upper steel roller sleeve with a surface pattern were rotated at 0.25 rpm such that a constant rolling surface speed of 1.4 mm s−1 was achieved Al strips were placed in between the bottom and top rollers Once rotation started, the normal load was increased until a pre-set load level was reached The metal strip was then rolled in steady state, with the normal loading force continuously recorded Multiple experiments were carried out by varying the normal loading force applied to the Al strips, while keeping all other parameters unchanged No damage was observed on the roller sleeve after multiple roll molding runs Microhardness measurements on as-received and annealed Al strips were conducted on a Future-Tech® FM-1E tester, using a diamond Vickers indenter The height of roll molded features was measured on a VanGuard optical microscope with a calibrated focal depth dial by focusing on the microchannel top and bottom, and confirmed with additional scanning electron microscopy (SEM) measurements For one specimen, at least five independent depth measurements were carried out at random locations on the specimen, from which the average feature height and its standard deviation were calculated Morphological examinations of roll molded Al specimens were conducted through SEM on a FEI Quanta3D FEG DaulBeam focused ion beam (FIB) instrument 2.2. Pool boiling heat transfer performance testing To determine how roll-molded Al alloy micro fin arrays perform as heat sinks within a pool boiling environment, a series of experiments were performed on un-patterned P A Primeaux et al J Micromech Microeng 27 (2017) 025012 Figure 3. A typical 1D micro fin array structure fabricated by room temperature roll molding: (a) an optical overview, with markings on the ruler in mm; (b) a low magnification cross-sectional SEM image, with micro fin dimensions measured and marked 3. Results and discussion surface orientation angles The interior of the sample stage was a 25 mm × 32 mm × 6 mm C101 oxygen-free Cu block with four embedded cartridge heaters (Omega) connected in parallel to a 2000 VA variable AC power transformer (Philmore) A smaller 10 mm × 13 mm × 6 mm C101 block was situated atop this main heater block with solder at the interface One of the thermocouples was embedded within the center of the smaller C101 block via a drilled cavity and held in place with thermally conductive adhesive The sample of interest was mounted on top of the mesa using a thin layer of thermally conductive graphite/polylactic acid composite All surfaces except the mounted sample were surrounded by 6 mm thick low thermal conductivity Teflon to provide high temperature-compatible insulation from the surrounding pool All seams/crevices around the sample were sealed with a high temperature RTV silicone to prevent spurious heat transfer/bubble nucleation At the maximum heater power (~200 W), the upper limit of uncertainty on heat flux originates from heat loss through the Teflon layers and via thin insulated metallic lead wires This upper limit is