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1. Two-phase Impingement Cooling using a Trapezoidal Groove Microchannel Heat Sink and Dielectric Coolant HFE 7000

   https://doi.org/10.1109/ITherm51669.2021.9503283  

   Hoang, Cong Hiep; Rangarajan, Srikanth; Radmard, Vahideh; Fallahtafti, Najmeh; Tradat, Mohammad; Arvin, Charles; Schiffres, Scott; Sammakia, Bahgat (June 2021, 2021 20th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (iTherm))

   This paper focuses on two-phase flow boiling of dielectric coolant HFE 7000 inside a copper multi-microchannel heat sink for high heat flux chip applications. The heat sink is composed of parallel microchannels, 200 μm wide, 2500 μm high, and 20 mm long, with 200-μm-thick fins separating the channels. The copper heat sink consists of almost 100 channels connected by a longitude groove with a nearly trapezoidal cross section. Coolant impinges down to the base at the groove and then goes along the microchannels. A copper block heater arrangement was used to mimic a computer chip with a footprint of 1”x1” (6.45 cm2). The base heat flux was varied from 7.75 W/cm2 to 96.1 W/cm2 and the mass flux from 547.6 to 958.4 kg/m2s, at a nominal saturation temperature of 54 °C. Heat transfer coefficients as high as 57.5 kW/m2K were reached, keeping the base temperature under 66 °C with a maximum of 21.9 kPa of pressure drop, for inlet subcooling of 5 degree and a coolant flow rate of 958.4 kg/m2. Effects of inner diameter of tubing on thermal performance and pressure drop are also discussed. It was observed that an increase of tubing inner diameter by 60 % can result in increase of heat transfer coefficient by 47.8 % and reduction in pressure drop by 63 %. 

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2. Physical mechanisms for delaying condensation freezing on grooved and sintered wicking surfaces

   https://doi.org/10.1063/5.0105412  

   Stallbaumer-Cyr, Emily M.; Derby, Melanie M.; Betz, Amy R. (August 2022, Applied Physics Letters)

   Heat pipes are passive heat transfer devices crucial for systems on spacecraft; however, they can freeze when exposed to extreme cold temperatures. The research on freezing mechanisms on wicked surfaces, such as those found in heat pipes, is limited. Surface characteristics, including surface topography, have been found to impact freezing. This work investigates freezing mechanisms on wicks during condensation freezing. Experiments were conducted in an environmental chamber at 22 °C and 60% relative humidity on three types of surfaces (i.e., plain copper, sintered heat pipe wicks, and grooved heat pipe wicks). The plain copper surface tended to freeze via ice bridging—consistent with other literature—before the grooved and sintered wicks at an average freezing time of 4.6 min with an average droplet diameter of 141.9 ± 58.1  μm at freezing. The grooved surface also froze via ice bridging but required, on average, almost double the length of time the plain copper surface took to freeze, 8.3 min with an average droplet diameter of 60.5 ± 27.9  μm at freezing. Bridges could not form between grooves, so initial freezing for each groove was stochastic. The sintered wick's surface could not propagate solely by ice bridging due to its topography, but also employed stochastic freezing and cascade freezing, which prompted more varied freezing times and an average of 10.9 min with an average droplet diameter of 97.4 ± 32.9  μm at freezing. The topography of the wicked surfaces influenced the location of droplet nucleation and, therefore, the ability for the droplet-to-droplet interaction during the freezing process. 

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3. Interrogation of Mid-build Internal Temperature Distributions within Parts Being Manufactured via the Powder Bed Fusion Process

   Wood, N; Mendoza, H; Boulware, P; Hoelzle, D.J. (August 2019, Proceedings of the 2019 Annual International Solid Freeform Fabrication Symposium)

   This work reports on the measurement of the internal temperature distributions of parts being manufactured via the Powder Bed Fusion (PBF) process. Eight test coupons were machined from a piece of wrought 304 stainless steel (SS). Thermocouples were inserted into the test coupon interiors to sample internal thermal history. The coupons were then placed into the open architecture laser PBF machine housed at EWI and covered to their uppermost surfaces with 316 SS powder.Three tests were executed: First, the laser rastered over the coupons without inducing melting.Second, the laser rastered over the coupons while melting the exposed faces. Lastly, five layers of316 SS were built atop the coupons. The main result is a comprehensive data set of a multitude of measured physical inputs and outputs under typical build conditions: embedded thermocouple temperatures, laser centroid, laser power, and infrared imagery of the exposed coupon faces. 

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4. Pushing the Limit of Beetle‐Inspired Condensation on Biphilic Quasi‐Liquid Surfaces

   https://doi.org/10.1002/adfm.202211113  

   Boylan, Dylan; Monga, Deepak; Shan, Li; Guo, Zongqi; Dai, Xianming (January 2023, Advanced Functional Materials)

   Abstract Massive studies concern the development of low‐carbon water and energy systems. Specifically, surfaces with special wettability to promote vapor‐to‐liquid condensation have been widely studied, but current solutions suffer from poor heat transfer performances due to inefficient droplet removal. In this study, the limit of condensation on a beetle‐inspired biphilic quasi‐liquid surface (QLS) in a steam environment is pushed, which provides a heat flux 100 times higher than that in atmospheric condensation. Unlike the beetle‐inspired surfaces that have sticky hydrophilic domains, the biphilic QLS consists of PEGylated and siloxane polymers as hydrophilic and hydrophobic quasi‐liquid patterns with the contact angle hysteresis of 3° and 1°, respectively. More importantly, each hydrophilic slippery pattern behaves like a slippery bridge that accelerates droplet coalescence and removal. As a result, the condensed droplets grow rapidly and shed off. It is demonstrated that the biphilic‐striped QLS shows a 60% higher water harvesting rate in atmospheric condensation and a 170% higher heat transfer coefficient in steam condensation than the conventional beetle‐inspired surface. This study provides a new paradigm to push the limit of condensation heat transfer at a high heat flux, which sheds light on the next‐generation surface design for water and energy sustainability. 

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5. Electrowetting-based coalescence of droplets during dropwise condensation of humid air

   Wikramanayake, Enakshi; Bahadur, Vaibhav (July 2019, Summer Heat Transfer Conference)

   Dropwise condensation yields higher heat transfer coefficients by avoiding the thermal resistance of the condensate film, seen during filmwise condensation. This work explores further enhancement of dropwise condensation heat transfer through the use of electrowetting to achieve faster droplet growth via coalescence of the condensed droplets. Electrowetting is a well understood microfluidic technique to actuate and control droplets. This work shows that AC electric fields can significantly enhance droplet growth dynamics. This enhancement is a result of coalescence triggered by various types of droplet motion (translation of droplets, oscillations of three phase line), which in turn depends on the frequency of the applied AC waveform. The applied electric field modifies droplet condensation patterns as well as the roll-off dynamics on the surface. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation rates under the influence of electric fields. It is noted that this study deals with condensation of humid air, and not pure steam. Results show that increasing the voltage magnitude and frequency increases droplet growth rate and overall condensation rate. Overall, this study reports more than a 30 % enhancement in condensation rate resulting from the applied electric field, which highlights the potential of this concept for condensation heat transfer enhancement. 

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