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1. Geometric Optimization on an Impinging Cold-Plate with a Trapezoidal Groove Used for Warm Water Cooling

   Hadad, Y.; Pejman, R.; Ramakrishnan, B.; Chiarot, P.R.; Sammakia, B.G. (June 2018, ITHERM)

   Due to their lower pressure drop, impinging cold-plates are preferred over parallel flow cold-plates when there is no strict space limitation (i.e. when flow can enter perpendicular to the electronic board). Splitting the flow into two branches cuts the flow rate and path in half, which leads to lower pressure drop through the channels. A groove is used to direct the flow exiting the diffuser into the channels. The number of the geometric design parameters of the cold-plate will vary depending on the shape of the groove. In this research, the response surface method (RSM) was used to optimization the fin geometry of an impinging cold-plate with a trapezoidal cross section groove. The cold plate is used for warm water cooling of electronics. Three fin parameters (thickness, height, and gap) and three groove parameters were optimized to reach minimum values for hydraulic and thermal resistances at fixed values of coolant inlet temperature, coolant flow rate, and electronic chip power. 

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2. Design and Thermal Analysis of a 3D Printed Impingement Pin Fin Cold Plate for Heterogeneous Integration Application

   https://doi.org/10.1109/TCPMT.2022.3185401  

   Hoang, Cong Hiep; Azizi, Arad; Fallahtafti, Najmeh; Rangarajan, Srikanth; Radmard, Vahideh; Arvin, Charles; Sikka, Kamal; Schiffres, Scott; Sammakia, Bahgat (June 2022, IEEE Transactions on Components, Packaging and Manufacturing Technology)

   Miniaturization and high heat flux of power electronic devices have posed a colossal challenge for adequate thermal management. Conventional air-cooling solutions are inadequate for high-performance electronics. Liquid cooling is an alternative solution thanks to the higher specific heat and latent heat associated with the coolants. Liquid-cooled cold plates are typically manufactured by different approaches such as: skived, forged, extrusion, electrical discharge machining. When researchers are facing challenges at creating complex geometries in small spaces, 3D-printing can be a solution. In this paper, a 3D-printed cold plate was designed and characterized with water coolant. The printed metal fin structures were strong enough to undergo pressure from the fluid flow even at high flow rates and small fin structures. A copper block with top surface area of 1 inch by 1 inch was used to mimic a computer chip. Experimental data has good match with a simulation model which was built using commercial software 6SigmaET. Effects of geometry parameters and operating parameters were investigated. Fin diameter was varied from 0.3 mm to 0.5 mm and fin height was maintained at 2 mm. A special manifold was designed to maximize the surface contact area between coolant and metal surface and therefore minimize thermal resistance. The flow rate was varied from 0.75 L/min to 2 L/min and coolant inlet temperature was varied from 25 to 48 oC. It was observed that for the coolant inlet temperature 25 oC and aluminum cold plate, the junction temperature was kept below 63.2 oC at input power 350 W and pressure drop did not exceed 23 Kpa. Effects of metal materials used in 3D-printing on the thermal performance of the cold plate were also studied in detail. 

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3. Experimental characterization and geometrical optimization of a commercial two-phase designed cold plate

   https://doi.org/10.1016/j.icheatmasstransfer.2024.107457  

   Fallahtafti, Najmeh; Hosseini, Farzaneh; Hadad, Yaser; Rangarajan, Srikanth; Hoang, Cong Hiep; Sammakia, Bahgat (June 2024, International Communications in Heat and Mass Transfer)

   The increasing prevalence of high-performance computing data centers necessitates the adoption of cutting-edge cooling technologies to ensure the safe and reliable operation of their powerful microprocessors. Two-phase cooling schemes are well-suited for high heat flux scenarios because of their high heat transfer coefficients and their ability to enhance chip temperature uniformity. In this study, we perform experimental characterization and deep learning driven optimization of a commercial two-phase cold plate. The initial working design of the cold plate comprises a fin height of 3mm, fin thickness of 0.1 mm, and a channel width of 0.1 mm.A dielectric coolant, Novec /HFE 7000, was impinged into microchannel fins through impinging jets. A copper block simulated an electronic chip with a surface area of 1˝ × 1˝. The experiment was conducted with three different coolant inlet temperatures of 25◦ C, 36◦ C, and 48◦ C with varying heat flux levels ranging from 7.5 to 73.5 W cm2. The effects of coolant inlet temperatures and flow rate on the thermo-hydraulic performance of the cold plate were explored. In two-phase flow, increasing coolant inlet temperature results in more nucleation sites and improved thermal performance consequently. Thermal resistance drops with flow rate in single-phase flow while it is not affected by flow rate in nucleate boiling region. An improvement in the design of the cold plate was carried out, with the goal of increasing the number of bubble sites and flow velocity at the root fins, by cutting the original fins and creating channels perpendicular to the original channels. Three design parameters, fin height, width of machined channels, and height of short fins preserved through machined channels, were defined. It was observed that widening the machined channels and cutting fins to some point can improve the thermal performance of the cold plate. However, removing fins excessively adversely affects the thermal performance of the cold plate because of loss of heat transfer surface area. Moreover, preserving the short fins through the machined channels decreases thermal resistance as they increase heat transfer surface area and nucleation sites. Furthermore, a deep learning-based compact model is demonstrated for the two-phase cold plate design in the specific range of geometry and flow conditions. The developed compact model is utilized to drive the single and multi-objective optimization to arrive at global optimal results. 

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4. Thermal-Hydraulic Analytical Models of Split-Flow Microchannel Liquid-Cooled Cold Plates With Flow Impingement

   https://doi.org/10.1115/IPACK2021-73283  

   Kisitu, Deogratius; Ortega, Alfonso (October 2021, Proceedings of the 2021 ASME InterPACK)

   Abstract Impingement split flow liquid-cooled microchannel cold plates are one of several flow configurations used for single-phase liquid cooling. Split flow or top-in/side-exit (TISE) cold plates divide the flow into two branches thus resulting in halved or reduced flow rates and flow lengths, compared to traditional side-in /side-exit (SISE) or parallel flow cold plates. This has the effect of reducing the pressure drop because of the shorter flow length and lower flow rate and increasing the heat transfer coefficient due to thermally developing as opposed to fully developed flow. It is also claimed that the impinging flow increases the heat transfer coefficient on the base plate in the region of impingement. Because of the downward impinging and turning flow, there are no exact analytical models for this flow configuration. Computational and experimental studies have been performed, but there are no useful compact analytical models in the literature that can be used to predict the performance of these impingement cold plates. Results are presented for novel physics-based laminar flow models for a TISE microchannel cold plate based on an equivalent parallel channel flow approach. We show that the new models accurately predict the thermal-hydraulic performance over a wide range of parameters. 

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5. Numerical modeling and optimization of a V-groove warm water cold-plate

   https://doi.org/10.1109/SEMI-THERM.2017.7896948  

   Hadad, Yaser; Ramakrishnan, Bharath; Alkharabsheh, Sami; Chiarot, Paul R.; Sammakia, Bahgat (January 2017, Thermal Measurement, Modeling & Management Symposium (SEMI-THERM), 2017 33rd)

   In electronics cooling, water is increasingly replacing air for applications requiring high heat flux. Water is the ideal substitute due to its high specific heat capacity and density. Indeed, high values of heat capacity (high density and specific heat capacity) enable water to receive, store and carry higher amounts of energy compared to air. Water's incompressibility and very low specific volume also requires smaller amounts of mechanical work for fluid circulation. Using warm water instead of chilled water makes the cooling process more economical, but requires more efficiently designed cold-plates. Our current work focuses on modeling and optimization of a V-groove mini-channel cold-plate using warm water as the coolant. Our results show that the performance of an impinging channel heat sink is significantly different compared to parallel channel designs. Dividing the flow into two branches cuts the fluid velocity and flow path in half for the impinging design. This reduction in the fluid velocity and flow length affects the developing thermal boundary layer and is an important consideration for a shorter length heat exchanger (where the channel length is comparable to the thermal entrance length). Distributing the coolant uniformly to every channel is a challenge for impinging cold-plates where there are strict limitations on size. 

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