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1. A Numerical Heat Transfer Investigation of Lattice Structures As an Alternative AM-Enabled Design for Cooled sCO2 Airfoils

   https://doi.org/10.1115/GT2023-104180  

   Otto, Marcel; Wardell, Ryan; O’Neal, Parker; Veselý, Ladislav; Kapat, Jayanta (June 2023, ASME Turbo Expo 2023)

   Abstract Supercritical Carbon dioxide (sCO2) power cycles are rapidly developing and gaining popularity in waste heat recovery systems, as primary power cycles for a variety of heat sources such as nuclear, or as a stand-alone power cycle where fossil fuels are combusted. Akin to conventional gas turbines, sCO2-powered systems are pushing the boundaries for firing temperatures for higher efficiencies. Direct oxy-fired sCO2 systems will demand internal cooling of the airfoils for safe and reliable operations. Gas turbine cooling technology can be leveraged for that purpose. However, two key differences exist. First, the coolant medium is sCO2 instead of air, and second, sCO2 airfoils are much smaller compared to power-generation gas turbines. Novel AM manufacturing techniques promise advanced internal cooling geometries. This paper investigates a novel trailing edge cooling design to replace conventional pin fin arrays. Here, a lattice structure with microchannels is introduced. The study presents the changes in heat transfer due to the substitution of the heat transfer medium and the new geometry. The component is assumed to be printed Inconel 718. Based on an oxy-fired combustion sCO2 power cycle, coolant temperature and pressure and hot gas path temperature and pressure are chosen. The converging trailing edge duct is simulated in StarCCM+ using COOLPROP for sCO2 properties as a conjugate heat transfer model. 

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2. Experimental Investigation of Supercritical Carbon Dioxide in Horizontal Micro Pin Arrays with Non-Uniform Heat Flux Boundary Conditions

   https://doi.org/10.1615/TFEC2019.fnd.027547  

   Jajja, Saad A.; Sequeira, Jessa M.; Fronk, Brian M. (April 2019, 4th Thermal and Fluids Engineering Conference)

   This study is part of the preliminary experimental investigations designed to assess the feasibility of using supercritical carbon dioxide (sCO2) in the vicinity of its critical point for thermal management applications. In the present study, sCO2 was used as a working fluid in a diffusion bonded 316/316L stainless steel test section having staggered micro pin fin array flow passages of hydraulic diameter 679 µm (0.679 mm). The test section was subjected to a single wall non-uniform heat flux boundary condition and was operated in a horizontal orientation. The primary objective was to characterize the heat transfer performance of sCO2 as it flows through the staggered pin fin array for experimental conditions that span its critical and pseudocritical point. Data analysis methods employing 2-D and 3-D heat transfer models of the test section were used to calculate the average heat transfer coefficients for a given set of experimental conditions. Experiments were conducted by varying the inlet temperature (18 ≤ T_in ≤ 50 °C) and for fixed mass flux (300 kg m-2 s-1), heat flux (40 W cm-2), and reduced pressure (1.1). Experimental data were also compared against the predictions of a correlation proposed for single phase flows in microchannel staggered diamond pin arrays. The correlation predicted the data within 4.3 % when the ratio, T_Bulk/T_PC exceeded 1. It was also found that the enhancement in the heat transfer, a result of employing staggered pin array flow geometry instead of microchannels, carries a commensurate penalty in pressure drop. 

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3. LES Simulation of turbulent supercritical CO2 heat transfer in microchannels

   Nabil, M.; Rattner, A.S. (March 2018, 6th International Supercritical CO2 Power Cycles Symposium)

   Although supercritical CO2 (sCO2) heat transfer has been employed in industrial process since the 1960s, the underlying transport phenomenon in high-flux microscale geometries, as could be employed in concentrating solar receivers, is poorly understood. To date, nearly all experimental studies and simulations of supercritical convective heat transfer have focused on large diameter vertical channel and tube bundle flows, which may differ dramatically from microscale supercritical convection. Computational studies have primarily employed Reynolds averaged (RANS) turbulence modeling approaches, which may not capture effects from the sharply varying property trends of supercritical fluids. In this study, large eddy simulation (LES) turbulence modeling techniques are employed to study heat transfer characteristics of sCO2 in microscale heat exchangers. The simulation geometry consists of a microchannel of 750×737 μm cross-section and 5 mm length, heated from all four sides. Simulation cases are evaluated at reduced pressure P_r = 1.1, mass flux G = 1000 kg/m^2-s, heat flux q'' = 1.7 − 8.9 W/cm^2 , and varying inlet temperature: 20 − 100℃. Computational results reveal thermal transport mechanisms specific to microscale sCO2 flows. Results have been compared with available supercritical convection correlations to identify the most applicable heat transfer models for engineering of microchannel sCO2 heat exchangers. 

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4. Investigation of Buoyancy Effects in Asymmetrically Heated Near-Critical Flows of Carbon Dioxide in Horizontal Microchannels Using Infrared Thermography

   https://doi.org/10.1115/HT2021-63004  

   Randle, Lindsey V.; Fronk, Brian M. (June 2021, ASME 2021 Heat Transfer Summer Conference collocated with the ASME 2021 15th International Conference on Energy Sustainability)

   In this study, we use infrared thermography to calculate local heat transfer coefficients of top and bottom heated flows of near-critical carbon dioxide in an array of parallel microchannels. These data are used to evaluate the relative importance of buoyancy for different flow arrangements. A Joule heated thin wall made of Inconel 718 applies a uniform heat flux either above or below the horizontal flow. A Torlon PAI test section consists of three parallel microchannels with a hydraulic diameter of 923 μm. The reduced inlet temperature (TR = 1.006) and reduced pressure (PR = 1.03) are held constant. For each heater orientation, the mass flux (520 kgm−2s−2 ≤ G ≤ 800 kgm−2s−2) and heat flux (4.7 Wcm−2 ≤ q″ ≤ 11.1 Wcm−2) are varied. A 2D resistance network analysis method calculates the bulk temperatures and heat transfer coefficients. In this analysis, we divide the test section into approximately 250 segments along the stream-wise direction. We then calculate the bulk temperatures using the enthalpy from the upstream segment, the heat flux in a segment, and the pressure. To isolate the effect of buoyancy, we screen the data to omit conditions where flow acceleration may be important or where relaminarization may occur. In the developed region of the channel, there was a 10 to 15 percent reduction of the local heat transfer coefficients for the upward heating mode compared to downward heating with the same mass and heat fluxes. Thus buoyancy effects should be considered when developing correlations for these types of flow. 

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5. Design Improvement of a Commercial Impingement Two-Phase Cold Plate Used for High Heat Flux Applications

   Fallahtafti, N. (April 2022, 2022 38th Semiconductor Thermal Measurement, Modeling & Management Symposium (SEMI-THERM))

   Heat fluxes in the data center have been increasing significantly due to the rise in advanced technologies such as Artificial Intelligence (AI), 5G, high-performance computing (HPC), and machine learning. The traditional air-cooling technology cannot handle high heat fluxes and requires a bigger heat sink; therefore, hindering high heat flux and high density in the data center. Two-phase cooling schemes are particularly appropriate for high heat flux situations because of their enhanced heat transfer coefficients and the non-linear relationship between heat flux and surface-to-fluid temperature difference. In this study, an experimental setup was developed to characterize and optimize the thermo-hydraulic performance of two-phase cooling cold plates intended for high heat flux applications. An improvement of 12% in thermal performance was obtained by cutting the original fins and creating mini-channels perpendicular to the original microchannels without a significant pressure drop penalty. 

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