Computational Thermal Fluid Dynamics Research Articles

Manipulating scanning strategies towards controlled microstructure of laser remelted Mg–3Al–1Zn alloy

Various laser scanning strategies of multitrack products, consisting of a default Raster pattern (R1), a Raster pattern with a changed line order (R2), and a default Zigzag pattern (Z1), with different overlap rates ranged from 0.7 to 0.5, have been conducted to investigate microstructure evolution for Mg–3Al–1Zn alloy. As the overlap rate changes from 0.7 to 0.5, the average grain size decreases and the texture is slightly concentrated, except for R2 pattern. For effect of different scanning pattern, a relatively small average grain size can be obtained using the R1, R2 patterns compared to using the Z1 pattern; a relatively weak texture can be obtained using the R1, Z1 patterns compared to using the R2 pattern. The utilization of Z1 pattern results in a further texture weakening compared to that of the R1 pattern. Microstructure evolution of scanning strategy is mainly determined by low-undercooling-required epitaxial growth along temperature gradient directions at the solid/liquid interface. Combined with computational thermal-fluid dynamics simulation, a strong fluid flow within the molten pool promotes side expansion of the molten pool, resulting in the larger overlap area, the corresponding three-times remelting domain and continuous deflections of grain major axes.

Computational Thermal-Fluid Dynamics analyses of borated water distribution in the Vacuum Vessel of the Divertor Tokamak Test facility

This work presents the full set of 3D Computational Thermal-Fluid Dynamics (CFD) analyses of the Divertor Tokamak Test (DTT) Vacuum Vessel (VV). The VV D-shaped double shell structure, divided into 18 Regular and Special sectors, is actively maintained at the operating temperature of 60 °C by borated water in forced flow to counteract the thermal loads and the heat transfer with the surroundings. Due to manufacturing and integration constraints, the different sectors cannot share the same design: the water will flow in the free space between the two shells composing the VV but, given the complexity of the geometry, a careful hydraulic design is mandatory, to avoid local stagnation points which may cause either overheating or freezing. The different hydraulic paths are separately analysed with the Star-CCM+ software, with a SST k−ω turbulence closure, proving the effectiveness of their design (pressure drop, temperature constraints and stagnation/backflow). In addition, the borated water mass flow rate distribution among the different paths is assessed. To conclude, results from CFD analyses are exploited to approximate the hydraulic characteristic of each sector and to develop a system-level model of the full VV with the Modelica language. The overall VV pressure drop of Δp=2810 Pa, the outlet mixing temperature of Tout,mix=333.06 K and a mass flow rate distribution close to the homogeneous condition (≈2.22 kgs−1 in each sector) confirm that no relevant issues are found in the current VV design from the thermal-hydraulic point of view.

Solute segregation in a rapidly solidified Hastelloy-X Ni-based superalloy during laser powder bed fusion investigated by phase-field and computational thermal-fluid dynamics simulations

Solute segregation significantly affects material properties and is a critical issue in the laser powder-bed fusion (LPBF) additive manufacturing (AM) of Ni-based superalloys. To the best of our knowledge, this is the first study to demonstrate a computational thermal-fluid dynamics (CtFD) simulation coupled multi-phase-field (MPF) simulation with a multicomponent-composition model of Ni-based superalloy to predict solute segregation under solidification conditions in LPBF. The MPF simulation of the Hastelloy-X superalloy reproduced the experimentally observed submicron-sized cell structure. Significant solute segregations were formed within interdendritic regions during solidification at high cooling rates of up to 1.6 × 106 K s−1, a characteristic feature of LPBF. Solute segregation caused a decrease in the solidus temperature (TS), with a reduction of up to 38.4 K, which increases the risk of liquation cracks during LPBF. In addition, the segregation triggers the formation of carbide phases, which increases the susceptibility to ductility dip cracking. Conversely, we found that the decrease in TS is suppressed at the melt-pool boundary regions, where re-remelting occurs during the stacking of the layer above. Controlling the re-remelting behavior is deemed to be crucial for designing crack-free alloys. Thus, we demonstrated that solute segregation at the various interfacial regions of Ni-based multicomponent alloys can be predicted by the conventional MPF simulation. The design of crack-free Ni-based superalloys can be expedited by MPF simulations of a broad range of element combinations and their concentrations in multicomponent Ni-based superalloys.

Inverse Columnar-Equiaxed Transition (CET) in 304 and 316L Stainless Steels Melt by Electron Beam for Additive Manufacturing (AM)

According to Hunt’s columnar-to-equiaxed transition (CET) criterion, which is generally accepted, a high-temperature gradient (G) in the solidification front is preferable to a low G for forming columnar grains. Here, we report the opposite tendency found in the solidification microstructure of stainless steels partially melted by scanning electron beam for powder bed fusion (PBF)-type additive manufacturing. Equiaxed grains were observed more frequently in the region of high G rather than in the region of low G, contrary to the trend of the CET criterion. Computational thermal-fluid dynamics (CtFD) simulation has revealed that the fluid velocity is significantly higher in the case of smaller melt regions. The G on the solidification front of a small melt pool tends to be high, but at the same, the temperature gradient along the melt pool surface also tends to be high. The high melt surface temperature gradient can enhance Marangoni flow, which can apparently reverse the trend of equiaxed grain formation.

Melting and Solidification Behavior of 316L Steel Induced by Electron-Beam Irradiation for Additive Manufacturing

Additive manufacturing（AM）technologies have attracted much attention because it enables us to build 3D parts with complicated geometry easily and control material properties significantly via the control of microstructures. In the powder- bed fusion（PBF）type AM process, 3D parts are fabricated by repeating a process of melting and solidifying metal powders by laser or electron beams. In general, the solidification microstructures can be predicted from solidification conditions defined by the combination of temperature gradient G and solidification rate R on the basis of columnar-equiaxed transition（CET）theory proposed by Hunt [Mater. Sci. Eng. 65, 75（1984）]. However, it is unclear whether the CET theory can be applied to the PBF type AM process because of the high G and R, even for general 316L stainless steel. In this study, to reveal relationships between microstructures and solidification conditions, we have analyzed solidification microstructures of 316L steel induced by electron- beam irradiation and evaluated solidification conditions at the solid/liquid interface using a computational thermal-fluid dynamics （CtFD）method. It was found that equiaxed grains were often formed under high G conditions contrary to the CET theory. CtFD simulation revealed that there is a fluid flow up to a velocity of about 400 mm s -1 , and suggested that equiaxed grains are formed owing to the effect of fragmentations and migrations of dendrites.

Role of operating and environmental conditions in determining molten pool dynamics during electron beam melting and selective laser melting

Abstract Electron beam melting (EBM) and selective laser melting (SLM) are representative powder bed fusion additive manufacturing methods. Because EBM and SLM have different operating and environmental conditions, such as ambient pressure of the chamber, initial temperature, and heat source, they have different molten pool dynamics. In this study, single-bead melting experiments using EBM and SLM were performed in conjunction with computational thermal-fluid dynamics simulations in high-energy conditions to highlight the differences in the molten pool dynamics of EBM and SLM. The experimental results reveal that SLM is more likely to melt in the keyhole mode than EBM under nominally identical line energy. The simulations showed that the instantaneous maximum temperature of the SLM molten pool is much lower than that of the EBM molten pool. An increase in the preheating temperature is found to strengthen the vapor recoil pressure; however, the vapor recoil pressure under vacuum is maintained at a considerably low level in EBM. Compared to EBM, the high atmospheric pressure and multiple laser reflections during SLM significantly enhance the effect of the vapor recoil pressure on the melt surface. The findings of this study can be useful for the formulation of appropriate processing strategies for the two processes.

Effects of plasma rotating electrode process parameters on the particle size distribution and microstructure of Ti-6Al-4 V alloy powder

The starting powder quality significantly influences the process window optimization and properties of parts fabricated by additive manufacturing. In this study, we investigated the influence of plasma rotating electrode process (PREP) parameters on the particle size distribution and microstructure of Ti-6Al-4 V alloy powder. The martensite size in the powder decreased with increasing rotating electrode speed owing to the higher cooling rate. Numerical simulations using computational thermal fluid dynamics were found to be feasible for quantitative evaluation of the temperature variation, cooling rate, and powder size during PREP, which provide a new strategy to study the mechanism of powder formation. In addition, a statistical model combining principal component analysis and the Monte Carlo methods was proposed to evaluate the relationships between PREP parameters and average powder diameter based on the limited collected experimental data. The proposed statistical model can also be applied in research fields where multivariable problems exist.

Data-Driven Microstructure and Microhardness Design in Additive Manufacturing Using a Self-Organizing Map

Abstract To design microstructure and microhardness in the additive manufacturing (AM) of nickel (Ni)-based superalloys, the present work develops a novel data-driven approach that combines physics-based models, experimental measurements, and a data-mining method. The simulation is based on a computational thermal-fluid dynamics (CtFD) model, which can obtain thermal behavior, solidification parameters such as cooling rate, and the dilution of solidified clad. Based on the computed thermal information, dendrite arm spacing and microhardness are estimated using well-tested mechanistic models. Experimental microstructure and microhardness are determined and compared with the simulated values for validation. To visualize process–structure–properties (PSPs) linkages, the simulation and experimental datasets are input to a data-mining model—a self-organizing map (SOM). The design windows of the process parameters under multiple objectives can be obtained from the visualized maps. The proposed approaches can be utilized in AM and other data-intensive processes. Data-driven linkages between process, structure, and properties have the potential to benefit online process monitoring control in order to derive an ideal microstructure and mechanical properties.

Investigation of water cooled aluminium foam heat sink for concentrated photovoltaic solar cell

Concentrated photovoltaic cell (CPV) is a popular renewable source nowadays. It has high efficient yet cost effective energy harvesting system available in the market. The performance of the CPV depends strongly on the temperature of the solar cell. The efficiency reduces as the surface temperature of the CPV increases. It is crucial to reduce the temperature of the CPV and maintains a small temperature variation along the surface of the CPV. Metal foam is an ideal solution for a CPV thermal management system. It helps to enhance the mixing of coolant due to its porous characteristics and lead to higher heat transfer rate and reduce the overall average temperature of a CPV. The main objective of this study is to enhance the heat removal rate along of the CPV. Aluminium foam is used as a heat sink to removal the heat generated. The effects of porosities and pores density (PPI) of the aluminium foam are measured against the thermal performance. Computational thermal fluid dynamics is conducted to study the thermal performance of aluminium foam heat sink. The parameters required for the simulations are extracted from literature. The results suggested that aluminum foam is able to enhance the heat removal and maintains better temperature uniformity of the CPV. 10PPI aluminum foam with porosity 0.682 provides the most optimum results with average temperature of 55.1 ˚C and temperature different of 7.4 °C at flow rate of 40 g/s. This approach is suitable to promote the efficiency of the CPV and prolong the cycle life.

Comprehensive study on mechanisms for grain morphology evolution and texture development in powder bed fusion with electron beam of Co–Cr–Mo alloy

Grain morphology, size, and growth direction are crucial in determining the performances of metallic implant components. Understanding the effects of the manufacturing characteristics of powder bed fusion with electron beam (PBF-EB), an additive manufacturing process, on microstructure formation and anisotropy development during solidification is essential to achieve flexible microstructure control. In this study, PBF-EB was employed to fabricate a Co–Cr–Mo alloy and the grain morphology and texture formation with different process parameters were analyzed by experimental characterization with the aid of computational thermal–fluid dynamics simulations. It was found that the epitaxial growth with resulting columnar grain and near-cubic texture tended to be dominant in the solidification process, due to competitive grain growth and the heat flow characteristics in the snake-scanning strategy. However, the molten pool connection between adjacent melt tracks resulted in the random orientation of <001> in the horizontal plane, producing a fiber-like texture. In addition, nucleation and new grain growth rather than extensive epitaxial growth could be achieved by manipulating the molten pool geometry and overlap between adjacent melt tracks. Increasing the slope of the solid/liquid interface of the molten pool and decreasing the remelting fraction of adjacent melt tracks favored the formation of new grains with random orientations to restrict the extensive epitaxial growth of the columnar grains and attenuate the microstructural anisotropy.

Numerical study of the geometrically graded metal foam for concentrated photovoltaic solar cell cooling

Abstract Concentrated photovoltaic cell (CPV) had gained much attention recently due to high efficiency at a competitive cost. However, efficiency of CPV is inversely proportional to the temperature. Hence, it is important to reduce the maximum temperature and variation of temperature across the CPV. Metal foam with its high specific surface area, thermal conductivity and tortuous flow path to promote mixing is an ideal candidate for thermal management for CPV. However, the thermal performance of the metal foam may drop from upstream to downstream and lead to poor cooling performance near the outlet. In this study, functionally graded metal foam is attached on the CPV to extract the heat generated. Functionally graded aluminum foam with gradual variation porosity are modelled to investigate the thermal performance and flow field using computational thermal fluid dynamics analysis. Heat transfer correlation, permeability and resistance loss coefficient are extracted from the literature and used in the simulation. The results showed that functionally graded metal foam with gradual reducing porosity offered a better temperature uniformity for the CPV. Therefore, this approach will further extend the cycle life as well as improve the overall efficiency of the CPV.

Grain Morphology and Texture Development in a Co-Cr-Mo Alloy Fabricated by Powder Bed Fusion with an Electron Beam

Grain morphology, size, and growth direction are crucial in determining the performances of metallic implant components. Understanding the effects of the manufacturing characteristics of powder bed fusion with an electron beam (PBF-EB), an additive manufacturing process, on microstructure formation and anisotropy development during solidification is essential to achieve flexible microstructure control. In this study, PBF-EB was employed to fabricate a Co-Cr-Mo alloy and the grain morphology and texture formation with different process parameters were analyzed by experimental characterization with the aid of computational thermal-fluid dynamics simulations. It was found that the epitaxial growth with resulting columnar grain and near-cubic texture tended to be dominant in the solidification process, due to competitive grain growth and the heat flow characteristics in the snake-scanning strategy. However, the molten pool connection between adjacent melt tracks resulted in the random orientation of in the horizontal plane, producing a fiber-like texture. In addition, nucleation and new grain growth rather than extensive epitaxial growth could be achieved by manipulating the molten pool geometry and overlap between adjacent melt tracks. Increasing the slope of the solid/liquid interface of the molten pool and decreasing the remelting fraction of adjacent melt tracks favored the formation of new grains with random orientations to restrict the extensive epitaxial growth of the columnar grains and attenuate the microstructural anisotropy.

Molten pool behavior and effect of fluid flow on solidification conditions in selective electron beam melting (SEBM) of a biomedical Co-Cr-Mo alloy

Selective electron beam melting (SEBM) is a type of additive manufacturing (AM) that involves multiple physical processes. Because of its unique process conditions compared to other AM processes, a detailed investigation into the molten pool behavior and dominant physics of SEBM is required. Fluid convection involves mass and heat transfer; therefore, fluid flow can have a profound effect on solidification conditions. In this study, computational thermal-fluid dynamics simulations with multi-physical modeling and proof-of-concept experiments were used to analyze the molten pool behavior and resultant thermal conditions related to solidification. The Marangoni effect of molten metal primarily determines fluid behavior and is a critical factor affecting the molten pool instability in SEBM of the Co–Cr–Mo alloy. The solidification parameters calculated from simulated data, especially the solidification rate, are sensitive to the local fluid flow at the solidification front. Combined with experimental analysis, the results presented herein indicate that active fluid convection at the solidification front increase the probability of new grain formation, which suppresses the epitaxial growth of columnar grains.

Improving Accuracy of Temperature Distribution and Energy-Saving Technology of Air Conditioners in Data Centers

Reducing the energy consumption of a data center has recently become more important because the data center market is rapidly expanding. We developed an “IT-facility linkage system” to deal with this energy saving requirement. This system reduces the amount of energy consumed in a data center by linking two systems. One is an optimized server load allocation system, and the other is an air conditioning optimization control system. One of the key technologies of the “IT-facility linkage system” is the precise prediction of the server inlet temperature. If we can comprehend future temperature distributions, we can reduce the amount of energy consumed in a data center by consolidating the IT load to more effectively cooled servers. Then, we carried out computational thermal fluid dynamics to predict the server inlet temperatures and evaluated the prediction precision by comparing the measured temperature data. As a result, we found that when we use a detailed rack model and reproduce the characteristic airflow of a data center such as the recirculation, we can predict the server inlet temperature to within less than 1.5°. Next, we carried out an experiment to verify the optimal server consolidation and confirmed that the effect of optimal consolidation can be predicted by simulation.

관성관 맥동관 극저온 냉동기의 유동 특성 모델링

The flow characteristics of inertance pulse tube cryocooler(IPTC) was investigated with a computational thermal fluid dynamics for the reciprocating flow in IPTC including the piston movement of linear compressor. Two dimensional axisymmetric modeling was applied for the flow in an IPTC with a clearance between the piston and cylinder wall of linear compressor. The pressure, velocity, and temperature distribution were examined for the steady state. These were compared with previous results to confirm the validity in the modeling and computational results. The leakage between piston and cylinder wall affect the cooling capacity seriously. The dependence on mesh numbers were also examined to obtain a proper mesh numbers to improve the accuracy of calculation, which showed significant effect on the results. The user-defined function was used for the process of compression and expansion of piston.

Computational thermal-fluid dynamics analysis of the laminar flow regime in the meander flow geometry characterizing the heat exchanger used in high temperature superconducting current leads

The Karlsruhe Institute of Technology and the Politecnico di Torino have developed and validated a computational thermal-fluid dynamics (CtFD) strategy for the systematic analysis of the thermal-hydraulics inside the meander flow heat exchanger used in high-temperature superconducting current leads for fusion applications. In the recent past, the application of this CtFD technique has shown that some operating conditions occurring in these devices may not reach the turbulent regime region. With that motivation, the CtFD analysis of the helium thermal-fluid dynamics inside different meander flow geometries is extended here to the laminar flow regime. Our first aim is to clarify under which operative conditions the flow regime can be considered laminar and how the pressure drop as well as the heat transfer are related to the geometrical parameters and to the flow conditions. From the results of this analysis, correlations for the pressure drop and for the heat transfer coefficient in the meander flow geometry have been derived, which are applicable with good accuracy to the design of meander flow heat exchangers over a broad range of geometrical parameters.

CtFD-based correlations for the thermal–hydraulics of an HTS current lead meander-flow heat exchanger in turbulent flow

The Karlsruhe Institute of Technology is responsible for the design, construction and testing of the high temperature superconductor (HTS) current leads for the Wendelstein 7-X stellarator and for the JT-60SA tokamak. These HTS current leads mount a heat exchanger of the meander-flow type, in which the helium flows between the fins and is forced to cross flow with respect to the central Cu bar, which actually carries the current. Since an important issue in the operation of the HTS current lead is the optimization of the cooling power consumption, the helium thermal–hydraulics in such complex geometry becomes rather important.In this paper we extend a computational thermal fluid dynamics (CtFD) technique, previously introduced by the same authors and validated on short samples of meander flow heat exchanger and on the W7-X HTS current lead prototype, to a systematic analysis of the helium thermal-fluid dynamics inside different meander flow geometries. The first aim is to clarify under what operative conditions the flow regime can be considered turbulent and how the pressure drop as well as the heat transfer are related to the geometrical parameters and to the flow conditions. From the results of this analysis, correlations for the pressure drop and the heat transfer in the meander flow geometry have also been derived, which are applicable with good accuracy for the design of HXs over a broad range of geometries.

CtFD Analysis of HTS Current Lead Fin-Type Heat Exchanger for Fusion Applications

We apply a recently proposed Computational thermal Fluid Dynamics (CtFD) strategy to the analysis of a meander-flow path (MF) fin-type heat exchanger (HX), to be used in the HTS current leads for the LTS coils of both the W7-X stellarator and the JT-60SA tokamak. A mock-up of the HX was tested at the Karlsruhe Institute of Technology providing the database for the validation of the computational model. The hydraulic characterization of the mock-up is considered first, and then the heat transfer characteristic is analyzed.