Thermal-fluid Simulation Research Articles

Determination of Turbulent Prandtl Number for Thermal Fluid Dynamics Simulation of HVAC Unit by Data Assimilation

Abstract Regarding high-precision temperature field prediction of a heating ventilation and air conditioning (HVAC) unit using thermal fluid dynamics simulation, the determination of the turbulent Prandtl number (Prt) was a key issue. In this study, we present an attempt to improve the accuracy of thermal fluid dynamics simulations in HVAC units by adjusting the Prt using data assimilation techniques. First, we simultaneously measured the velocity and temperature in the hot and cold air mixing zone of a simple HVAC model using particle image velocimetry and thermocouples. Second, we coupled data assimilation to the thermal fluid simulation model to determine the Prt in the mixing field. Finally, we proposed two functions of the Prt for high velocity and low velocity regions using multidimensional analysis. In the future, we believe that the Prt functions can be applied to thermal fluid simulations of actual HVAC unit to accurately predict performance without conducting prototype experiments, thereby contributing to reducing development cost and time of HVAC unit.

The influence mechanism of pre-combustion chamber orifice structure on natural gas engines: Combustion, emissions, and thermofluid analysis

The pre-combustion chamber (PCC) can enhance ignition stability, promote flame propagation and thus improve thermal efficiency. However, the mechanism by which the structure of the PCC orifice affects the combustion characteristics of engines is still unclear. In this study, thermal fluid simulation was used to investigate the effects of different volume ratios (ratio of PCC to main combustion chamber), apertures, and orifice structures (under stoichiometric and lean-burn conditions) on a PCC-type natural gas engine. The results showed that as the volume ratio and pore size increased, the indicated thermal efficiency (ITE) of the engine first increased and then decreased. Furthermore, the ITE of the engine reached its highest value when a PCC structure with a volume ratio of 1.5% and a pore diameter of 1.6 mm was adopted. As the orifice taper increased, the flame propagation velocity in the W-shaped orifice (Din > Dout) and N-shaped orifice (Din < Dout) decreased. The flame produced by the direct orifice (Din = Dout) had the longest penetration distance and the highest propagation velocity, which increased the cylinder pressure. In addition, under lean-burn conditions, in-cylinder combustion was more complete, with the lowest CO and THC emissions.

Quality Evaluation of 150 mm 4H-SiC Grown at over 1.5 mm/h by High-Temperature Chemical Vapor Deposition Method

To reduce manufacturing costs, high-quality 150 mm 4H-SiC wafers were grown at over 1.5 mm/h by high-temperature chemical vapor deposition. The dislocations in the initial growth stage did not increase compared with those in the seed crystal. The dislocation densities decreased during crystal growth, and the densities of threading dislocations and basal plane dislocations at the growth thickness of 7.1 mm were 1186 and 211 /cm2, respectively. The resolved shear stress, which is the cause of the increase in dislocations during growth, was calculated based on thermal fluid simulations; the shear stress of the grown crystal with a flat surface was small compared with that of the convex-shaped crystal. The dislocations did not increase likely because the crystals grown at high speeds were relatively flat. In addition, the decrease in dislocations was attributed to the frequent annihilation of dislocations due to the growth at a high temperature (2490 °C).

Factorial design analytics on effects of material parameter uncertainties in multiphysics modeling of additive manufacturing

A bottleneck in Laser Powder Bed Fusion (L-PBF) metal additive manufacturing (AM) is the quality inconsistency of its products. To address this issue without costly experimentation, computational multi-physics modeling has been used, but the effectiveness is limited by parameter uncertainties and their interactions. We propose a full factorial design and variable selection approach for the analytics of main and interaction effects arising from material parameter uncertainties in multi-physics models. Data is collected from high-fidelity thermal-fluid simulations based on a 2-level full factorial design for 5 selected material parameters. Crucial physical phenomena of the L-PBF process are analyzed to extract physics-based domain knowledge, which are used to establish a validation checkpoint for our study. Initial data visualization with half-normal probability plots, interaction plots and standard deviation plots, is used to assess if the checkpoint is being met. We then apply the combination of best subset selection and the LASSO method on multiple linear regression models for comprehensive variable selection. Analytics yield statistically and phyiscally validated findings with practical implications, emphasizing the importance of parameter interactions under uncertainty, and their relation to the underlying physics of L-PBF.

Research on Bionic Fish Scale Channel for Optimizing Thermal Performance of Liquid Cooling Battery Thermal Management System

Liquid cooling battery thermal management systems (BTMSs) are prevalently used in electric vehicles (EVs). With the use of fast charging and high-power cells, there is an increasing demand on thermal performance. In this context, a bionic fish scale (BFS) channel structure optimization design method is proposed to optimize the thermal performance. The effects of different structural parameters of the liquid cooling plate in BTMS on its cooling performance, including BFS notch diameter (D), BFS notch depth (H), and BFS notch spacing (S), are investigated. To minimize the maximum temperature (Tmax) and the maximum temperature difference (ΔTmax) as optimization indicators, experimental tests and numerical calculations are performed for a battery pack consisting of 36 square cells. Sixteen sets of thermal performance are discussed for different structural parameters in the transient thermal fluid simulation by using orthogonal tests. Under the optimal structural parameters, Tmax decreases by 1.61 °C (10.8%) and ΔTmax decreases by 0.43 °C (16.7%). In addition, the maximum increase in outlet flow velocity is 2.72% and the pressure is reduced by 4.98%. Therefore, the proposed BTMS will have effective cooling performance in high-power dissipation.

Development of Basic Technology for Oil-Cooled Motor Coil End Thermal-Fluid Simulation based on Thermal Experiments of Heated Rectangular Columns

Development of Basic Technology for Oil-Cooled Motor Coil End Thermal-Fluid Simulation based on Thermal Experiments of Heated Rectangular Columns

Numerical Investigation of the Initial Charging Process of the Liquid Hydrogen Tank for Vehicles

Liquid hydrogen has been studied for use in vehicles. However, during the charging process, liquid hydrogen is lost as gas. Therefore, it is necessary to estimate and reduce this loss and simulate the charging process. In this study, the initial charging process of a vehicle liquid hydrogen tank under room temperature and atmospheric pressure conditions was numerically investigated. A transient thermal-fluid simulation with a phase-change model was performed to analyze variations in the volume, pressure, mass flow rate, and temperature. The results showed that the process could be divided into three stages. In the first stage, liquid hydrogen was actively vaporized at the inner wall surface of the storage tank. The pressure increased rapidly, and liquid droplets were discharged into the vent pipe during the second stage. In the third stage, the mass flow rates of liquid and hydrogen gas at the outlet showed significant fluctuations, owing to complex momentum generated by the evaporation and charging flow. The temperatures of the inner and outer walls, and insulation layer, decreased significantly slower than that of the gas region because of its high heat capacity and insulation effect. The optimal structure should be further studied because the vortex, stagnation, and non-uniform cooling of the wall occurred near the inlet and outlet pipes.

Impact of a mechanical attachment on the preheating temperatures of a central receiver tube

The receiver tubes of Solar Power Tower plants are typically attached through clips and sliding rods to the reradiating wall of the receiver in order to reduce the thermal deflection of the tubes. However, as a side effect, these mechanical attachments increase heat dissipation from the tube to the exterior and can reduce the local temperature of the tube during the receiver start-up, an operation performed every morning or after a severe drop of solar irradiation due to passing clouds. In the start-up operation, the empty tubes of molten salt receivers must be preheated with a careful aiming sequence of the heliostat field until their temperature is high enough to avoid the freezing of the incoming flow of the molten salt. This work numerically addresses for the first time the influence of the mechanical attachment on the preheating temperature of a receiver tube. To achieve this goal, a set of three-dimensional thermal fluid simulations of the temperature distribution in the tube and the mechanical attachment, described with different levels of detail, has been performed for the receiver of the Gemasolar Solar Power Tower plant. The results of this work show that the mechanical attachment locally affects the tube temperature, leading to a temperature overestimation above 25.1% when the tube is modeled without a mechanical attachment. In addition, the temperature measured separated from the attachment at the rear face of the tube to control the preheating sequence may not be very representative of the minimum tube temperature, which occurs in the internal face of tube just beneath the mechanical attachment. The results indicate that accumulation of heat not only by the clip but also by the guide and the rod of the mechanical attachment plays a significant role in the dissipation of heat from the tube to the attachment. This reveals the need of considering all the elements of the mechanical attachment if an accurate modeling of the preheat temperatures of the tube is needed. Besides, the use of a simplification in the modeling of the air and radiative heat transfer outside the tube reduces by a factor of 4.9 the computational cost involved in a full simulation of these transfer processes, with a temperature discrepancy between models below 14.3%. Finally, the effect of the size of the attachment clip on the minimum tube temperature is investigated, revealing that the minimum tube temperature of the preheated tube can decrease below the freezing temperature of the molten salt for clip heights larger than 170 mm . • Preheat temperatures in a receiver tube with a mechanical attachment are simulated. • The attachment clip, guide and rod need to be modeled to ensure accuracy. • The heat absorbed by the tube is partially transferred to the attachment. • The attachment creates a cold spot in the tube close to the clip during the preheating. • The control thermocouple temperature is not representative of minimum temperature.

Differentiation of Numerical Simulation Result of Direct Current Circuit Breaker Interruption Process Using Artificial Intelligence

AbstractFor realizing the widespread application of fast charging of electric vehicles, a direct current circuit breaker (DCCB) is required to interrupt a large charging current. For developing a next‐generation DCCB that can satisfy this requirement, a three‐dimensional electromagnetic thermal fluid simulation was developed to mimic the interruption process of the DCCB, and an artificial intelligence (AI) model was constructed for differentiating the simulation result. As a result, successful and failed interruption of the DCCB were both simulated, and the constructed AI model successfully differentiated the interruption result and located the extinguish timing of arc plasma in case of a successful interruption. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Numerical simulation of external magnetic field effect on arc plasma after molten metal bridge breaking under different electrode opening velocities

In this study, a three-dimensional electromagnetic thermal fluid simulation program was developed to simulate arc movement numerically while considering metal vapor generated from a molten metal bridge along with an electrode opening under external magnetic field application. The change in the physical properties of the arc and the increment rate of the arc voltage per unit length under different electrode opening velocities were analyzed by applying an external magnetic field with different intensities. It was found that the current path of the arc became dispersed when the electrode opening velocity was high because the metal vapor generated from the molten metal bridge remained dominant and highly concentrated. Before the completion of the electrode opening process, the difference in the increment rate of the arc voltage per unit length at different electrode opening velocities became more significant when an external magnetic field was applied. This was because under high and low electrode opening velocities, the cross-sectional area of the arc became narrow and broad, respectively.

Evaluation of heat dissipation characteristics of quantum cascade laser with diamond submount using structure function and three-dimensional thermal fluid simulation

The heat dissipation of a quantum cascade laser (QCL) for a mounted structure with and without a diamond submount was evaluated by temperature and structure function measurements and three-dimensional simulation. From the structure function, it was shown that the thermal resistance between the QCL on the InP substrate and the CuW mount was reduced from 5.0 K W−1 without the submount to 2.5 K W−1 with the diamond submount. In the 3D simulation, it was confirmed that the heat flux transmitted horizontally through the diamond mount is larger than that without the submount. It is considered that the heat flux in the horizontal direction improved the heat dissipation from the InP substrate to the CuW mount. As a result, the output of the QCL with the submount was 1.15 times that of the QCL without the submount.

Cathode Spot Velocity of Atmospheric Arc Plasma after Arc Ignition Affected by Self and External Electromagnetic Force between Parallel Electrodes Using 3D Numerical Simulation

For visually and quantitatively elucidating the contribution of self and external electromagnetic forces of the cathode spot area to the cathode spot movement, a three‐dimensional electromagnetic thermal fluid simulation program was used to simulate the arc movement between parallel electrodes. According to the analysis results, the cathode spot first step occurred when the electromagnetic force balance of cathode spot area was broken, and the direction of total electromagnetic force at the right side control volume pointed forward. When the current path changes from straight to reentry model, the cathode spot movement became faster because of the self electromagnetic force derived from electrodes. The rapid of cathode spot first step significantly increased along with the arc current increasing, and the increment of self electromagnetic force was more significant than the external electromagnetic force. In conclusion, the application of external magnetic field or the self magnetic field derived from the parallel electrodes benefit to break the electromagnetic force balance of cathode spot area and promote the rapid of cathode spot first step. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Thermal Control by Thermal Fluid Simulation Based on Inverse Analysis

Thermal Control by Thermal Fluid Simulation Based on Inverse Analysis

The Design and Investigation of a Cooling System for a High Power Ni-MH Battery Pack in Hybrid Electric Vehicles

High power cylindrical Ni-MH battery cells have a heavy heat load because of their high discharge rate and large equivalent internal resistance. This heavy heat load, together with an imbalanced flow in parallel liquid cooling systems, can lead to variances in the temperature of each cell in the entire battery pack, thereby reducing the life cycle of the battery pack. In this paper, a parallel-series combined liquid cooling system for a 288V Ni-MH battery pack was designed, and several parameters that influence the flow balance of the system by heat transfer and fluid dynamics were calculated. Then, a thermal-fluid simulation was executed with different parameters using StarCCM+ software, and the simulation results were validated by a battery pack temperature experiment on a bench and in a vehicle. The results indicate that the cell’s temperature and temperature differences can be kept within an ideal range. We also determined that within the battery power requirements and structural spacing limits, the total flow rate of the cooling liquid, the cross-sectional area ratio of the main pipe to the branch pipes, and the number of internal supporting walls in each branch pipe need to be large enough to minimize the cell’s maximum temperature and temperature differences.

Three‐dimensional electromagnetic thermal fluid simulation of re‐strike phenomenon in magnetically driven arc between parallel electrodes with edges

A magnetically driven arc does not advance smoothly between parallel electrodes with edges. The anode spot stagnates on the anode surface when the arc column and cathode spot move forward. According to experimental results, when the arc column moves closer to the anode surface, or a high‐temperature gas comes into contact with the anode, a new anode spot appears and electrical breakdown occurs. Meanwhile, the original anode spot disappears. It is well known as the re‐strike phenomenon. Also, it can be observed using a bandpass filter (520 nm) that a large amount of Cu vapor is generated whenever a new anode spot appears. Because the Cu vapor considerably increases the arc's radiative emission and has a dissimilar effect on the gas's electrical conductivity at different concentrations, a three‐dimensional electromagnetic thermal fluid simulation program that considers the metal vapor from the electrodes is proposed in this paper. Based on the experimental figures, it is able to simulate the re‐strike process by controlling the electrical conductivity of the control volume at the anode surface. In addition, the physical properties of the arc plasma, the original anode spot area, and the re‐strike point area are analyzed, in order to elucidate the occurrence conditions of the re‐strike phenomenon. The results show that the arc form is affected by the electrode jets, which leads to an increase in the electric field density between the re‐strike point and the arc column, thereby causing the re‐strike phenomenon to occur. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.

Study of the thermal erosion, ejection and solidification processes of electrode materials during EDM

Due to the complexity of electrical discharge machining (EDM), there is no perfect theory to explain the mechanisms of material erosion, ejection and solidification during the machining process. Therefore, this paper analyzed the micro-process of a single-pulse discharge during EDM and studied the thermal erosion, ejection and solidification processes of the electrode materials. A thermal-fluid simulation model that considered melting-solidification was established. A mathematical model of vapor flow considering the impact pressure was developed, and it was compiled and loaded as a material ejection force in simulations with a user defined function (UDF). The changes in the material ejection, crater formation and material solidification processes at different discharge times were studied. Finally, the simulation results were verified by single-pulse material discharge experiments, which provided a reference for additional understanding of the micro material discharge processes during EDM.

Reduced-order thermomechanical modeling of multibody systems using floating frame of reference formulation

In this study, a reduced-order thermomechanical coupling model, which accounts for the inertia coupling of the thermoelastic deformation and the large reference body motion, is proposed using the floating frame of reference formulation for the transient thermomechanical analysis of constrained multibody systems. In this approach, the reduced-order heat equations are fully embedded in the final form of the equations of motion. Accordingly, the transient thermal response as well as the resulting thermoelastic behavior of constrained multibody system can be predicted within the general multibody dynamics computer algorithm. It is demonstrated that appropriate selection of the thermal interface coordinates is crucial for describing the thermal modes (i.e. temperature distribution) induced by external heat sources using the Craig–Bampton component mode synthesis approach generalized for thermomechanical systems. Furthermore, a systematic procedure for imposing prescribed surface temperature given, for example, from thermal-fluid dynamics simulations is proposed for the thermomechanical floating frame of reference formulation. Using several numerical examples, simulation capabilities of the thermomechanical floating frame of reference formulation model are demonstrated for multibody dynamics applications. Numerical results show good agreement with the nonlinear thermomechanical finite element solutions considering the large rotational motion with substantial reduction in the model dimensionality and computational time.

Classification of machine learning frameworks for data-driven thermal fluid models

Thermal fluid processes are inherently multi-physics and multi-scale, involving mass-momentum-energy transport phenomena at multiple scales. Thermal fluid simulation (TFS) is based on solving conservative equations, for which – except for “first-principles” direct numerical simulation – closure relations (CRs) are required to provide microscopic interactions or so-called sub-grid-scale physics. In practice, TFS is realized through reduced-order modeling, and its CRs as low-fidelity models can be informed by observations and data from relevant and adequately evaluated experiments and high-fidelity simulations. This paper is focused on data-driven TFS models, specifically on their development using machine learning (ML). Five ML frameworks are introduced including physics-separated ML (PSML or Type I ML), physics-evaluated ML (PEML or Type II ML), physics-integrated ML (PIML or Type III ML), physics-recovered (PRML or Type IV ML), and physics-discovered ML (PDML or Type V ML). The frameworks vary in their performance for different applications depending on the level of knowledge of governing physics, source, type, amount and quality of available data for training. Notably, outlined for the first time in this paper, Type III models present stringent requirements on modeling, substantial computing resources for training, and high potential in extracting value from “big data” in thermal fluid research.The current paper demonstrates and investigates ML frameworks in three examples. First, we utilize the heat diffusion equation with a nonlinear conductivity model formulated by convolutional neural networks (CNNs) and feedforward neural networks (FNNs) to illustrate the applications of Type I, Type II, Type III, and Type V ML. The results indicate a preference for Type II ML under deficient data support. Type III ML can effectively utilize field data, potentially generating more robust predictions than Type I and Type II ML. CNN-based closures exhibit more predictability than FNN-based closures, but CNN-based closures require more training data to obtain accurate predictions. Second, we illustrate how to employ Type I ML and Type II ML frameworks for data-driven turbulence modeling using reference works. Third, we demonstrate Type I ML by building a deep FNN-based slip closure for two-phase flow modeling. The results show that deep FNN-based closures exhibit a bounded error in the prediction domain.

Multi-physics steady-state analysis of OECD/NEA modular high temperature gas-cooled reactor MHTGR-350

ABSTRACTThis paper presents the application results of MCS/GAMMA+ to multi-physics analysis of OECD/NEA modular high temperature gas-cooled reactor (MHTGR) benchmark Phase I Exercise 3. It is a part of international R&D efforts lead by the Next Generation Nuclear Plant (NGNP) US project to improve the neutron-physics and thermal-fluid simulation of (high temperature gas-cooled reactors) HTGRs, one of the next generations of safer nuclear reactors. Accurate and validated analysis tools are indeed a crucial requirement for safety analysis and licensing of nuclear reactors. To guide this effort, a numerical benchmark on the MHTGR was created by the NGNP project and formally approved in 2012 for international participation by the OECD/NEA. The benchmark defines a common set of exercises and the comparison of solutions obtained with different analysis tools is expected to improve the understanding of simulation methods for HTGRs. The coupled neutronics/thermal-fluid solution presented in this paper was obtained with the neutron transport Monte Carlo code MCS developed by Ulsan National Institute of Science and Technology and the thermal-fluid code GAMMA+ developed by Korean Atomic Energy Research Institute. The purpose of this paper is to present the GAMMA+/MCS coupled system, the calculation methodology, and the obtained solutions.

FSW Numerical Simulation of Aluminium Plates by SYSWELD - Part II

Abstract Friction Stir Welding (FSW) is one of the most effective solid state joining processes and has numerous potential applications in many industries. The simulation process can provide the evolution of physicals quantities such as temperature, metallurgical phase proportions, stress and strain which can be easily measured during welding. The numerical modelling requires the modelling of the complex interaction between thermal, metallurgical and mechanical phenomena. The aim of this paper is to describe the thermal-fluid simulation of FSW using the finite element method. In the theoretical part of paper heating is provided by the material flow and contact condition between the tool and the welded material. Thermal-mechanical results from the numerical simulation using SYSWELD are also presented for aluminium alloy.