Computational Fluid Dynamics Code Research Articles

Influences of optimizing the turbulator arrangement on the heat transfer and hydraulic characteristics of the tubular heat exchanger

The Two-Tube heat exchangers due to their uncomplicated structure and high performance, have extensive use in the thermal engineering industries. Therefore, raising the performance and efficiency and the heat transfer capability is always a popular concept. Thus, the present paper has focused on the simulation and optimization of the application of the spiral-shaped turbulator in this kind of heat exchanger. A Computational Fluid Dynamic (CFD) code, which is 3D, is developed to solve the flow governing equations. So, 12 different cases (c1 to c12) are modeled with different spiral radii (R = 2 mm, 6 mm, and 9 mm) and pitches (500 mm, 250 mm, 100 mm, and 50 mm) to find the optimal position and configuration of the turbulator. The results indicated that c10 and c11 with R = 9 and with pitch = 250 mm and 100 mm can rise 168 % and 183.5 %, respectively, in Nusselt while increasing the pressure slippage compared to the base case. In the next part, it is endeavored to find the optimal base profile shape for the turbulator. Various profile shapes are examined and as a result, it is found that increasing the edge of the base shape increases the friction coefficient. On the other hand, the triangular base profile shape has the best thermal and hydraulic performance.

On the application of overset meshing to numerical studies of roll damping of hulls in complex scenarios

The application of computational fluid dynamics (CFD) and overset meshing, to predict the roll response of ships is of significant interest to ocean engineering. Despite its growing popularity, it remains essential to validate the efficacy of computations using overset meshing for modelling strong vortex flows, particularly in complex geometric scenarios involving relative motion and narrow gaps, and for deriving roll damping coefficients. This study presents a new series of results in which the solvers and overset meshing methods in the open-source CFD code, OpenFOAM, were applied to compute the vortex flow from sharp edges and the roll damping of hulls, including the particularly complex scenario of a hull rolling next to a solid vertical wall. It has been demonstrated that the methods can effectively model the intricate flow physics associated with flow separation, including the shedding of primary and secondary vortices, and reliably estimate damping coefficients. Further, the study provides suitable numerical configurations for overset mesh applications and new insights to explain the discrepancies between experimental and numerical results. These new results not only contribute to the understanding of wave-induced roll response but also reinforce the value of the overset mesh technique for the prediction of related problems in fluid mechanics.

CFD and machine learning based hybrid model for passive dilution of helium in a top ventilated compartment

Abstract For hydrogen management in the containment of Nuclear Power Plants (NPPs), besides the Passive autocatalytic Recombiners (PAR), the passive dilution of lighter gas plays an important role. This could be an attractive option to optimize the containment design and to estimate the extent of dilution. Passive dilution has many other applications in nuclear industry. The experimental studies of air entrainment in the upward rising helium plume and the resulting dilution of helium gas by the Canadians in terms of Volume Flow Magnification Factor (VFMF) have been utilized for Computational Fluid Dynamics (CFD) validation. The CFD based Fire Dynamics Simulator (FDS) predicted values of VFMF found to be in good agreement with the test data. After FDS code validation, parametric study has been carried out to generate a data base of VFMF for range of hydrogen injection, side opening area and opening height. In present study various Machine Learning (ML) models are evaluated based on two-parameter relationship i.e. non dimensional hydrogen injection and VFMF using the CFD code generated database. The trained ML models were used for the predictions of the mass flow rate of gas entrainment (through opening) in the rising buoyant plume in terms of VFMF. The ML predictions were in good agreement with the predictions against test data. Multivariate Adaptive Regression Splines (MARS) based ML model found to performed best and discussed in the paper. The paper highlights details of methodology of numerical simulation, results of the CFD studies and machine learning based predictions.

Data-Driven Modeling for Transonic Aeroelastic Analysis

Aeroelasticity in the transonic regime is challenging because of the strongly nonlinear phenomena involved in the formation of shock waves and flow separation. In this work, we introduce a computationally efficient framework for accurate transonic aeroelastic analysis. We use dynamic mode decomposition with control to extract surrogate models from high-fidelity computational fluid dynamics (CFD) simulations. Instead of identifying models of the full flowfield or focusing on global performance indices, we directly predict the pressure distribution on the body surface. The learned surrogate models provide information about the system’s stability and can be used for control synthesis and response studies. Specific techniques are introduced to avoid spurious instabilities of the aerodynamic model. We use the high-fidelity CFD code SU2 to generate data and test our method on the benchmark supercritical wing. Our Python-based software is fully open source and will be included in the SU2 package to streamline the workflow from defining the high-fidelity aerodynamic model to creating a surrogate model for flutter analysis.

Heat transfer enhancement using combined microchannel with vertical rectangular micro fins

This paper focuses on the numerical optimisation of a combined hybrid microchannel heat sink with rectangular solid fins. The axial length and volume are fixed, and external structure is allowed to vary. The simulation was performed on an elemental unit cell of the microchannel heat sink . The purpose of the optimisation is to discover an optimal geometric arrangement in internal and external configurations that minimises peak temperature in the microchannel heat sink. A high-density uniform heat flux of 250 W/cm2 is assumed to be dissipated on the bottom wall of the unit cell by microelectronics circuit boards devices. Computational fluid dynamic code was used to discretized the fluid domain and solve a set of governing equations. The influence of hydraulic diameter, external structural shape and fluid velocity on peak temperature and global thermal resistance, is discussed. Coolant or water of Reynolds number range 400 to 500 in a forced convection laminar flow is introduced through the inlet of the computational domain to remove the heat at the bottom of the rectangular block microchannel. The results show that when the fluid velocity is increased from 9.8 to 12.3 m/s across the axial length of the micro heat sink, more heat is removed from the bottom of the combined heat sink. The results revealed that the pump power increased by 37.1% in combined microchannels with fins and from by 27.2% in finless micro heat sink. The result of the study is validated with what is documented in open literature for a traditional micro heat sink with circular flow channel and the trends agree.

Forced convection heat transfer in micro heat sinks with square and circular configuration

This paper reports the results of three-dimensional numerical optimisation of microchannel heat exchanger with square and circular cooling channels. The objective of the optimisation is to maximise the global thermal conductance or minimise global thermal resistance. Response surface optimisation methodology (RSM) is used in the numerical optimisation. A high-density heat flux (2.5×106W/m2) is imposed at the bottom surface of the unit cell microchannel and numerical simulation carried out using ANSYS Fluent commercial software package.The elemental volume and axial length N=10mm of the microchannel were all fixed, while the width was free to morph. The cooling technique employs single-phase water which flows through the rectangular block microchannel heat sink to remove the heat at the bottom of the microchannels in a forced convection laminar flow regime. The velocity of the fluid pumped across the microchannel axial length is the range 400≤Rew≤500. Finite volume method (FVM) is used to descretised the computational domain and computational fluid dynamic (CFD) code employed to solve a series of governing equations. The effect of channel hydraulic diameter and Reynolds number of water-flow on peak wall temperature and minimised temperature are investigated and reported. The numerical results show that the microchannel with square cooling channel has the highest maximised global thermal conductance than the micro heat sink with circular configuration. The result of the numerical study agrees with what is in the open literature.

Novel approach for modeling a full-sized air-cooled heat exchanger using porous media and iterative processes

Air-Cooled Heat Exchangers (ACHE) have gained importance through their extensive use in the oil and gas industry. Many researchers studied ACHE extensively and proposed analysis methods. Most of them investigate part of the ACHE in their modeling. Others consider only pressure drop in their calculations in simulating the porous media (fins and surrounding air). These assumptions lead to unprecise modeling results. In contrast to existing literature, by incorporating computational fluid dynamics (CFD) with real geometry design, this paper includes a study of a full-size ACHE with precise geometric measurements, commencing at the hot water's input and going all the way to the ACHE's exit through a U-path tube. By incorporating the complete geometry dimensions of the U-path tube length from the hot water inlet to the outlet, this methodology has the potential to produce more accurate predictions of temperature distribution, heat flow rate, logarithmic mean temperature difference (LMTD), and fin efficiency for each tube along the entire length of the ACHE. Further, unlike other works of literature, the porous media in this research includes calculations for pressure drops and heat transmission in place of real geometry design which gives more sensible and accurate modeling results. Moreover, a novel verification method is proposed by presenting an iterative process for verification of entire geometry design calculations by comparison with the Real Geometric Design. The incorporation of Real Geometry Design and Computational Fluid Dynamics (CFD) Code programs can lead to more precise and efficient methodologies for modeling air-cooled exchangers, offering valuable insights that can ultimately enhance their design and performance. In this work, the convergence with the actual geometrical design is less than 1%, demonstrating the superiority of the suggested strategy.

Quantitative Validation of Gas-Liquid Flow Regime Transition Using Eulerian-Eulerian CFD Models

Quantitative validation of a computational fluid dynamics (CFD) code is about making point-to-point comparisons over fields of data to make statements about predictive fidelity. By the very nature of CFD, these comparisons are high-performance computing and data intensive. This presentation provides an overview of workflow development toward quantitative development of two-phase CFD codes. The focus here is on multifield two-fluid model capability implemented in the commercial CFD code ANSYS CFX applicable to boiling gas-liquid two-phase flows. Applications to validating three-dimensional predictions that span two-phase flow regimes are discussed.

Accuracy of viscosity regularization models employed by computational fluid dynamics codes

Accuracy of viscosity regularization models employed by computational fluid dynamics codes

Heat transfer enhancement of phase change materials PCMs using innovative fractal H-shaped fin configurations

Despite their limited thermal conductivity, phase change materials (PCMs) have been extensively utilized in numerous engineering applications as a means of energy storage. Using the biomimicry method, an innovative configuration was proposed and investigated numerically. It was inspired by trees' structures aiming to improve the melting performance of triplex concentric tube heat exchangers.Based on the Computational Fluid Dynamics (CFD) code, this study examines the impact of integrating innovative fractal H-shaped solid fins, as well as various geometrical parameters such as the number of patterned fins, arrangement type, and stretched fins, on enhancing the PCM's performance. Compared with previous experimental work, the numerical method was validated.Through two-dimensional 2D simulations, the main objective of this work was to identify the optimal configuration that effectively maximizes the heat transfer performance during the charging process and significantly enhances the overall efficiency of an energy storage system filled with RT82.The obtained results demonstrate the positive impact of applying the bio-inspired fractal H structures to enhance the melting process. The configuration with four fin patterns achieved an improvement of 69.14 % compared with the thermal energy storage TES system equipped with uniform plate fins.

Experimental investigations and three-dimensional computational fluid dynamics modeling of sediment transport in tanks influenced by cavities

Sediment transport is a very complex process in urban water systems, and sediment movement mechanisms are poorly understood. However, it is necessary to comprehend sediment transport to optimize stormwater and sewer tank designs for better performance in trapping particulate pollutants. This work focuses on the numerical simulation and experimental investigation of tank sediment transport. In the numerical simulation, a discrete phase model is applied to calculate particle trajectories, and the particle tracking calculation is coupled with the steady-state flow calculation. A user-defined function is developed in the Fluent computational fluid dynamics (CFD) code to implement the settling condition and improve the particle motion predictions. In the experimental work, a back-scattered ultrasonic method is used to measure the velocity field, and the accumulation of particles at the bottom is reproduced by transforming the raw experimental data. The sediment deposition is also recorded photographically, and the trap efficiency is calculated. The CFD and experimental results are compared to estimate the accuracy of the modified settling boundary condition, showing an improvement in the trap efficiency and deposition zone predictions. High water level in the tank corresponds to higher sediment trap efficiency, and sediment deposition type is strongly affected by the water level. Tank length and the presence of cavity alter the sediment deposition zone, and the presence of cavity increases the trap efficiency of tank.

Wake interaction between two spar-type floating offshore wind turbines under different layouts

Limited by layout space and manufacturing costs, wake interaction in offshore wind farms is inevitable and can have adverse effects on the performance of downstream wind turbines. To gain a better understanding of the wake interaction between Floating Offshore Wind Turbines (FOWTs), this paper conducts coupled aero-hydrodynamic simulations for two spar-type FOWTs under different layouts. The Unsteady Actuator Line Model (UALM) is used to analyze the unsteady aerodynamic loads of the wind turbine, while the hydrodynamic responses of the floating support platform are obtained using the Computational Fluid Dynamics (CFD) method. To predict the aero-hydrodynamic performance of the FOWT under combined wind and waves, an in-house CFD code developed at Shanghai Jiao Tong University (SJTU), called FOWT-UALM-SJTU solver, is utilized. First, grid convergence test and time step sensitivity study are performed to determine appropriate simulation parameters. Subsequently, numerical simulations of two FOWTs under tandem and offset layouts are conducted to investigate the influence of wake interaction on the performance of downstream FOWT. The dynamic responses of the FWOT, including aerodynamic loads, platform motions, and wake characteristics, are analyzed in detail. From the simulation results and discussions, several conclusions are drawn. Both platform motions and wake interaction contribute to an increased variation range of inflow wind speed experienced by the downstream FOWT, thereby exacerbating the instability of its aerodynamic loads. Under the tandem layout, the platform motions of the upstream FOWT and the downstream FOWT exhibit opposite effects on the aerodynamic loads of the downstream FOWT. Moreover, platform motions increase turbulence intensity in the wake region, accelerating wake velocity recovery and widening the wake width.

A Review of Numerical Simulation and Analytical Calculation Methods for the Study of Flow over a Wedge

Abstract: The study of flow over a wedge holds immense significance in diverse domains, encompassing aerodynamics and heat transfer applications. This research paper presents a comprehensive investigation of essential flow parameters, including pressure, temperature, velocity, and shock wave characteristics. Moreover, it offers a comparative analysis between analytical and numerical approaches for studying flow over a wedge. Theoretical analyses involve calculating vital parameters like pressure ratio and temperature ratio using 1-D inviscid compressible flow equations coupled with conservation laws for energy, mass, and momentum. On the other hand, the numerical investigation employs Computational Fluid Dynamics (CFD) techniques, specifically the ANSYS Fluent CFD Code, to calculate a wide range of parameters and visualise the flow characteristics for the wedge geometry. In contrast, the ANSYS Fluent simulation employs 2-D inviscid compressible equations to conduct a detailed study of flow behaviour past the wedge geometry. Throughout the paper, the key flow parameters are meticulously analysed, taking into account both analytical and numerical results.

Realistic thermal analysis of the CANDU spent fuel dry storage canister

Thermal analysis of the CANDU spent fuel dry storage canister is very important to ensure the integrity of the spent fuel. The analyses have been conducted using a conservative approach, with a particular focus on the peak cladding temperature (PCT) of the fuel rods in the canister. In this study, we have performed a realistic thermal analysis using a computational fluid dynamics (CFD) code. The canister contains 9 fuel bundle baskets. A detailed analysis of even a single basket requires significant computational resources. To overcome this challenge, we replaced each basket with an equivalent heat conductor (EHC), of which effective thermal conductivity (ETC) is developed from the results of detailed CFD calculations of a fuel bundle basket. Then, we investigated the effects of some conservative models, ultimately aiming at a realistic analysis. The results revealed: (i) The influence of convective heat transfer in the basket cannot be ignored, but it's less significant than expected. (ii) Modeling of the lifting rod is crucial, as it plays a decisive role in axial heat transfer at the center of the canister and significantly reduces the PCT. (iii) Convection within the canister is very important, as it not only reduces the PCT but also shifts its location upwards.

Applications of Advanced CFD Codes for Aircraft Design and Development at NAL

Development of Computational Fluid Dynamics (CFD) has now reached a state of maturity, which enables it to simulate the flow past practical aircraft configurations. It can thus play a very useful role in aircraft design and development. CFD codes can be used to determine the aerodynamic loads on aircraft and thus provide a necessary input for the structural design. They can also be used to study and optimize the flow patterns and thus help design an aerodynamically efficient shape. Advanced level multi-block Euler and Navier-Stokes codes with multi-block grid generation package developed at National Aerospace Laboratories (NAL) are introduced here and their applications in analysis and design & development of practical aerospace configurations are described in detail.

Large Eddy Simulation of Convective Heat Transfer in a Random Pebble Bed Using the Spectral Element Method

Abstract The development of fluoride-cooled high-temperature reactors has drastically increased the demand for an in-depth understanding of the heat transfer (HT) in packed beds cooled by liquid salts. The complex flow fields and space-dependent porosity found in a pebble bed require a detailed understanding to ensure the proper cooling of the reactor core during normal and accident conditions. As detailed experimental data are complicated to obtain for these configurations, high-fidelity simulation such as large eddy simulation and direct numerical simulation (DNS) can be employed to create a high-resolution heat transfer numerical database that can assist in addressing industrial-driven issues associated with the heat transfer behavior of fluoride-cooled high-temperature reactors. In this paper, we performed a series of large eddy simulation using computational fluid dynamics (CFD) code NekRS to investigate the heat transfer for a bed of 1741 pebbles. The characteristics of the flow, such as average, rms, and time series of velocity and temperature, have been analyzed. Porous media averages have also been performed. The simulation results show a good agreement between non-conjugate heat transfer and conjugate heat transfer. The generated data will be used to benchmark heat transfer modeling methods and local maxima/minima of heat transfer parameters. It will also be used for supporting convective heat transfer quantification for Kairos Power and benchmarking lower fidelity models.

Evaluation of hydrogen recombination characteristics of a PAR using SPARC PAR experimental results

Passive auto-catalytic recombiners (PARs) are widely used to mitigate a hydrogen hazard. The first step to evaluate the hydrogen safety by PARs is to obtain qualified test data of the PARs for validation of their analytical model. SPARC PAR tests SP8 and SP9 were conducted to evaluate the hydrogen recombination characteristics of a honeycomb-shaped catalyst PAR. To obtain the hydrogen recombination rate from the PAR test data, two methods, Method-1 and Method-2, introduced by the THAI project, were applied. Since a large gradient of hydrogen concentration developed during hydrogen injection can cause a large error in the hydrogen mass obtained by integrating the measured hydrogen concentrations, a gate was installed at the PAR inlet to homogenize hydrogen in the test vessel before the PAR operation in the tests. A computational fluid dynamics (CFD) code with a PAR model was also applied to evaluate the characteristics of the PAR recombination according to the PAR inlet conditions, and the results were compared with those from Method-1 and Method-2. It was confirmed that the recombination rates from Method-1 require a correction factor to be compatible with results from Method-2 and the CFD simulation in the case of the SPARC-PAR tests.

Simulating the Effect of Electric Bias Voltages on the Electrical Characteristics of Oxyfuel Preheat Flame Using Reduced Combustion Mechanism

Abstract A three-dimensional computational model is presented in this paper that illustrates the detailed electrical characteristics, and the current–voltage (i–v) relationship throughout the preheating process of premixed methane-oxygen oxyfuel cutting flame subject to electric bias voltages. As such, the equations describing combustion, electrochemical transport for charged species, and potential are solved through a commercially available finite volume computational fluid dynamics (CFD) code. The reactions of the methane-oxygen (CH4–O2) flame were combined with a reduced mechanism, and additional ionization reactions that generate three chemi-ions, H3O+, HCO+, and e−, to describe the chemistry of ions in flames. The electrical characteristics such as ion migrations and ion distributions are investigated for a range of electric potential, V ∈ [−5 V, +5 V]. Since the physical flame is comprised of twelve Bunsen-like conical flames, inclusion of the third dimension imparts the resolution of fluid mechanics and the interaction among the individual cones. It was concluded that charged “sheaths” are formed at both torch and workpiece surfaces, subsequently forming three distinct regimes in the i–v relationship. The i–v characteristics obtained from this study have been compared to the previous experimental and two-dimensional computational model for premixed flame. In this way, the overall model generates a better understanding of the physical behavior of the oxyfuel-cutting flames, along with more validated i–v characteristics. Such understanding might provide critical information toward achieving an autonomous oxyfuel-cutting process.

Multiscale Thermal-Hydraulic Analysis of the ATHENA Core Simulator

In the framework of the ALFRED research and development program, the ATHENA facility will be constructed for thermal-hydraulic analysis of full-scale ALFRED components and systems. The source system of the facility is the core simulator, which aims to be representative of an ALFRED average fuel assembly. Computational fluid dynamics (CFD) codes are gaining attention for the analysis of complex systems in pool-type reactors since they are able to reproduce three-dimensional phenomena. In this paper, a multiscale approach based on porous media is proposed to reduce the computational cost of the core simulator CFD model. The multiscale approach starts with the detailed simulation of the infinite lattice domain of the fuel assembly to characterize the porous media hydraulic behavior. Then the porous media are applied in the system model. Three different approaches are investigated: (1) adopting a single porous media for the entire fuel assembly, (2) representing the bundle with two porous domains, and (3) adopting the so-called hybrid medium. The results have been compared with the reference detailed CFD simulation for performance evaluation. The first step of the analysis is the application of the multiscale approach on the CIRCE fuel pin simulator to carry out a turbulence model validation against experimental data and a comparison of the three approaches with a proven CFD model. Then the approach is applied on the ATHENA core simulator exploiting the CIRCE results. The results obtained with the porous media models are compared with a detailed CFD simulation of the core simulator to evaluate the performance of the three approaches. Eventually, the best solution is applied on a model of the entire ATHENA core simulator integrated with the feeding region. The model is tested also in transient conditions. The numerical experiment demonstrates the effectiveness of the multiscale approach in reducing the computational cost while maintaining high accuracy in representing the quantities of interest.

Verification & validation of CFD predictions regarding pressurized thermal shock (PTS) situations in ROCOM installation: Comparison with IAEA benchmark

The current paper documents the Computational Fluid Dynamics (CFD) code validation activity, carried out at the Nuclear Research Center of Birine relevant of Atomic Energy Commission of Algeria as part of International Atomic Energy Agency (IAEA) Coordinated Research Project (CRP): Application of Computational Fluid Dynamics Codes for Nuclear Power Plant Design to assess the current capabilities of these codes and to contribute to technological progress in their verification and validation. A set of ROCOM CFD-grade test data of Pressurized Thermal Shock test (PTS) specifications was made available in the framework of this (CRP) by Helmholtz Zentrum Dresden-Rossendorf, (HZDR) Germany, to perform detailed calculations of the proposed test. The reference point is the injection of relatively cold core cooling water (ECC), which can induce buoyant stratification. The data obtained from the PTS experiment were compared with the results of Ansys-CFX calculations in this paper. Unsteady Reynolds-Averaged Navier-Stokes (URANS) model is used to examine the buoyancy-influenced flows in the reactor pressure vessel for condition where natural circulation is a dominant factor. The Shear Stress Transport (SST k-ω) turbulence model is used to take into account the turbulence effects on the mean flow. Calculation results show a good qualitative and quantitative agreement with the experiment data.