Achieving high parallel performance for an unstructured unsteady turbomachinery CFD code

CAPTools - An interactive toolkit for mapping CFD codes onto parallel architectures

In a reactor severe accident, molten jet breakup and solidification are important behaviors after large pours of molten material fall into the coolant in-vessel or ex-vessel. However, heat and mass transfer processes inside melt during jet breakup have not been studied sufficiently. Existing research on jet fragmentation is relatively macroscopic, and the micro interface condensation details are not well studied. In this paper, a two-dimensional multiphase computational fluid dynamics (CFD) code with the Volume of Fluid (VOF) method and solidification model is applied to simulate molten jet breakup with surface solidification. The VOF model is used to capture the interface, study the details, and add the influence of solidification. Solidification and instability can be seen at the interface. In order to simulate melt solidification, an energy equation is modeled using an enthalpy-based formulation, and viscosity variation during phase change is taken into account. The comparative results between the CFD code and jet breakup experiments show that melt jet front position histories, breakup length, and breakup time are in good agreement with the experiments. The simulation results show that crust formation of the jet surface suppresses surface instability and jet breakup behavior. As the interfacial temperature decreases, the droplet cumulative mass fraction decreases, and the solidified metal proportion increases. The simulation results by the CFD code with the solidification model are valuable and important for understanding the molten jet breakup mechanism.

Accurate prediction of bubble behavior in large diameter pipes is crucial for evaluating the performance of safety systems, steam generators, and heat exchangers in nuclear systems. Bubble behavior in large diameter pipes under two-phase flow significantly differs from that in small pipes. With the increasing use of computational fluid dynamics (CFD) codes, predicting interfacial area concentration (IAC) is critical for understanding multi-dimensional bubble behavior. This study developed a two-group local bubble size model for bubbly, slug, and churn flows under adiabatic conditions. The model includes correlations for void fraction and bubble sizes of two groups, which were implemented into CFD codes and validated against experimental data from large diameter pipes with low-pressure air–water flow. Results show the model's prediction accuracy surpasses existing correlations. The developed correlations are applicable across a range of flow conditions covering pipe diameters in the range 0.05–0.152 m, pressures from atmospheric to 300 kPa, superficial liquid velocities from 0.25 m/s to 2.85 m/s, and superficial gas velocities from 0.04 m/s to 5.48 m/s. The model is expected to enhance the prediction capabilities of CFD codes for the adiabatic two-group two-phase flows in the large diameter pipes.

The Use of Computational Fluid Dynamic Code for Modelling Spray from a Mistblower

Thermal-hydraulic analysis of the TRADE spallation target

A computational fluid dynamics (CFD) code, developed at Argonne national laboratory to simulate turbulent mixing, combustion reaction, radiation heat transfer, and pollutant kinetics of combustion flow, has been used to study various melting furnaces. The code employs an integral approach to incorporate a lumped combustion reaction model in the flow calculation and a separate hybrid technique to perform pollutant kinetics calculations. The CFD code has been validated with experimental data collected from industrial furnaces and then used for a parametric study of various furnace geometries and operation conditions. The furnace configuration greatly effected the flow property distribution as well as the combustion efficiency. The air injection velocity effected the flow penetration and the species mixing. The injection angle also significantly effected the species mixing. And finally, the equivalence ratio effected the temperature and pollutant concentrations. The study demonstrates that CFD can be a useful tool for analyzing the flow field of the combustion space in industrial furnaces.

Transitional flow has a significant impact on vehicles operating at supersonic and hypersonic speeds. An economic way to simulate this problem is to use computational fluid dynamics (CFD) codes. However, not all CFD codes can solve transitional flows. This paper examines the ability of the Spalart–Allmaras one-equation BCM (SA-BCM) transitional model to solve hypersonic transitional flow, implemented in the open-source CFD code Eilmer. Its performance is validated via existing wind tunnel data. Eight different hypersonic flow conditions are applied. A flat plate model is built for the numerical tests. The results indicate that the existing SA-BCM model is sensitive to the freestream turbulence intensity and the grid size. It is not accurate in all the test cases, though the transitional length can be matched by tuning the freestream intensity. This is likely due to the intermittency term of the SA-BCM model not being appropriately calibrated for high-velocity flow, though if the model can be recalibrated it may be able to solve the general high-velocity flows. Although the current SA-BCM model is only accurate under certain flow conditions after one calibration process, it remains attractive to CFD applications. As a one-equation model, the SA-BCM model runs much faster than multiple-equation flow models.

Modelling aerosol number distributions from a vehicle exhaust with an aerosol CFD model

The Method of Manufactured Solutions is used to perform order of accuracy code verification tests on some features of a hypersonic computational fluid dynamics (CFD) code and a material response code. The CFD verification tests include results for a two species gas mixture in thermal nonequilibrium, and the material response results are for an anisotropic material. After verifying the two codes separately, a verification test is performed with the codes loosely coupled so that the material response code provides updated boundary conditions to the CFD code. In the cases tested, the observed order of accuracy of the CFD code is less than the theoretical order. The tests of the material response code, however, show the expected order of accuracy.

This chapter focuses on the description of a pyrolysis model which can be used for implementation within computational fluid dynamics (CFD) codes. The pyrolysis model is available from the literature and includes both thermal heat transfer in the solid phase as well as chemical kinetics. Besides the description of the model, the way how to obtain input parameters is discussed in this chapter and their sensitivity is shown. Finally validation results of the pyrolysis model are given.

The tilt pouring and gravity top pouring of an Al-4.5 pct Cu alloy have been studied. A computercontrolled rollover casting wheel was used to perform the tilt pouring. Filling sequences with tranquil or turbulent flow patterns have been visualized using real-time X-ray video radiography and modeled using a computational fluid dynamics (CFD) code. The area of the free surface film entrained into the bulk of liquid metal in different filling conditions has been calculated using a filling sequence free from surface turbulence as a baseline. The tensile properties of the castings have been quantitatively assessed for reliability using a two-parameter Weibull distribution function. The study reveals that the liquid metal flow in the mold filling process can be accurately simulated using a CFD code. In addition, the computed total surface area of the entrained surface film can be used as a criterion to judge the deterioration of reliability. The high Weibull modulus achieved by filling a mold without surface turbulence was reduced by a factor of 2.5 of its original value by entrained surface films. Entrainment of bubbles required surface turbulence, but folded films could be entrained simply by contraction of the free surface, creating excess surface film that necessarily folds inward.

Decrease of high-carbon-ash landfilling by its Co-firing inside a cement calciner

A project is underway to develop, verify and validate an advanced two-phase flow modeling capability for the highly-scalable, high-performance CFD code NEK5000 [1]. The goal of the project is to develop a new two-phase version of the NEK5000 code, named NEK-2P, to simulate the two-phase flow and heat transfer phenomena that occur in a Boiling Water Reactor (BWR) fuel bundle under various operating conditions. The NEK-2P two-phase flow models follow the approach used for the Extended Boiling Framework (EBF) [2-3] previously developed at Argonne, but include more fundamental physical models of boiling phenomena and advanced numerical algorithms for improved computational accuracy, robustness, and computational speed. The development of the NEK-2P two-phase solver and the implementation of the Extended Boiling Framework two-phase models were initially supported by Argonne National Laboratory (Argonne) through a Laboratory Directed Research and Development (LDRD) project during FY2014-2016. The development and validation of the two-phase models through analyses of selected two-phase boiling flow experiments was supported by the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program in FY2017-2019. This report focuses on the FY2019 development and validation of the Conjugate Heat Transfer (CHT) model for the NEK-2P Two-Phase, Computational Fluid Dynamics (CFD) code. The NEK-2P CHT model calculates the coupled behavior of the fluid and solid domains. In the fluid domain NEK-2P calculates the vapor and liquid phase temperatures, velocities and mass fractions. In the solid domain only the temperatures are calculated. The fluid and solid domains are coupled with a heat flux boundary condition at the fluid-solid interface. The heat flux and the wall temperatures at the fluid-solid interface are updated using the NEK-2P wall heat-flux partitioning model. A wall heat flux boundary condition is also applied to the exterior surface of the solid domain. The CHT model validation was illustrated through the analysis of three of the Becker Critical Heat Flux (CHF) tests. Reasonably good agreement with measured data was obtained in predicting the CHF location and post CHF wall temperature behavior illustrating the ability of the NEK-2P CHT model to simulate the CHF phenomena for a wide range of thermal-hydraulic conditions. Apart from the CHT model development and validation, initial simulations were performed for the Virginia Tech. (VT) 3x3 bare rod bundle experiments including both cold and boiling flow simulations.

The electrolytic capacitor is one of the most important components for the thermal analysis of electronic equipment. To predict component and system temperatures, the thermal flow simulation technique has been applied to thermal design in the development phase of electronic equipment. In this study, we examined a compact modeling method for electrolytic capacitors in order to simulate thermal flow based on the computational fluid dynamics (CFD) code. To obtain fundamental data for the thermal modeling method, first, we conducted experiments to identify the major thermal path of electrolytic capacitors in actual electronic equipment by using a switch mode power supply unit. Next, to verify the validity of the thermal model, a benchmark experiment was conducted to obtain actual measurement data of the temperature rise of electrolytic capacitors under various operating conditions. The thermal model of the electrolytic capacitor was presented based on the CFD code, which is a commercially available thermal flow simulation tool. In this paper, we describe in particular the snap-in type electrolytic capacitor.

Mechanistic prediction of ash deposition in a pilot-scale test facility