Performance Analysis of Nek5000 for Single-Assembly Calculations

The ExasSMR project focuses on the exascale application of single and coupled Monte Carlo (MC) and computational fluid dynamics (CFD) physics. Work is based on the Shift MC depletion, OpenMC temperature-dependent MC, and Nek5000 CFD codes. The application development objective is to optimize these applications for exascale execution of full-core simulations and to modularize and integrate them into a common framework for coupled and individual execution. Given the sheer scale of nuclear systems, the main algorithmic driver on the CFD side is weak scaling. The focus for the first four years of the project is on demonstrating scaling up to a full reactor core for high-fidelity simulations of turbulence. Full-core fluid calculations aimed at better predicting the steady-state performance will be conducted with a hybrid approach in which large eddy simulation is used to simulate a portion of a core and unsteady Reynolds-averaged Navier-Stokes handles the rest. This zonal hybrid approach provides an additional scaling dimension besides the number of assemblies. The present manuscript focuses on performance assessment using assembly-level simulations with Nek5000. We discuss the development of two benchmark problems: a subchannel (single-rod) problem to assess internode performance and a larger full-assembly problem representative of a small modular reactor (SMR). We note that current SMR assemblies are considerably simpler than pressurized water reactor assemblies since they contain no mixing vanes. This feature allows for considerable reduction in the degrees of freedom required to simulate the full core. We discuss profiling and scaling results with Nek5000, describe current bottlenecks and potential limitations of the approach, and suggest optimizations for future investigation.

Effect of Green Wall Modules on Air Temperature and Humidity

Green or living walls are active bio-filters developed to enhance air quality. Often, these walls form the base from which plants are grown; and the plant-wall system helps to remove both gaseous and particulate air pollutants. They can be classified as passive or active systems. The active systems are designed with ventilators which force air through the substrate and plant rooting system, therefore the air is purified and filtered through a bio-filtration process which also acts as a natural cooling system. Their benefits include temperature reduction, improvement of air quality and reduction of air pollution, oxygen production as well as the social and psychological wellbeing. They can produce changes in the ambient conditions (temperature and humidity) of the air layers around them which create an interesting insulation effect. The effect of green wall modules on the air temperature and on humidity is investigated in this work. A closed chamber made of acrylic sheets is used to monitor the temperature and humidity variation caused by a green wall module placed at its center. A fan positioned at the back center of the module drives air at ambient conditions and direct it into the module. Temperature and humidity are measured at different locations inside the chamber during operation for different modules with different plant species. The effect of changing the surrounding ambient conditions is also investigated.

Study of Transmissivity of a Fracture Under Shearing

A Marcellus shale rock fracture was subjected to four shearing steps and at the end of each shearing step CT (computed tomography) scans with resolution of 26.8 μm were obtained. The CT images were used to generate full aperture maps of the fracture configuration at the end of each shearing phase. The pressure drops along the fracture were also measured for different water flow rates through the fracture. The aperture map of the fracture was used to generate the geometry of the fracture for use in numerical simulations. The water flows and pressure drops in the fracture were simulated with different computational methods that included the full Navier-Stokes simulation, Modified Local Cubic Law (MLCL), and Improved Cubic Law (ICL) methods. Full 3-D Navier-Stokes simulation is the most accurate computational approach which was done with use of the ANSYS-Fluent software for each shear step and different flow rates. The MLCL is a 2-D relatively fast method which is commonly used for prediction of transmissivity of fractures. ICL is a 1-D method proposed in this study in which the effects of surface roughness and tortuosity were included in calculation of the effective aperture height of fractures. To provide an understanding of the accuracy of each of these models their predictions were compared with each other and with the experimental data. Also, to examine the effects of resolution of CT scans and the surface roughness on prediction of fractures transmissivity, similar simulations were performed on average aperture maps. Here the fracture of the full resolution data was averaged over 10 × 10 pixels. Comparing the results of the average aperture maps with those of the full maps showed that the lower resolution of CT scans led to underestimation of the fracture pressure drop due to missing the small features of the fracture surfaces and smoothing out their roughness.

Characterization of Cavitation During Pump Operation Based on Hilbert-Huang Transform

Cavitation has negative influence on pump operation. In order to detect incipient cavitation effectively, experimental investigation was conducted to through acquisition of current and vibration signals during cavitation process. In this research, a centrifugal pump was modeled for research. The data was analyzed by HHT method. The results show that Torque oscillation resulted from unsteady flow during cavitation process could result in energy variation. Variation regulation of RMS of IMF in current signal is similar to that in axial vibration signal. But RMS of IMF in current signal is more sensitive to cavitation generation. It could be regarded as the indicator of incipient cavitation. RMS variation of IMF in base, radial, longitudinal vibration signals experiences an obvious increasing when cavitation gets severe. Such single variation regulation could be selected as the indicator of cavitation stage recognition. Hilbert-Huang transform is suitable for transient and non-stationary signal process. Time-frequency characteristics could be extracted from results of HHT process to reveal pump operation condition. The contents of current work could provide valuable references for further research on centrifugal pump operation detection.

Numerical and Experimental Investigation of Heat Transfer Enhancement Using Heat Pipes for Electronic Cooling

Computers are crucial to nearly every endeavor in the modern world. Some computers, particularly those used in military applications, are required to endure extreme conditions with limited maintenance and few parts. Units such as these will hereafter be referred to as “rugged computers.” This series of experiments aims to produce improvements to rugged computers currently in service. Using heat pipes and finned heat sinks on an enclosed box, a computer’s Central Processing Unit (CPU) is able to reject heat without suffering contamination from unforgiving environments. A modular prototype was designed to allow for three distinct cases; a case with no heat pipes and fins, a cast with heat-pipes mounted internally with exterior fins and a case with heat-pipes extended externally with exterior fins. Each case was tested at three different heat loads, with a copper plate heated by a silicone heat strip simulating the heat load generated by a CPU. Each case/load combination was run many times to check for repeatability. The aim of this research is to discover the ideal case for maximum heat transfer from the CPU to the external environment. In addition to the experiments, numerical simulation of these modular prototypes with different designs of heat pipes were conducted in this research. Creating an accurate model for computer simulations will provide validation for the experiments and will prove useful in testing cases not represented by the modular prototype. The flow and heat transfer simulations were conducted using Autodesk CFD. The aim here is to create a model that accurately reflects the experimentally-verified results from the modular prototype’s cases and loads, thereby providing a base from whence further designs can branch off and be simulated with a fair degree of accuracy.

Aerodynamic Drag Measurement in a High-Enthalpy Shock Tunnel

Shock tunnels create very high temperature and pressure in the nozzle plenum and flight velocities up to Mach 20 can be simulated for aerodynamic testing of chemically reacting flows. However, this application is limited due to milliseconds of its test duration (generally 500 μs–20 ms). For the force test in the conventional hypersonic shock tunnel, because of the instantaneous flowfield and the short test time [1–4], the mechanical vibration of the model-balance-support (MBS) system occurs and cannot be damped during a shock tunnel run. The inertial forces lead to low frequency vibrations of the model and its motion cannot be addressed through digital filtering. This implies restriction on the model’s size and mass as its natural frequencies are inversely proportional the length scale of the model. As to the MBS system, sometimes, the lowest natural frequency of 1 kHz is required for the test time of typically 5 ms in order to get better measurement results [2]. The higher the natural frequencies, the better the justification for the neglected acceleration compensation. However, that is very harsh conditions to design a high-stiffness MBS structure, particularly a drag balance. Therefore, it is very hard to carried out the aerodynamic force test using traditional wind tunnel balances in the shock tunnel, though its test flow state with the high-enthalpy is closer to the real flight condition.

Careful Parameter Study to Enhance the Effect of Injecting Heavy Fuel Oil Into a Crossflow Using Numerical Approaches

The flow and spray parameters can have noticeable roles in heavy fuel oil (HFO) spray finesse. As known, the interaction between droplets and cross flow should be considered carefully in many different industrial applications such as the process burners and gas turbine combustors. So, it would be so important to investigate the effect of injecting HFO into a crossflow more subtly. In this work, the effects of various flow and spray parameters on the droplet breakup and dispersion parameters are investigated numerically using the finite-volume-element method. The numerical method consists of a number of different models to predict the droplets breakup and their dispersion into a cross flow including the spray-turbulence interaction one. An Eulerian–Lagrangian approach, which suitably models the interaction between the droplets and turbulence, and also models the droplets secondary breakup is used to investigate the interactions between the flow and the droplet behaviors. After validating the computational method via comparing them with the data provided by the past researches, four test cases with varying swirl number, air axial velocity, droplet size, and fuel injection velocity are examined to find out the effects of preceding parameters on some spray characteristics including the droplets path, sauter mean diameter (SMD), and dispersed phase mass concentration. The results show that the droplets inertia and the flow velocity magnitude have significant effects on spray characteristics. As the droplets become more massive, the deflection of spray in flow direction becomes less. Also, increasing of flow velocity causes more deflection for sprays with the same droplet sizes.

Flow Control Around a SUV Simplified Vehicle

The research on the external aerodynamics of ground vehicles can nowadays be related to sustainable development strategies, confirmed by the worldwide CO2 regulation target. Automotive manufacturers estimate that a drag reduction of 30% contributes to 10g/km of CO2 reduction. However, this drag reduction should be obtained without constraints on the design, the safety, comfort and habitability of the passengers. Thus, it is interesting to find flow control solutions, which will remove or remote recirculation zones due to separation edges with appropriate control techniques. In automotive sales, the SUV, 4x4 and compact cars represent a large part of the market share and the study of control approaches for this geometry is practically useful. In this work, appropriate control techniques are designed to decrease the drag forces around a reduced scale SUV car benchmark called POSUV. The control techniques are based on the DMD (Dynamic Mode Decomposition) algorithms generating an optimized drag reduction procedure. It involves independent transient inflow boundary conditions for flow control actuation in the vicinity of the separation zones and time resolved pressure sensor output signals on the rear end surface of the mockup. This study, that exploits dominant flow features behind the tailgate and the rear bumper, is performed using Large Eddy Simulations on a sufficient run time scale, in order to minimize a cost function dealing with the time and space average pressure coefficient. The resulting dynamic modal decomposition obtained by these LES simulations and by wind tunnel measurements has been compared for the reference case, in order to select the most appropriate run time scale. Analysis of the numerical results shows a significant pressure increase on the tailgate, for independent flow control frequencies. Similar decomposition performed in the wake with and without numerical flow control help understanding the flow modifications in the detachment zones.

Flame Speed and Deflagration-Detonation Analysis in Flare Stack and Header

This paper examines safety concerns related to flame speeds when warm relief gas snuffs out the pilot at the flare stack and pulls in ambient air and a spark ignites the vapor in the header. The flame speed essentially determines if the propagating flame speed is a deflagration or a detonation based on whether its subsonic or supersonic. While pipes are sized for deflagrations, they need to be analyzed and tested for detonation pressures and temperatures. Transient CFD calculations help determine the flame speeds, deflagration to detonation transition, pressures and temperatures are compared to pipe specifications and help determine if a detonation leads to a Loss of Containment and suggests mitigations.

Design of a Thermal Mass Flow Meter for Measurements Within the Rotor Rim Ducts of a Hydroelectric Generator

To ensure the proper operation of hydroelectric generators, their cooling must be well understood. However, the airflow within such machines is difficult to characterize, and although Computational Fluid Dynamics (CFD) can be a reliable engineering tool, its application to the field of hydroelectric generators is quite recent and has certain limitations which are, in part, due to geometrical and flow complexities, including the coexistence of moving (rotor) and stationary (stator) components. For this reason, experimental measurements are required to validate CFD simulations of such complex flows. Of particular interest is the quantification of the flow within the rotor rim ducts, since it is directly responsible for cooling the poles (one of the most critical components of a hydroelectric generator). Thus, to measure the flow therein, an anemometer was designed. The anemometer had to be accurate, durable, cost-effective, easy to install, and able to withstand the extreme conditions found in hydroelectric generators (temperatures of 45°C, centrifugal forces of 300 g, etc.). In this paper, a thermal mass flow meter and a method for validating its performance, using hot-wire anemometry and a static model of a rotor rim, are described. Preliminary tests demonstrate that the thermal mass flow meter is capable of i) measuring the mass flow rate in the rotor rim ducts with an accuracy of approximately 10%, ii) fitting inside small rectangular ducts (12.2 mm by 51 mm), and iii) resisting forces up to 300 g.