Fluid Dynamics Calculations Research Articles

Acoustic Behavior Prediction and Analysis of Resonators in the Presence of Low Mach Number Flow

The frequency-domain linearized Navier–Stokes equations (LNSEs) are used to describe the sound field of Helmholtz resonators and concentric perforated tube resonators in the presence of low Mach number flow. The numerical procedure of LNSEs method is performed in three steps, computational fluid dynamics (CFD) calculation, data transfer and acoustics calculation. The transmission loss predictions of the resonators exhibit good agreement with measurements published in the literature. The results show that the low Mach number flow shifts the resonance frequencies of resonators and changes the acoustic attenuation behavior, which may be attributed to the change of acoustic impedance of the opening and orifices. In order to weaken the effect of flow on the resonance frequency, the modified configurations of resonators are proposed by using the conical tubes to reduce the flow velocity passing the opening and orifices. Numerical results demonstrated that the influence of flow velocity on the resonance frequency of the modified resonators is less sensitive than the original configurations.

The Effect of a Conical Inner Electrode on the Mass-transfer Behavior in a Cylindrical Electrochemical Reactor under Single-Phase and Two-Phase (Gas-Liquid) Swirling Flow

The influence of a continuous change in the cross-section area on mass-transfer performance is analyzed in an electrochemical reactor under swirling flow. Thus, a conical frustum is used as an inner electrode in a cylindrical arrangement. A convergent flow is obtained by placing the conical frustum with its smaller base at the bottom of the reactor, or conversely, it produces a divergent flow. Experimental local mass-transfer coefficients along the electrode length are reported using the reduction of ferricyanide as a test reaction. The mass-transfer distribution as well as the global mass-transfer coefficient are both discussed in three geometric arrangements, i.e. constant interelectrode gap, convergent and divergent cases under single-phase and two-phase (gas-liquid) flow. The highest global mass-transfer coefficients are obtained for the convergent arrangement in the middle range of volumetric flow rates, giving out values 7.6% higher than those for the constant interelectrode gap case. Divergent geometry presents, as its main asset, the most uniform mass-transfer distribution in comparison with the other two configurations. The presence of the gas phase also has little influence on the mass-transfer performance of the reactor. Computational fluid dynamics calculations are in good agreement with experimental measurements.

The effects of changing coil radius and tube diameter in the simulation of oil-water separation in a coil tube separator.

Separating oil and water is a frequent task in the oil and gas industry, and new methods and devices are constantly being designed to accomplish this more effectively. One of the most prevalent devices used is the coil tube separator, in which the separation process takes place between the two-phase fluids, each with its own density, utilising eccentricity acceleration and gravity. In this paper, a numerical simulator based on fluid dynamics calculations is used to survey and study the coil tube separator, modelling the two-phase flow using fluent software based on the Eulerian-Eulerian method. The subject variables such as tube diameter and radius of regression are taken as a focus in order to optimise the device’s efficiency and function. For such optimisation, two targets were taken into consideration. The first is pressure flow drop and the second is rate of separation. To attain maximum performance, the pressure drop in this device should be low, while a high rate of separation is required. The research results showed that every variable has an optimum value: from this work, a tube with a 50 mm diameter and a coil with a 500 mm radius are defined as the most appropriate and optimum options.

Driven topological transitions in active nematic films.

The topological properties of many materials are central to their behavior. In intrinsically out-of-equilibrium active materials, the dynamics of topological defects can be particularly important. In this paper, local manipulation of the order, dynamics, and topological properties of microtubule-based active nematic films is demonstrated in a joint experimental and simulation study. Hydrodynamic stresses created by magnetically actuated rotation of disk-shaped colloids in proximity to the films compete with internal stresses in the active nematic, influencing the local motion of +1/2 charge topological defects that are intrinsic to the nematic order in the spontaneously turbulent active films. Sufficiently large applied stresses drive the formation of +1 charge topological vortices through the merger of two +1/2 defects. The directed motion of the defects is accompanied by ordering of the vorticity and velocity of the active flows within the film that is qualitatively unlike the response of passive viscous films. Many features of the film's response to the stress are captured by lattice Boltzmann simulations, providing insight into the anomalous viscoelastic nature of the active nematic. The topological vortex formation is accompanied by a rheological instability in the film that leads to significant increase in the flow velocities. Comparison of the velocity profile in vicinity of the vortex with fluid-dynamics calculations provides an estimate of the film viscosity.

Tool for Rapid Transient Transpiration-Cooled Reentry Simulation

This Paper presents a numerical model intended for a system study of transpiration-cooled thermal protection systems. The code PIRATE enables rapid transient calculations for thermal protection systems of hypersonic flight vehicles. In the code, semi-empirical aerodynamic correlations are employed. However, the code could also be used by applying higher-fidelity inputs from computational fluid dynamics calculations for more complicated vehicle geometries. The wall temperature distribution is calculated by using the thermal impulse and step responses of porous media, increasing the calculation speed significantly. A heat flux balance is considered for a one-dimensional porous material. The code is validated with experimental data of plasma wind tunnel tests and the SHEFEX II flight experiment. The transient heating of a flat plate model in a steady-state flow condition in the Arc-heated wind tunnel 2 (L2 K) plasma wind tunnel is assessed for both uncooled and transpiration-cooled cases. The SHEFEX II flight simulation accounts for transient external flow conditions and is used to assess the downstream coolant film effect after the injection point. The trends of experimental temperature measurements are replicated well by the simulations. The predicted and measured temperatures are in good agreement and within a margin of 10% with maximum deviations of 40 K.

Design of a high power production target for the beam dump facility at CERN

The Beam Dump Facility (BDF) project is a proposed general-purpose facility at CERN, dedicated to beam dump and fixed target experiments. In its initial phase, the facility is foreseen to be exploited by the Search for Hidden Particles (SHiP) experiment. Physics requirements call for a pulsed 400 GeV/c proton beam as well as the highest possible number of protons on target (POT) each year of operation, in order to search for feebly interacting particles. The target/dump assembly lies at the heart of the facility, with the aim of safely absorbing the full high intensity Super Proton Synchrotron (SPS) beam, while maximizing the production of charmed and beauty mesons. High-Z materials are required for the target/dump, in order to have the shortest possible absorber and reduce muon background for the downstream experiment. The high average power deposited on target (305 kW) creates a challenge for heat removal. During the BDF facility Comprehensive Design Study (CDS), launched by CERN in 2016, extensive studies have been carried out in order to define and assess the target assembly design. These studies are described in the present contribution, which details the proposed design of the BDF production target, as well as the material selection process and the optimization of the target configuration and beam dilution. One of the specific challenges and novelty of this work is the need to consider new target materials, such as a molybdenum alloy (TZM) as core absorbing material and Ta2.5W as cladding. Thermo-structural and fluid dynamics calculations have been performed to evaluate the reliability of the target and its cooling system under beam operation. In the framework of the target comprehensive design, a preliminary mechanical design of the full target assembly has also been carried out, assessing the feasibility of the whole target system.

Numerical Investigation on the Scale Effect of a Stepped Planing Hull

This article discusses the scale effects on a planing boat, utilizing the computational fluid dynamics method. The simulation is compared with a tank test for verification and validation. The planing boat sails use both aerodynamics and hydrodynamics. Studying the performances and wave patterns of different dimensions of the models is the best way to investigate the scale effect without using experimental data. The resistance is discussed in two parts, namely residuary resistance and friction resistance, and is compared to the calculated data using the international towing tank conference (ITTC) formula. The computational fluid dynamics (CFD) calculations of the model are increased by 4.77% on average, and the boat computations are also increased by 3.57%. The computation shows the scale effect in detail. The residuary resistance coefficients at different scales are approximately equal, and the friction resistance coefficients show the scale effect. The scale effect for longitudinal steadiness is also captured for the period of the porpoising behavior. The rational for the full-scaled boat oscillation period and the model is the root of the scales.

Numerical simulation of the rotor winding temperature field with self-ventilation from the sub-slot duct

This paper describes the evaluation of rotor winding self-ventilation system with the radial ducts fed from sub-slot ducts for air and hydrogen cooled turbogenerators. The verification of the computational fluid dynamic (CFD) calculation has been carried out with the experiment. The conjugate heat transfer (CHT) is performed with the finite-volume method for rated excitation current. The analysis of the temperature field of the rotor winding with different configuration of the sub-slot duct was carried out.

A SEARCH OF AN INDUCER GEOMETRY THAT IS BENEFICIAL FOR LIFTING PARAMETERS OF A LIFTED OBJECT OF A SELECTED SHAPE

The literature describes acoustic levitation phenomena with the utilization of air squeeze film between the vibrating inducer and the lifted object. The objective of the study is to determine the shape of the inducer with vibration characteristics that would allow the levitation of an object of the assumed geometry. In this paper, the influence of the dimension ratio of the inducer on the frequency of the first mode of vibration was presented. CFD calculations for a selected dimension series were performed with the goal of the determination of lifting conditions. The data obtained from the analysis will be used to manufacture an inducer that will serve as an experimental verification for the fluid dynamics calculation.

Study on Propulsion Performance by Varying Rake Distribution at the Propeller Tip

This study provides a comparison of propulsion performance, with a particular focus on efficiency, by varying rake distribution at the tips of propellers. Owing to increased attention to environmental pollution, there is a significant interest in reducing the energy efficiency design index (EEDI) and SOx emissions by improving the performance in the field of shipbuilding. The forward (Kappel) [1] and backward tip rake propellers have been widely used to improve efficiency, as well as to reduce fluctuating pressure from the tip vortex cavitation. As there is almost no parametric and design research on tip rake propellers, this systematic parametric study was conducted to identify the optimal configuration by the potential code. For this performance comparison the KP505 (KCS propeller) was chosen as the reference propeller as the tips of that propeller have no rake. The model test and computational fluid dynamics (CFD) calculation confirmed the result by comparing the open water performances for the three optimally selected propellers (forward, backward, KP505). The differences of efficiency obtained from the potential analysis and the model test exhibit similar tendencies, but the result for the CFD is different. The difference would be investigated by changing the grid system around the tip as well as the turbulence model in the CFD analysis. An analysis of self-propulsion and pressure fluctuation is also expected to be conducted in the near future.

Experimental visualization of flow structure inside subchannels of a 4 × 6 rod-bundle

A fuel element geometry frequently used in nuclear reactors is the rod-bundle. In it, coolant flows axially through subchannels formed between the rods. The mixing of cooling fluid in a rod-bundle reduces the temperature differences in the coolant and along the perimeter of the rods. To ensure thermal performance of a nuclear reactor, detailed information about heat transfer and turbulent mixing taking place within subchannels is required. To improve the prediction ability of the subchannel analysis using computer code, a considerable amount of experimental data should be utilized for developing a new model for subchannel analysis. Experimental work has been conducted in the PRIUS-I (in-PWR Rod-bundle Investigation of Undeveloped mixing flow across Subchannel) test facility, which has adopted the matched index-of-refraction (MIR) technique to visualize the multi-dimensional velocity and turbulence fields in an unheated 4 × 6 rod-bundle geometry as well as to provide benchmark data for computer code validation. The 4 × 6 rod-bundle array has the same pitch-to-diameter (P/D) as the prototype fuel assembly. PRIUS-I test facility can simulate the uniform and non-uniform inlet flow conditions in order to validate the effect in crossflow between the fuel assemblies of PWR. In this study, the crossflow and mixing phenomena that exist between two adjacent subchannels through the gaps are visualized in any cross section of the rod-bundle geometry using a MIR technique. For non-uniform inlet velocity condition, the flow characteristics near the inlet where the flow mixing occurs actively is compared with computational fluid dynamics (CFD) calculation results. The experimental data can be used as benchmark data for the system safety analysis code or CFD code validation.

Benefits of time dependent ignition probability models within the framework of Quantitative Risk Analysis

The evaluation of ignition probabilities is a fundamental step within the calculation of hazardous phenomena frequencies associated to a loss of containment of a flammable product (explosion, pool fire, jet fire). An error on this step can substantially change quantitative risk analysis conclusions.It is important to keep in mind that quantitative risk analysis are increasingly used to design facilities. In other words, facilities (equipment, structures and buildings) are designed to withstand accidental loads up to down a certain frequency. Three main approaches are commonly used for ignition probabilities calculation: tabulated values ??from databases, semi-quantitative methods or more or less sophisticated mathematical models.Nowadays, time dependent models are implemented to predict the evolution of the ignition probability as function of time. These models are particularly useful when combined with Computational Fluid Dynamics (CFD) calculations to provide flammable gas volume build up as function of time. Thus, it is possible to predict the evolution of the delayed ignition probability as function of time. Main interest of these models is to propose a discretization by time intervals of the delayed ignition probability. This approach is particularly relevant for performing explosion risk analysis.

Large cemented gibbsite agglomerates in alkaline nuclear waste at the Hanford site and the impacts to remediation

Recent work with the remediation of legacy alkaline nuclear waste has focused on nanometer and micrometer particle sizes, emphasizing how these small particles can impact efforts to treat the waste. Building upon this work, we present here findings that show very large particles (several centimeters in size) also exist in these waste which likewise play an important role in the remediation process. While large cemented gibbsite nodules have been periodically reported in acid soils in the literature, this study found similar large gibbsite agglomerates (7 cm in diameter) in alkaline nuclear waste, the first time that such large agglomerates have been identified in an alkaline environment. The morphology of the gibbsite in the agglomerates that were grown over more than 40 years of storage in the waste tank were similar to the much smaller agglomerates that have been reported in previous shorter term studies. Fluid dynamics calculations indicate that these cemented particles would be difficult to mobilize with standard jet slurry technologies, which is consistent with their persistence in the waste heel after jet sluicing of the tank.

Calculation of Droplet Coalescence in Binary Liquid–Liquid Systems: An Incompressible Cahn–Hilliard/Navier–Stokes Approach Using the Non-Random Two-Liquid Model

Due to their lower costs regarding money and time compared to performing experiments, computer simulations grow in importance when developing industrial separation processes. On plant scales, process simulators are widely applied, and on particular scales, computational fluid dynamics (CFD) calculations can support design. In this work, the incompressible Cahn–Hilliard/Navier–Stokes equations and the non-random two-liquid (NRTL) model were applied to perform numerical simulations in order to investigate droplet interactions of extraction systems. In contrast to other CFD approaches, no assumptions regarding the evolution of drop size distributions are required. The impact of the influence parameter and mobility coefficient within this framework was studied for the first time. While the mobility coefficients show a low impact on the flow field, different values of the influence parameter can lead to completely different behavior, with low influence parameters causing small morphologies and requiring larger...

Design Method and Performance Prediction for Radial-Inflow Turbines of High-Temperature Mini-Organic Rankine Cycle Power Systems

The realization of commercial mini organic Rankine cycle (ORC) power systems (tens of kW of power output) is currently pursued by means of various research and development activities. The application driving most of the efforts is the waste heat recovery from long-haul truck engines. Obtaining an efficient mini radial inflow turbine, arguably the most suitable type of expander for this application, is particularly challenging, given the small mass flow rate, and the occurrence of nonideal compressible fluid dynamic effects in the stator. Available design methods are currently based on guidelines and loss models developed mainly for turbochargers. The preliminary geometry is subsequently adapted by means of computational fluid-dynamic calculations with codes that are not validated in case of nonideal compressible flows of organic fluids. An experimental 10 kW mini-ORC radial inflow turbine will be realized and tested in the Propulsion and Power Laboratory of the Delft University of Technology, with the aim of providing measurement datasets for the validation of computational fluid dynamics (CFD) tools and the calibration of empirical loss models. The fluid dynamic design and characterization of this machine is reported here. Notably, the turbine is designed using a meanline model in which fluid-dynamic losses are estimated using semi-empirical correlations for conventional radial turbines. The resulting impeller geometry is then optimized using steady-state three-dimensional computational fluid dynamic models and surrogate-based optimization. Finally, a loss breakdown is performed and the results are compared against those obtained by three-dimensional unsteady fluid-dynamic calculations. The outcomes of the study indicate that the optimal layout of mini-ORC turbines significantly differs from that of radial-inflow turbines (RIT) utilized in more traditional applications, confirming the need for experimental campaigns to support the conception of new design practices.

A numerical study of the effect of nozzle diameter on diesel combustion ignition and flame stabilization

The role of nozzle diameter on diesel combustion is studied by performing computational fluid dynamics calculations of Spray A and Spray D from the Engine Combustion Network. These are well-characterized single-hole sprays in a quiescent environment chamber with thermodynamic conditions representative of modern diesel engines. First, the inert spray evolution is described with the inclusion of the concept of mixing trajectories and local residence time into the analysis. Such concepts enable the quantification of the mixing rate, showing that it decreases with the increase in nozzle diameter. In a second step, the reacting spray evolution is studied focusing on the local heat release rate distribution during the auto-ignition sequence and the quasi-steady state. The capability of a well-mixed-based and a flamelet-based combustion model to predict diesel combustion is also assessed. On one hand, results show that turbulence–chemistry interaction has a profound effect on the description of the reacting spray evolution. On the other hand, the mixing rate, characterized in terms of the local residence time, drives the main changes introduced by the increase of the nozzle diameter when comparing Spray A and Spray D.

Actively lubricated hybrid journal bearings based on magnetic fluids for high-precision spindles of machine tools

The research work reported in this article is focused on the use of magnetic fluids as active lubricant for improving the performance of hybrid journal bearings, with application to high-precision machine tools. Prototype design was optimized following numerical computation of Reynolds equation and computational fluid dynamics calculations, in both cases with Herschel–Bulkley model for the magnetorheological fluid. This fluid (LORD Corp. MRF 122-2ED) was experimentally characterized in detail. The improvement of the hydrodynamic effect in journal bearings was demonstrated with 50% higher load capacity and stiffness, mainly at half of shaft eccentricity 0.4 < ε < 0.7. Active hydrostatic lubrication achieved quasi-infinite stiffness within working limits (load and speed), at low frequencies. For high dynamic response, the active lubrication based on magnetorheological valves did not show good response. The feasibility of using magnetic fluids for developing high performance machine tool spindles and the validity of the simulation models was demonstrated experimentally.

Design of Two Experimental Mock-Ups as Proof-of-Concept and Validation Test Rigs for the Enhanced EU DEMO HCPB Blanket

Within the framework of EUROfusion activities, the helium cooled pebble bed (HCPB) breeding blanket is under development in Karlsruhe Institute of Technology (KIT). The enhanced HCPB, using a fissionlike fuel breeder pin assembly configuration, is proposed as the near-term breeding blanket for the European Union DEMOnstration power plant (EU DEMO). The helium gas with a pressure of 8 MPa is used as the coolant, EUROFER is used as the structural material, advanced ceramic breeder pebbles are used as the tritium breeder, and Be12Ti is used as the neutron multiplier material. In contrast to the former HCPB cooling plate configuration, the fuel breeder pin assemblies greatly reduce the pressure drop, reducing the circulating power to the level where state-of-the-art helium turbomachinery can be used. However, because of the reduced coolant velocity in the breeder zone, the heat transfer performance is compromised, especially in the annular channel of the fuel breeder pins. An increased surface roughness is therefore proposed as a heat transfer augmentation technique for the fuel breeder pins. Although heat transfer augmentation using artificial roughness is common, it is relatively novel for small annular gaps with moderate velocity as the ones in the fuel breeder pins. Currently, a dedicated correlation for the small annular rough-wall channel is available to predict the Nu number. This correlation is wished here to be benchmarked and validated experimentally. Therefore, an experimental investigation on a fuel breeder pin mock-up (mock-up 1) is planned. Additionally, based on computational fluid dynamics calculation, unsteady, nonuniform flow patterns were found at the return flow after the jet impingement in the first-wall region. Another upscaled mock-up (mock-up 2) to investigate the nonuniform flow patterns of the return flow in the annular channel of the fuel breeder pin is planned. The dedicated experimental campaigns are foreseen at the Helium Loop Karlsruhe (HELOKA) in KIT as validation and proof-of-concept test rigs for this enhanced pin design. In this paper, the motivation and the preliminary design of these two mock-ups of the enhanced EU DEMO HCPB blanket are shown, together with the plan for the foreseen experiments.

Water transport law of fuel cell membranes at different current densities

Proton exchange membrane fuel cell (PEMFC) is considered to be one of the most promising technical for automotive power. Because it has high power density, low operating temperature and environmental friendliness. During the operation of the fuel cell, hydrogen protons accompanying water was transferred from the anode to the cathode by electro-osmotic drag. At the same time, the water will be produced at the cathode, and diffused from the cathode to the anode by a certain concentration difference. When the electro-osmotic drag is greater than the back diffusion, the anode needs to be continuously humidified to prevent the water loss in the membrane. When the back diffusion is greater than the electro-osmotic drag, the water generated in the cathode is continuously transferred to the anode. Therefore, the law of water transfer in the membrane is an important issue in fuel cell research. In the present work, it adapted such thicker homogeneous membranes such as Nafion 117. It was found that at low current densities (0.2 A/cm2), the anode was more likely to be flooded. While at high current densities, they believed that a higher electroosmotic drag will drag the anode water to the cathode, thereby reducing the anode water content. However, the used more and more thin membranes become a trend in fuel cell technology. For example, in the past, DuPont′s most thin membrane Nafion 211 was 25 μm, but now the 15 μm membrane represented by Gore was widely used, and Toyota has used 10 μm membrane. these thin membranes are different from the conventional thick membrane with uniform structure. Generally, the membrane was a sandwich structure with a perfluorosulfonic acid resin on both sides of the porous PTFE reinforced matrix. Therefore, the law of water transport was different from the homogeneous membrane. The water transport flux of the actual situation was tested by designing a dew point meter water balance test to analyze the water transfer law in the fuel cell. At the same time, the three-dimensional PEM fuel cell was simulated by using the fluid dynamics calculation software. The distribution of water content, anode water molar concentration and hydrogen mole fraction in the membrane was obtained. The results of experiment were compared to simulation results. The simulation results show that under the working condition of 75°C, 150 kPa, and cathode non-humidification conditions, the anode moisture of 15 μm membrane below 0.4 A/cm2 decreases with the increase of current. In this case, electroosmosis drags counteract partial back diffusion; The anode water above 0.4 A/cm2 increases with the increase of current, and the back diffusion played a leading role in transferring a large amount of water from cathode to the anode; the higher the current density, the more water was transferred to the anode. The actual test results showed that as the current increases, the more liquid water could be observed by the anode, which was consistent with the simulation results. When the membrane thickness was 15 μm, a thinner membrane increases the amount of water transferred from the cathode to anode. When the current was higher than 3.0 A/cm2, the anode water volume of 25 μm membrane will decrease due to electroosmotic drag. But the anode water of 15 μm membrane will still increase, and the effect of back diffusion will increase with the decrease of membrane thickness.

Multidisciplinary Design and Verification of the HB Flatback Airfoil Family

This paper presents the design, multidisciplinary optimization, and verification of 6 different flatback airfoils with different maximum thickness ranging from 35 to 50% with various trailing edge gap using 2D flow conditions for large multi-megawatt wind turbine blades. Because the design of the airfoil sections is a multidisciplinary field, involving several aerodynamic and structural parameters, the Simulated Annealing algorithm was used to design the optimized shape of the flatback airfoil sections. Computational fluid dynamics calculations in the optimization process were performed using the commercial Fluent code. The design Reynolds number was and the optimized airfoil sections have 35, 40, 45, and 50% maximum thickness with a trailing edge gap ranging from 12.5 up to 20.0%. The wind tunnel test was performed on the HB-FB-350-150 airfoil at a Reynolds number of . The results show that the optimized flatback airfoils have several benefits compared with currently applied flatback airfoils. The results for these new optimized flatback airfoils are mainly a higher lift coefficient, higher stall angle of attack, higher sectional moment of inertia, and lower sensitivity to leading edge roughness compared with the current flatback airfoils.