bluuebunny/crossref_metadata_2025 huggingface dataset:
Void Fraction Measurements for Air-Water Pipe Flows Using Quick-Closing Valves and Wire-Mesh Sensors
Authors: Lessard Étienne , Yang Jun
Published: October 2018

In support of a header/feeder phenomena study, an adiabatic, near-atmospheric, air-water flow loop was commissioned simulating a single feeder of a Pressurized Heavy Water Reactor’s primary heat transport system under a postulated Loss of Coolant Accident scenario. An extensive database in representative two-phase flow conditions was collected, 750 tests in total, in order to create a two-phase flow map to be used in the more complex geometries such as header/feeder systems. The flow loop consists of two vertical test sections, for upwards and downwards flow, and one horizontal test section, each with an inner diameter of 32 mm and at least 120 diameters in length. Superficial velocities extended up to 6 m/s for the water and 10 m/s for the air. Void fraction was measured by means of quick-closing valves and a pair of wire-mesh sensors (WMS) in each test section.

Two-phase repeatability tests showed that the liquid and gas superficial velocities varied by 1.1% and 0.6% at reference conditions of 2.0 and 2.8 m/s, respectively. The corresponding void fraction measurements varied for the quick-closing valves by at most 6.8%, which indicates a low sensitivity to the closure time of the valves and an appropriate axial distance between them, and 2.3% for the WMS. For both measurement techniques, the largest variations occurred in the vertical downwards test section.

For the formal two-phase tests, over 600 distinct flow conditions were performed. The results showed that the two measurement techniques agreed within 5% at high void fractions and low liquid flow rates in vertical flow. For all other cases corresponding to the transitional or dispersed bubbly flow regime, the WMS over-estimated the void fraction by a consistent bias. An empirical correction is proposed, with a root-mean-square error of 6.6% across all tests. The void fraction map resulting from this database provides validation for the WMS measurements, a quantitative assessment of its uncertainty and range of applicability, and will be used as a reference in future tests under similar scale and flow conditions.
A Computational Modelling for Hemodynamic Conditions Following Flow-Diverting Treatment in Cerebral Aneurysms
Authors: Fu Gen , Untaroiu Alexandrina
Published: October 2018

Cerebral aneurysms are abnormal dilations of blood vessels within the skull that, in some cases, may rupture and bleed. The rupture of an aneurysm can cause significant bleeding into or around the brain (a stroke). Flow diverters are specially designed low porosity stents that are deployed into the parent artery to cover the neck of the aneurysm. The dense mesh-like structure of flow diverters aims at redirecting flow from the aneurysm to the parent artery and vice versa, resulting in flow stasis in the aneurysm and promoting thrombus formation conditions. The thrombosed aneurysm is then resorbed by the body’s wound healing mechanisms-the end result of which is a remodeled vessel returned to its normal physiological state. Most previous studies have been focused on correlating the hemodynamic conditions with the outcome of the flow diverters. On the other hand, the effects of the location of the stents have not been addressed. In this study, a numerical simulation of an idealized side wall aneurysm model is used to predict the hemodynamic conditions for different flow diverter stent locations. The CFD model of the aneurysm is developed based on data from the literature and the geometrical parameters are set according to the test data. Pulsatile boundary conditions are chosen according to the normal physiological conditions. The entire stent geometry is used to model the effect of the stent on the flow characteristics. The hemodynamic conditions in the aneurysm corresponding to different stent locations are compared. The results show that the average velocity and vorticity are significantly different depending on different stent locations. Marked reduction in average velocity, average vorticity, and mean wall shear stress within the aneurysm sac have been observed even in malposition cases. The results of this study can be further used to guide the deployment of the flow diverter stent in clinical application.
Effect of Pipe Inclination on Settling Slurry Flow Near Deposition Velocity
Authors: Matoušek Václav , Kesely Mikoláš , Konfršt Jiři , Vlasák Pavel
Published: October 2018

Inclined slurry flows occur often in industrial applications such as mining and dredging. Pipelines transporting slurries contain inclined sections of various lengths and slopes. If the transported slurry is settling slurry then pipe inclination considerably affects flow structure and behavior.

We discuss settling slurry flow near and at the deposition limit at which stationary deposit starts to be formed at the bottom of the pipe. In particular, we focus on the effect of the pipe slope on the deposition velocity, and on the solids distribution and manometric hydraulic gradient in flow round the deposition limit. We introduce our new layered model for inclined settling slurry flows and demonstrate its predictive capabilities. Model predictions are verified by our experiment in a laboratory loop. We also introduce our new experimental approach to a detection of the deposition velocity based on radiometric sensing of the change of local concentration of solids at the bottom of a pipe.

Our experiments cover a broad range of flow slopes and contain measurements of solids distribution in a pipe cross section. Experimental results show that the degree of flow stratification and frictional pressure drop decrease with the increasing angle of inclination in the ascending pipe while the opposite applies in the descending pipe, which affects the deposition velocity and other related flow parameters.

A comparison with model predictions demonstrates that experimentally observed effects of pipe inclination are reproduced well by the layered model. Predicted deposition velocities, pressure drops and solids distributions are in a good agreement with the experimental results and indicate suitability of the model for engineering practice.
Evaluating the Ability of SLW Model in Numerical Simulation of Radiative Turbulent Reacting Flow in Industrial Application
Authors: Darbandi Masoud , Barezban Mohammad Bagher , Schneider Gerry E.
Published: October 2018

In this paper, the turbulent reacting flow in an industrial furnace is numerically simulated using the RANS equations. The two-equation standard k-ε and the eddy dissipation models are used respectively to close the turbulent closure problem and to consider the turbulence-chemistry interaction. The radiation transfer equation is solved using the discrete ordinates method (DOM). To calculate the radiation absorption coefficient in participating combustion gases, we use the spectral line-based weighted sum of grey gases (SLW) model and compare the achieved results with famous gray-based model, i.e., the weighted-sum-of-gray-gases (WSGG) model. The results of this research show that using the SLW model, the predicted heat transfer from the flame to the furnace walls is reduced due to the thermal radiation. So, the predicted temperature filed increases up to 5% near the outlet of furnace in comparison with the results of WSGG model, which is in more agreement with the experimental data. These results indicate that if one wishes to accurately predict the temperature field and the temperature sensitive quantities such as the NOx emission, one should use the spectral-based models to calculate the radiation absorption coefficient. The details are discussed in the results section.
CFD Analysis of Natural Gas Cooling System Through a Conventional Aircooler Improved by a Fogging System
Authors: Pérez Jhon , Asuaje Tovar Miguel
Published: October 2018

Nowadays, under unstable prices scenarios, the oil and gas industry is looking for improvement in its production processes, either by increasing the production and/or lowering the operational costs. Aircoolers, particularly, are key equipments in the natural gas industry, and are frequently the bottleneck of gas conditioning processes.

To improve air cooling efficiency and increase their gas volume capacity, several solutions are commonly implemented such as: fan blade angle change, air inlet section modification, fogging cooling system, among others.

The present study shows the CFD (Computational Fluid Dynamics) analysis of an air cooler, under an air flow with an evaporative cooling system, in order to quantify the effect of this cooling process on its overall equipment performance.

Simulations were carried out using sing the ANSYS CFX v-14® software, in a simplified multidomain which consider fluid and solid blocks:

1.- External two-phase flow (air + water droplets) with heat and mass transfer

2.- Heat transfer through the pipe wall

3.- Single phase natural gas flow inside the tubes

In order to stablish an operational range of the fogging system, the influence of parameters like: inlet temperature and relative humidity of the air, water flow rate, water droplets mean diameter, water injection position, were studied [9, 10].

The results show a good agreement (around 5%) respect to the reported values on the literature. The best performance for the equipment was reported with a droplet diameter of 20 μm and for low relative humidity (less than 65%), which guarantees the complete evaporation of the droplets within the studied domain. For the analyzed operating conditions, a reduction of the gas outlet temperature of up to 1.5°C can be achieved.
Flow Control Around a SUV Simplified Vehicle
Authors: Edwige Stéphie , Gilotte Philippe , Mortazavi Iraj , Eulalie Yoann , Holst David , Nayeri Christian N. , Aider Jean-Luc , Varon Eliott
Published: October 2018

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
Authors: Diwakar Philip , Valappil Jaleel
Published: October 2018

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.
Vibration Characteristics of a 50MW Francis Type Hydro Power Plant
Authors: Cho Y. , Oh Y. R. , Choi J. W. , Kim Y. J. , Novotny J. , Loub R.
Published: October 2018

Vibration characteristics of the rotating machinery has been usually managed to facilitate the deterioration of the equipment and to prevent accident in advance. In a hydropower turbine, pressure pulsation characteristics to induce vibration is investigated during the model testing with a small scale model turbine for various operating conditions, and a prototype turbine is constructed to operate stably on the site. However, the model test has limitation that can’t be considered together with the vibration characteristics of a generator itself and of civil engineering structures for the building that support a turbine and a generator. Therefore, field tests of vibration for a hydroelectric power plant are carried out periodically, thereby maintaining reliability for safe operation of power generation facilities.

In the study, the vibration of a Francis type hydroelectric power plant operated over 30 years and overhauled a year and a half ago was measured and its characteristics has been investigated. Displacement and velocity sensors were installed at appropriate positions to measure the vibration of the rotating shaft and bearing support of a turbine and a generator, and the vibration characteristics of a typical hydroelectric power plant have confirmed by analyzing the measurement results. The vibration characteristics of rotating shaft and non-rotating parts of the hydroelectric turbine have been analyzed to confirm the degree of aging of the plant. Vibration in the power plant building depending on the operating conditions was also measured to be large enough to sense, and its frequency characteristics were analyzed.
Extending a Hybrid Continuum-Molecular Simulation Method to Solve the Micro/Nanoscale Gas Mixing Problems
Authors: Sabouri Moslem , Darbandi Masoud , Schneider Gerry E.
Published: October 2018

Despite vast efforts in developing hybrid continuum-molecular methods, there has been no specific work focused on the gas mixture flow simulations including the mixing and/or separation of species. In present study, we extend a hybrid method to analyze such phenomena suitably and study the gas mixing problems in micro/nano length scales reliably. The results of current hybrid simulations are compared against the results of full-molecular simulations to evaluate the physical accuracy of developed hybrid method. The effect of continuum breakdown criterion is investigated to find out the achieved accuracy of developed hybrid simulation method from different perspectives. The current results indicate that using a reasonable breakdown parameter can result in very good physical accuracy. The results also indicate that using the hybrid simulation can be quite effective to avoid the statistical fluctuations, which are inherent to the molecular simulation methods.
Numerical Analysis of Lateral Skirts Performance on Drag Force of a Semi-Trailer Truck
Authors: Lateb M. , Fellouah H.
Published: October 2018

This work performs computational fluid dynamics (CFD) simulations using a transient URANS (unsteady Reynolds averaged Navier–Stokes) turbulence model to investigate the influence of lateral skirts — located in the lower part of a semitrailer truck — in terms of reducing the total drag force and fuel consumption savings. The total drag force values are calculated for three semi-trailer trucks speeds (i.e. 60, 70 and 100 km/h), compared, and then validated against experimental results carried out in a wind tunnel reduced model scale (1:28). The relative errors of the aerodynamic drag force parameter are assessed in order to quantify the accuracy and the reliability of the numerical modeling results with regard to the experimental results. In addition, the flow pattern around the semi-trailer truck is then investigated to determine how the induced flow field is channeled, and where the recirculating zones are modified and developed when using the additional skirt device.
Experimental and Numerical Analyses of Unsteady Flow Regimes and Mixing in a Micro T-Mixer
Authors: Salvetti Maria Vittoria , Mariotti Alessandro , Galletti Chiara , Brunazzi Elisabetta
Published: October 2018

Despite the very simple geometry, T-shaped micro-mixers are characterized by different and complex laminar flow regimes. In the present work, experiments and direct numerical simulations are employed jointly to investigate unsteady periodic flow regimes, viz. the asymmetric and symmetric regimes. The first is characterized by a periodic dynamics of the three-dimensional structures and by a high degree of mixing, while in the second the flow always maintains a double mirror symmetry in the mixing channel, which causes a dramatic decrease of the mixing performance. A methodology, allowing us to quantitatively compare the numerical predictions with experimental flow visualizations, is used to investigate these unsteady regimes and to evaluate the relevant frequencies and the degree of mixing. In both regimes the characteristic non-dimensional frequency, based on the bulk velocity and hydraulic diameter of the mixing channel, increases with the Reynolds number, but a significant discontinuity is found at the transition from the first to the second regime.
Remarks on the Recirculation at the Outlet of a Radial Impeller Near Shut-Off
Authors: Palamarchuk Evgenii , Mengdehl Tino , Thamsen Paul Uwe
Published: October 2018

Centrifugal pumps are characterized by the performance curves, showing the dependency between its main operating parameters for a range of external conditions. One of the most important curves is the head curve, which shows the dependency between the volume flow rate and the pressure head, induced by the pump. Stable operating conditions occur when the pump curve has a negative slope for the point of intersection with the system resistance curve (dH/dQ < 0). Most of the pump curves have a “stable” negative slope of the head curve for the most part of the possible operating range. Nevertheless, centrifugal pumps of low specific speed display a tendency to generate an unstable pump performance curve, especially if designed with a high head coefficient at the best efficiency point [1]. These curves are characterized by lowering the pressure head near the shut-off area (drooping curves). Operating range of such pumps could be significantly limited in the case of high static head component for the system resistance curve (firefighting applications) or parallel operation.

The flow pattern for the part load conditions differs from the one by the optimum conditions. There are two recirculation zones, occurring at the inlet and at the outlet of the impeller [2]. This paper concentrates on the investigation of the pressureside recirculation and its influence on the curve slope. The pressure-side recirculation may have various position and intensity in the case of a radial impeller with an axis-parallel trailing edge [3, 4]. There are a few measures that influence positively the stability of the performance curve [3, 5].

The tests contain an experimental setup with comparison of experimental data to numerical simulations. Subject of the experimental investigations is a single volute radial centrifugal pump with a specific speed nq = 35 min−1. The casing was modified in order to provide optical access to the impeller outlet area. Radial velocity components are measured by using the Particle Image Velocimetry (hereafter: PIV) methods in order to define the backflow zones for the part load regimes. Measured operating points are then compared to numerical simulations carried out by Computational Fluid Dynamics (hereafter: CFD). The flow pattern gained by CFD allows analyzing the phenomena of the pressure side recirculation in detail, also in areas, where the access with measuring instruments is limited.

The investigation is performed on two modifications of the pump impeller — applying the slots opening in the back shroud [2] and diagonal trimming of the impeller [5], as well as for the original (unmodified) case. The combination of the described methods gives a clear vision on the 3D recirculating structures for the cases of stable and unstable behavior of the curve.
Effect of Liquid Viscosity on Two-Phase Flow Development in a Vertical Large Diameter Pipe
Authors: Ibrahim Abubakr , Hewakandamby Buddhika , Yang Zhilin , Azzopardi Barry
Published: October 2018

Effect of injector geometry on two phase flows is of profound importance to industry. If the injection method is found to vary the flow characteristics dramatically, it can be employed to obtain desirable two phase flow regimes/attributes and avoid rather unsought conditions. This could potentially save a lot of costs in the extraction and transportation of oil and gas as well as in many other applications. Moreover, the issue of flow development and dependency on the injection conditions is essential when modelling two phase flows. A lot of experimental data and empirical models may have been developed based on systems that may not be fully developed. Therefore, inaccurate modelling of the physical interactions of the flow gets adopted, and hence large divergence between models and experimental data produced by different researchers often transpires.

Most of the published studies on entrance effect were conducted on air-water or steam-water systems because of their relevance to heat transfer units in the nuclear industry. This paper presents an extensive experimental investigation into the issue of flow development using two approaches; measuring void fraction at five axial stations along the test section, and using different geometries for bubble injection into the base of the pipe. The study focuses on how the entrance effect is influenced by the liquid viscosity. The experiments were conducted in a 127 mm diameter vertical pipe. The investigation is achieved by contrasting 180 runs produced using three different injector geometries, the runs were repeated using 4 different oil viscosities, making 2160 experimental run. Gas superficial velocity (Ugs) was varied between 0.01–5.40 m/s, while liquid superficial velocity (Uls) between 0.07–0.86 m/s. The viscosities investigated span between 4.0 cP up to 104.6 cP. The void fraction was measured using Electrical Capacitance Tomography (ECT) and the Wire Mesh Sensor (WMS). That in addition to differential pressure measurements.
Calculation of Friction Factors and Nusselt Numbers for Twisted Elliptical Tube Heat Exchangers Using Nek5000
Authors: Shaver Dillon R. , Carasik Lane B. , Merzari Elia , Salpeter Nate , Blandford Edward
Published: October 2018

The development of fluoride salt-cooled high-temperature reactors (FHRs) for nuclear power generation relies on the development of new technologies. Of the potential options being explored, twisted elliptical tube geometries for heat exchanger design are promising based on usage in other industries. They are expected to offer significant enhancement in heat transfer with only a marginal increase in frictional losses. This allows them to be deployed in relatively compact designs that are well suited for FHRs. The presented work focuses on the computational fluid dynamics (CFD) simulations of heated molten salt flows through various twisted elliptical tube geometries at low modified Froude numbers. The objectives of this work are to evaluate the available correlations at lower Froude numbers and to determine the impact of using non-zero tube to tube spacing to resolve contact points or numerical singularities for future CFD simulations efforts The spectral element CFD code Nek5000 was used for all simulations, which were performed in periodic domains of triangular (hexagonal) and square unit cells surrounding a single tube through a complete twist using an explicit filtering large eddy simulation (LES) method. Simulations were used to para-metrically test the effects of tube-to-tube spacing for laminar and turbulent flow regimes on frictional pressure drop and heat transfer. The tested Reynolds numbers covered both laminar flow and fully developed turbulent flow (90 < Re < 12200). The tested SL/dmax ratios cover the range of 1.02 to 1.08 for both unit cell types.

At moderate Reynolds number and comparitively high modified Froude number, excellent agreement for the Nusselt number was observed between simulations and the applicable correlation. As Froude number was decreased towards the bounds of the correlation, the agreement worsened. Cases were then simulated at low Froude number, testing the effects of tube spacing. It was determined that the laminar case for the square unit cell is the most affected by increasing SL/dmax and the gap size should be minimized to mitigate this. Whereas in the triangular unit cell the laminar flow regime is more significantly impacted by increasing SL/dmax compared to the turbulent flow regime which was only marginally impacted.
Wing Tip Vortex Development Under a Grid Generated Turbulent Flow
Authors: Ben Miloud Kamal , Dghim Marouen , Fellouah Hachimi , Ferchichi Mohsen
Published: October 2018

The interaction of a NACA 0012 wingtip vortex with a grid-generated flow was investigated in this paper. The experiments were conducted in the near and mid-wakes regions at three free stream turbulence (FST) levels of 0.5% (empty wind tunnel), 3% and 6%, and at two Reynolds numbers, based on the wing chord length, of 2 × 105 and 3 × 105. Stereoscopic Particle Image Velocimetry (SPIV) and hot wire measurements were carried out at four downstream positions, namely x/c = 0.5, 2.5, 5 and 7. Streamwise velocity contours showed that the wingtip vortex decayed with increased FST and downstream distance. In the vortex core region, the streamwise velocity decelerated while the vortex adopted a wake-like profile. FST was found to decrease the vortex circulation, to increase the vortex radius, and to increase the vortex meandering amplitude. By increasing the Reynolds number, the grid cases showed a small variation of the vortex radius and vorticity peak, particularly at downstream positions of 5 and 7. With meandering correction, the turbulence level within the vortex core were found to be reduced as the artificial turbulence induced by the vortex meandering was removed.
Computational Analysis of a Microfluidic Magnetophoretic Device for DNA Isolation
Authors: Hale C. , Darabi J.
Published: October 2018

A continuous flow magnetophoretic DNA isolation device was modeled using COMSOL Multiphysics and MATLAB to study and optimize the fluidic flow and geometric parameters of the device. The optimization was performed with the intention of increasing the volumetric flow rate and thus increasing the sample processing capabilities of the device. Key features of the device include oppositely oriented magnets that provide extremely strong magnetic field gradients and improve the capture rate of the magnetic particles. Another feature of the device is a nickel grid arrayed on the bottom surface of the channel to create localized amplified magnetic field gradients, and to provide even spreading and capture of the particles on the bottom surface of the device. These features in conjunction with the optimization of the fluid flow and geometric parameters provide the ability for higher velocity sampling and increased throughput. The simulation was conducted in two steps. First the fluid flow and the magnetic field simulations were performed in COMSOL Multiphysics. Next, the values of velocity fields and magnetic field gradients throughout the computational domain were exported to MATLAB to calculate the motion and trajectory of the particles. The particle trajectory analysis was performed until the particle either reached the bottom of the channel or was not captured within the set length and flowed out to the outlet channel.
Characteristics of Large Vortical Structures in the Shallow Wake Flow With a Gap Near the Bed
Authors: Shinneeb Monsif , Balachandar Ram
Published: October 2018

PIV measurements were made to investigate the turbulent wake flow generated by a vertical sharp-edged flat plate suspended in a shallow channel flow with a gap near the bed. The purpose of this study is to investigate the behaviour of large vortical structures in the wake flow. The investigation focused on the horizontal velocity field in the mid-distance between the bottom bed and the top free surface. Two different gap heights between the channel bed and the bottom edge of the bluff body was studied. These two cases were compared to the no-gap flow case which is considered as a reference case. The Reynolds number based on the water depth was 45,000. The large vortical structures were exposed by analyzing the PIV velocity fields using the proper orthogonal decomposition (POD) method. Only few modes were used for the POD reconstruction of the velocity fields to recover ∼50% of the turbulent kinetic energy. A vortex identification algorithm was then employed to quantify the number, size, circulation, and direction of rotation of the exposed vortices. A statistical analysis of the distribution of number, size, and strength of the identified vortices was carried out to explore the characteristics of the vortical structures.

The results revealed that the number of vortical structures increased as a result of the gap flow with a corresponding decrease in the vortex size and strength. This behaviour is attributed to the production of new vortices and the enhancement of the tearing process.
Investigation of Interaction Between Tip Leakage Flow Generated by Unshrouded Impeller and Diffuser Internal Flow
Authors: Ichinose Asuma , Kimura Norio , Yoshimura Mamiko , Hayashi Tomoyuki , Miyagawa Kazuyoshi
Published: October 2018

Rocket turbo pumps and industrial pumps such as water feed pumps are required to work under high pressure conditions, therefore low specific speed pumps are needed in spite of high rotational speed. In recent years, unshrouded impellers were used because of easy manufacturing and cost reduction. However, when low specific speed unshrouded impellers are used in such conditions, complex tip leakage flow occur and decrease impeller performance. In addition, splitter blades are often used, the internal flow becomes even more complicated. Therefore, such the internal flow of the unshrouded impeller must be clarified. In this research, we have studied such a centrifugal pump, and we have analyzed the internal flow using experiments and CFD (Computational Fluid Dynamics) simulations.

The experimental verification was carried out by measuring the total pressure distribution on the outlet of the impeller and the diffuser. The unsteady static pressure distribution at the shroud side of the impeller was measured to confirm pump performance. We used two types of CFD simulation to evaluate the internal flow in detail. In the first CFD simulation, the unsteady internal flow of an impeller was evaluated by carrying out DES (Detached Eddy Simulation) with a periodic boundary condition that does not contain the diffuser. In the second CFD simulation, interaction between the impeller leakage flow and the diffuser internal flow was evaluated by DES with the whole impeller and diffuser.

From the experimental verification and CFD simulation, it was confirmed that a large-scale vortex structure caused by the tip leakage flow and the secondary flow was observed in the impeller blade-to-blade. And the influence of the impeller leakage flow on the diffuser internal flow and the diffuser performance was evaluated.

From the above studies, it was confirmed that the tip leakage flow has a large influence on the impeller internal flow and the diffuser performance.
Multi-Plane Characterization of the Turbulent Boundary Layer
Authors: Dennis Kadeem , Siddiqui Kamran
Published: October 2018

The boundary layers are known for their significance in several engineering systems. In particular, the inner region of the turbulent boundary layer has been shown to play a significant role in controlling the dynamics of turbulent structures that are responsible for the transport of mass, heat and momentum. While substantial work has been done in the past to characterize the structure of turbulent flow in this region, the characterization of the three-dimensional turbulent flow structure is limited. This study reports a multi-plane particle image velocimetry (PIV) approach to investigate three-dimensional dynamics of the turbulent boundary layer in the near-wall region. Planar PIV is used to capture two-dimensional fluid velocity fields in several planes with respect to the fluid flow direction. These results are used to describe three-dimensional turbulent events given by key quantities such as mean and turbulent velocities and turbulent kinetic energy.
Analysis of Unsteady Flow Characteristics in Spool Valves for Mobile Hydraulics Machine Control
Authors: Oo S.-J. , Kim G. , Do J. , Lee S.
Published: October 2018

The main control valve is a key part of mobile hydraulic control systems. The main control valve consists of several types of spool valves. A notch is usually introduced to the end of the spool valve to reduce the influence of the flow force generated by the movement of fluid. In this study the flowrate from a spool valve combined with a servo-valve system is controlled based on an accurate prediction of transient pressure-flow relations by CFD. The transient analyses inside the spool valves with three typical types of notch are performed using a commercial CFD code of Fluent™. The flow characteristics such as flow pattern, discharge coefficient, and flow force, which depend on the notch shapes and their openings, are analyzed via vorticity distributions. Hysteresis of flow coefficients is observed for opening and closing motions of the spool valve, which should be compensated by the servo-valve in order to control the designed rate change of flowrate.
The Numerical and Experimental Investigation Into Hydraulic Characteristics of a No-Load Running Check Valve due to Fluid-Structure Interaction
Authors: Zhang Xiang-yuan , Shuai Zhi-jun , Jiang Chen-xing , Li Wan-you , Jian Jie
Published: October 2018

Valve is a very important unit in pipeline system. The valve flow fluctuation brings about structural vibration and unpopular noise, and even leads to the safety problems and disasters. In this paper, a special no-load running check valve is investigated. The check valve is structural complex with one inlet and two outlets. It can be simplified as a spring-mass system which manipulates the flow rate by combine action of the ambient pressure of medium and the spring deformation.

The three-dimensional model of the valve is established and also the relationship between pressure drops and flow rate of the valve is obtained in various openings and operating conditions. The structure modals were verified by the field tests and thus its fixing boundaries are obtained correctly.

The mechanism causing self-excited vibration of a piping system is determined using a dynamic model which couples the hydraulics of internal flow with the structural motion of a three-ports passive check valve. The coupling is obtained by making the fluid flow coefficient at the check valve to be a function of valve plug displacement. The results are compared with the experimental data, which verifies the correctness of the theoretical results.

It is shown that the special valve has its own hydraulic characteristics, which greatly influence its flow distribution as it has two outlets. It was also testified that the coupling between fluid and structure changes its natural frequencies and has a non-negligible impact on the pressure fluctuation while working.
Effects of Wettability on Capillary Flow of Non-Newtonian Fluid in Microchannels
Authors: Keshmiri Kiarash , Nazemifard Neda , Huang Haibo
Published: October 2018

In this study, capillary filling of diluted bitumen was evaluated using glass etched microchannel. Glass microchannel was treated using Trichloro(1H,1H,2H,2H-perfluorooctyl) silane that makes the microchannel lyophobic (not favorable for neither hydrophilic nor hydrophobic liquids). Water contact angle, as a degree of hydrophilicity, was changed from 15° for untreated microchannel to 115° for treated microchannel. Measured Capillary filling speed of bitumen in hexane (10% to 60%) was experimentally monitored using white light microscope and compared with Washburn theoretical model. For all samples, a linear relation between square of propagation distance and time was found. However, a deviation between experimental and theoretical values of penetration as a function of time was recorded. Experimental results indicated slower velocity compared to theoretical prediction due to simplifications of the Washburn model. Advancing dynamic contact angle of capillary-driven flow was measured and compared with static contact angle using MATLAB®. It was found that dynamic contact angle was increasing during the penetration in microchannel and application of a constant contact angle leads to higher deviation between experimental and theoretical results.
Operational Impacts and Consequences of Piping Component Failure: A Review of Operating Experience Data As Recorded in CODAP
Authors: Carr Braedon , Lydell Bengt , Riznic Jovica R.
Published: October 2018

Water chemistry plays an important part in maintaining corrosion resistance in water transport systems throughout nuclear power plants (NPP’s). Small changes in liquid chemistry such as pH, borate concentration, or build-up of crud in reactor cooling water can result in rapid degradation or damage to components and lead to unexpected failures. The Chemical and Volume Control System (CVCS) and Reactor Water Cleanup System (RWCU) are responsible for maintaining these parameters at appropriate levels, and so failure of either of these systems can result in unnecessary stresses on many other reactor systems due to resulting transients. While the major components of these systems all have sufficient redundancy to prevent major accidents, failure of components in these systems can result in failure of other redundant components and affect plant safety [1]. The CVCS and RWCU systems have experienced aging related degradations and failures in the past, and although they have not affected the system’s emergency functions, they have resulted in unnecessary actuation of related systems, and reactor shutdowns [1]. Reactor shutdowns can result in large changes in reactor coolant chemistry such as oxygen and borate concentration transients, and the build-up of corrosion products which can’t be as easily removed during periods of reactor shutdown [2].

In the following analysis of Component Operational Experience Degradation and Ageing Program (CODAP) experience data; causes, impacts, and preventative actions as recorded in CODAP are examined for degradation events which took place in the CVCS and RWCU, of PWRs and BWRs, respectively. The analysis will demonstrate the usefulness of CODAP in examining reactor component failure trends, as well as discuss insights on improvement for the program.
Thermal Hydraulic Analyses for PTS Evaluation: Comparison of Temperature Fields at RPV Predicted by System TH Code and CFD Code
Authors: Kral Pavel , Vyskocil Ladislav
Published: October 2018

The paper describes methods used for evaluation of pressurized thermal shock (PTS) at UJV Rez and selected results. The UJV Rez participated on the one hand in a number of international programs focused on development of PTS methodology, benchmark testing etc. On the other hand UJV has worked in several projects evaluating PTS for nuclear power plants in the Czech Republic and abroad. The introductory part of the paper also briefly mentions a development of PTS methodology in the world, its current status and trends.

The core of the paper describes thermal hydraulic part of the PTS evaluation process, which usually contains system thermal hydraulic (TH) analysis and mixing calculation. Already at the system TH calculation level, a simple 2D model of reactor downcomer (DC) is used in UJV Rez analyses — as this approach enables the prediction of 2D temperature and velocity fields in DC and improves the prediction of natural circulation flow or its stagnation in individual loops. For the subsequent mixing calculation the Computational Fluid Dynamics (CFD) computer codes are used nowadays. Comparison of 2D temperature fields from 2D system TH analysis and CFD calculation shows surprisingly good agreement for most cases. The paper shows such comparison for several representative cases and takes a more detailed look into one of them.
Numerical Study of Deteriorated Convection Heat Transfer of Supercritical Fluid Flowing Through Vertical Mini Tube at Relatively Low Reynolds Numbers
Authors: Zhao Chen-Ru , Zhang Zhen , Liu Qian-Feng , Bo Han-Liang , Jiang Pei-Xue
Published: October 2018

Numerical investigations are performed on the convection heat transfer of supercritical pressure fluid flowing through vertical mini tube with inner diameter of 0.27 mm and inlet Reynolds number of 1900 under various heat fluxes conditions using low Reynolds number k-ε turbulence models due to LB (Lam and Bremhorst), LS (Launder and Sharma) and V2F (v2-f). The predictions are compared with the corresponding experimentally measured values. The prediction ability of various low Reynolds number k-ε turbulence models under deteriorated heat transfer conditions induced by combinations of buoyancy and flow acceleration effects are evaluated. Results show that all the three models give fairly good predictions of local wall temperature variations in conditions with relatively high inlet Reynolds number. For cases with relatively low inlet Reynolds number, V2F model is able to capture the general trends of deteriorated heat transfer when the heat flux is relatively low. However, the LS and V2F models exaggerate the flow acceleration effect when the heat flux increases, while the LB model produces qualitative predictions, but further improvements are still needed for quantitative prediction. Based on the detailed flow and heat transfer information generated by simulation, a better understanding of the mechanism of heat transfer deterioration is obtained. Results show that the redistribution of flow field induced by the buoyancy and flow acceleration effects are main factors leading to the heat transfer deterioration.
Uncertainty Evaluation and Sensitivity Analysis Under Accident Scenarios
Authors: Gomes Daniel de Souza , Silva Antonio Teixeira e
Published: October 2018

Nuclear power units need to operate conditioned the lowest risk possible. Safety analysis must use paired models, combining probabilistic and deterministic methods. In this study, FRAPCON and FRAPTRAN codes were used to simulate an idealized test based on IFA-650 series, carried out within Halden program. Nuclear systems work to depend on uncertainty values that must be quantified and propagated. The sources of uncertainties can be divided among physical models, boundary conditions, and mechanical tolerances. Eight physical models that can be configured, such as thermal conductibility, and fission gas release. Mechanical tolerances introduced by fuel fabrication are deviations that must propagate throughout of the system. To measure the effects produced by uncertainties were used correlation coefficients between entry and exit. Uncertainties contained on input values are spread to measure the impact created on safety limits. The method adopted used 96 samples to achieve the 95% of probability and 95% of confidence level.
Synthesis of a CFD Benchmark for the Thermal Mixing in a Sharp Corner T-Junction With a Wall
Authors: Shams Afaque , Edh Nicolas , Angele Kristian
Published: October 2018

This article reports a CFD-benchmark with the purpose of validating different turbulence modelling approaches for the transient heat transfer due to mixing of hot and cold flow in a T-junction including the wall. This validation exercise has been carried out within the MOTHER project. In the framework of the project, new experiments were performed with a novel measurement sensor allowing the measurements of the fluctuating wall temperature inside the solid pipe wall. The tests were performed for two different Reynolds numbers (Re) 40000 and 60000 and for two different T-junction geometries; a sharp corner and a round corner. The present article reports the synthesis of the CFD validation for a sharp corner T-junction for Re = 40 000. The CFD validation study has been performed using four different CFD softwares, namely STAR-CCM+, Code_Saturne, LESOCC2 and Fluent. In addition, five different turbulence models i.e. wall-function Large Eddy Simulation (LES), Deatched Eddy Simulation (DES), Partially Resolved Numerical Simulation (PRNS), Unsteady Reynolds Averaged Navier-Stokes URANS and RANS were used to perform the CFD computations.

The validation exercise has shown that LES gives the best agreement with the experimental data followed by hybrid (LES/RANS), URANS and RANS models, respectively. The velocity and the thermal fields in the fluid region are correctly predicted by the proper use of the LES modelling, whereas, the accurate prediction of the thermal field in the solid requires very long sampling time in order to achieve a statistically converged solution, which of course requires an enormous computational power. Therefore, the statistical convergence of the thermal field in the solid has been found to be a bottleneck in order to accurately predict the temperature fluctuations in the wall. However, measuring the small amplitude temperature fluctuations is also associated with an uncertainty so the disagreement between CFD and measurements (of the order of 10 %) can also be attributed, in part, to uncertainties in the measurements.
Study on the Influence of the Heaving Condition on the Pressure Drop Characteristics of the Two-Phase Flow in the Circular Tube
Authors: Xie Tianzhou , Xu Jianjun , Chen Bingde , Bao Wei
Published: October 2018

The experimental study on the pressure drop characteristics of the gas-liquid two-phase flow under heaving condition is carried out. The influence of the heaving condition on the transient and time-averaged two-phase pressure drop is obtained. On this basis, the calculation model of two-phase transient pressure drop under heaving condition is constructed, which is in good agreement with the experimental results. According to the experimental data and the results of the model calculation, the influence of heaving condition on the pressure drop components is analyzed. It is found that the transient total pressure drop is fluctuating periodically, while the time-averaged value is nearly the same as that under static condition. Further analysis shows that the friction pressure drop and the gravity pressure drop are basically the same under heaving condition. The fluctuation of additional pressure drop introduced by heaving motion is the reason for the periodic fluctuation of the total pressure drop.
Experimental Study of a Micrometer-Sized Droplet Impinging on a Smooth Heated Surface
Authors: Zhang Zhen , Chen Kai , Jiang Pei-Xue , Yang Xing-Tuan
Published: October 2018

The micrometer-sized (∼100 μm) droplet impinging on a surface at 46°C or 120°C was experimentally investigated for spray cooling system in nuclear plants and other high heat flux applications. Photographs were taken continuously by a high speed camera with 256 × 128 pixel to 300, 000 fps and a silicon surface was used as the test surface, heat source and temperature sensor. The time sequences of the droplet impingement cooling process on a surface were presented and analyzed. The effects of the droplet size and velocity were studied. The droplet spreading and evaporating on a 120°C surface with or without boiling were shown and discussed.