Flow Recirculation Zone Research Articles

Numerical Analysis of Flow Structure Above Dimpled Surfaces with Different Depths in a Channel

Turbulent air flow characteristics in channels of three different dimple depth (δ/D = 0.1, 0.2, and 0.3) placed on the bottom wall are numerically predicted using FLUENT for Reynolds number based on the channel height, Re H = 20,000, and the ratio of channel height to dimple print diameter, H/D = 1.0. The turbulence model employed is a realizable k − ϵ model with no wall function. Steady-state analyses of fluid within and near different dimple depths demonstrate the existence of different vortex pairs. These vortex pairs are investigated at the central position of dimples as well as near the spanwise edge of individual dimples, and become stronger as dimple depth increases. In addition, steady-state results show that the streamwise vorticity distributions, which are associated with the flow recirculation zones positioned within the upstream halves of each dimple, are higher in magnitude and cover larger spatial extents as the dimple depth increases. Magnitudes of eddy diffusivity for momentum and eddy diffusivity for heat are also augmented by the vorticity concentrations within and downstream of individual dimples as the dimple depth increases. Such characteristics are due to the advection of reattaching and recirculating flow within the dimple cavities, as well as to the strong instantaneous secondary flows and mixing within the vortex pairs.

Argon Blowing Technology Using in the LF Refining

The process of LF Refining is introduced in this paper. Because the LF ladle furnace has the advantages are widely used and developed in many companies. The argon is blown into the ladle, so the purpose of refining is achieved. The argon stirring can accelerate the chemical reaction between slag and steel. In the circulation process, large inclusion and deoxidation product in the lower region transfer into the slag. The material transfer between steel and slag can be accelerated. At the same time, argon stirring can make molten steel composition and temperature uniformity. Thus, the ladle of blowing argon can be roughly divided into three important flow region has been introduced. Two-phase region, the top horizontal flow zone and recirculation zone exists in the ladle.

Research Status of Physical Simulation on LF Furnace Refining Ladle

The research status of physical simulation on LF furnace refining ladle is introduced in this paper.Since 1970, the research on the gas-liquid two-phase region has begun. The ladle of blowing argon can be roughly divided into three important flow region has been introduced. Two-phase region, the top horizontal flow zone and recirculation zone exists in the ladle.The establishment of physical model should include similarity principle and flow field measurement technique

Experimental Evaluation of the Effectiveness of Online Water-Washing in Gas Turbine Compressors

An investigation of the effectiveness of online combustion turbine axial compressor washing using various purity grade waters and commercial washing detergents was performed. For this project, blade surface fouling dirt was obtained from gas turbine axial compressor blades installed at various field sites. The dirt was analyzed to determine consistency of typical blade surface fouling materials. A representative dirt formula and blade coating procedure was developed so that comparative tests could be performed using various cleaning fluids. Dirt coated blades were installed in a wind tunnel capable of simulating compressor operating conditions. A spray nozzle upstream of the blade test section was used for washing blades with five different test liquids to determine the effectiveness or advantages of any liquid. Once this testing was completed, a similar test setup was then utilized to inject a mixture of formulated fouling dirt and the various online cleaning liquids upstream of the blade into the wind tunnel to assess redeposit characteristics. The effect of high-purity water versus regular water on fouling dirt was also studied in separate residue experiments. Results indicate that spraying cleaning fluid into a flowing air stream is a viable means of cleaning a compressor blade. Each of the fluids was able to clean the test blade at both low and high air velocities and at different blade incident angles. Within the parameters/fluids tested, the results indicate that: (1) The blade cleaning is primarily a mechanical function and does not depend on the type of fluid used for cleaning. The results showed that most of the cleaning occurs shortly after the cleaning fluid is introduced into the flow stream. (2) Dirt removed from the blades may redeposit in other areas as the cleaning fluid is evaporated. Redeposit occurred in flow recirculation zones during the cleaning tests, and heated flow tests demonstrated dirt deposit in the presence of a cleaning fluid. In addition, the type of fluid used for cleaning has no effect on the redeposit characteristics of the dirt. (3) Blade erosion was not found to be a significant issue for the short durations that online water-washing was performed. However, uncontrolled water-washing (or overspray) for extended periods of time did result in measureable leading and trailing edge blade erosions.

Inverse diffusion and partially premixed flames with elliptical/swirling- and cross-flows

The combustion performance of inverse diffusion and partially premixed flames was optimized by employing concentric elliptical ports with variable relative angular orientation, co-swirling, and cross-flow opposing jets of different inclination angles. The flexible and favorably active control of combustion was pronounced by changing the nozzle relative angular orientation. The optimum angular shift was found to be 20° where the flame length was shortened by 39% from the case of 0° and the NOx levels were reduced to about 0.51 g/kg of fuel for the inverse partially premixed flames. At such optimum angular shift, the minimum CO and NOx emissions respectively decreased to 17.1 and 1.52 g/kg of fuel for the inverse diffusion flames. The elliptical jets increased the size of the swirling flow recirculation zone by about 28%. Increasing the inner aspect ratio consistently increased the combustion efficiency, where the CO emissions reach minimum values of 12.3 and 6.22 g/kg of fuel respectively for the inverse diffusion and partially premixed flames. The correspondingly minimum NOx emissions were thus 0.89 and 0.51 g/kg of fuel at a cross-flow momentum flux ratio of 2.0. Splitting the opposing cross-flow air jets by increasing their inclination angle above 0° duplicated the stagnation zone vortical impact to enrich the average turbulent energy at the major axis tips. The inclination angle of 15° was the optimum at no swirl while increasing the outer swirl angle shifted the optimum inclination angle to 25°. The inner/outer co-swirl of 45°/60° thus provided a minimum CO emission of 11.1 g/kg of fuel for the inverse diffusion flame and 2.65 g/kg of fuel for the inverse partially premixed flame.

Effect of adding a swirl on flow pattern and recirculation zone in ADS windowless spallation target

Aiming the key issues in the accelerator driven system (ADS), windowless spallation target focus on the minimization of the recirculation zone and on the stability of the free surface, an innovation has been made by numerically adding swirl to the fluid at the inlet. At first, two phase flow pattern in the simulation is compared with the experiments and numerical method is employed correctly. The results reveal that the recirculation zone and the flow pattern are greatly influenced when the swirl strength is changed from 1.0rad/s to 2.5rad/s. The height of the recirculation zone decreases with increase in swirl strength and completely disappears when the swirl strength reaches 2.0rad/s. In addition, larger swirl strength leads to different flow pattern and a new cavitation zone is generated under the recirculation zone. The Bernoulli's equation and angular momentum conservation are applied to make it clear that this phenomena is due to the decrease of the axial pressure caused by the radial velocity. Moreover, the new cavitation zone totally links to the vapor area above the recirculation zone when the swirl strength is 2.5rad/s. The results are very helpful to the design and optimization of the ADS windowless spallation target.

Vector Projectile Imaging: Time-Resolved Dynamic Visualization of Complex Flow Patterns

Achieving non-invasive, accurate and time-resolved imaging of vascular flow with spatiotemporal fluctuations is well acknowledged to be an ongoing challenge. In this article, we present a new ultrasound-based framework called vector projectile imaging (VPI) that can dynamically render complex flow patterns over an imaging view at millisecond time resolution. VPI is founded on three principles: (i) high-frame-rate broad-view data acquisition (based on steered plane wave firings); (ii) flow vector estimation derived from multi-angle Doppler analysis (coupled with data regularization and least-squares fitting); (iii) dynamic visualization of color-encoded vector projectiles (with flow speckles displayed as adjunct). Calibration results indicated that by using three transmit angles and three receive angles (–10°, 0°, +10° for both), VPI can consistently compute flow vectors in a multi-vessel phantom with three tubes positioned at different depths (1.5, 4, 6 cm), oriented at different angles (–10°, 0°, +10°) and of different sizes (dilated diameter: 2.2, 4.4 and 6.3 mm; steady flow rate: 2.5 mL/s). The practical merit of VPI was further illustrated through an anthropomorphic flow phantom investigation that considered both healthy and stenosed carotid bifurcation geometries. For the healthy bifurcation with 1.2-Hz carotid flow pulses, VPI was able to render multi-directional and spatiotemporally varying flow patterns (using a nominal frame rate of 416 fps or 2.4-ms time resolution). In the case of stenosed bifurcations (50% eccentric narrowing), VPI enabled dynamic visualization of flow jet and recirculation zones. These findings suggest that VPI holds promise as a new tool for complex flow analysis.

건축자재로부터 방출되는 라돈의 실내 확산에 대한 수치해석적 연구

Growing concerns about harmful influence of radon on human body, many efforts are being made to decrease indoor radon concentration in advanced countries. To develop an indoor radon reduction technology, it is necessary to develop a technology to predict and evaluate indoor inflow and emission of radon. In line with that, the present study performed computational modelling of indoor dispersion of radon emitted from building materials. The computational model was validated by comparing computational results with analytical results. This study employed CFD (Computational Fluid Dynamics) analysis to evaluate the radon concentration and the airflow characteristics. Air change rate and ventilation condition were changed and several building materials having different radon emission characteristics were considered. From the results, the indoor radon concentration was high at flow recirculation zones and inversely proportional to the air change rate. For the different building materials, the indoor radon concentration was found to be highest in cement bricks, followed by eco-carats and plaster boards in the order. The findings from this study will be used as a method for selecting building materials and predicting and evaluating the amount of indoor radon in order to reduce indoor radon.

219 Rock Inhibits Endothelial Migration in Disturbed flow Condition: A Potential Therapeutic Target in PCI

Stent deployment to treat coronary artery disease causes damage and loss of endothelial cells (EC). Wall shear stress (WSS), a mechanical force exerted by flowing blood on the vessel wall,...

Heat transfer coefficient distribution on the pole face of a hydrogenerator scale model

This paper focuses on the effect of rotation on heat transfer mechanisms in rotating machines with the purpose to improve the understanding of thermal phenomena and cooling of hydrogenerators. Using a simplified scale model equipped with a heated pole, it was possible to measure the temperature distribution on the pole surface and to deduce, through numerical simulations, the heat transfer coefficients. The results show an asymmetric profile in the tangential direction since lower h values are found closer to the trailing edge due to the presence of a flow recirculation zone. Furthermore, the heat transfer profiles indicate that, although fans improve cooling at the top and bottom ends of the pole, the highest h values are found in an intermediate region. This is due to the flow from the fans that enters the interpole space and only after penetrating a certain distance in the axial direction it exits through the air gap and goes around the pole face. The study also shows that the heat transfer coefficients along the pole face at 300 rpm average about four times those at 50 rpm.

Macro- and microscale variables regulate stent haemodynamics, fibrin deposition and thrombomodulin expression

Drug eluting stents are associated with late stent thrombosis (LST), delayed healing and prolonged exposure of stent struts to blood flow. Using macroscale disturbed and undisturbed fluid flow waveforms, we numerically and experimentally determined the effects of microscale model strut geometries upon the generation of prothrombotic conditions that are mediated by flow perturbations. Rectangular cross-sectional stent strut geometries of varying heights and corresponding streamlined versions were studied in the presence of disturbed and undisturbed bulk fluid flow. Numerical simulations and particle flow visualization experiments demonstrated that the interaction of bulk fluid flow and stent struts regulated the generation, size and dynamics of the peristrut flow recirculation zones. In the absence of endothelial cells, deposition of thrombin-generated fibrin occurred primarily in the recirculation zones. When endothelium was present, peristrut expression of anticoagulant thrombomodulin (TM) was dependent on strut height and geometry. Thinner and streamlined strut geometries reduced peristrut flow recirculation zones decreasing prothrombotic fibrin deposition and increasing endothelial anticoagulant TM expression. The studies define physical and functional consequences of macro- and microscale variables that relate to thrombogenicity associated with the most current stent designs, and particularly to LST.

Volumetric intake flow measurements of an IC engine using magnetic resonance velocimetry

Magnetic resonance velocimetry (MRV) measurements are performed in a 1:1 scale model of a single-cylinder optical engine to investigate the volumetric flow within the intake and cylinder geometry during flow induction. The model is a steady flow water analogue of the optical IC-engine with a fixed valve lift of \(9.21\) mm to simulate the induction flow at crank-angle \(270^{\circ }\)bTDC. This setup resembles a steady flow engine test bench configuration. MRV measurements are validated with phase-averaged particle image velocimetry (PIV) measurements performed within the symmetry plane of the optical engine. Differences in experimental operating parameters between MRV and PIV measurements are well addressed. Comparison of MRV and PIV measurements is demonstrated using normalized mean velocity component profiles and showed excellent agreement in the upper portion of the cylinder chamber (i.e., \(y \ge -20\) mm). MRV measurements are further used to analyze the ensemble average volumetric flow within the 3D engine domain. Measurements are used to describe the 3D overflow and underflow behavior as the annular flow enters the cylinder chamber. Flow features such as the annular jet-like flows extending into the cylinder, their influence on large-scale in-cylinder flow motion, as well as flow recirculation zones are identified in 3D space. Inlet flow velocities are analyzed around the entire valve curtain perimeter to quantify percent mass flow rate entering the cylinder. Recirculation zones associated with the underflow are shown to reduce local mass flow rates up to 50 %. Recirculation zones are further analyzed in 3D space within the intake manifold and cylinder chamber. It is suggested that such recirculation zones can have large implications on cylinder charge filling and variations of the in-cylinder flow pattern. MRV is revealed to be an important diagnostic tool used to understand the volumetric induction flow within engine geometries and is potentially suited to evaluate flow changes due to intake geometry modifications.

Abstract 521: Endothelial Cell Forward Migration in a Disturbed Wall Shear Stress Environment is Promoted by Rho-Kinase Inhibition

Stent deployment to treat coronary artery disease causes damage and loss of endothelial cells (EC). Repair of injured arteries by EC migration is co-ordinated by Rho family GTPases. It is also regulated by wall shear stress (WSS), a mechanical force exerted by flowing blood on the vessel wall, via mechanisms that are poorly understood. It was hypothesised that stent struts may impede repair of injured endothelium by inducing localised disturbances in WSS. To simulate a stented artery in vitro, chamber slides were fabricated with ridges (100 μm high) positioned perpendicularly to the flow direction. Confluent EC monolayers seeded on one side of either ridged or non-ridged (control) chambers were exposed to flowing culture medium (Ibidi® system). Flow patterns were determined by computational fluid dynamic (CFD) modelling and particle velocimetry. Live cell imaging and analysis using ImageJ® software enabled quantification of EC migration velocity and directional persistence (DP). EC on the non-ridged chamber slide were exposed to a uniform WSS of 13 dyn/cm2 and migrated relatively uniformly in parallel with the flow direction (average velocity 1.13 μm/min; DP 0.59). By contrast, CFD and particle velocimetry revealed spatial WSS differences over the ridged chamber, with significant spikes above physiological levels (>70 dyn/cm2) at the corners of the ridges and distinctive flow recirculation zone immediately downstream from ridges (-4 dyn/cm2). While EC were able to migrate over the ridges, cells that reached the recirculation zone displayed non-uniform directionality (DP 0.25) and reduced velocity (0.78 μm/min). Inhibition of the RhoA signalling pathway with ROCK inhibitors promoted EC forward migration within the recirculation zone by elevating DP (0.41) and velocity (1.16 μm/min). It was concluded that disturbed WSS downstream from stent strut-like ridges prevented the forward migration of EC. Inhibition of the RhoA/ROCK signaling pathway promoted EC migration and re-population of these sites. Our data suggest that treatment using a ROCK inhibitor may promote re-endothelialisation of stented arteries.

Thrombus Formation Patterns in the HeartMate II Ventricular Assist Device

Postimplant device thrombosis remains a life-threatening complication and limitation of continuous-flow ventricular assist devices (VADs). Using advanced computational fluid dynamic (CFD) simulations, we successfully depicted various flow patterns, recirculation zones, and stagnant platelet trajectories which promote thrombus formation and observed that they matched actual thrombus formation patterns observed in Thoratec HeartMate II VADs explanted from patients with pump thrombosis. Previously, these small eddies could not be captured by either digital particle image velocimetry or CFD due to insufficient resolution. Our study successfully demonstrated the potential capability of advanced CFD to be adopted for device optimization, leading to enhanced safety and efficacy of VADs for long-term destination therapy.

Investigation of the velocity field in a full-scale model of a cerebral aneurysm

Due to improved and now widely used imaging methods in clinical surgery practise, detection of unruptured cerebral aneurysms becomes more and more frequent. For the selection and development of a low-risk and highly effective treatment option, the understanding of the involved hemodynamic mechanisms is of great importance. Computational Fluid Dynamics (CFD), in vivo angiographic imaging and in situ experimental investigations of flow behaviour are powerful tools which could deliver the needed information. Hence, the aim of this contribution is to experimentally characterise the flow in a full-scale phantom model of a realistic cerebral aneurysm. The acquired experimental data will then be used for a quantitative validation of companion numerical simulations. The experimental methodology relies on the large-field velocimetry technique PTV (Particle Tracking Velocimetry), processing high speed images of fluorescent tracer particles added to the flow of a blood-mimicking fluid.First, time-resolved planar PTV images were recorded at 4500fps and processed by a complex, in-house algorithm. The resulting trajectories are used to identify Lagrangian flow structures, vortices and recirculation zones in two-dimensional measurement slices within the aneurysm sac. The instantaneous inlet velocity distribution, needed as boundary condition for the numerical simulations, has been measured with the same technique but using a higher frame rate of 20,000fps in order to avoid ambiguous particle assignment. From this velocity distribution, the time-resolved volume flow rate has been also derived. In this manner, a direct comparison between numerical simulations and PTV measurements will be possible in the near future, opening the door for highly accurate computational predictions.

A simulation and experimental study of fluid flow and heat transfer on cylindrical oblique-finned heat sink

A novel cylindrical oblique fin minichannel heat sink was proposed to fit over cylindrical heat sources in the form of an enveloping jacket. The periodic cylindrical oblique fin causes the hydrodynamic boundary layer development to be reinitialized at the leading edge of the next downstream fin. This decreases the average thermal boundary layer thickness, enhances the heat transfer performance and yields negligible pressure drop penalty due to combined effect of thermal boundary layer re-development and flow mixing. Its cooling effectiveness is compared with conventional straight fin minichannel heat sinks through experimental and numerical approach for the Reynolds number ranged from 50 to 500. The results showed that the averaged Nusselt number, Nuave for the cylindrical oblique-cut fin minichannel heat sink increases up to 75.6% and the total thermal resistance decreases up to 59.1% when compared with the conventional straight fin minichannel heat sink. It is also found firstly that a flow recirculation zone will form at larger Reynolds number in the secondary channel however this recirculation is insignificant in the present low Reynolds number study. Heat transfer enhancement (ENu) and pressure drop penalty (Ef) show that a significant improvement of the cylindrical oblique fin minichannel over conventional straight fin minichannel overall.

Internal wall-jet film cooling with compound angle cylindrical holes

The flow field associated with cylindrical coolant jets inclined in tangential and azimuthal direction employed inside a circular pipe has been studied. Numerical results are compared with in-house experimental data for a row of circumferential film cooling holes with two distinct geometric configurations. Results provide insight into the film cooling performance and the heat transfer characteristics associated with this type of film-cooling jets. Secondary flow recirculation zones are found near the jet exit in the regimes close to the wall and the center. Its occurrence and length is found to be controlled by the geometric configuration of the coolant hole. This secondary flow structure and the jet impingement on the wall are found responsible for developing a maximum local Nusselt number downstream of coolant injection. Reverse heat transfer regimes are noted for conjugate walls with higher thermal conductivity. The spreading of the coolant around the circumference is mainly due to the asymmetric vorticity levels present at the jet exit plane. Higher tangential orientation of the coolant jet with the mainstream has resulted in lower injectant concentration near the test section wall and lower effectiveness throughout the test section. The results indicate that through the use of optimal coolant injector configurations, reduction in coolant requirement can be achieved.

Numerical Analysis on the Influence of the Unsteady Effect upon the Flow Characteristics in a Human Airway

The focus is on the unsteady effect on the flow characteristics considering the real respiration in a human airway. In order to capture the unsteady effect, the flow characteristics were investigated and compared with that of steady flow by using the computational fluid dynamics technique. The real respiratory flow rate in a light workload was appliedto the inlet of airway. The unsteady effects especially appear when the flow rate accelerates in the inspiratory and the expiratory phase than other times in a breath cycle. The size of the flow recirculation zone, the turbulence fluctuation and the secondary flow intensity decrease because the unsteady effect reduces the strength of jets passing through the constriction of airway by interfering with the flow. These results imply that the unsteady effect should be considered to investigate the flow characteristics and the particle movement in a human airway.

Numerical Investigation of the Flow Characteristics Considering the Human Workloads in a Human Airway

The flow characteristicscan be sensitively changed by the respiration flow rate due to the complex geometries, e.g., bends, constrictions in a human airway. Also, caused by the switching the flow direction, the disturbance destabilizes the flow when a human breathes. Thus, the real breathing conditions, e.g., peak flow rate and the breath cycle time related to workload, should be considered to investigate on the flow characteristics in a human airway. In this study, realated to the human workloads, the effect of the change of the respiratory flow rate on the flow characteristics is investigated with the numerical method at the 4-specific normalized times in a breath cycle. In almost time of a breath cycle, as the workload increases, the flow recirculation zone decreases slightly because the energy transport toward the wall increases due to the turbulence fluctuation and the secondary motion.However, when the flow acclecrates in the expiratory phase, the flow recirculation zone increases because of the unsteady effect and the glottal jet. Thus, respiratory flow rate considering the human workloads and the unsteady effect caused by the real respiration should be considered to investigate the flow characteristics and the particle movement in a human airway.

Flow recirculation zone length and shear rate are differentially affected by stenosis severity in human coronary arteries

Flow recirculation zones and shear rate are associated with distinct pathogenic biological pathways relevant to thrombosis and atherogenesis. The interaction between stenosis severity and lesion eccentricity in determining the length of flow recirculation zones and peak shear rate in human coronary arteries in vivo is unclear. Computational fluid dynamic simulations were performed under resting and hyperemic conditions on computer-generated models and three-dimensional (3-D) reconstructions of coronary arteriograms of 25 patients. Boundary conditions for 3-D reconstructions simulations were obtained by direct measurements using a pressure-temperature sensor guidewire. In the computer-generated models, stenosis severity and lesion eccentricity were strongly associated with recirculation zone length and maximum shear rate. In the 3-D reconstructions, eccentricity increased recirculation zone length and shear rate when lesions of the same stenosis severity were compared. However, across the whole population of coronary lesions, eccentricity did not correlate with recirculation zone length or shear rate (P = not signficant for both), whereas stenosis severity correlated strongly with both parameters (r = 0.97, P < 0.001, and r = 0.96, P < 0.001, respectively). Nonlinear regression analyses demonstrated that the relationship between stenosis severity and peak shear was exponential, whereas the relationship between stenosis severity and recirculation zone length was sigmoidal, with an apparent threshold effect, demonstrating a steep increase in recirculation zone length between 40% and 60% diameter stenosis. Increasing stenosis severity and lesion eccentricity can both increase flow recirculation and shear rate in human coronary arteries. Flow recirculation is much more sensitive to mild changes in the severity of intermediate stenoses than is peak shear.