Recirculation Flow Research Articles

Effect of Blade Number on Internal Flow and Performance Characteristics in Low-Head Cross-Flow Turbines

Cross-flow turbines are widely used in microhydropower systems because of their cost-effectiveness, environmental sustainability, adaptability, and robust design. However, their relatively lower efficiency than other turbine types limit their application in large-scale projects. Previous studies have identified poor flow profiles as a significant factor contributing to inefficiency, with the number of blades playing a critical role in the flow behavior, efficiency, and structural stability. This study employed numerical simulations to analyze how varying the number of blades affects the internal flow characteristics and performance of the turbine at, and off, its best operating points. Configurations with 16, 20, 24, 28, 32, 36, 40, and 44 blades were investigated under constant low-head conditions, fully open valve settings, and varying runner speeds. Simulations were performed using ANSYS CFX, incorporating steady-state conditions, a two-phase flow model with a movable free surface, and a shear stress turbulence model. The results indicate that the 28-blade configuration achieved a maximum hydraulic efficiency of 83%, outperforming the preset 24-blade setup by 6%. Flow profiles were examined using pressure and velocity gradients to identify regions of adverse pressure. Due to the impulse nature of the turbine, the flow profile is more sensitive to changes in the flow speed than to pressure. The flow trajectory showed stability in the first stage but exhibited discrepancies in the second stage, which were attributed to turbulence, recirculation, and shaft flow impingement. The observed performance improvements were linked to reduced hydraulic losses due to flow separation and friction, emphasizing the significance of the number of blades and the regions of optimal efficiency under low-head conditions.

Simulation of Flow Around a Finite Rectangular Prism: Influence of Mesh, Model, and Subgrid Length Scale

This study investigates the flow field around a finite rectangular prism using both experimental and computational methods, with a particular focus on the influence of the turbulence approach adopted, the mesh resolution employed, and different subgrid length scales. Ten turbulence modelling and simulation approaches, including both ‘scale-modelling’ Reynolds-Averaged Navier–Stokes (RANS) models and ‘scale-resolving’ Delayed Detached Eddy Simulation (DDES), were tested across six different mesh resolutions. A case with sharp corners allows the location of the flow separation to be fixed, which facilitates a focus on the separated flow region and, in this instance, the three-dimensional interaction of three such regions. The case, therefore, readily enables an assessment of the ‘grey-area’ issue, whereby some DDES methods demonstrate delayed activation of the scale-resolving model, impacting the size of flow recirculation. Experimental measurements were shown to agree well with reference data for the same geometry, after which particle image velocimetry (PIV) data were gathered to extend the reference dataset. Numerical predictions from the RANS models were generally quite reasonable but did not show improvement with further refinement, as one would expect, whereas DDES clearly demonstrated continuous improvement in predictive accuracy with progressive mesh refinement. The shear-layer-adapted (SLA) subgrid length scale (ΔSLA) displayed consistently superior performance compared to the more widely used length scale based on local cell volume, particularly for moderate mesh resolutions commonly employed in industrial settings with limited resources. In general, front-body separation and reattachment exhibited greater sensitivity to mesh refinement than wake resolution. Finally, in order to correlate the observed DDES mesh requirements with the observations from the converged RANS solutions, an approximation for the Taylor microscale was explored as a potential tool for mesh sizing.

Enhanced Biomass Combustion via Bluff Body- and Impingement-Stabilized Natural Gas-Air Premixed Flames

ABSTRACT The preheating, flow recirculation and impingement effects of trapezoidal bluff bodies increased the biomass heating value-based combustion efficiency to 94% at a firing intensity of 78 MW/m3. A sawdust-laden air stream was combined to bluff body-stabilized natural gas-air premixed flames in three different configurations. A common impingement of opposing fuel-lean mixture jets with a sawdust-laden air cross-flow upstream of a solid bluff body provided a peak turbulent kinetic energy of 11.8 m2/s2. Using opposing jets with ports of multiple sharp corners minimized the flame length/combustor diameter ratio to 4.2. The combination of premixed flame impingement on the combustor base with the flow stagnation effect inside hollow bluff bodies provided a maximum blow-off velocity of 19.5 m/s at Ф = 0.8. Delaying the biomass interaction with the premixed flames decreased the peak exhaust NOx concentration to 0.17 g/kW.hr. Confronting the premixed flame jets by the sawdust-air laden jet provided a superior flow configuration in which the premixed flame high temperatures provided effective tar cracking, thus minimizing the exhaust total tar content to 50 mg/Nm3. A simultaneous peak exhaust soot volume fraction of 7 × 10−8 and a peak exhaust CH4 concentration of 0.1% were thus combined to a peak CO concentration of 14 mg/MJ.

Study of Internal Flow in a Liquid Nitrogen Flow Decelerator Through Swirl Effect Consisting of a Jet-Type Cryogenic Injection System for Food Freezing

This article addresses the study of internal flow dynamics within a cryogenic chamber designed for freezing food using liquid nitrogen injection. The chamber features a circular section with strategically placed jet-type atomizers for this purpose. The primary objective is to extend the residence time of the cryogenic fluid within the chamber to ensure uniform and effective freezing of the passing food items. This is achieved by inducing a swirl effect through strategic deceleration of the flow using the atomizers. The meticulous placement of these atomizers at periodic intervals along the internal walls of the cylindrical chamber ensures prolonged recirculation of the internal flow. Internal temperature analysis is crucial to ensure the freezing process. The study is supported by numerical analysis in CFD ANSYS to assess the dynamics of the swirl effect and parameters associated with the nitrogen–air interface, from which we obtain a sophisticated analysis thanks to the design of a hexahedral mesh made in greater detail in ICEM CFD. This approach aims to understand internal flow behavior and its correlation with the complexity of cryogenic system design, utilizing variable nitrogen-injection pressures and strategic atomizer placement as fundamental parameters to optimize system design.

Production of hydroxyectoine from biogas by an engineered strain of Methylomicrobium alcaliphilum using a novel Taylor‐flow bioreactor

AbstractBACKGROUNDThe production of compatible solutes, such as ectoine and hydroxyectoine, is of great interest due to their industrial and biotechnological applications. Methylomicrobium alcaliphilum was genetically engineered to replace a native gene with a heterologous one, aiming to enhance ectoine production. This study focuses on the optimization of bioreactor conditions to maximize the microbial production of these metabolites from methane.RESULTSThe engineered strain (M. alcaliphilum PstEctD) was cultured in a Taylor flow bioreactor under varying gas recirculation flow rates. Increased flow rates enhanced methane consumption, biomass concentration, and ectoine production. The highest production of ectoine (32 mg/g‐VSS) and hydroxyectoine (272 mg/g‐VSS) was observed at a flow rate of 0.7 L min−1, while methane removal efficiency improved from 30% to over 60% as flow rates increased.CONCLUSIONSOptimizing bioreactor conditions, particularly gas recirculation flow rates, significantly improved both the efficiency of methane consumption and the production of ectoine derivatives. This work provides a scalable approach for the sustainable production of compatible solutes from methane, offering potential applications in biotechnological processes utilizing renewable carbon sources. © 2024 The Author(s). Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

Investigating the Effects of Labeled Data on Parameterized Physics-Informed Neural Networks for Surrogate Modeling: Design Optimization for Drag Reduction over a Forward-Facing Step

Physics-informed neural networks (PINNs) are gaining traction as surrogate models for fluid dynamics problems, combining machine learning with physics-based constraints. This study investigates the impact of labeled data on the performance of parameterized physics-informed neural networks (PINNs) for surrogate modeling and design optimization. Different training approaches, including physics-only, data-only, and several combinations of both, are evaluated using fully connected (FCNN) and Fourier neural network (FNN) architectures. The test case focuses on reducing drag over a forward-facing step through optimal placement and sizing of an upstream obstacle. Results demonstrate that the inclusion of labeled data significantly enhances the accuracy and convergence rates of FCNNs, particularly in predicting flow separation and recirculation regions, and improves the stability of design optimization outcomes. Conversely, FNNs exhibit inconsistent responses to parameter changes when trained with labeled data, suggesting limitations in their applicability for certain design optimization tasks. The findings reveal that FCNNs trained with a balanced integration of data and physics constraints outperform both data-only and physics-only models, highlighting the importance of optimizing the training approach based on the specific requirements of fluid mechanics applications.

Effects of Cerium Oxide on Kidney and Liver Tissue Damage in an Experimental Myocardial Ischemia-Reperfusion Model of Distant Organ Damage.

Background and Objectives: Ischemia-reperfusion (I/R) injury is a process in which impaired perfusion is restored by restoring blood flow and tissue recirculation. Nanomedicine uses cutting-edge technologies that emerge from interdisciplinary influences. In the literature, there are very few in vivo and in vitro studies on how cerium oxide (CeO2) affects systemic anti-inflammatory response and inflammation. Therefore, in our study, we aimed to investigate whether CeO2 administration has a protective effect against myocardial I/R injury in the liver and kidneys. Materials and Methods: Twenty-four rats were randomly divided into four groups after obtaining approval from an ethics committee. A control (group C), cerium oxide (group CO), IR (group IR), and Cerium oxide-IR (CO-IR group) groups were formed. Intraperitoneal CeO2 was administered at a dose of 0.5 mg/kg 30 min before left thoracotomy and left main coronary (LAD) ligation, and myocardial muscle ischemia was induced for 30 min. After LAD ligation was removed, reperfusion was performed for 120 min. All rats were euthanized using ketamine, and blood was collected. Liver and kidney tissue samples were evaluated histopathologically. Serum AST (aspartate aminotransferase), ALT (alanine aminotransaminase), GGT (gamma-glutamyl transferase), glucose, TOS (Total Oxidant Status), and TAS (Total Antioxidant Status) levels were also measured. Results: Necrotic cell and mononuclear cell infiltration in the liver parenchyma of rats in the IR group was observed to be significantly increased compared to the other groups. Hepatocyte degeneration was greater in the IR group compared to groups C and CO. Vascular vacuolization and hypertrophy, tubular degeneration, and necrosis were increased in the kidney tissue of the IR group compared to the other groups. Tubular dilatation was significantly higher in the IR group than in the C and CO groups. TOS was significantly higher in all groups than in the IR group (p < 0.0001, p < 0.0001, and p = 0.006, respectively). However, TAS level was lower in the IR group than in the other groups (p = 0.002, p = 0.020, and p = 0.031, respectively). Renal and liver histopathological findings decreased significantly in the CO-IR group compared to the IR group. A decrease in the TOS level and an increase in the TAS level were found compared to the IR group. The AST, ALT, GGT, and Glucose levels are shown. Conclusions: CeO2 administered before ischemia-reperfusion reduced oxidative stress and ameliorated IR-induced damage in distant organs. We suggest that CeO2 exerts protective effects in the myocardial IR model.

Inertia-induced mixing and reaction maximization in laminar porous media flows

Solute transport and biogeochemical reactions in porous and fractured media flows are controlled by mixing, as are subsurface engineering operations such as contaminant remediation, geothermal energy production, and carbon sequestration. Porous media flows are generally regarded as slow, so the effects of fluid inertia on mixing and reaction are typically ignored. Here, we demonstrate through microfluidic experiments and numerical simulations of mixing-induced reaction that inertial recirculating flows readily emerge in laminar porous media flows and dramatically alter mixing and reaction dynamics. An optimal Reynolds number that maximizes the reaction rate is observed for individual pore throats of different sizes. This reaction maximization is attributed to the effects of recirculation flows on reactant availability, mixing, and reaction completion, which depend on the topology of recirculation relative to the boundary of the reactants or mixing interface. Recirculation enhances mixing and reactant availability, but a further increase in flow velocity reduces the residence time in recirculation, leading to a decrease in reaction rate. The reaction maximization is also confirmed in a flow channel with grain inclusions and randomized porous media. Interestingly, the domain-wide reaction rate shows a dramatic increase with increasing Re in the randomized porous media case. This is because fluid inertia induces complex three-dimensional flows in randomized porous media, which significantly increases transverse spreading and mixing. This study shows how inertial flows control reaction dynamics at the pore scale and beyond, thus having major implications for a wide range of environmental systems.

Analysis of MHD slip blood flow through a constricted artery filled with a porous medium of variable permeability: Applications in Biomedical Engineering

The primary goal of this research is to investigate the impact of partial slip and variable permeability on the dynamics of blood flow in a constricted artery filled with a porous medium, and to assess the influence of an externally applied magnetic field on the blood flow. The blood is modeled as an incompressible Newtonian fluid. By employing a stream function formulation, the coupled nonlinear flow equations are transformed into a single dimensionless equation, which is solved using the Adomian decomposition method (ADM). Key parameters such as the slip parameter, permeability parameter, Hartmann number, and Reynolds number are examined in relation to their effects on the flow field. The results reveal significant changes in blood flow dynamics within the stenosed section of the artery, particularly due to the incorporation of variable permeability in the porous medium and the partial slip condition along the arterial wall in the presence of the magnetic field. Notable outcomes include the escalation in axial velocity with increasing permeability and slip, as well as the formation of flow separation and recirculation zones in regions of high stenosis height. Additionally, the magnetic field was found to suppress axial velocity while increasing shear stress, particularly near the throat of the stenosis. These findings provide deeper insights into how variable permeability and slip conditions influence blood flow in clinical scenarios such as magnetic resonance imaging (MRI). Importantly, the results in specific cases align with established findings from existing literature, validating the approach and offering new contributions to the understanding of MHD flow in constricted arteries.

Spatiotemporal visualization of instantaneous flame structure in a hydrogen-fueled axisymmetric supersonic combustor

Spatiotemporal visualization of instantaneous flame structures in a hydrogen-fueled axisymmetric supersonic combustor was investigated using multiview planar laser-induced fluorescence of the hydroxyl radical, coupled with high-speed photography and pressure measurement. The axisymmetric cavity generates a loop-shaped recirculation flow and shear layer that sustains the flame. An irregular and wrinkled flame loop with a central hole is formed near the loop-shaped region. Due to turbulent disturbances, multiple small-scale holes and fragmented flames are randomly distributed in the flame loop or near the wrinkled flame front. The combustion near the cavity shear layer is more likely to be stronger and sustained. As the thickness of the cavity shear layer increases along the axial direction, the flame loop is expanded toward the core flow and the cavity. The flame base anchors near the cavity leading edge with a low global equivalence ratio (GER). The increased GER expands the flame loop to compress the high-speed core flow dramatically, promoting the flame base to propagate upstream along the hydrogen jet wake. The flame base is unable to anchor near the thin boundary layer. Consequently, it propagates reciprocally to enhance the combustion oscillation that disturbs the flame structure dramatically. The flame structure becomes more complex and tendentially fragmented, which increases the fractal dimension, especially near the middle part of the combustor. In comparison, the flame structure near the ramp is more resistant to disturbances due to the dramatic expansion of local flame loop, extending the favorable combustion environment. Despite the instantaneous flame structure being severely wrinkled and even tendentially fragmented, it is primarily sustained within a relatively regular loop region near the cavity recirculation flow and the cavity shear layer.

Topologic-thermal synergism analysis for wedge-shaped channels leveraging data mining and self-organization geometries

Heat transfer enhancement is particularly difficult to achieve within narrow space and turning channels due to strong structural constraints plus flow recirculations. Traditional designs usually follow periodic topology layout of simple geometric features, such as pin-fin arrays, to distribute cooling for narrow turning channels, lacking alignment with specific design objectives and the constraints. This is attributed to three main issues: the absence of control methods for complex structures, the lack of data for variable topologies, and insufficient understandings of Topologic-thermal Synergism. To address these challenges, the integration of self-organization geometry and the constrained Bayesian optimization method is employed. The channel is divided into 15 regions: the left region is subjected to solid presence control and anisotropy determination, while other regions are assigned control parameters, enabling self-organizing parameterization in design region. The parameters then are optimized using a constrained Bayesian optimization approach, aimed at maximizing the thermal performance factor (TPF) while constraining the area of low heat transfer regions. The optimization process begins with 216 samples to build the initial surrogate model, followed by 30 optimization iterations, resulting in the creation of high-performance complex structures and the establishment of a topology database. Furthermore, the SHAP method is utilized to extract topology design guidelines based on the most influential parameters and their beneficial variation directions. The guideline serves to guide the topology layout of other cooling structures in similar design scenarios. Results demonstrate that optimized self-organized structure enhanced the overall thermal parameter by 70% and decreased the low heat transfer zones by 74% when compared with pin fin structures. Data mining identified material orientation and generation on the left-side region as critical factors in topological structures, significantly influencing overall heat transfer performance. Leveraging the obtained design guidelines, a topological layout for a guiding pin fin structure is redesigned in wedge-shaped channels with varying contraction ratios, achieving performance improvements without the need for additional computational resources. The outcomes of this work hold scientific significance and industrial application value in enabling rapid design of high performance heat transfer structures.

Hydrodynamic performance of swimming fish in the wake region of a semi-cylinder

The characteristics of environmental vortices significantly influence the behavioral strategies of fish. This study investigated how wake vortices from obstacles affected propulsion and stability disturbances in fish behavior by numerically quantifying hydrodynamic effects related to different scales of semi-cylinders. The strong reverse flow in the recirculation regions of semi-cylinders generated trailing edge vortices near the tail, which reduced thrust and induced instability upon wake vortex impingement. Decreasing the gap between the semi-cylinder and the fish increased the duration of disruption. Additionally, passing wake vortices induced drag on fish tails and destabilized the fish when the gap between the vortices was small. Notably, for fish behind small semi-cylinders, wake vortex impingement did not cause drag and instability; rather, the primary source of drag was the shed wake vortices. These factors extended the disruption region laterally from the edges of the semi-cylinder to twice its diameter outside the wake region. Only the far downstream area, beyond the recirculation flow, could provide a beneficial turbulence environment with low-momentum flow. The findings of this study may enhance the understanding of vortex-fish interactions and offer valuable insights for the design and path planning of bio-inspired underwater vehicles.

Natural convective heat transfer analysis of a vertical tapered inner annulus tube with dual inverted frusto-conical turbulator using CFD

Vertical tube heat exchangers play a pivotal role in various industrial processes, where efficient heat transfer is essential for optimal system performance. This research introduces a novel heat exchanger design, the Dual Inverted Frusto-Conical Turbulator Attached Tapered Inner Annulus Tube, aimed at enhancing natural convection in vertical annulus heat exchangers using nano-coolants. Traditional designs often suffer from non-uniform heat transfer due to varying boundary layer thicknesses along the height of the annulus. The proposed design mitigates this by incorporating a tapered inner annulus tube, which has a larger diameter at the bottom and gradually decreases towards the top. This taper reduces wall friction and boundary layer thickness, enhancing thermal diffusivity and improving the heat transfer rate. To further optimize heat transfer, Dual Inverted Frusto-Conical Turbulators are strategically placed around the tapered tube, generating controlled turbulence that minimizes drag and avoids unwanted flow reversals and recirculation. Simulation results show a Nusselt number of 321, a friction factor of 0.023, and a pressure drop of 154 Pa at a Reynolds number of 5000. The design also achieved a thermal conductivity of 0.3 kW/mK, thermal efficiency variation of 0.8658%, and a heat transfer coefficient of 26,000 W/m2K.

Two-phase flows downstream, upstream and within Plate Heat Exchangers

Air-water flows were assessed within Plate Heat Exchangers (PHE) with the aid of fast camera imaging. Tests occurred in transparent setups with three chevron angle arrangements (30o/30o, 30o/60o and 60o/60o), representative of low, in-between and high pressure drop channels. Evaluation upstream the PHE inlet happened with Electrical Capacitance Tomography. Three patterns were tested: bubbly, slug and stratified. The effects of flow direction, superficial fluid velocities, two-phase pattern, and chevron angle arrangement on air-water distributions were assessed. The PHE channel outlet is characterized by intense flow recirculation. Bubble entrapment occurs in the core of the recirculation zones. Energy dissipation processes along the PHE channel flow affect the inlet gaseous content, intensifying the mixing process of air and water phases, particularly at flow distribution areas owing to the occurrence of flow acceleration and deceleration. Bubble distribution is wide since the break-up process is rather heterogeneous. Prediction of the maximum bubble diameter was obtained with a modification to Hinze's model. Coalescence can occur with small liquid superficial velocities. At the exit manifold, the recirculation zones affect the two-phase pipe flow. In addition to swirling decay, two-phase flow features and gravitational forces need to be accounted to determine the necessary pipe length to attain stationary process.

Study of unsteady shear flow near stall in a shrouded supercritical carbon dioxide centrifugal compressor

This study investigates the unsteady shear flow patterns and their relation with the aerodynamic instability of a shrouded supercritical carbon dioxide (SCO2) centrifugal compressor impeller using the dynamic mode decomposition method combined with detached eddy simulation. The results demonstrate that discrete shedding vortex array in the blade tip region is the dominant mode of the SCO2 shrouded centrifugal compressor impeller, with a frequency of 0.5 times the blade passing frequency. In contrast, the main flow structures in the identical impeller using ideal air consist of small-scale shedding vortices near the suction surface and the separated secondary flow. Further analysis of flow field details indicates that the shedding vortex structure inside the shrouded centrifugal impeller originates from the Kelvin–Helmholtz instability generated on the shear interface between the mainstream and the recirculation flow. The greater shear intensity within the SCO2 centrifugal impeller is the reason for the evident occurrence of vortex shedding phenomena in its flow field. Additionally, during the transition from near-stall to stall, vortex pairing phenomena happens at the throat of the centrifugal impeller due to evolutional behaviors of the vortexes. The occurrence of vortex pairing leads to faster blockage of the blade tip region and then the secondary flow spillage over to adjacent passage, ultimately resulting in reverse flow in the blade tip region and extensive blockage at the impeller inlet. The unsteady shear flow evolution clearly demonstrates the processes of stall in SCO2 shrouded impellers.

Hydrogen as a direct heat exchange fluid in room temperature hydride systems: Numerical study on the desorption process

Abstract Hydrogen, as an energy carrier, is a promising candidate to foster decarbonization. However, its storage poses significant challenges. Common methods, such as compressed gas and liquid hydrogen, have high energy consumption and safety concerns. Recently, solid hydrogen storage in materials like metal hydrides has gained attention for their ability to store hydrogen safely at low pressures and low temperatures. This study aims to develop a numerical model to simulate the performance of metal hydrides using hydrogen as a direct fluid heat exchanger during desorption. The model, formulated as a system of partial differential equations, is implemented in MATLAB with the ODE15s solver and applied to a disk-type lanthanum nickel reactor to minimize pressure drops. Performance is investigated by varying design parameters, including reactor length and diameter, bed porosity, hydride particle diameter, operating pressure and temperature, and hydrogen mass flow rate at the reactor inlet. Additionally, the energy consumption of auxiliary equipment, such as pumping and thermal power, is evaluated. Results show that the system energy requirement is about 8-9% of the hydrogen lower heating value, with most desorption occurring in less than 300 seconds. The reactor dimensions are crucial for fast desorption and low pressure drops, with pumping power under 1 W given the small thickness and flow rate. Particle diameter and porosity have minor impacts, while pressure, temperature, and flow rate are fundamental. High temperatures, low pressures, and high recirculating flow rates favor the reaction, though a trade-off between performance and energy consumption is necessary since all high temperatures high recirculated mass flow rate allows for high consumption.

Aerodynamic interference effect and flow field mechanism of two tandem rectangular columns with a small width–thickness ratio at a high Reynolds number

This paper conducted wind tunnel tests and large eddy simulations to study the aerodynamic interference effect and flow field mechanism of two tandem rectangular columns with a small width–thickness ratio (B/D = 0.25) at a high Reynolds number (Re = 2.1 × 105). The spacing ratio (L/B) varied from 0.2 to 20. Results showed that single-bluff body, reattachment, and co-shedding regimes occur at 0.2 ≤ L/B &amp;lt; 3, 3 ≤ L/B &amp;lt; 10, and 12 &amp;lt; L/B ≤ 20, respectively. In the single-blunt body regime, the mean drag coefficient of the upstream column, the fluctuating lift coefficient of the downstream column, and the Strouhal number of both columns are significantly amplified compared to a single column. These amplification effects are linked to the reattachment of the recirculation flow between columns and a reduced wake recirculation length. In the reattachment regime, the amplification effects in the mean drag coefficient and the fluctuating lift coefficient are diminished, but the Strouhal number still shows a marked amplification due to the short wake recirculation length. In the co-shedding regime, the amplification effects in aerodynamic force coefficients disappear. In addition to the three classic flow regimes, a bistable flow regime was identified at 10 ≤ L/B ≤ 12, where the aerodynamic characteristics observed in the reattachment and the co-shedding regimes alternate randomly at irregular time intervals.

Simulation and validation of a Francis turbine load rejection procedure using OpenFOAM CFD code

Abstract The effect of off-design operation on the health of hydraulic turbines is known to be adverse, with substantial compromise in efficiency, due to evolvement of flow instability in the draft tube and consequent pressure fluctuation. The worst-case scenario may entail resonance between developed pressure fluctuation and system natural frequencies and therefore lead to power swings, possible mechanical vibrations and machine wear and tear. Present global energy ambition, however, necessitates frequent off-design operation and as such development of accurate numerical methods to determine the anomalous flow field in the turbomachine is mandated. To this end, a load rejecting transient operation of a model high head Francis turbine from design operating point to part load condition is simulated using a hybrid turbulence modelling approach. A detailed analysis of the draft tube velocity field is carried out after validation of the test case with experimental investigations that have discerned similar results. Spatio-temporal features, such as central flow stagnation, flow separation, central flow recirculation and development of a precessing vortex core, which are embodiments of the vortex breakdown phenomenon are thoroughly analysed and discussed. Finally, the draft tube flow is visualized using streamlines on the meridional plane and tracked as the turbine transitions from design to part load operating conditions.

Flow Field Analysis within Industrial Quenching Tanks Operated with Aqueous Polymer Solutions in the Steel Industry

Abstract Quenching with polymer solutions for steel heat treatment offers adjustable performance and improved ecological impact compared to conventional water or oil quenching. The submersion cooling operation poses challenges due to the intense vapor/polymer film collapse on the component surface, potentially destabilizing batch components. Therefore, controlled recirculation is critical to preventing tank and specimen damage during cooling and ensuring effective steel hardening. However, flow structure analysis is often overlooked and disregarded in industrial tank design. Thus, this study evaluates the tank geometries of two partner industries to determine if they provide the necessary homogeneous flow for optimal quenching. The analysis combines experimental velocity measurements with a numerical model, with the identification of different flow intensity regions being the main outcome of this work. The results suggest that geometric modifications could improve flow recirculation, enhancing quenching performance.

Analysis of Flow Pattern and Bubble Behavior in Slab Continuous Casting with Mold Electromagnetic Stirring

The electromagnetic stirring (EMS) and argon blowing are the widely recognized and extensively employed technologies to control the flow pattern in slab continuous casting, and their interaction is directly linked to the final product. To investigate the flow pattern, bubble characteristics under mold EMS and argon blowing, a mathematical model coupling the electromagnetic field, two‐phase flow is constructed. The population balance model (PBM) under the Eulerian frame is applied to describe the bubbles breakage and coalescence. The results show that the flow regime under the interaction of argon blowing and EMS transfers from bubble ascending flow to intermediate flow to horizontal recirculation flow as EMS current increases, and the large argon flow rate could suppress the formation of horizontal recirculation flow. Meanwhile, EMS can promote the coalescence of bubbles, with the increasing EMS current, the bubble numbers decrease first and then increase, and the bubble mean diameter increases first and then decreases. For obtaining the horizontal recirculation flow, the suitable argon flow rate and EMS current should be matched suitably.