Blade Channel Research Articles

Numerical Study on the Blade Channel Vorticity in a Francis Turbine

A relevant way to promote the sustainable development of energy is to use hydropower. Related systems heavily rely on the use of turbines, which require careful analysis and optimization. In the present study a mixed experimental-numerical approach is implemented to investigate the related mixed water flow. In particular, particle image velocimetry (PIV) is initially used to verify the effectiveness of the numerical model. Then numerical results are produced for various conditions. It is shown that an increase in the guide vane opening can reduce the extension of the region where the fluid velocity is 0 at the inlet of the runner blade, i.e., it can counteract the generation of the channel vortex; an increase in the guide vane opening also contributes to mitigate the pressure acting on the runner blade; no matter what the working conditions are, the surface pressure is usually higher than that on the suction surface, and there is a cliff-like drop of pressure at the tail of the blade, which indicates that the runner blade tail is more prone to develop backflow.

Effect of Unsteady Secondary Vortices on Performance of a 1.5-Stage High-Pressure Turbine

In modern high-load high-pressure turbine, the secondary flow in the blade channel is very strong and occupies a large spanwise region. Although high-quality experimental data at the stage interfaces have been obtained in previous research, the influence of the clocking position on the secondary flow patterns is not fully understood. This paper investigates the clocking effect in a 1.5-stage high-pressure turbine and focuses on the variations of secondary flow patterns and their effect on the turbine performance. The detailed flow fields of various clocking positions were obtained by carrying out unsteady flow simulations using an in-house code. Among the four clocking positions in this work, the highest entropy generation was observed when the wakes from stator 1 hit the leading edges of stator 2, which is opposite to the well-known conclusion for the turbine with high-aspect-ratio blades. Detailed flow analysis showed that the wakes and the near tip secondary vortices from stator 1 showed different traces when entering the stator 2 channel and the secondary vortices clearly have a more important influence in determining the performance. The different behaviors of the secondary vortices explained the performance variations due to the clocking effect.

Про вплив форми меридіонального контуру на енергетичні характеристики відцентрового колеса компресора

This work is concerned with the development of approaches to the optimal aerodynamic design of centrifugal compressor wheels, which is due to the use of centrifugal stages in compressors of modern aircraft gas turbine engines and power plants. The aim of this work is a computational study of the effect of the meridional contour shape of a centrifugal compressor wheel on its power characteristics. The basic method is a numerical simulation of 3D turbulent gas flows in centrifugal wheels on the basis of the complete averaged Navier¬–Stokes equations and a two-parameter turbulence model. The computational study features: varying the shape of the hub and tip part of the meridional contour over a wide range, formulating quality criteria as the mean integral values of the wheel power characteristics over the operating range of the air flow rate through the wheel, and a systematic scan of the independent variable range at points that form a uniformly distributed sequence. As a result of multiparameter calculations, it was shown that in the case of a flow without separation in the blade channels of a wheel with a given starting shape of the meridional contour, varying that shape has an insignificant effect on the wheel power characteristics. It is pointed out that in similar cases it seems to be advisable to aerodynamically improve centrifugal wheels by varying the shape of their blades in the circumferential direction rather than in the meridional plane. This conclusion was made using rather a “coarse” computational grid, which, however, retains the sensitivity of the computed results to a variation in the centrifugal wheel geometry. On the whole, this work clarifies ways of further aerodynamic improvement of centrifugal compressor impellers in cases where the starting centrifugal wheel is a well-designed wheel with a flow without separation in the blade channels. The results obtained may be used in the aerodynamic optimization of centrifugal stages of aircraft gas turbine engines.

Investigation on Vortex Characteristics of a Multi-Blade Centrifugal Fan near Volute Outlet Region

The origins and effects of the complex vortex structure near the volute outlet of a multi-blade centrifugal fan are investigated in this paper. Due to the wide blade and short blade channel, the airflow maintains a large radial velocity during the blade channel. This continuous radial partial velocity causes vortices to be generated at the region of volute outlet. Then, the secondary flow close to the impeller generate from the center to the sides in volute. It is obtained that the streamlines are divided into two parts (backflow and outflow) at volute outlet. Although the vortices near volute outlet region are complex, the main features of flow behavior caused by the vortex are understandable.

3D simulation of gas-laden liquid flows in centrifugal pumps and the assessment of two-fluid CFD methods

An assessment of a two-fluid model assuming a continuous liquid and a dispersed gas phase for 3D computational fluid dynamics (CFD) simulations of gas/liquid flow in a centrifugal research pump is performed. A monodisperse two-fluid model, in conjunction with a statistical eddy-viscosity turbulence model, is utilized. By a comprehensive measurement database, a thorough assessment of model inaccuracies is enabled. The results on a horizontal diffuser flow reveal that the turbulence model is one main limitation of simulation accuracy for gas/liquid flows. Regarding pump flows, distinctions of single-phase and two-phase flow in a closed and semi-open impeller are figured out. Even single-phase flow simulations reveal challenging requirements on a high spatial resolution, e.g., of the rounded blade trailing edge and the tip clearance gap flow. In two-phase pump operation, gas accumulations lead to coherent gas pockets that are predicted partly at wrong locations within the blade channel. At best, a qualitative prediction of gas accumulations and the head drop towards increasing inlet gas volume fractions (IGVF) can be obtained. One main limitation of two-fluid methods for pump flow is figured out in terms of the violation of the dilute, disperse phase assumption due to locally high disperse phase loading within coherent gas accumulations. In these circumstances, bubble population models do not appear beneficial compared to a monodisperse bubble distribution. Volume-of-Fluid (VOF) methods may be utilized to capture the phase interface at large accumulated gas cavities, requiring a high spatial resolution. Thus, a hybrid model, i.e., a dispersed phase two-fluid model including polydispersity for flow regions with a dilute gas phase, should be combined with an interphase capturing model, e.g., in terms of VOF. This hybrid model, together with scale-resolving turbulence models, seems to be indispensable for a quantitative two-phase pump performance prediction.

The investigation of runner blade channel vortices in two different Francis turbine models

The present study aims to acquire deeper understanding of the runner blade channel vortices phenomenon by numerical simulations of the whole turbine passage. Several operating points along different specific discharge were calculated by a hybrid turbulence model, Very Large Eddy Simulation, which was proved that it was able to capture the blade channel vortices similarly. The results showed that the blade channel vortices structure and intensity were various for different runners at incipient point. The flow pattern revealed that the direct relationship of a backflow region and the blade attacking angle near the hub had a distinct variation in vortex structure and intensity. Furthermore, this paper referred a new method to evaluate the strength of blade channel vortex quantitatively, basing on the evolution of specific energy gradient over the stream-wise location. Therefore, this new method enabled the researchers to understand deeply the flow characteristics associated with the blade channel vortices formation and carry out efficiently the hydraulic optimization design.

Numerical analysis of the vortical structure and its unsteady evolution of a centrifugal pump

To investigate the complex flow and vortical structure of a centrifugal pump with low specific speed, the numerical simulation approach DDES is used to capture the unsteady flow fields when the blade periodically sweeps the volute tongue. Attentions are attracted on unsteady vortical structures on the mid-span of the model pump, and quantitative data of z-vorticity(axial component of vorticity) at the impeller exit are extracted to discuss the corresponding evolution process. Besides, three dimensional vortices within the impeller are also identified at different working conditions. Results show that at the nominal and high working conditions, evident vorticity sheets are generated within the pump. High vorticity sheets are developed on the blade suction side and the trailing edge, which are also significant in the volute area caused by the vortex shedding from the impeller. The interactions between the shed wake flow and the tongue are not identical at various working conditions, and it is more evident at high working condition. With the blade moving away from the tongue, high vorticity is generated around the blade suction side. Especially at high working condition, several vorticity sheets with alternative negative and positive values are formed near the blade suction side. The vorticity sheet tends to stretch towards the blade pressure side with the impeller passing the tongue. Based on the 3D vortical structures, it is found that the hairpin vortices are generated within the blade channels, which could not be captured at low working condition. Finally, the obtained vorticity sheet and 3D vortex results will contribute to the understanding of the complex flow structures in pumps.

Video Laryngoscopic Intubation Using the King VisionTM Laryngoscope in a Simulated Cervical Spine Trauma: A Comparison Between Non-Channeled and Channeled Disposable Blades.

Videolaryngoscopes may reduce cervical spine movement during tracheal intubation in patients with neck trauma. This manikin study aimed to compare the performance of disposable non-channeled and channeled blades of the King Vision™ videolaryngoscope in simulated cervical spine injury. Fifty-eight anesthesiologists in training intubated the TruMan manikin with the neck immobilized using each blade in a randomized order. The primary outcome was the time needed for tracheal intubation, secondary aims included total success rate, the time required for visualization of the larynx, number of attempts, view of the vocal cords, and subjective assessment of both methods. Intubation time with the channeled blade was shorter, with a median time of 13 s (IQR 9–19) vs. 23 s (14.5–37.5), p < 0.001, while times to visualization of the larynx were similar in both groups (p = 0.54). Success rates were similar in both groups, but intubation with the non-channeled blade required more attempts (1.52 vs. 1.05; p < 0.001). The participants scored the intubation features of the channeled blade significantly higher, while visualization features were scored similarly in both groups. Both blades of the King Vision™ videolaryngoscope are reliable intubation devices in a simulated cervical spine injury in a manikin model when inserted by non-experienced operators. The channeled blade allowed faster intubation of the trachea.

DDES analysis of unsteady flow evolution and pressure pulsation at off-design condition of a centrifugal pump

In the present paper, to investigate unsteady flow structures and pressure pulsations at off-design conditions, a low specific speed centrifugal pump is investigated by the DDES (Delayed Detached Eddy Simulation) method. Emphasis is laid on the unsteady evolution of flow structure and pressure pulsation at the stalled condition. Results show that the jet-wake flow pattern could be captured by the current numerical simulation method, and it shows a high calculation precision for the jet flow. Relative velocity decreases rapidly in the wake region towards the blade suction side. Significant inflection point exists between the jet and wake flow regions, and it almost keeps unchanged at the identical impeller-volute position under various flow rates. At the stalled status, the inflection point moves towards the blade pressure side with the blade passing the volute tongue, and evident non-uniform flow distributions are formed in the blade channels characterized by the multiple vortices. The blade channel is gradually blocked by the corresponding stall cell when the blade approaches the volute tongue, and it indicates that the volute tongue has a significant effect on the evolution process of the stall structure. By combination analysis of the unsteady evolution of the stall structure and pressure spectrum, the stall frequency at 0.25fn (being fn the impeller rotating frequency) is captured for the current model pump.

Evolutions of flow patterns and pressure fluctuations in a prototype pump-turbine during the runaway transient process after pump-trip

The runaway after pump-trip is a fast transient process, during which unit vibrations, water-hammer fluctuations, pressure pulsation changes and flow pattern transitions happen violently, endangering pump-turbine units of pumped-storage power stations. In this paper, based on validated CFD data from 1D-3D coupled simulation, we describe the evolutions of flow patterns and pressure fluctuations in a prototype pump-turbine during this working point fast sliding process, and try to reveal the influence mechanism of flow pattern transitions on pressure fluctuation changes. The results show that the general evolutions of flow patterns are dominated by the changes of the runner velocity triangles, which are controlled by the changes of the discharge and rotational speed. Inhomogeneous local flow also contributes to abundant local vortex structures, such as rotating-stall vortices in the vane and runner regions, skirt and helical vortices in the draft-tube, separation vortices in the blade channels, and the impinging jets on the blade surfaces. These unstable vortices appear, change, and vanish in a fast transitional and uneven manner, leading to great differences in pressure and runner force fluctuations in different working regions.

НАПОРНАЯ ХАРАКТЕРИСТИКА ЦЕНТРОБЕЖНОГО КОМПРЕССОРА МАЛОРАЗМЕРНОГО ТУРБОРЕАКТИВНОГО ДВИГАТЕЛЯ НА ОСНОВЕ ЧИСЛЕННОГО МОДЕЛИРОВАНИЯ

Shown the possibility of using the standard ANSYS CFX hydrodynamic software package for calculating the gasdynamic characteristics of the centrifugal compressor impeller of micro turbojet engines with different options for profiling blades which based on physical and numerical modeling. Presented a methodology for designing the impeller of a centrifugal compressor based on solving the inverse problem of gas dynamics. As a result of a numerical study, the head coefficient of various forms of the impeller was obtained and presented the dependences of the head coefficient and efficiency on the blade back sweep angle 2 β . b The article discusses the effect of the blade back sweep angle 2 β b on the compressor efficiency and the head characteristic for three different values of the blade back sweep angle 2 β b for example, the impeller with the back sweep angle  2 β b and with the radial blades  2 β 90 b and with blades bent forward  2 β b The centrifugal compressor was designed using Vista CCD programs in one-dimensional computing and Fluid flow CFX in three-dimensional computing. For blade profiling, the BladeGend program was used with different profiling options in order to improve compressor efficiency. The computational grid and the construction of a structured hexahedral mesh for the impeller was carried out in Ansys Turbogrid and the SST model of turbulence was selected in the calculation, which, with sufficient grinding of the mesh at the walls, adequately simulates separated flows at the channel walls, as well as the flow in the flow core. When constructing a grid along the walls between the channel blades, the parameter y + was controlled, which should not exceed 2. It is permissible to use a coarser grid in the flow core compared to the grid near the walls. The design grid of the impeller consists of 350000 elements.

Time-dependent inception of vortex rings in a Francis turbine during load variation: large eddy simulation and experimental validation

The present study investigates a transient condition, i.e. load variation from the best efficiency point to the part load, of a Francis turbine. A complete turbine is considered for the study. The dynamic mesh approach is implemented to close the guide vanes and to perform load variation numerically. The primary goal is to investigate the inception, propagation and interaction of vortex rings as load on the turbine changes. The secondary goal is to determine characteristic frequencies. The continuous change in flow angle, due to the closing of the guide vanes, induces strong boundary layer separation in the runner. Flow decelerates along the blade and establishes an adverse pressure gradient due to steep curvature of the blades. Separated flow from the boundary layer creates two swirling regions attached to the blade suction side. Fully developed vortex rings exiting from the adjacent blade channels interact and induce high-amplitude stochastic fluctuations at the runner outlet.

Non-ideal compressible flows in supersonic turbine cascades

Flows in the close proximity of the vapour–liquid saturation curve and critical point are examined for supersonic turbine cascades, where an expansion occurs through a converging–diverging blade channel. The present study illustrates potential advantages and drawbacks if turbine blades are designed for operating conditions featuring a non-monotonic variation of the Mach number through the expansion process, and non-ideal oblique shocks and Prandtl–Meyer waves downstream of the trailing edge. In contrast to ideal-gas flows, for a given pressure ratio across the cascade, the flow field and the turbine performance are found to be highly dependent on the thermodynamic state at the turbine inlet, in both design and off-design conditions. A potentially advantageous design, featuring stationary points of the Mach number at the blade trailing edge, is proposed, which induces a nearly uniform outlet Mach number distribution in the stator–rotor gap with a low sensitivity to slight variations in the outlet pressure. These findings are relevant for turbomachines involved in high-temperature organic Rankine cycle power systems, in particular for supercritical applications.

Synergy Methodology for Internal Flow of Turbomachinery

The complex curvature of turbomachinery rotor blade channels combined with strong rotational effect and clearance leakage brings on intricate internal flow phenomenon. It is necessary to study the internal flow and energy loss mechanism to reveal the influence law of the key parameters and to achieve its optimal design. Considering features of flow and temperature fields in rotor passage, the concept of synergy analysis derived from equation of energy conservation was put forward. Typical NASA low-speed centrifugal compressor (LSCC) rotor was chosen for analysis using CFD. Numerical results showed remarkable agreement with experiment datum in both the tendency of the performance characteristics and quantitative pressure values. Under different flow rates and inlet total temperatures conditions, thermal-fluid interaction effect and losses were studied by synergy analysis. Results showed that peak synergy positive value zones located around blade leading edge, across the shroud wall and hub wall, and at the position where tip-leakage flow was mixing with the bulk flow and high entropy zones existed. Increasing flow rate from design condition, positive and negative synergy areas both changed tiny around leading edge and trailing edge. Reducing flow rate, positive synergy areas tended to increase and negative areas decreased at same positions. The relationship between flow separation, heat transfer and losses in turbomachinery rotor can be revealed based on synergy analyses.

Secondary Flow and Endwall Optimization of a Transonic Turbine

A detailed numerical analysis of secondary flows in a transonic turbine is presented in this paper. The turbine stage is optimized by mitigating secondary flow through the method of non-axisymmetric endwall design. An automated optimization platform of NUMECA/Design3D was coupled with Euranus as a flow solver for the numerical investigation. The contoured endwalls of the stator and the rotor hub were designed based on equidistant Bézier curves along the camber line in the blade channel. The initial design samples were ten times the number of the design variables, and were generated through the LHS method for database generation. The optimization of the endwalls was achieved by using a state-of-the-art multi-objective optimization algorithm, NSGA-II, connected with the BPNN to increase the isentropic efficiency and decrease the secondary kinetic energy, while the mass flow and the degree of reaction were constrained to remain on the datum value as in the original geometry. The individual optimization of the hub endwalls of the stator and the rotor produced an increase in the efficiency of 0.27% and 0.25%, respectively, resulting in a cumulative improvement of 0.46% in the efficiency. The increase in the performance was analyzed at part-load conditions, and it was further confirmed through unsteady simulations.

Development of a method for geometrical modeling of the airfoil profile of an axial turbomachine blade

A method for geometric modeling of the contours of the suction and the pressure side profiles of axial turbomachine blades described by compound curves and formed by two sections has been proposed. Each section of the profile contour was modeled by a curve described in a natural parameterization. The cubic law of curvature distribution was applied to the leading sections of the profile and square law was used for the trailing section. Juncture of the leading and trailing sections of the profiles of the suction and the pressure side was provided with smoothness of the third order which assumes equality of values of functions, function derivatives, curvature and curvature derivatives in the junction point. When modeling the blade profile, thirteen kinematic and geometric parameters were used. Unknown coefficients of the square and cubic laws of distribution as well as lengths of the profile arcs of the suction and the pressure side sections were determined in the process of modeling the profile cascade for specified parameters. The problem was solved by minimizing deviations of the plotted curves from the reference points of the modeled profile located in the throat of the blade channel and on the circle, which determines maximum thickness of the profile.Based on the proposed method, a program code was developed which in addition to the digital information on the modeled turbomachine blade profile displays the results in a graphical form on a computer screen. The calculation studies confirmed feasibility of the proposed advanced method of modeling the suction and the pressure side profiles of axial turbine blades. The developed method can be useful in organizations engaged in the design and manufacture of blades of axial gas turbines of gas turbine engines

Numerical Analysis of Working Processes in the Blade Channels of the Highly-Loaded Turbine of a Marine Gas Turbine Engine, Using a Refined Finite Element Model

Numerical Analysis of Working Processes in the Blade Channels of the Highly-Loaded Turbine of a Marine Gas Turbine Engine, Using a Refined Finite Element Model

CFD-Aided Design of a Transonic Aeroelastic Compressor Rig

Abstract This paper presents the results of computational fluid dynamics (CFD)-aided design calculations of a transonic linear cascade wind tunnel. The purpose of the wind tunnel is to generate data for the validation of numerical methods employed to calculate aerodynamic damping for forced response cases in transonic compressors. It is common for transonic wind tunnels to use transonic walls (perforated walls with controlled suction) to adjust the transonic flow in the experiment. Unfortunately, perforated walls are difficult to model in CFD simulations, and they complicate the validation process. One of the goals of the new tunnel is not to use perforated walls. The main difficulty in the design of a transonic linear cascade is achieving periodic flow for the central blades due to shock reflections from the side walls and the sensitivity of transonic flow to small changes in geometry. Other design constraints are the maximum available mass flow of 4.5 kg/s and the minimum required blade thickness of 2 mm for instrumentation. The purpose of the current CFD simulations is to determine the optimum geometry (sidewalls, tailboards, and throttle) of the tunnel with the goal of achieving near periodic flow conditions for the central blade channels at the similar condition in a typical transonic compressor.

Influence of the guide vanes design on stress parameters of Francis-99 turbine

The frequencies with predominant amplitudes in low specific speed Francis turbines are related to rotor-stator interaction and they are calculated on the basis of the runner rotational speed and the number of guide vanes and runner blades. Pressure pulsations in the blade channels can be a reason for noise and vibration in the turbine above allowed level. High pressure pulsations can be caused by certain combination of runner blades and guide vanes number and/or resonance with one of the runner’s natural frequencies. The stress parameters of the Francis-99 turbine guide vanes and their modifications are analysed in this paper. The main aim is to determine the impact of the geometry modification (thinner for increased efficiency) of the guide vanes on the Francis turbine stresses by performing numerical simulations. The original Francis-99 turbine guide vane geometry and three modifications consisting of new guide vane shapes are being considered. The numerical investigation of the flow field is based on the k-ω SST turbulence model with ‘frozen rotor’ approach selected, constituting a quasi-steady state analysis, without taking into account the physical rotation of the runner to obtain Rotor-Stator Interaction (RSI). Pressure distribution on one guide vane determined by a Computational Fluid Dynamics (CFD) simulation of the turbine is coupled to a Finite Element Method (FEM) simulation in order to analyse the stresses. The results from the one-way fluid-structure interaction analysis give the stresses distribution and deformations of the guide vanes. Moreover, modal-acoustics analysis is conducted to obtain the natural frequencies of the guide vanes in water and comparison is made with the calculated vortex shedding frequencies to estimate the risk of resonance.

Numerical simulation of a centrifugal slurry pump handling solid-liquid mixture: Effect of solids on flow field and performance

The effect of solids on a centrifugal slurry pump performance is a major concern to the design of slurry transportation system. In the present study, the multiphase modeling of centrifugal slurry pump is performed using two models, Mixture and Eulerian-Eulerian multiphase. Sliding mesh approach is employed for unsteady simulation of the pump. The accuracy of the simulations is ascertained by comparing the performance characteristics of the pump obtained numerically and experimentally. Experimental results are obtained by measurements in a pilot plant test rig with three different mean size sand particulate slurries. The Eulerian-Eulerian multiphase model predicted the effect of the solids on pump performance close to the experimental results as compared to Mixture model. The obtained accuracy with Eulerian-Eulerian model for predicting the effect of solids on head and efficiency is around ±2% and ±3%, respectively. The predicted results using Eulerian-Eulerian model confirm that the head and efficiency of the pump decrease with the increase in particle size and concentration. The particles of high specific gravity show less reduction in head and efficiency of the pump. Further, the effect of variation in particle size and concentration on the flow field in the impeller and casing has also been analyzed at best efficiency point operation. Non-homogeneous suspension of particles inside the blade channels and casing passages is examined. The particulate concentration is observed higher near the impeller back shroud, pressure side of the blades, and non-suction side of the casing as compared to other locations.