Tandem Cascade Research Articles

The Matching Mechanism and Flow Mechanisms of Supersonic Through-Flow Variable-Pitch Tandem Cascade

Abstract The geometric structure of the supersonic through-flow variable-pitch tandem fan poses challenges in ensuring optimal relative blade positions across various operating modes. This study employed numerical simulations to investigate the impact of axial overlap and percent pitch on the aerodynamic performance of fan in typical modes. Furthermore, it delved into the intricate interplay between the front and rear blades, offering detailed insights into the selection principles governing the relative blade positions in the supersonic through-flow variable-pitch tandem fan. The findings underscored the pivotal role of percent pitch based on chord direction in fan's aerodynamic performance. Specifically, supersonic and transonic modes exhibited superior performance when chord direction percent pitch was set at 10% and 100%, respectively. The key factor contributing to these lower losses was the influence of low-speed fluid from the wake of the front blade, which weakened the shock boundary layer interaction on the rear blade's surface. In the high-speed windmilling mode, within the confines of geometric constraints, higher percent pitch based on chord direction values corresponded to reduced losses. This outcome was primarily attributable to the gap-blocking effect, which diminished the shock boundary layer interaction on the rear blade's pressure side and improved the wake of the rear blade. However, in this mode, the loss demonstrated a positive correlation with the pressure ratio. Consequently, the selection of the relative blade position required a delicate balance between the competing demands of these two factors.

Numerical investigations of aerodynamic characteristics in highly loaded tandem cascades with end-bend

Tandem cascades and end-bend blades can reduce compressor losses and promote energy conservation. In this study, a vortex model tailored for tandem cascades is proposed. Numerical investigations are conducted to elucidate the influence of end-bend heights and angles on aerodynamic characteristics. A quantitative assessment of the loss contribution for each vortex is provided based on loss weight coefficients. The findings demonstrate that end-bend angles exert a more pronounced impact on aerodynamic characteristics than end-bend heights. Although the implementation of end-bend increases the loss of the front blade trailing edge shedding vortex, it reduces losses by weakening the passage vortex and the rear blade trailing edge shedding vortex, resulting in a net decrease of the total loss. The end-bend blade optimizes the loading profile near the endwall, characterized by an expanded low-pressure zone behind the gap. A comparative analysis is performed among four tandem cascades: straight, curved, end-bend, and combined curved-end-bend. The correlation between losses, vortices, and topological structures is established by using topological analysis together with the vortex model. It is a crucial research methodology for elucidating the mechanisms of loss generation and vortex dynamics in tandem cascades with end-bend.

Numerical investigation of aerodynamic characteristics in tandem cascades with various curving strategies

Tandem configurations with curved blades can improve the flow field in compressor cascades. This study conducts a numerical investigation into the impact of various curving strategies on the losses and flow structures within tandem cascades. Curving strategies of three distinct configurations (curving both blades, solely the rear blade, and solely the front blade) along with three curved angles (5°, 10°, and 15°) are compared. Four primary vortex structures are causing losses in the tandem cascades. Loss weight coefficients, derived from each vortex, are adopted to quantify the proportion of losses. All curving strategies mitigate the accumulation of low-energy fluid near the endwalls, weakening the passage vortex. Various curving strategies have different effects on losses of the endwall spanwise vortex and trailing edge shedding vortices. The influence of curved angles on the three configurations indicates that curving both blades at 5° and curving the front blade at 5° or 10° can mitigate losses. An increasing loss is observed when the rear blade curves at any angle alone. Curving both blades or only the front blade mitigates the corner separation while the curved rear blade exacerbates it. The novelty of this study lies in applying topology analysis to establish correlations between losses, vortices, and topological configurations. This approach is a pivotal research methodology for elucidating the underlying mechanisms of loss generation and vortex dynamics within curved tandem cascades.

Analysis of the factors influencing the guide vane hydraulic torque of Francis turbine

Abstract The model test is commonly used to measure the guide vane hydraulic torque. With the development of numerical simulation technology, it is possible that numerical simulation can substitute for model test to study guide vane torque. In order to study the characteristic of guide vane torque, numerical simulation of tandem cascade based on CFX software are performed for operation points under different guide vane openings. In this paper, numerical calculation of guide vane torque takes singly periodic tandem cascade with the refined mesh of the boundary layer as the research object, and hydraulic force and torque characteristic of guide vane are computed under different guide vane openings. Furthermore, the influence of stay vane profile, turbine rotational speed, guide vane periodicity and mass flow rate on guide vane torque has been researched and analysed. Considering all relative influences, the calculation results of guide vane torque with singly periodic tandem cascade were modified. The result shows the modified method is more reasonable to analyse the guide vane torque by numerical simulation.

Design strategy and relevant flow mechanisms of highly loaded 3D compressor tandem cascades

To investigate the design strategy of highly loaded tandem cascades at both the midspan and endwall, the overall performance and flow mechanisms of four typical tandem cascades based on the optimization were analyzed from multiple perspectives numerically. The results show that the interference effects on the Front Blade (FB) and Rear Blade (RB) should not be overlooked during the design phase, and the design strategies at the midspan and endwall are completely different. At the midspan, the optimization aims to increase the interference effects and the strength of the gap jet while maintaining the same load on the FB and RB. However, the endwall optimal airfoil exhibits weakening interference effects, advancement of the gap jet location, and load transfer from the FB to RB. Through further analysis of flow characteristics, the midspan optimal airfoil is beneficial for inhibiting the low-energy fluid from interacting with the suction surface of RB under the design condition, but results in earlier occurrence of corner stall. The endwall optimal airfoil helps suppress the development of the secondary flow and delay the onset of corner stall. Furthermore, by combining the benefits of these two design approaches, additional forward sweep effects are achieved, further enhancing the performance of the tandem cascade.

Coupling optimization design of configuration and swept-bowed effects in highly loaded subsonic compressor tandem cascades with tip clearance

The coupling optimization design for swept-bowed effects and configuration of tandem cascade is conducted in a highly loaded subsonic compressor tandem cascade, in which the novelty lies in considering the spanwise distribution of configuration parameters and exploring the coupling relationship between them and swept-bowed design. Research results show that, compared with the original one, the loss coefficient of the optimal tandem cascade is decreased by 38.7% at the 4° incidence angle and the range of available incidence angle expands by 25%. For the distribution of the configuration parameters, parameters at the hub are significantly different from those near the tip, which confirms the tandem blade design in compressors is complex and needs to consider 3D flow effect. Forward swept and positive bowed design are beneficial for reducing loss at tip and alleviating the blockage, which cannot be achieved by the optimization only for configuration parameters. In terms of the weak coupling relationship between configuration parameters distribution and swept-bowed effects, the former should be considered first in practical engineering.

Influence of Gap Shape on Flow Characteristics of Highly Loaded Compressor Tandem Cascades

Abstract Gap shape is an important factor affecting the tandem cascade performance. This paper uses the gap expansion angle (Kb−b) to define gap shape and studies the effect of Kb−b on the two-dimensional flow field and flow characteristic near the endwall in a highly-loaded subsonic tandem cascade. By changing the incidence angle from −20 to 10°, the Kb−b value increases, and the two-dimensional tandem cascade performance improves at positive incidence angles, while it descends at zero and negative incidence angles. Due to the influence of Kb−b on the camber of front and rear blades (RB), the optimal Kb−b should provide appropriate load distribution and sufficient gap flow strength. The corner stall more likely appears on the front blade near the endwall, which can suppress the flow separation on the rear blade to a certain extent. The Kb−b value near the endwall should be slightly larger than in the midspan to ensure a sufficient operating range for the front blade. For a highly loaded subsonic tandem cascade, the overall optimal range of Kb−b is between −12 and 0°.

Complicated flow in tip flow field of a compressor tandem cascade using delayed detached eddy simulation

The complex flow characteristics in the tip region of a tandem cascade with tip clearance have been calculated and analyzed using Delayed Detached Eddy Simulation (DDES). The coherent mechanism of the vortex structures near the blade tip was discussed, and the unsteady behaviors and features in the tip flow field were analyzed. Additionally, the interaction between the tip leakage flow and the gap jet was revealed. The results show that, compared to the datum cascade, the blade tip load of the rear blade increases while that of the front blade decreases. Unsteady fluctuations of the tandem cascade are mainly caused by the interaction between the tip leakage flow and gap jet, and by the mixing of the vortex structures, but there is no essential change in the spectrum feature of the tip leakage flow. Finally, a detailed analysis of the development of vortices in the tip region is conducted by the topological structures of the flow field. Combined with the three-dimensional vortex structures, the schematic diagram of the vortex system of the datum single-row cascade and tandem cascade is summarized.

Effects of curved vanes on aerodynamic performance and flow structures in highly loaded tandem cascades

Combining tandem cascades and curved vanes can mitigate the separation in highly loaded compressor cascades. Flow loss mechanisms in curved tandem cascades are crucial for fundamental design and optimization. This study investigates the flow losses and vortex structures in curved tandem cascades with different curved angles and an original straight tandem cascade. Numerical simulations are conducted at variable incidence angles. The results indicate no consistent effect of curved angles on the aerodynamic performance of curved tandem cascades. The tandem cascade with a curved angle of 5° effectively reduces losses at all incidence angles, while the tandem cascade with a curved angle of 15° extends the critical incidence angle to 3°. The comparative analyses reveal that curved vanes increase spanwise pressure gradients, causing low-energy fluid near endwalls to move toward the midspan. The weighting coefficient of losses quantitatively describes that curved vanes weaken the trailing edge shedding vortices of both vanes and passage vortex while enhancing the end wall spanwise vortex of the front vane, thus affecting the loss distribution. Finally, this study innovatively applies topological analysis methods to curved tandem cascades, establishing the relationship between topological structures and vortex structures in corner regions of tandem cascades, which provides a valid research approach to reveal the vortex dynamics mechanisms of the loss distribution in tandem cascades influenced by curved vanes.

A Study of the Hydrodynamic Characteristics of Two-Dimensional Tandem Cascades

In comparison to single-row cascades, tandem cascades offer the advantages of reduced losses and enhanced operational capabilities, making them widely employed in compressor applications. However, current research on tandem cascades in hydraulic equipment remains relatively limited. In order to explore the potential application of two-dimensional tandem cascade structures in hydrodynamics and investigate their performance differences from single-row cascades, this study proposes a design scheme for a tandem cascade based on an existing single-row cascade design. Numerical simulation technology is utilized to compare and analyze the impact of these two designs on various flow losses under identical working conditions. The results indicate that compared to single-row vanes, the vane configuration of a serial-row design can better reduce losses and increase the pressure difference between the upper and lower surfaces of the vanes, thereby enhancing their load-bearing capacity and stability. This research finding is expected to provide valuable insights for future water pump design and optimization.

Design criteria of load split and chord length ratio for highly loaded compressor tandem cascades

To investigate the design strategy for load split (LS) and chord length ratio (CR) of highly loaded compressor tandem cascades, the parameterization study of the tandem cascades with different LS and CR was carried out. The parameterization results show that the LS is a crucial design parameter that can be used to regulate performance under off-design conditions. Specifically, lower LS enhances performance at positive incidence angles but comes at the cost of degraded performance at negative incidence angles. Moreover, relatively large CR (2–4) can improve performance under off-design conditions and also plays a role in achieving robust design, but it comes at the expense of optimal performance. To further explore the LS and CR effects on the flow field of the tandem cascades, three additional tandem cascades with LS = 0.5 and different CR were conducted. At large negative incidence angles (LS < 0.5), large CR (2.828) helps mitigate the accumulation of low-energy fluid on the pressure surface of the front blade and enhances the strength of the gap jet. Identically, at large positive incidence angles (LS > 0.5), the large CR mitigates the risk of corner stall and induces a transition in the stall mode of the front blade from corner stall to boundary layer separation over the entire span, further improving the tandem cascade performance. Based on the flow field analysis, the overall design strategy for LS and CR was summarized.

I2-Catalyzed three-component synthesis of 3-selenylated pyrazolo[1,5-a]pyrimidines

A one-pot strategy has been developed for the synthesis of 3-selenylated pyrazolo[1,5-a]pyrimidines from amino pyrazoles, chalcones/enaminones, and diselenides through I2-catalyzed tandem cascade annulations/C–H selenylation.

Recent advances in catalytic hydrogenation of CO2 into value-added chemicals over oxide–zeolite bifunctional catalysts

Thermo-catalytic CO2 hydrogenation with renewable energy has become one of the promising alternatives to fossil carbon-based routes for the production of bulk chemicals and has been extensively researched in recent years. And the heterogeneous oxide–zeolite (OX–ZEO) bifunctional catalysts have received much attention due to their high product selectivity. However, the need for multifunctional active sites, the complexity of tandem cascade reactions and the difficulty in understanding reaction mechanisms pose significant challenges for the design of effective catalysts. In this review, we present an overview of recent advances in the catalytic hydrogenation of CO2 to value-added products over OX–ZEO bifunctional catalysts, with particular emphasis on the role of the oxide. The nature of the metal oxide, the modification of oxides to improve catalytic performance in CO2 hydrogenation and the reaction mechanisms are discussed. Finally, the current challenges and future perspectives of the catalytic conversion of CO2 to hydrocarbons are briefly presented.

Study on flow separation and suction control of a compressor tandem cascade

To control the boundary layer separation on the front blade suction surface in a compressor tandem cascade at a high incidence angle, three suction schemes at different axial positions were introduced and the cascade performance improvement and separation flow control effects were investigated. Based on the experimental and numerical simulations, the aerodynamic performance of the prototype tandem cascade reaches the peak at the incidence angle of 5°. Continue to increase the incidence angle, the loss increases sharply, and the static pressure rise coefficient and diffusion factor begin to decrease. With the boundary layer suction on the suction surface of the front blade, fluid energy in the upstream boundary layer of the suction slot is increased, and the adverse pressure gradient downstream is decreased. By suction, the development and separation of the boundary layer are inhibited, and the loss in the tandem cascade is significantly reduced. The flow control effect of the suction scheme at the onset location of boundary layer separation is the best, and the flow loss can be reduced by about 34.9% with a suction flow rate of 1% at the incidence angle of 5°. The suction schemes have good flow control effects in a wide working range and broaden the available range of incidence angles of the tandem cascade. This study provides a reference for the design of tandem blades considering flow control and has a positive significance for improving compressor efficiency and pressure ratio.

Research on the Influence of Inflow Conditions on the Aerodynamic Performance of a Tandem Cascade

We perform a thorough numerical analysis of the impact of inflow conditions on the aerodynamic performance of a tandem cascade. In particular, we investigate the effects of the incidence angle and the inlet boundary layer (IBL) thickness on the three-dimensional flow field structure and aerodynamic performance. Our results show that the gap flow strength of the tandem cascade decreases with the increase of incidence angle, and it can effectively reduce the mixing of the wakes of the forward blade (FB) and rear blade (RB). In turn, this prevents the passage vortex (PV) in the RB passage from developing along the circumferential direction. The occurrence of IBL does not modify the effects of the incidence angle on the tandem cascade, however, it reduces the load of the RB and the gap flow strength near the endwall. Under all incidence angles, IBL increases the total pressure loss of the tandem cascade, and decreases the static pressure rise (except for an incidence angle equal to -6°). The maximum loss increment is at 2° incidence angle, and the maximum static pressure rise decrement is at 6° incidence angle (Thick-IBL condition) or 7° incidence angle (Thin-IBL condition). Furthermore, we found that the presence of IBL changes the minimum loss condition from 0° (design condition) to -2° incidence angle. Our results thus indicate that in the practical engineering application of the tandem cascade, the reality that IBL degrades the tandem cascade performance in the full incidence angle range should be considered. And the strong endwall secondary flow effect caused by IBL should be considered in the tandem cascade three-dimensional design, so that the tandem cascade two-dimensional performance advantage can be better played.

Numerical Investigation of Inlet Boundary Layer in an Axial Compressor Tandem Cascade

To explore the inlet boundary layer (IBL) influence on the tandem cascade aerodynamic performance, this paper took the high subsonic compressor NACA65 K48 cascade and its modified tandem cascade as the research object. The effects of the IBL thickness and the skewed IBL on the aerodynamic performance of the original cascade and tandem cascade were analyzed based on the numerical method. The results show that the tandem cascade effective design makes it better than the original cascade in the aerodynamic performance under different IBL conditions. Compared with the collateral IBL, the skewed IBL can effectively improve the aerodynamic performance of the original cascade and tandem cascade by suppressing the endwall cross flow, but an increase in the IBL thickness will suppress this advantage. In addition, the increase of incidence angle or the IBL thickness will make the tandem cascade forward blade corner separation more serious and cause the flow passage to be blocked, which seriously affects the rear blade diffusing capacity. In general, the IBL thickness is positively correlated with the tandem cascade total pressure loss and negatively correlated with the static pressure rise (except for the −6° incidence angle). The skewed IBL can effectively reduce the total pressure loss and increase the static pressure rise within −4°~7° incidence angle, but the law is opposite at a −6° incidence angle. At a 0° or 2° incidence angle, the performance improvement effect of the skewed IBL on the tandem cascade is the best, and this positive effect diminishes as it tends towards a larger positive incidence angle or a smaller negative incidence angle.

Intermittent cavitating flow field in tandem cascades of Kaplan turbines

Within the framework of the classical turbine design setup, we present a numerical investigation of unsteady cavitating flow in tandem cascades of guide vanes and rotor blades. The practical relevance of the present investigation is ensured by choosing real blade profiles and domain geometry for the large Kaplan turbines (9.5 m rotor diameter) from the Iron Gates I hydropower plant on the Danube River. The 2D tandem cascade setup is obtained with a conformal mapping of the circular guide vane cascade into a straight cascade, while the rotor blade cascade is by default a straight one for axial turbines. Both single-phase and two-phase cavitating flows results are presented and analysed. The single-phase liquid flow displays rather modest pressure and tangential force fluctuations, with the characteristic blade passing frequency, due to the interaction of the rotor blades with the wakes of guide vanes. The two-phase cavitating flow displays a completely different dynamics in comparison to the liquid flow, with periodic cavitation growth and collapse at a specific frequency lower than the blade-passing one. Moreover, the tangential force on the rotor blade displays severe intermittency and the force fluctuations are almost one order of magnitude larger than in the single-phase flow.

Zn(OTf)2-Promoted Isocyanide-Based Three-Component Reaction: Direct Access to 2-Oxazolines and β-Amino Amides.

An efficient one-pot reaction of propargylamides, isocyanides, and water catalyzed by zinc was developed for the rapid construction of 2-oxazolines with a wide functional group tolerance. The methylene-3-oxazoline was proven to play a vitally important role to start the tandem cascade transformation through unfunctionalized alkynes with sequential nucleophilic addition approaches of isocyanide and water. Notably, with a slight alteration of the reaction temperature and the addition of one molecule of water, various β-amino amide derivatives were synthesized in good to excellent yields.

Direct conversion of CO2 into aromatics over multifunctional heterogeneous catalysts

ABSTRACT The conversion of CO2 into fuels and value-added chemicals over heterogeneous catalysts may help realize a carbon-neutral or carbon-negative society. Recently, the direct conversion of CO2 to aromatics has been extensively explored. However, the requirement of multifunctional active sites, complexity of tandem cascade reactions, and difficulty in understanding reaction mechanisms present significant challenges to the design of effective heterogeneous catalysts. In this review, catalytic systems used for the direct production of aromatics from CO2 via the methanol-mediated and modified Fischer–Tropsch synthesis pathways are critically discussed. The initial section discusses the role of acid sites in HZSM-5 for the formation of aromatics. This is followed by a thorough review of recent progress on aromatic syntheses in terms of their catalytic performance, Si/Al ratios of zeolites, proximity between metal oxide and zeolite, and reaction conditions. The reaction mechanisms and the stability of existing catalysts are also discussed. Finally, the current challenges and future prospects of the catalytic conversion of CO2 to aromatics are highlighted.

Performance Enhancement of a Centrifugal Compressor by Designing a Tandem Cascade Diffuser

The performance of a vane diffuser plays an important role in transforming the high kinetic energy into pressure energy at the impeller outlet of a centrifugal compressor. In this study, the performance enhancement of a centrifugal compressor is achieved by designing a tandem cascade diffuser. An optimum value of the total bending angle of a tandem cascade diffuser is obtained through numerical simulation. The total bending angle is defined as the angle between the tangent line of the front blade arc line at the leading edge point and the tangent line of the rear blade arc line at the trailing edge point. The range of the total bending angle increases from zero to 21°. The simulation results show that the variation of the total bending angle has a great impact on the performances of the compressor stage. The best performance is achieved by the 10° model, by which the minimum total pressure loss coefficient and the maximum static pressure recovery coefficient are obtained. The mechanism of performance improvement by choosing a reasonable total bending angle is that the separated flow zone in the diffuser is constrained and the distribution of radial velocity at the inlet and outlet of the diffuser is more uniformed. With the 10° model, the separated zone almost completely disappears in the diffuser and a distribution of more uniform radial velocity along the meridional section is obtained. Compared with the stage of the prototype with a single row of vane diffuser, the stage with the newly designed tandem vane diffuser (10° model) achieved performance improvement, the efficiency increased about 6%, and the total pressure ratio increased about 3.5% at the flow coefficient of 0.15.