Secondary Flow Losses Research Articles

Investigation of the effects of main geometric parameters and flow characteristics on secondary flow losses in a turbine cascade

This paper presents numerical simulation results of a three-dimensional (3D) transitional flow in a stator cascade of an axial turbine. The influences of the main geometric parameters and flow characteristics including, the blade aspect ratio, pitch-to-chord ratio, inlet flow angle, and exit Mach number, on secondary flows development and end-wall losses, were studied. The numerical results were validated by the results of experiments conducted in the laboratory of the steam and gas turbine faculty of the Moscow Power Engineering Institute. The maximum difference between computed and experimental results was 2.4 %. The total energy losses decrease by 20 % when the exit Mach number changes from 0.38 to 0.8. Numerical results indicated that the blade aspect ratio had the most effect on secondary flow losses. The total energy losses increase by 46.6 % when the aspect ratio decreases from 1 to 0.25. The total loss of energy by 13.2 % decreases by increasing the inlet flow angle from 60 degrees to 90 degrees. Then by increasing the inlet flow angle from 90 to 110 degrees, the total loss rises by 3.6%. As the pitch-to-chord ratio increases from 0.7 to 0.75, the total energy losses are reduced by 12.2 %. Then by increasing the pitch-to-chord ratio from 0.75 to 0.8, the total energy losses increase by 6 %. As with experimental data, the numerical results showed that the optimal inlet flow angle and relative pitch for the cascade are 90 degrees and 0.75, respectively.

Loss Characterization of Advanced VIGV Configurations With Adjustable Blade Geometry

Abstract Variable inlet guide vanes (VIGV) are the main control element to adjust the flow rate of industrial centrifugal compressors by customized pre-swirl in the inlet plane of the impeller. The efficient working range of VIGVs is however restricted due to open flow separation occurring at critical stagger angles. In order to overcome the narrow limitations of current blade geometries and to enhance the operating range of the compressor, split blades consisting of a separate front and tail blade segment proved to be particularly promising in previous linear-cascade measurements. Each blade segment is thereby individually staggered. This enables a gradual flow deflection along the chord length. Secondary flow losses, however, were not considered in the previous investigations with linear cascades. To highlight the potential of the split blade concept under more application-oriented conditions including all relevant flow effects, highly resolved field measurements were conducted in the wake of annular VIGVs. Four different blade configurations, a customary reference case and three variations of the split blade with full, partial and missing sealing in the gap between the segments were assessed using five-hole probe measurements. By investigating a wide range of stagger angles, the coverage of the full low-loss working range of the VIGV could be ensured. Especially, the fully sealed split blade configuration proved its capacity to extend the efficient operational range significantly.

Loss Characterization of a Conventional Variable Inlet Guide Vane

Variable inlet guide vanes (VIGVs) are most commonly used as the major control unit of integrally geared centrifugal compressors (IGCCs). In order to enhance the efficient operating range of the compressor, the loss mechanisms and utilization limits of state-of-the-art VIGVs need to be better understood. Field measurements in the wake of a typical, commercially used configuration were therefore conducted at the VIGV test facility of the Bundeswehr University Munich. The investigations were carried out at application oriented subsonic flow conditions and stagger angles from 50∘ to 90∘ covering the full low-loss operating range, including the limits of efficient operation. For a precise local loss characterization, an inflow correlation was developed and applied to consider total pressure inhomogeneities caused by the radial inflow velocity profile and minor circumferential velocity deviations. Contrary to previous research efforts, not only the profile losses, but also the secondary flow losses induced by the open blade tips and wall-blade interactions were resolved in full detail. For this reason, a more precise and comprehensive loss assessment of realistic VIGV cascades is acquired.

Investigation of the flowfield on a vaneless counter-rotating turbine with seal cavity flow ejection

The sealing of the rotor-rotor gap and rotor disk cooling are vital to the safe operation of the vaneless counter-rotating turbine(VCRT). In order to quantifies the influence of the wheel-space cavity flow on the VCRT aerodynamic performance, and to improve turbine efficiency of the VCRT at certain rim seal ejection rates, numerical studies which considered the effects of rotor-rotor rim seal flow ejection are carried out in this paper. The three dimensional unsteady computational fluid dynamic analysis of a VCRT at the engine conditions are performed, and the seal flow ejected downstream of the high pressure rotor row at six sealing flow rate are modeled. The interaction among the high pressure rotor trailing shock wave, the downstream secondary flow and the seal flow has been studied and quantitatively characterized as a function of the purge ejection rate. Numerical results show that seal flow- mainstream flow interaction is entirely dominated by the high pressure rotor trailing edge shock at the hub, low pressure hub passage vortex and the mixing of the sealing flow from wheelspace and mainstream. When the mass flow rate of the coolant is smaller than some threshold value, the shock loss of the high pressure rotor and hub secondary flow loss of the low pressure rotor are decreased with the increasing of the coolant mass flow rate. It causes that the VCRT efficiency is gradually increased. On condition that the amount of the seal flows is beyond the threshold value, the key roles in modification of the VCRT performance are changed. The increment of the hub secondary flow loss and the mixing loss are gradually larger than the decrement of the shock loss. As a result, the turbine efficiency gradually decreases.

A Study on the Secondary Flow Loss Reduction in a Gas Turbine Cascade by Combining Optimized Fence and Inclined Blades

A Study on the Secondary Flow Loss Reduction in a Gas Turbine Cascade by Combining Optimized Fence and Inclined Blades

Design of a rear-stage subsonic axial compressor with casing treatments

The aerodynamic design of a 3.5-stage high-pressure axial compressor, representing a modern rear stage design with low aspect ratio blades and large relative tip gaps is presented. The subsonic rotor and stator airfoils feature a controlled-diffusion (CDA) type of sectional design, obtained from 2D viscous-inviscid simulations using the MISES flow solver. Improvements of the main gas path, as well as the 3D blade design, are based on 3D steady Reynolds-averaged Navier–Stokes simulations (ANSYS CFX). Sweep and dihedral were applied to improve the design and off-design performance of the compressor. The matured set of blading, with smooth casing, still exhibits large rotor tip vortices and double leakage, limiting the compressors operability towards stall point. Consequently, the application and development of axial slot casing treatments (CT) into the smooth casing is performed, aimed at improving local stage and overall stall margin with particular emphasis on avoiding efficiency penalties within stages. In order to investigate and optimise the geometric properties of the CTs with respect to efficiency a DoE (Design of Experiments) is executed. Based on the DoE a CT-geometry is deduced and applied to the critical rotor. Due to the fact, that the stage operability with casing treatment is limited by the downstream stator, a first adaption of the existing blade design is considered. In a first step, the downstream stator’s incidence is adjusted, resulting in a slightly increased stall margin. The second step incorporates the modification of rotor blade count, following the idea of trading rotor operability for an efficiency increase, due to reduced friction. However, no notable efficiency benefit was achieved, as the reduced friction loss is counterbalanced by increased secondary flow loss resulting from higher blade loadings.

Reducing rotor secondary losses in a turbine stage through endwall airfoil profiling

The rotor blade of an experiment turbine for low Reynolds number flow conditions is recontoured on the hub endwall to reduce the hub’s secondary flow losses. The baseline rotor blade is radially stacked with airfoils of free vortex twisting. The hub section airfoil of the rotor blade is reprofiled for three considerations. The first is to reduce the inlet metal angle to fit with the vane’s exit flow angle near the hub endwall. The second is to reduce the airfoil’s stagger angle to reduce the pitchwise pressure gradient on the hub endwall. The third is to extend the airfoil’s axial chord to a limited distance before the baseline one to build a leading edge fillet. A steady RANS flow computation is carried out to compare the flow fields inside the baseline and the new contoured rotor passages. Flow comparison reveals that the passage vortex intensity is reduced and that the total pressure loss is reduced by 8.9 % in the hub region. The new contoured rotor shows potential application in reducing rotor secondary losses through the stacking of two-dimensional sectional airfoils.

Advances in axial turbine blade profile aerodynamics

The aerodynamic performance of axial turbines depends significantly on profile losses, secondary flow losses, and clearance gap losses of vanes and blades. In modern high-efficiency turbomachinery operating at various working conditions, profile losses are very important criteria for the development of vanes and blades, and turbine designers strive to minimize the losses, based on better understandings of flow and loss characteristics at various working conditions. This paper summarizes recent advances in the field of turbine blade profile aerodynamics, and covers: (1) flow and loss characteristics of blade profiles, (2) flow structure and loss mechanism for transonic blade profiles, (3) off-design performance, (4) flow control, (5) design and optimization, (6) engineering design considerations, and (7) research methods of blade profile aerodynamics. The emphasis is placed on flow characteristics and loss control methods, and present insights regarding the current research trends and the prospects for future developments.

Characteristics of pressure drop, mass flow distribution and flow asymmetry in three‐dimensional branching networks based on model human bronchial tree

AbstractThe complex flow field in the three‐dimensional internal passages of a model human bronchial tree is studied computationally. Up to five generations of branches, both in‐plane and out‐of‐plane configurations, two velocity profiles and varying inlet Reynolds number are considered. There exists substantial non‐uniformity of the fluid dynamic features (e.g. velocity field, pressure field, mass flow distribution or viscous dissipation) over any longitudinal plane or cross‐sectional plane of a particular branch as well as that between different branches of any generation. The flow asymmetry is explained here by attributing the fluid dynamics to three important features ‐ curvature of flow path, flow division at bifurcations and inertia of the flow. A synthesis of the present comprehensive study leads to the generalization that the out‐of‐plane configuration fosters greater degree of uniformity in the mass flow distributions in the various branches of any generation, but gives rise to greater flow non‐uniformity on any particular cross‐sectional plane as compared to the in‐plane configuration. The out‐of‐plane configuration also causes greater secondary flow and greater loss in total pressure in the network. The effectiveness of previously developed modular computational approaches is assessed through separate simulations for networks comprising two, three, four and five generations (containing 3, 7, 15 and 31 branches respectively). The inadequacy of Hagen‐Poiseuille type 1D model is demonstrated ‐ it can neither predict the flow asymmetry (captured by the 3D computations) nor determine the overall magnitude of loss in total pressure.

Optimization of an axial turbine for a small scale ORC waste heat recovery system

Low power Organic Rankine Cycle (ORC) turbines are typically characterised by flow efficiencies much smaller than those of large power steam or gas turbines. The paper concentrates on a possibility of improving the flow efficiency of an ORC axial turbine working on toluene using several optimization algorithms such as a simplex method of Nelder-Mead, a hybrid of genetic algorithm with the Nelder-Mead method, as well as implicit filtering. Values of the maximized objective function, which is the isentropic efficiency of the turbine stage, are found from 3D RANS computation of the flowpath geometry changing during the optimization process. Among the optimized geometric parameters are stator and rotor profile parametrization variables, rotor blade twist angle, circumferential lean and axial sweep angles, as well as parameters characterising the shape of endwall contours within the stator and rotor domain. The process of optimization leads to new 3D designs featuring increased efficiency compared to the original design, mainly due to a reduction in secondary and tip leakage flow losses as well as boundary layer, separation and leaving energy losses. The results especially highlight a large potential of the method of implicit filtering in optimizing multi-parameter objective functions. • A 45 kW turbine prepared for a small ORC CHP is shown. • Deterministic and hybrid methods of optimization are applied. • A wide range of variation of ORC turbine flowpath are investigated. • Performance maps for the best results are shown. • Turbine mechanical qualification is performed.

Experimental Investigation of the Effect of Purge Flow and Main Flow Interaction in a Low-Speed Turbine Cascade Passage.

In order to protect the vulnerable turbine components from extreme high temperature, coolant flow is introduced from the compressor to the disk cavity, inevitably interacting with the main flow. This paper describes an experimental investigation of the interaction between the main flow and the purge flow in a low-speed turbine cascade with three purge flow rates, Cm = 0, Cm = 1%, and Cm = 2%. In order to study the effect of the interaction between the main flow and the purge flow on the secondary flows, a Rortex method developed by Liu Chaoquan is introduced to identify the vortex in the flow field. In the meantime, a method to calculate the mean entropy production rate based on the particle image velocimetry (PIV) result is adopted to investigate the flow loss. The PIV result indicates that the purge flow has a prominent impact on the flow field of the cascade passage, changing the velocity distribution that induces a local blockage area. The results of vortex identification show that the purge flow promotes the generation of the passage vortex near the suction side. In addition, the purge flow makes the passage vortex migrate to the tip wall direction, enlarging the region affected by the secondary flow. The mean entropy production (MEP) result shows that the flow loss is mainly caused by the passage vortex. The coincidence of the high-MEP region and the location of the passage vortex indicates that the purge flow increases the secondary flow loss by affecting the formation and the migration of the passage vortex.

Design and Analysis of Energy-Efficient Low-Flow Centrifugal Compressors

Abstract With the increasing demands of energy-saving from industries, the low-flow coefficient and low specific speed centrifugal compressors have gained more attention. The design of this type of compressor faced many challenges, for example, high secondary flow losses, high tip leakage losses, and low exit width based on a Reynolds number. The design also lacks a reliable database for preliminary studies. The impeller design studies were limited. Most designs for low-flow coefficient and low specific speed compressor follow the traditional methods. This paper presents design studies and discusses some unique design features to improve performance of this type of compressor. The detail computational fluids dynamics (CFD) results are presented to demonstrate the success of the design strategies. A prototype compressor for fuel cell applications was built, and performance tests were performed. The test results are compared with those of the computational analysis, and the agreement is reasonably satisfactory. The compressor meets the customer's performance goals. The design features can be used for future low-flow coefficient and low specific speed centrifugal compressor design.

Effect of Camber on Non-axisymmetric End-wall Contouring Performance in Compressor Linear Cascades

In order to explore non-axisymmetric end-wall’s mechanism of secondary flow control in compressor cascade, based on three-dimensional numerical calculation, this paper analysed effects of camber on non-axisymmetric end-wall contouring performance in compressor linear cascades. Results showed that non-axisymmetric end-wall reduces the corner separation loss mainly by suppressing the interaction between the crossflow and the suction surface boundary layer and decreasing the adverse pressure gradient on suction surface. Meanwhile, non-axisymmetric end-wall partly enhances the cross flow, increasing the secondary flow loss. At the incidence of 0°, the increase of the secondary flow loss caused by the non-axisymmetric end-wall plays the leading role when the camber angle is 20°; with the increase of camber angle, the reduction of corner separation loss becomes the leading role. At the incidence of 7°, the hub-corner stall is removed by the non-axisymmetric end-wall when the camber angle increases to 40°, decreasing the loss remarkably.

The Design of Curved Endwall with Leading-edge Deformation

The inlet boundary layer separates in front of the leading edge of the blade on the endwall and forms the pressure side leg of horseshoe vortex and the suction side one. The pressure side leg of the horseshoe vortex immediately moves toward to the suction side and form a stronger vortex, ”passage vortex”, in the cascade. These vortices mentioned above are called secondary flows which will result in an increase of secondary flow losses and a reduction of stage efficiency. In this paper, the flow characteristics are analyzed in the leading edge region and inside the cascade based on the numerical simulation results of the Langston cascade. A new type endwall design method, curved endwall structure combined with the deformation in the leading edge region, is established and optimized. It can be observed that the new structure can efficiently reduce the strength of the horseshoe vortex and suppress the generation of the leading edge separation line and saddle point. The uses of the new structure also decrease the pressure gradient between the pressure side and the suction side in the streamline direction, which suppresses the deviation of the pressure side horseshoe vortex from the pressure side of the endwall to the suction side and delays the formation position of the passage vortex. The rate of increase in the total pressure loss coefficient along the mainstream direction also decreases 25.34% in the exit of the cascade.

Unsteady effects of periodic wake passing frequency on aerodynamic performance of ultra-high-lift low pressure turbine cascades

Flow losses and flow field data downstream of an ultra-high-lift aft-loaded low pressure turbine cascade were comprehensively measured for different incoming wake passing frequencies, and the mechanisms through which incoming wakes influence secondary flow were examined via numerical calculations. Reynolds numbers ranging from 25 000–100 000 (based on the axial chord and inlet velocity) were considered in both the presence and absence of the wakes. At low Reynolds numbers of 25 000 and 50 000, increasing wake passing frequency gradually suppressed the suction surface separation bubble and increased the cross-passage pressure gradient, and the unsteady wakes clearly improved the throughflow characteristics of the cascade passage. Furthermore, the larger separation bubble on the suction surface hindered the migration of the secondary vortices from the endwall to span. Consequently, incoming wakes were not beneficial for suppressing secondary flow at low Reynolds numbers. At the high Reynolds numbers of 80 000 and 100 000, increasing wake passing frequency afforded stronger inhibition of the secondary flow owing to the reduction in blade loading originating from the “negative jet” influence of the wakes. Transport of incoming wakes in the cascade passage caused the downstream migration of the position of the saddle point, which is also advantageous for decreasing secondary flow losses.

Study of combined flow control strategies based on a quantitative analysis in a high-load compressor cascade

Micro-vortex generator and a jet flow control technique are combined to improve the flow performance of a high-load compressor cascade, and the injection position is further investigated. The calculated results showed that the reverse flow region of baseline grows with the increment of incidence, and an unstable flow phenomenon rapidly occurs near the leading edge when the incidence rises to +7.9°. Meanwhile, two critical points occur on the end-wall, that is considered to be the main reason for occurring stall. When an injection tube is applied to remove the low energy fluid, the total pressure loss appears drastic reduction, and the occurrence of stall shows a slight delay from +7.9° to +8.7° incidence compared with the baseline. With the micro-vortex generator is introduced in the baseline, the occurrence of stall is delayed from +7.9° to +10.0° incidence due to the mixture of low energy fluid and main flow. The combined application shows an excellent effect in delaying the occurrence of stall and reducing the total pressure loss, and the sudden deterioration of the cascade performance occurs at +9.8° incidence. A quantitative analysis of total pressure loss shows that the secondary flow loss and wake loss contribute to the reduction of total pressure loss, the former is reduced by 31.0% and the latter is decreased by 24.1% at the near stall condition. Moreover, the spanwise injection position can affect both the total pressure loss and stall incidence, and the wider operating range is accompanied by the increment of total pressure loss.

Reducing Secondary Flow Losses in Low-Pressure Turbines: The “Snaked” Blade

This paper presents an innovative design for reducing the impact of secondary flows on the aerodynamics of low-pressure turbine (LPT) stages. Starting from a state-of-the-art LPT stage, a local reshaping of the stator blade was introduced in the end-wall region in order to oppose the flow turning deviation. This resulted in an optimal stator shape, able to provide a more uniform exit flow angle. The detailed comparison between the baseline stator and the redesigned one allowed for pointing out that the rotor row performance increased thanks to the more uniform inlet flow, while the stator losses were not significantly affected. Moreover, it was possible to derive some design rules and to devise a general blade shape, named ‘snaked’, able to ensure such results. This generalization translated in an effective parametric description of the ‘snaked’ shape, in which few parameters are sufficient to describe the optimal shape modification starting from a conventional design. The “snaked” blade concept and its design have been patented by Avio Aero. The stator redesign was then applied to a whole LPT module in order to evaluate the potential benefit of the ‘snaked’ design on the overall turbine performance. Finally, the design was validated by means of an experimental campaign concerning the stator blade. The spanwise distributions of the flow angle and pressure loss coefficient at the stator exit proved the effectiveness of the redesign in providing a more uniform flow to the successive row, while preserving the original stator losses.

Aerodynamic design and optimization of a high-loaded axial fan stage using a curvature control method

This study designs and optimizes a high-loaded transonic fan stage using a curvature control method. A fan stage with a total pressure ratio of 2.49 and adiabatic efficiency of over 87 % at the design point is obtained after 3D optimizations. The geometric curvature of the blade passage has important driving and adjusting effects on the mainstream and on the flow in the boundary layer. The reasonable optimization of camber line curvature can weaken the radial secondary flow on the blade surface and decrease shock losses effectively. It can also postpone or suppress boundary layer separation by reorganizing aerodynamic load. Hence, secondary flow losses are reduced, and the overall stall margin is expanded. This method features high precision, high operability, and short design cycle, hence providing highloaded fans/compressors with new freedom to improve design ability and offering new possibility to design system development.

Evaluation of an algebraic model for laminar-to-turbulent transition on secondary flow loss in a low-pressure turbine cascade with an endwall

The predictive qualities of a recently developed algebraic intermittency model for laminar-to-turbulent transition are analysed for the flow through a linear cascade of low-pressure turbine blades with an endwall. Both steady RANS (Reynolds-averaged Navier–Stokes) and time-accurate RANS (URANS) simulations are performed. The results are compared with reference LES (Large Eddy Simulation) by Cui et al. (2017, Numerical investigation of secondary flows in a high-lift low pressure turbine, Int. J. of Heat and Fluid Flow, vol. 63) and results by the local correlation-based intermittency transport model (LCTM) by Menter et al. (2015, A one-equation local correlation-based transition model. Flow Turbul. Combust., vol. 95) for laminar and turbulent endwall boundary layers at the cascade entrance. Good agreement is obtained with the reference LES and with results by the LCTM for the evolution through the cascade of the mass-averaged total pressure loss coefficient and for profiles of pitchwise-averaged total pressure loss coefficient at the cascade exit.

Effects of endwall profiling near the blade leading edge on the sealing effectiveness of turbine rim seal

Endwall profiling designed to reduce secondary flow loss may change the local pressure distribution which has an impact on the sealing effectiveness of a rim seal. This paper presents a numerical comparison of the sealing effectiveness of the rim seal and the aerodynamic performance of the blade with five different endwall profiling near the blade leading edge. Three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) equations coupled with a fully developed shear stress transport (SST) turbulent model are utilized to investigate the sealing effectiveness and the flow characteristics of turbine rim seal. The numerical method for the pressure field and sealing performance of turbine rim seal is validated on the basis of published experimental data. The total-to-static efficiency of the blade and the minimum sealing rates of the rim seal with five endwall profiling near blade leading edge are compared. The baseline, convex and concave cases are selected to investigate the transient variation of the sealing effectiveness and the flow field in the disc cavity. In comparison with baseline case, the convex endwall makes the high pressure area move forward, increases the mainstream circumferential pressure fluctuation, and reduces the sealing effectiveness. The concave endwall reduces the local pressure and the mainstream circumferential pressure fluctuation, and increases the sealing effectiveness. However, the concave endwall profiling enhances the vortex in the blade passage and increases the secondary flow loss. The flow field near the rim seal with different endwall profiling is illustrated and analyzed.