Vane Stagger Research Articles

Effects of endwall clearances of variable diffuser vane on centrifugal compressor performance

To ensure the variable diffuser vane can rotate around a pivot fluently in practical applications, there have to be clearances between vanes and diffuser walls. As the compressor pressure ratio increases, the effects of the vane endwall clearances are enhanced and cannot be ignored. This paper investigates the effects of both-side vane endwall clearances on the centrifugal compressor performance. Results show that the existence of endwall clearances mainly enlarges the flow capacity of the vane diffuser and decreases the compressor efficiency, and these effects are more significant as the vane stagger angle decreases. A one-side endwall clearance of 2.7% of the diffuser width increases the choke mass flow rate by as much as 20% and decreases the compressor peak efficiency by 2.7%. Due to the non-uniform flow along span at the impeller outlet, the shroud-side vane endwall leakage plays a more important role than the hub-side in affecting the compressor performance. In additions, the respective contributions of the vane throat, the hub-side and the shroud-side endwall clearances to the increases of the vane choke mass flow rate are quantitatively studied. The flow physics and loss mechanisms of the endwall leakage are revealed.

Inlet Guide Vane Stagger Effects on Multistage Compressor Performance and Stall Inception

The effects of variable inlet guide vanes on compressors have been evaluated throughout the turbomachinery industry for decades. These vanes improve both the performance and stability of a machine—the extent to which is highly dependent on the individual machine and the conditions of operation. Evaluating their effects on compressor stall inception via experimental investigations can inform model development and validation since commonly used RANS and URANS tools face limitations in accurately predicting and modeling stall events. The current literature on variable inlet guide vane restaggering, however, is dominated by numerical investigations. This study offers a unique experimental investigation into restaggering effects on compressor stall behavior by evaluating experimental results from different variable inlet guide vane stagger angles in a high-pressure axial compressor. Previous investigations have shown that variable inlet guide vane restaggering may change the stall inception mechanism of axial flow compressors. This investigation found a distinct change in the stage matching of the machine without altering the stall inception mechanism, further demonstrating the need for experimental investigations to inform the specific effects that restaggering inlet guide vanes offers comparable compressors.

Performance analysis of a novel small-scale radial turbine with adjustable nozzle for ocean thermal energy conversion

Ocean thermal energy is acknowledged as one of the most promising ocean renewable energy sources in low latitude sea areas. In the ocean thermal energy conversion system, the turbine plays a significant role, and it is responsible for converting the working medium enthalpy into the shaft output power. The present study is focused on the performance analysis of a novel radial inflow turbine with an adjustable nozzle in the OTEC system in order to adapt to the changing operating conditions of the turbine, which vary with the change in seawater temperature. At the design point, the predicted overall isentropic efficiency is 86.5%, and the shaft output power is 15.3 kW, slightly higher than the expected 15 kW. Furthermore, a parametric study is performed, respectively, for the nozzle vane stagger angle and the nozzle-impeller radial clearance to explore the favorable geometric parameters for different conditions. The turbine’s overall efficiency increases slightly with deceasing nozzle-impeller radial clearance, and the variation of the nozzle vane stagger angle is much more influential on the turbine shaft power and overall efficiency. The optimum stagger angle point moves from 32° to 36° gradually with the increase in nozzle-impeller clearance. Finally, the feasibility of an adjustable nozzle for the turbine under off-design conditions was verified by combining the radial clearance and nozzle stagger angle.

Unsteady Analysis of a Pulsating Alternate Flow Pattern in a Radial Vaned Diffuser

In centrifugal compressors, Mild Surge (MS) leads to unstable operation. Previous experimental work on a centrifugal compressor designed and built by Safran Helicopter Engines (SafranHE) showed that MS corresponds to the pulsation of an alternate stall pattern at the Helmholtz frequency of the test rig on two channels in the radial diffuser. The present contribution experimentally investigates the impact of the Inlet Guide Vane (IGV) stagger angle on this alternate flow and numerically studies the topology of this pulsating alternate flow. The experimental investigation is performed with unsteady pressure sensors, and shows that the IGV stagger angle only impacts the pulsation frequency of the alternate flow pattern. This change is explained by the dependence of the Helmholtz frequency on the compressor inlet section. The topological analysis of the average flow field, computed from wall-resolved Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations, demonstrates that the saddle point (major critical point) in the corner hub/suction side of the stalled blade migrates upstream while staying in the corner if the mass flow rate decreases. One main blade over two is stalled on both sides because the flow originating from this corner separation circumvents the trailing edge and migrates upstream along the pressure side. In the simulation, the pulsation of the alternate stall is coupled with the reflection of acoustic waves on the inlet and outlet planes, regarded as an environmental effect.

Forced Response Due to Vane Stagger Angle Variation in an Axial Compressor

Abstract Recent developments in turbomachinery design require an improved prediction accuracy of blade vibrations to maintain safe operations. This article aims to investigate the accuracy of numerical aeroelastic approaches for the calculation of blade vibrations. For validation, extensive aerodynamic and forced response measurements in an 1.5-stage axial compressor with a blade integrated disk (Blisk) are presented. The excitation intensity of the vibration is controlled by varying the stagger angle of the inlet guide vane (IGV). In addition, a second engine order is imposed by a nonsymmetric circumferential vane angle distribution to simulate a multistage behavior. Experimental validated Reynolds-averaged Navier–Stokes (RANS) simulations in both the frequency and the time domain are compared to assess the prediction accuracy of the numerical approaches. The numerical results agree with the experiments for low and intermediate vane angles. However, at high IGV stagger angles and when exciting multiple engine orders, the inaccuracy in the prediction of flow separation by the RANS simulations leads to an overprediction of vibration amplitudes. This exaggeration becomes even more pronounced in the frequency domain simulations. Time domain methods with a time lag formulation tend to be efficient and more accurate approaches for large separated flow regimes.

Experimental study on thermoacoustic instability and emission characteristics of premixed swirl flame in a longitudinal combustor

To investigate thermoacoustic instability and emission characteristics in premixed swirl combustor, several low-emission swirlers with different vane stagger angles were designed. At normal temperatures and pressures, the flow field, flame shape, thermoacoustic instability and emissions as well as their relationships were investigated. The flame shapes and swirling flow dynamics were characterized by chemiluminescence (CL) imaging and particle image velocimetry (PIV), the pressure oscillations and emissions were obtained using microphone and flue gas analyzer. The results indicate that the position and size of recirculation zone varies abruptly with the swirl number. When combustion equivalence ratio is 0.8–1.5, three flame modes, including attached flame, W-shape and V-shape detached flame, arise above the combustor as equivalence ratio is reduced. The V-shape detached flame shows the weakest sound pressure, which suggests the best stability and lowest emissions. Additionally, emissions are also affected by swirling intensity. In general, the inside stage of swirler with higher swirling intensity generates lower NOx emission. For the two-stage swirler C3, the maximum CO and NOx emissions are about 2 ppm and 40 ppm respectively. In the same flame conditions, the three-stage swirler C5 has a lower NOx emission, less than 8 ppm.

Investigation of Unsteady Aerodynamic Excitation on Rotor Blade of Variable Geometry Turbine

To investigate the aerodynamic excitations in variable geometry turbines, the full three-dimensional viscous unsteady numerical simulations were performed by solving N-S equations based on SAS SST method. The aerodynamic excitations at varied expansion ratios with six different vane stagger angles that cause the unsteady pressure fluctuation on the rotor blade surface are phenomenologically identified and quantitatively analyzed. The blade pressure fluctuation levels for turbines with different vane stagger angles in the time and frequency domain are analyzed. As the results suggest, the blade excitation mechanisms are directly dependent on the operating conditions of the stage in terms of vane exit Mach numbers for all test cases. At subsonic vane exit Mach numbers the blade pressure fluctuations are simply related to the potential filed and wake propagation; at transonic conditions, the vane trailing edge shock causes additional disturbance and is the dominating excitation source on the rotor blade, and the pressure fluctuation level is three times of the subsonic conditions. The pressure fluctuation energy at subsonic condition concentrates on the first vane passing period; pressure fluctuation energy at higher harmonics is more prominent at transonic conditions. The variation of the aerodynamic excitations on the rotor blade at different vane stagger angles is caused by the varied expansion with stator and rotor passage. The aerodynamic excitation behaviors on the rotor blade surface for the VGT are significantly different at varied vane stagger angle. Spanwise variation of the pressure fluctuation patterns on is also observed, and the mechanism of the excitations at different spans is not uniform.

Numerical Investigation of Low Solidity Vaned Diffuser Performance in a High-Pressure Centrifugal Compressor—Part III: Tandem Vanes

As Part III, following the authors’ previous studies, the aerodynamic performance of two different tandem LSDs (low solidity diffusers), Tandem (A) and (B), in a high-pressure centrifugal compressor was numerically investigated over flow rates from impeller choke to minimal flows available in computation. Tandem (A) was of conventional design where the first row came directly from the authors’ previous studies (Part I Oh and Agrawal, 2007, “Numerical Investigation of Low Solidity Vaned Diffuser Performance in a High-Pressure Centrifugal Compressor - Part I : Influence of Vane Solidity,” ASME Paper No. GT2007-27260, and Part II: Oh et al., 2008, “Numerical Investigation of Low Solidity Vaned Diffuser Performance in a High-Pressure Centrifugal Compressor - Part II : Influence of Vane Stagger,” ASME Paper No. GT2008-50178) selected as the highest efficiency vane at design flow, and the second row was designed to be added considering flow conditions at the exit of the first row vane. Tandem (B) followed a creative patent-pending concept where the number of the first row vanes was doubled with much smaller vane chord keeping a low solidity. A position parameter of RCP (relative circumferential position) was introduced to see the effect of the relative location of the second row vane. Using an in-house Navier-Stokes solver with finite volume time marching methods, overall performance was predicted to be compared with each other. Detailed investigation on the behavior of the static pressure recovery and the total pressure loss coefficient in both diffuser designs helps determine why Tandem (A) design is better and the case of RCP = 0.3 gives the best performance.

Impact of stagger angle nonuniformity on turbine aerodynamic performance

The assembling error may lead to variation in stagger angles, which would affect the aerodynamic performance of the turbine. To investigate this underlying effect, two parallel numerical experiments on two turbines with the same profile, but uniform and nonuniform vane stagger angle respectively, were conducted in both steady and unsteady methods. The results indicate that certain changes in the detailed flow field of the turbine occur when the stagger angles are nonuniform, further, the blade loading distribution of the vane and rotor become markedly different from that in uniform vane stagger angle situation. Then these consequences caused by nonuniformity mentioned above enhance the unsteadiness of the flow, finally, the aerodynamic performance changes dramatically. It also shows that, compared with steady simulation, the unsteady numerical simulation is necessary in this investigation.

Active Flow Separation Control of a Stator Vane Using Embedded Injection in a Multistage Compressor Experiment

Active flow control has been applied to the suction surface of stator vanes in a low speed axial compressor. Injection from the suction surface has been shown to reduce separation on vanes that were induced to separate by increasing the vane stagger angle by 3° deg. Various configurations were investigated including injector geometry (slots versus holes) and type of injection (steady versus unsteady). Unsteady injection was realized using two different approaches; external actuation through a high frequency valve and embedded actuation using a fluidic device internal to the vane. Using total pressure loss through the vane passage as a metric, reductions in area-averaged loss of 25% were achieved using injected mass flow rates on the order of 1% of compressor throughflow. The development of a tracking control algorithm was also explored for the purpose of closed-loop control. A reliable method of detecting surface separation was implemented using unsteady pressure measurements on the compressor casing near the vane suction surface.

Axial compressor performance modelling with a quasi-one-dimensional approach

This paper describes a technique for the numerical modelling of axial compressors using the simplest form of representation—a mean-line one-dimensional method. Although this type of flow analysis is a gross simplification of a complex three-dimensional system, which can now be modelled more accurately by many of today's computational fluid dynamics (CFD) packages, it does offer the advantages of simple input requirements and fast convergence times. These factors were major considerations when choosing a prediction method to be employed within an optimization program developed at Cranfield for the optimization of stator vane stagger angles, where detailed stage data were not required. The objective was to develop a technique capable of simply modelling the flow in multistage compressors to provide performance data including overall pressure ratio, efficiency and flow range at various speeds, as well as providing interblade data so that restaggering of blades could be accounted for. The salient issues presented here deal firstly with the construction of the compressor model and its incorporation into a FORTRAN program. A discussion of the results then follows, where four very different compressors have been modelled. Finally, the stability limit (or surge point) prediction methods are reviewed. Results have shown that consistent agreement can be obtained with such a performance prediction model, providing the loss and deviation correlations used are applicable to the blade profiles. It was also apparent that the two simplistic methods of surge prediction, which relied upon either stage or overall characteristic gradients, gave better agreement than the more rigorous discrete stage-to-stage model. This was thought to be a result of the continuous nature of the empirical rules utilized, which predict a smooth stage characteristic and hence falsely delay the estimated instability point.

The Use of Circumferentially Varying Stagger Guide Vanes in an Axial Flow Pump or Compressor

An actuator disk analysis is given of the flow through a guide vane and rotor combination. It is shown that changes in total pressure across the rotor are in general related to circumferential variations in guide vane outlet angle. In particular known variations in inlet total pressure may be eliminated by suitable circumferential changes in guide vane stagger.

Vane Stagger Angle and Camber Effects in Fan Noise Generation

The problem of sound generated by the interaction of velocity disturbances with stator vanes in an annular duct is considered theoretically. The duct carries a uniform subsonic main flow and is assumed to be anechoic. In this problem it seems consistent to model the vanes by flat plates parallel to the duct axis. However, this modeling may yield an unrealistically low acoustic power. In the present paper, a nonplanar lifting surface approximation of the vanes shows that at frequencies prevailing in current turbofans even a small vane inclination significantly affects the sound generation process. The lower the circumferential periodicity \m of the cut-on sound field, the more pronounced are the acoustic effects of vane stagger angle and camber. Calculations for a typical example of a turbof an stator show at \m = 1 a rise of 23 dB in upstream acoustic power when the stagger angle is increased from 0 to 15 deg. Furthermore, even at \m - 16 there is still a rise of 5 dB. Besides the acoustic power, the modal distribution of the sound field is very sensitive to vane camber and stagger angle.