EFFECT OF AN AXIALLY TILTED VARIABLE STATOR VANE PLATFORM ON PENNY CAVITY AND MAIN FLOW

This paper presents the impact of an axially tilted variable stator vane platform on penny cavity flow and passage flow, with the aid of both optical and pneumatic measurements in an annular cascade wind tunnel as well as steady CFD analyses. Variable stator vanes in axial compressors require a clearance from the endwalls. This means that penny cavities around the vane platform are inevitable. Production and assembly deviations can result in a vane platform which is tilted about the circumferential axis.. Penny cavity and main flow in geometries with and without platform tilting were compared in an annular cascade wind tunnel. Detailed particle image velocimetry measurements were conducted inside the penny cavity and in the vane passage. Steady pressure and velocity data was obtained by two-dimensional multi-hole pressure probe traverses in the inflow and the outflow. Furthermore, pneumatic measurements were carried out using pressure taps inside the penny cavity. Additionally, oil flow visualization was conducted on the airfoil, hub, and penny cavity surfaces. Steady CFD simulations have been benchmarked against experimental data. The results show that tilting the vane platform reduces the penny cavity leakage mass flow. By decreasing penny cavity leakage, platform tilting also affects the passage flow where it leads to a reduced turbulence level and total pressure loss in the leakage flow region. In summary, the paper demonstrates the influence of penny platform tilting on cavity flow and passage flow and provides new insights into the mechanisms of penny cavity-associated losses.

Forced Response Excitation Due to the Stator Vanes of Two and Three Compressor Stages Away

State-of-the-art axial compressors of gas turbines employed in power generation plants and aero engines should have both high efficiency and small footprint. Thus, compressors are designed to have thin rotor blades and stator vanes with short axial distances. Recently, problems of high cycle fatigue (HCF) associated with forced response excitation have gradually increased as a result of these trends. Rotor blade fatigue can be caused not only by the wake and potential effect of the adjacent stator vane, but also by the stator vanes of two, three or four compressor stages away. Thus, accurate prediction and suppression methods are necessary in the design process. In this study, the problem of rotor blade vibration caused by the stator vanes of two and three compressor stages away is studied. In the first part of the study, one-way FSI simulation is carried out. To validate the accuracy of the simulation, experiments are also conducted using a gas turbine test facility. It is found that one-way FSI simulation can accurately predict the order of the vibration level. In the second part of the study, a method of controlling the blade vibration is investigated by optimizing the clocking of the stator vanes. It is confirmed that the vibration amplitude can be effectively suppressed without reducing the performance. Through this study, ways to evaluate and control the rotor blade vibration are validated.

Experimental Aerodynamic and Aeroelastic Investigation of a Highly-Loaded 1.5-Stage Transonic Compressor with Tandem Stator

This paper experimentally investigates a highly-loaded 1.5-stage transonic axial compressor, which comprises a variable inlet guide vane, a BLISK rotor, and a variable stator in tandem arrangement. A detailed comparison between the newly designed compressor stage and a reference stage with a conventional stator design was conducted by using extensive instrumentation. Thus, steady and unsteady phenomena—focusing on the aerodynamic and aeroelastic behavior—were analyzed. Due to the new stator vane design, a higher aerodynamic stator vane loading was pursued, while the vane count was reduced. This, in turn, allowed a rotor design with an increased work coefficient. This experimental study revealed several effects of the optimized compressor stage in terms of both performance and the corresponding aerodynamics, as well as the aeroelastic behavior.

Effect of an Axially Tilted Variable Stator Vane Platform on Penny Cavity and Main Flow

This paper presents the impact of an axially tilted variable stator vane platform on penny cavity flow and passage flow, with the aid of both optical and pneumatic measurements in an annular cascade wind tunnel as well as steady CFD analyses. Variable stator vanes (VSVs) in axial compressors require a clearance from the endwalls. This means that penny cavities around the vane platform are inevitable. Production and assembly deviations can result in a vane platform which is tilted about the circumferential axis. Due to this deformation, backward facing steps occur on the platform edge. Penny cavity and main flow in geometries with and without platform tilting were compared in an annular cascade wind tunnel, which comprises a single row of 30 VSVs. Detailed particle image velocimetry (PIV) measurements were conducted inside the penny cavity and in the vane passage. Steady pressure and velocity data was obtained by two-dimensional multi-hole pressure probe traverses in the inflow and the outflow. Furthermore, pneumatic measurements were carried out using pressure taps inside the penny cavity. Additionally, oil flow visualization was conducted on the airfoil, hub, and penny cavity surfaces. Steady CFD simulations with boundary conditions, according to the measurements, have been benchmarked against experimental data. The results show that tilting the VSV platform reduces the mass flow into and out of the penny cavity. By decreasing penny cavity leakage, platform tilting also affects the passage flow where it leads to a reduced turbulence level and total pressure loss in the leakage flow region. In summary, the paper demonstrates the influence of penny platform tilting on cavity flow and passage flow and provides new insights into the mechanisms of penny cavity-associated losses.

Uncertainty Analysis of Film Cooling of Fan-Shaped Holes on a Stator Vane Under Realistic Inlet Conditions

Uncertainty due to operating conditions in gas turbines can have a significant impact on film cooling performance, or even the life of hot-section components. In this study, uncertainty quantification technique is applied to investigate the influences of inlet flow parameters on film cooling of fan-shaped holes on a stator vane under realistic engine conditions. The input parameters of uncertainty models include mainstream pressure, mainstream temperature, coolant pressure and coolant temperature, and it is assumed that these parameters conform to normal distributions. Surrogate model for film cooling is established by radial basis function neural network, and the statistical characteristics of outputs are determined by Monte Carlo simulation. The quantitative analysis results show that, on pressure surface, a maximum value of 61.6% uncertainty degree of laterally averaged adiabatic cooling effectiveness (ηad,lat) locates at about 4.0 diameters of hole downstream of the coolant exit; however, the maximum uncertainty degree of ηad,lat is only 4.5% on suction surface. Furthermore, the probability density function of area-averaged cooling effectiveness is of highly left-skewed distribution on pressure surface. By sensitivity analysis, the variation of mainstream pressure has the most pronounced effect on film cooling, while the effect of mainstream temperature is unobvious.

Full-Scale Vibration Testing of Nozzle Guide Vanes

An increasing number of turboexpanders are equipped with Nozzle Guide Vane (NGV) as the first stator stage. By varying the throat area of the first stator vane the NGV enables an additional control methodology to the line-up power output allowing higher operational flexibility and higher efficiency at partial load and partial speed. The design of this component might become critical for enabling high expander availability considering its exposure to high temperature, thermal loading, and fluid induced vibrations. This is especially true also considering that the vibration frequencies of this sub-assembly are influenced by internal clearances and by the value of the friction coefficient, which leaves a relevant margin of error when using numerical methods (such as FEM) for predicting the actual structural behavior of this component. In this paper, the design of a full-scale test bench for the determination of both friction coefficients and modal behavior of a nozzle guide vane geometry is described. The bench enables us to simulate the pre-load due to aerodynamic forces on the NGV airfoil simulating the actual working conditions of bushes and bearings.

Development of Choked Flow in Variable Nozzle Radial Turbines

In commonly applied one-dimensional choking models for radial turbines, choked flow is assumed to appear in the geometrical throat of each stator and rotor. Coupled and complex three-dimensional effects are not considered. In order to analyze the internal aerodynamic in a radial turbine at off design conditions and before carrying out experimental tests, which in the case of automotive turbocharger are limited by their compact size, computational fluid dynamics (CFD) simulations stand out as a useful tool. This paper presents the study of a variable geometry turbine (VGT) of a commercial turbocharger at off design conditions reaching choked flow, analyzing the presence of this limiting conditions in the stator and rotor under different operation points and VGT positions. Reynolds-averaged Navier-Stokes (RANS) and unsteady RANS simulation have been performed to obtain the flow structures in stator and rotor. The results reveal that the choked effective area mostly depends on the stator vane position and pressure ratio. For the closed VGT position a standing shock wave appears on the stator suction side and expands through the vaneless space. For the opened VGT position the flow is choked at the rotor outlet. However, the evolution of the choked area highly depends on the rotational speed and the secondary flow. A strong interaction with the tip leakage vortex has been identified.

A rough surface damping contact model and its application in spatial variable stator vane mechanism with dry friction joint

For the mechanical system without oil lubrication, the impact or collision often occurs in the joint clearance, such as the variable stator vane (VSV) mechanism. In the dry friction joint, the damping of the contact bodies has a significant effect on the simulation stability of the tribo-dynamics calculation process. In order to investigate the effect of contact damping and joint clearance on the VSV mechanism performance, this paper proposes a damping contact model on rough surfaces to calculate the clearance contact force between the trunnion and bushing, and the spatial tribo-dynamics of VSV is established by combining this model with spatial dynamics. In addition, the effect of clearance size on the tribo-dynamics is analyzed. The results show that the contact damping must be included in the contact force model of dry friction joints, otherwise the calculation process will oscillate or even not converge, but the contact damping effect can be ignored in the case of lubricating oil. The movement of the trunnion in the bushing is affected by the adjustment drive and the aerodynamic drag, which leads to the wear concentrated on the edge of the bushing. The clearance size affects the distribution of the damping forces and the rigid forces in the contact process, and the damping forces ensure the stability of the VSV tribo-dynamics simulation process. Moreover, with the increase of clearance, the adjustment accuracy of the VSV mechanism is reduced, and the wear of the bushing is intensified.

Performance Degradation Due to Fouling and Recovery After Washing in a Multistage Test Compressor

Land-based power units have to fulfill even more high levels of production and reliability. In harsh environments (desert and tropical installations, typically), the power unit ingests high amounts of dust that might deposit inside the compressor. In this paper, the analysis of a multistage compressor performance that operates under sandy and humid conditions has been assessed. The compressor units, which equips the Allison 250 C18 compressor, has been subjected to multiple runs under severe conditions of soil dust ingestion. The compressor has been operated according to subsequent runs, and at the end of each run, the performance curve was recorded; the performance losses, in terms of pressure ratio, have been measured during the operations. The characteristic curve of each run is representative of the level of contamination of the unit. Finally, the compressor has been washed, and the performance curve, in the recovered conditions, has been recorded. The results show the modification and the downward shift of the characteristic curves which lead to a gradual loss of the compressor performance. The curves realized after dust ingestion have been compared with the recovered curve after online washing. The measurement shows a promising recovery of the performances, even if the compressor flow path appears affected by localized deposits able to resist to the droplet removal action. Detailed photographic reports of the inlet guide vane (IGV) and the first compressor stages have been included in this analysis. After each run, the IGV, the rotor blade and stator vane of the first stage, and the hub and the shroud surfaces have been photographed. The pictures show the deposition patterns on the blades and the compressor surfaces. The comparison of the pictures of the internal surfaces, before and after the washing, highlights the parts that are more critical to clean and needy of attention during offline washing and overhaul.