Vane Pitch Research Articles

Impact of Turbine-Strut Clocking and Turbine Outlet Swirl on the Heat Transfer and Film Cooling in an Aggressive Turbine Center Frame

Abstract This study focuses on the thermal impact of the clocking position of the high-pressure turbine (HPT) vanes with respect to the turbine center frame (TCF) struts and the thermal impact of the HPT outlet swirl. The TCF is a stationary duct equipped with nonturning airfoils (struts), and it connects the HPT to the low-pressure turbine (LPT). The heat transfer coefficient and the purge film cooling effectiveness are measured in an aggressive TCF for two turbine-strut clocking positions and two HPT outlet swirl levels. The measurements are carried out in a product-representative 1.5-stage HPT-TCF-LPT vane configuration under Mach similarity. The unshrouded HPT is fully purged with four individually adjustable purge flows, and the film cooling effectiveness of these purge flows in the downstream TCF is investigated. The biggest influence of turbine-strut clocking was found for the heat transfer coefficient on the struts. By clocking the HPT vanes by half a vane pitch from the thermally least to the most favorable position, a significant heat transfer reduction was achieved for both the nominal and the increased HPT outlet swirl. Increasing the HPT outlet swirl increased the flow incidence of the TCF struts and affected the heat transfer along the struts significantly. On the TCF hub, the averaged heat transfer coefficient and the averaged purge film cooling effectiveness responded relatively robustly to all imposed operating point deviations with differences of less than 2%. However, the effects on the local heat transfer and film cooling distributions on the hub were more significant.

Modeling of the unsteady aerodynamic force of turbine blades considering nonuniform vane pitch

Modeling of the unsteady aerodynamic force of turbine blades considering nonuniform vane pitch

Effects of Asymmetric Vane Pitch on Reducing Low-Engine-Order Forced Response of a Turbine Stage

Asymmetric vane pitch is a key technique to suppress the forced response of downstream rotor blades. To address the problem of low-engine-order (LEO) excitation with high amplitude under an asymmetric configuration (half-and-half layout) widely recognized in the previous literature, we first apply the in-house computational fluid dynamics code Hybrid Grid Aeroelasticity Environment to perform full-annulus unsteady aeroelasticity simulations of the turbine stage, comparing the resonance response of rotor blades on different asymmetric configurations and analyzing the flow field at the vane exit, as well as the excitation force, modal force, and maximum vibrational amplitude on the rotor blades. Second, we reveal that the potential field of the vane row is the main source of the LEO excitation caused by asymmetric configuration on rotor blades, the vane wake and potential field jointly determine the LEO excitation strength of rotor blades, and the vane pitch difference ΔS can be used to regulate the strength of the LEO excitation. Finally, based on an in-depth understanding of flow physics under an asymmetric configuration, a more preferable and effective asymmetric configuration (non-half two-segment layout) is proposed. Our findings demonstrate that, with the proposed asymmetric configuration, the amplitude of the vane passing frequency was reduced by 48.32% compared to the uniform configuration; furthermore, the maximum vibrational amplitude of the three-nodal-diameter response of the rotor blade at the three-engine-order crossing decreased by 45.49% compared to the half-and-half layout. The non-half two-segment layout also significantly improves upon the half-and-half layout in terms of aerodynamic performance. The results presented in this paper provide a good theoretical basis for reducing blade vibration by applying asymmetric vane pitch in engineering practice.

A simultaneous use of a leading-edge fillet and a non-axisymmetrically contoured endwall in a turbine stage

Secondary flow minimization is a crucial problem in a turbine passage. In the present paper, three different strategies are investigated to reduce the secondary-flow-related total pressure loss. These are leading-edge fillet, non-axisymmetric endwall contouring, and combining these two approaches. An experimental investigation in a single-stage turbine facility and RANS-based computations are performed to evaluate the designs. The experiments are carried out in the large-scale Axial Flow Turbine Research Facility AFTRF. A reference Flat Insert is installed on the nozzle guide vane passage's hub surface to allow future endwall design implementation during the experimental phase. The stereolithography manufactured reference insert has a constant thickness with a cylindrical shape. This investigation also uses four different leading-edge fillets, and they are attached to cylindrical Flat Insert initially. The same fillets are also attached to a contoured endwall. Simultaneous use of a leading-edge fillet and a non-axisymmetric contoured endwall for secondary flow control is the main objective of this research. Total pressure measurements are taken at the rotor inlet plane with a Kiel probe. The probe traversing is completed along one vane pitch and from 8% to 38% span. The leading-edge fillets effectively managed to weaken the horseshoe vortex occurring in front of the leading-edge. In one of the fillet cases sitting on the Flat Insert, the computational results at the NGV exit showed that the mass-averaged loss value was reduced by 1.31%. Three fillet designs decreased the area-averaged loss with a maximum reduction of 15.06%.

Vibration Control of Vertical Turbine Pump by Optimization of Vane Pitch Tolerances of an Impeller Using Statistical Techniques

Vibration Control of Vertical Turbine Pump by Optimization of Vane Pitch Tolerances of an Impeller Using Statistical Techniques

Effects of inlet guide vane and blade pitch angles on the performance of a submersible axial-flow pump

This study investigates the effects of inlet guide vane (IGV) and blade pitch angles on the steady and unsteady performance of a submersible axial-flow pump. To analyze the interaction between the IGVs and the rotor blades, both steady and unsteady three-dimensional Reynolds-averaged Navier-Stokes equations were used with shear stress transport turbulence closure. Hexahedral meshes were used in the computational domain. The numerical results for performance curves showed good agreement with experimental data. The results showed that the steady and unsteady performance characteristics were dependent on both the IGV and blade pitch angles. Adjusting these angles affected the total pressure rise and thus caused variation in the efficiency in overload conditions. But adjusting these angles affected the unsteady pressure fluctuations in partial-load conditions. Detailed flow analyses were performed to find the root-cause of these phenomena.

Prediction of Flow-Induced Vibrations due to Impeller Hydraulic Unbalance in Vertical Turbine Pumps Using One-Way Fluid−Structure Interaction

Vertical turbine pumps are used for critical application in power plant. Apart from power plant, these are also used in irrigation, water supply, process industries and petrochemical industries. Centrifugal and vertical turbine pumps consist of rotor geometry and a structure that can vibrate in response to excitation forces. Mass unbalances associated with mechanical and hydraulic geometry of a pump are the two major factors which create dynamic effects in terms of pump vibrations. The generated hydraulic forces resulting due to hydraulic unbalance have similar effect as of mechanical unbalance. For satisfactory operation of the pump, the vibrations in the pumps must be within acceptable limits of applicable standards. Higher level of vibrations not only leads to operational loss, but also leads to down time due to premature failure. Therefore, it is of vital importance for product designers to understand the dominating cause of unbalanced force and its source. The estimation of vibrations using numerical methods can help a designer, especially vibrations arising due to the hydro dynamic forces, for a successful design. In any centrifugal pump, including vertical turbine pump, there is always interaction between fluid and structure. Solid and fluid interaction in present case can be completed by one-way coupling method. The one-way fluid−structure interaction approach is presented in the present paper to predict the vibrations at specific operating conditions which have significant correlation with the test data. Similar philosophy has been applied for an impeller geometry which has hydraulic unbalance to predict the impact of hydraulic unbalance. The benefit of reduction in computational effort and time in this approach can be applied during the initial design stage. Two case studies of one-way FSI approach of a vertical turbine pump are discussed. The approach can be used for evaluating the impact of geometrical deviation, especially vane pitch in an impeller in terms of vibration displacements.

Unsteady Flow Structures within a Turbine Rim Seal Cavity in the Presence of Purge Flow—An Experimental and Computational Unsteady Aerodynamics Investigation

Flow within the space between the rotor and stator of a turbine disk, and an area referred to as the rim seal cavity, develops azimuthal velocity component from the rotor disk. The fluid within develops unsteady structures that move at a fraction of the rotor speed. A test is designed to measure the number of unsteady structures and the rotational speed at which they are moving in the rim seal cavity of an experimental research rig. Data manipulation was developed to extract the speed, and the numbers of structures present using two fast-response aerodynamic probes measuring static pressure on the surface of the nozzle guide vane (NGV)-side rim seal cavity. A computational study is done to compare measured results to a transient unsteady Reynolds-averaged Navier–Stokes (URANS). The computational simulation consists of eight vanes and ten blades, carefully picked to reduce the error caused by blade vane pitch mismatch and to allow for the structures to develop correctly, and the rim seal cavity to measure the speed and number of the structures. The experimental results found 15 structures moving at 77.5% of the rotor speed, and the computational study suggested 14.5 structures are moving at 81.7% rotor speed. The agreement represents the first known test of its kind in a large-scale turbine test rig and the first known “good” agreement between computational and experimental work.

Parametric studying of low-profile vortex generators flow control in an aggressive inter-turbine duct

This paper presents a study of low-profile vortex generators flow control on the effect of casing boundary layer separation in an aggressive inter-turbine duct. Counter-rotating and co-rotating vortex generators configurations were tested parametrically to obtain the optimum low-profile vortex generators’ setup within the aggressive inter-turbine duct, in terms of the axial location, height, angle of attack and streamwise vortices per swirl vane pitch. Surface oil flow visualization was used to qualitatively examine the low-profile vortex generators eliminating the casing separation and subsequently the detailed seven-hole probe measurements were carried out to evaluate the loss reduction quantitatively. The inter-turbine duct examined for this study was more aggressive compared with the geometries found in the most current modern engine designs. Measurements were made inside the aggressive inter-turbine duct annulus at Reynolds number of 150,000. The flow structures within the aggressive inter-turbine duct were found to be dominated by counter-rotating vortices evolved by the swirl vanes and boundary layer separation in both casing and hub regions. A massive casing boundary layer separation extending from the duct first bend to the second bend was found within the aggressive inter-turbine duct. The primary source of pressure loss in the aggressive inter-turbine ducts was the result of the extensive casing boundary layer separation. Flow control by utilizing the low-profile vortex generators installed on the casing was conducted to eliminate the casing separation. Regardless of the vortex generator mitigating the casing separation associated with consequential loss reduction, a pair of casing counter-rotating vortices was always evident at the duct second bend in all cases. Among the tested low-profile vortex generator configurations, two of the most effective low-profile vortex generator configurations, one counter-rotating and one co-rotating low-profile vortex generator configurations, were acquired as a result of successfully eliminating the casing separation. Despite that the inter-turbine duct is aggressively designed, it is concluded that this duct associated with proper flow control techniques is considered as a potential candidate for weight saving in a high bypass ratio engine.

Regression analysis of a curved vane demister with Taguchi based optimization

Regression analysis was carried out using least squares method to develop correlations for the objective functions: friction factor (f) and droplet separation efficiency (η) of curved vane demisters used in seawater desalination. A Taguchi coupled CFD analysis performed earlier by the authors, to select the optimal geometrical parameters for a curved vane demister, was the baseline for this work. Responses of the chosen objective functions from the CFD analysis were fed to the statistical tool Minitab. ANOVA estimates were carried out for identifying the significant factors that control f, η and ranked. Linear model analysis was carried out for the signal-to-noise (S/N) ratios with ‘smaller the better’ approach for f and ‘larger the better’ approach for η. The R-square value for the correlation developed for f and η was found to be 98% and 87.9% respectively. The ranking of the parameters correlates very well with the numerical estimates. The factors like ‘vane pitch’ and ‘hook height above the vane’ were identified to have most significant influence when compared to other parameters.

Experimental Study of Ingestion in the Rotor–Stator Disk Cavity of a Subscale Axial Turbine Stage

This paper describes experiments in a subscale axial turbine stage equipped with an axially overlapping radial-clearance seal at the disk cavity rim and a labyrinth seal radially inboard which divides the disk cavity into a rim cavity and an inner cavity. An orifice model of the rim seal is presented; values of ingestion and egress discharge coefficients based on the model and experimental data are reported for a range of cavity purge flow rate. In the experiments, time-averaged pressure distribution was measured in the main gas annulus and in the disk cavity; also measured was the time-averaged ingestion into the cavity. The pressure and ingestion data were combined to obtain the discharge coefficients. Locations on the vane platform 1 mm upstream of its lip over two vane pitches circumferentially defined the main gas annulus pressure; in the rim cavity, locations at the stator surface in the radially inner part of the “seal region” over one vane pitch defined the cavity pressure. For the sealing effectiveness, two locations in the rim cavity at the stator surface, one in the “mixing region” and the other radially further inward at the beginning of the stator boundary layer were considered. Two corresponding sets of ingestion and egress discharge coefficients are reported. The ingestion discharge coefficient was found to decrease in magnitude as the purge flow rate increased; the egress discharge coefficient increased with purge flow rate. The discharge coefficients embody fluid-mechanical effects in the ingestion and egress flows. Additionally, the minimum purge flow rate required to prevent ingestion was estimated for each experiment set and is reported. It is suggested that the experiments were in the combined ingestion (CI) region with externally induced (EI) ingestion being the dominant contributor.

Vibration Analysis of Ship Propulsion Shaft with the Stern Bearing Characteristics

Efficiency improvements in ships lead often to increased size and speed. However, due to the increase in the agency's output, the weight of the propeller, propulsion shaft, vane pitch and the number is increased. Simultaneously, various. vibrations occur in the shaft. Thus, the design of shaft, and vibration analysis has become a challenging matter. In this study the vibration analysis of ship propulsion shaft has been performed, considering the stern bearing characteristics, and established the measures for shaft vibration with suggestion for shaft design method.

Showerhead Film Cooling Performance of a Turbine Vane at High Freestream Turbulence in a Transonic Cascade

This paper experimentally investigates the effect of blowing ratio and exit Reynolds number/Mach number on the film cooling performance of a showerhead film cooled first stage turbine vane. The vane midspan was instrumented with single-sided platinum thin film gauges to experimentally characterize the Nusselt number and film cooling effectiveness distributions over the surface. The vane was arranged in a two-dimensional, linear cascade in a heated, transonic, blow-down wind tunnel. Three different exit Mach numbers of Mex = 0.57, 0.76 and 1.0—corresponding to exit Reynolds numbers based on vane chord of 9.7 × 105, 1.1 × 106 and 1.5 × 106, respectively—were tested with an inlet free stream turbulence intensity (Tu) of 16% and an integral length scale normalized by vane pitch (Λx/P) of 0.23. A showerhead cooling scheme with five rows of cooling holes was tested at blowing ratios of BR = 0, 1.5, 2.0, and 2.5 and a density ratio of DR = 1.3. Nusselt number and adiabatic film cooling effectiveness distributions were presented on the vane surface over a range of s/C = −0.58 on the pressure side to s/C = 0.72 on the suction side of the vane. The primary effects of coolant injection were to augment the Nusselt number and reduce the adiabatic wall temperature downstream of the injection on the vane surface as compared to no film injection case (BR = 0) at all exit Mach number conditions. In general, an increase in blowing ratio (BR = 1.5 to 2.5) showed noticeable Nusselt number augmentation on pressure surface as compared to suction surface at exit Mach 0.57 and 0.75; however, the Nusselt number augmentation for these blowing ratios was found to be negligible on the vane surface for exit Mach 1.0 case. At exit Mach 1.0, an increase in blowing ratio (BR = 1.5 to 2.5) was observed to have an adverse effect on the adiabatic effectiveness on the pressure surface but had negligible effect on suction surface. The effectiveness trend on the suction surface was also found to be influenced by a favorable pressure gradient due to Mach number and boundary layer transition in the region s/C = 0.28 to s/C = 0.45 at all blowing ratio and exit Mach number conditions. An increase in Reynolds number from exit Mach 0.76 to 1.0 increased heat transfer levels on the vane surface at all blowing ratio conditions. A large increase in Reynolds number adversely affected adiabatic effectiveness on the pressure surface at all blowing ratio conditions. On the suction surface, a large increase in Reynolds number also affected adiabatic effectiveness in the favorable pressure gradient and boundary layer transition region.

Disk cavity purge air outflow into the main gas path of a model turbine stage

Purge (or secondary) air is supplied to rotor–stator disk cavities of gas turbine stages in order to reduce ingestion of hot main gas into the cavities and cool the disks. The air eventually flows out into the main gas path through gap between the disk rim seals, in the process altering the main gas path velocity field and its thermal signature on the airfoils and endwalls. The main gas flow possesses high circumferential velocity downstream of vanes and upstream of blades. This requires fast imaging of the outflowing purge air as it mixes with the main gas. In this work, particle image velocimetry was used to visualize the purge air outflow from the rim cavity into the main gas path of a model single-stage turbine; the rim cavity geometry was a simplification of an actual turbine rim cavity. The low-speed main gas flow had a Mach number of 0.11. The outflow trajectory and velocity field were found to depend on the rim seal geometry, the purge and main gas flow rates, as well as the circumferential position within a vane pitch.

Control of Tonal Fan Noise Using Flow Induced Secondary Sound Sources Generated by Air Jet Actuation

To reduce the tonal noise of axial fans loudspeakers are typically used to generate the required anti-phase sound field. The space requirement and the weight of the loudspeakers inhibit practical application of this method. In the present study the secondary sound field is generated by perturbing the flow in the blade tip regime by using high speed air jets. It has been shown, that this approach is very effective in controlling higher-order mode sound fields at blade passage frequency and its harmonics. The focus of the present work is to study the effect of various nozzle parameters on the generated secondary sound field. From the experimental data presented it can be concluded that when using circular nozzles an injection angle of α = 45° relative to the casing wall combined with an injection direction of β = 90° relative to the main flow (against the impeller rotation) are most efficient in reducing the tone levels. While it is principally possible to control acoustic modes at the blade passing frequency as well as at its harmonics, the injected mass flow rates and circumferential nozzle positions necessary for reducing two different blade tone harmonics are generally not the same. However, when using slit nozzles the differences between the circumferential nozzle positions where significant level reductions of two dominant acoustic modes can be observed can be reduced down to 4% of a stator vane pitch. Future studies will involve two rings of injection nozzles at close axial distance to reduce two dominant blade tone acoustic modes simultaneously. Finally, experiments are also planned with a model fan stage of a modern high bypass ratio aircraft engine.

Evaluation of Predicted Heat Transfer on a Transonic Vane Using v2-f Turbulence Models

In this study, three dimensional numerical simulations of fluid and heat transfer on the transonic vane have been performed using v2-f turbulence model. The objective of this paper is to evaluate the v2-f model’s ability to predict the heat transfer characters on the 3D showerhead film cooled vane. Exit Mach number of Mex=0.76, corresponding to exit Reynolds number based on vane chord of 1.1×106, is tested with an inlet free stream turbulence intensity (Tu) of 16% and integral length scale normalized by vane pitch (Λx/P) of 0.23. For the showerhead film cooling simulations, a new three simulations technique is discussed to calculate the recovery temperature, adiabatic wall temperature, and surface Nusselt number. CFD predictions show an overall good agreement with experimental results.

Effects of Large Scale High Freestream Turbulence and Exit Reynolds Number on Turbine Vane Heat Transfer in a Transonic Cascade

This paper experimentally and numerically investigates the effects of large scale high freestream turbulence intensity and exit Reynolds number on the surface heat transfer distribution of a turbine vane in a 2D linear cascade at realistic engine Mach numbers. A passive turbulence grid was used to generate a freestream turbulence level of 16% and integral length scale normalized by the vane pitch of 0.23 at the cascade inlet. The base line turbulence level and integral length scale normalized by the vane pitch at the cascade inlet were measured to be 2% and 0.05, respectively. Surface heat transfer measurements were made at the midspan of the vane using thin film gauges. Experiments were performed at exit Mach numbers of 0.55, 0.75, and 1.01, which represent flow conditions below, near, and above nominal conditions. The exit Mach numbers tested correspond to exit Reynolds numbers of 9×105, 1.05×106, and 1.5×106 based on a vane chord. The experimental results showed that the large scale high freestream turbulence augmented the heat transfer on both the pressure and suction sides of the vane as compared to the low freestream turbulence case and promoted a slightly earlier boundary layer transition on the suction surface for exit Mach 0.55 and 0.75. At nominal conditions, exit Mach 0.75, average heat transfer augmentations of 52% and 25% were observed on the pressure and suction sides of the vane, respectively. An increased Reynolds number was found to induce an earlier boundary layer transition on the vane suction surface and to increase heat transfer levels on the suction and pressure surfaces. On the suction side, the boundary layer transition length was also found to be affected by increase changes in Reynolds number. The experimental results also compared well with analytical correlations and computational fluid dynamics predictions.

Migration of Combustor Exit Profiles Through High Pressure Turbine Vanes

The high pressure turbine stage within gas turbine engines is exposed to combustor exit flows that are nonuniform in both stagnation pressure and temperature. These highly turbulent flows typically enter the first stage vanes with significant spatial gradients near the inner and outer diameter endwalls. These gradients can result in secondary flow development within the vane passage that is different than what classical secondary flow models predict. The heat transfer between the working fluid and the turbine vane surface and endwalls is directly related to the secondary flows. The goal of the current study was to examine the migration of different inlet radial temperature and pressure profiles through the high turbine vane of a modern turbine engine. The tests were performed using an inlet profile generator located in the Turbine Research Facility at the Air Force Research Laboratory. Comparisons of area-averaged radial exit profiles are reported as well as profiles at three vane pitch locations to document the circumferential variation in the profiles. The results show that the shape of the total pressure profile near the endwalls at the inlet of the vane can alter the redistribution of stagnation enthalpy through the airfoil passage significantly. Total pressure loss and exit flow angle variations are also examined for the different inlet profiles.

Effect of vane-rotor interaction on the unsteady flowfield downstream of a transonic high pressure turbine

This paper analyses the effect of the stator-rotor interference on the stage exit flow field of a transonic turbine operated under engine representative conditions in the von Karman Institute (VKI) compression tube facility. The test programme comprises three pressure ratios and two Reynolds numbers. The time-averaged radial distributions of total pressure and temperature downstream of the rotor are affected by the hub passage vortices and the interaction between the tip passage vortex and the rotor tip leakage vortex. The azimuthal distributions exhibit significant non-uniformities with a periodicity of one vane pitch. The amplitude of this non-uniformity is very sensitive to the pressure ratio. A simple model shows that, contrary to the common belief, the transport of the vane wake and secondary flows across the rotor is not enough to explain the magnitude of the variations. The pitch-wise vane exit static pressure distribution, which is significantly distorted owing to the transonic regime of the vane, should be taken into account. The amplitude of the time-resolved fluctuations of pressure and temperature, which are due to rotor blade passing events, also varies significantly depending on the probe location in a vane pitch. The measured time-accurate rotor relative inlet total pressure suggests that the rotor relative exit undergoes periodic excursions in the transonic regime.

Unbalanced Hydraulic Forces Caused by Geometrical Manufacturing Deviations of Centrifugal Impellers

When an impeller has geometrical manufacturing deviations, synchronous vibration occurs from an unbalanced hydraulic force rotating with the impeller. This phenomenon is termed “Hydraulic unbalance.” In this study, experiments and analyses were carried out to study the hydraulic unbalance. We made several model impellers with intentional “manufacturing deviations”: in vane angle, vane pitch, and with eccentricity. Hydraulic forces were estimated from the rotor vibrations. The results of the experiments showed that the magnitude of the hydraulic force increased as the increase of the extent of manufacturing deviations. It was also shown that the amplitude and the phase of the vibration due to the unbalanced hydraulic force depend on the flow rate. This character of the unbalanced hydraulic force is quite different from that of the mass unbalance force, which is independent of the flow rate. A two-dimensional inviscid flow analysis was carried out to clarify the characteristic of the unbalanced hydraulic force. It was found that the hydraulic force can be divided into three components which are constant, linear and parabolic function of the flow rate. Applications of the present results show that the “hydraulic unbalance” of a high-speed pump impeller manufactured within a geometrical tolerance recommended by a typical standard can be larger than the ordinary “mechanical unbalance.” It was thus shown that the hydraulic unbalance is one of the important factors in the balancing of high-speed pump rotors.