Blade Count Research Articles

Identification of Spinning Mode in the Unsteady Flow Field of a Low Pressure Turbine

This paper presents an experimental investigation of the vane-blade unsteady interaction in an unshrouded low pressure (LP) turbine research rig with uneven blade/vane count (72 blades and 96 vanes). The rig was designed in cooperation with MTU Aero Engines and considerable efforts were put on the adjustment of all relevant model parameters. In particular blade count ratio, airfoil aspect ratio, reduced mass flow, reduced speed, and Mach and Reynolds numbers were chosen to reproduce the full scale LP turbine at take off condition. Measurements by means of a fast-response pressure probe were performed adopting a phase-locked acquisition technique in order to provide the time resolved flow field downstream of the turbine rotor. The probe has been fully traversed both in circumferential and radial traverses. The rotor exit is characterized by strong perturbations due to the tip leakage vortex and the rotor blade wake. Circumferential nonuniformities due to the upstream vane wake and to the downstream exit guide vane potential effects are also identified. Furthermore, in the present configuration with an uneven blade/vane count the nonuniformities due to the stator and rotor row are misaligned along the whole turbine circumference and create a spinning mode that rotates in direction opposite to the rotor at a high frequency. The aeroacoustic theory is employed to explain such further unsteady pattern. The variations of the exit flow angle within a cycle of such pattern are not negligible and almost comparable to the ones within the blade passing period.

A method for the determination of turbulence intensity by means of a fast response pressure probe and its application in a LP turbine

This paper describes the measurements and the post-processing procedure adopted for the determination of the turbulence intensity in a low pressure turbine (LPT) by means of a single sensor fast response aerodynamic pressure probe. The rig was designed in cooperation with MTU Aero Engines and considerable efforts were put into the adjustment of all relevant model parameters. Blade count ratio, airfoil aspect ratio, reduced massflow, reduced speed, inlet turbulence intensity and Reynolds numbers were chosen to reproduce the full scale LP turbine. Measurements were performed adopting a phase-locked acquisition technique in order to provide the time resolved flow field downstream of the turbine rotor. The total pressure random fluctuations are obtained by selectively filtering, in the frequency domain, the deterministic unsteadiness due to the rotor blades and coherent structures. The turbulence intensity is derived from the inverse Fourier transform and the correlations between total pressure and velocity fluctuations. The determination of the turbulence intensity allows the discussion of the interaction processes between the stator and rotor for engine-representative operating conditions of the turbine.

Prime Geometry Solution of a Centrifugal Impeller Within a 3D Setting

This contribution deals with a numerical solution of a centrifugal impeller's prime geometry. A numerical description of impeller's geometry is transformed into a 3D CAD software. The numerical solution is based on a one dimensional theory with a correction to a finite blade count. The proposed prime geometry is supposed to be tested through a CFD simulation. The aim of the CFD simulation is to optimize the prime geometry of a pump. The numerical solution of the prime geometry varies according to specific speeds and a shape of the impeller's main streamline. An interface between the numerical solution and 3D CAD program is provided by a text document. Some examples of impellers with different specific speeds are being shown in the paper.

A Time-Domain Harmonic Balance Method for Rotor/Stator Interactions

In the absence of instabilities, the large deterministic scales of turbomachinery flows resulting from the periodic rotation of blades can be considered periodic in time. Such flows are not simulated with enough efficiency when using classical unsteady techniques as a transient regime must be bypassed. New techniques, dedicated to time-periodic flows and based on Fourier analysis, have been developed recently. Among these, harmonic balance methods cast a time-periodic flow computation in several coupled steady flow computations. A time-domain harmonic balance method is derived and adapted to phase lag periodic conditions to allow the simulation of only one blade passage per row regardless of row blade counts. Sophisticated space and time interpolations are involved and detailed. The test case is a single stage subsonic compressor. A convergence study of the present harmonic balance is performed and compared with a reference well-resolved classical unsteady flow simulation. The results show, on one hand, the good behavior of the harmonic balance and its ability to correctly predict global quantities as well as local flow pattern; on the other hand, the simulation time is drastically reduced.

Harmonic Balance Analysis of Blade Row Interactions in a Transonic Compressor

In this paper we apply the harmonic balance technique to analyze an inlet guide vane and rotor interaction problem, and compare the computed flow solutions to existing experimental data. The computed results, which compare well with the experimental data, demonstrate that the technique can accurately and efficiently model strongly nonlinear periodic flows, including shock/vane interaction and unsteady shock motion. Using the harmonic balance approach, each blade row is modeled using a computational grid spanning just a single blade passage regardless of the actual blade counts. For each blade row, several subtime level solutions that span a single time period are stored. These subtime level solutions are related to each other through the time derivative term in the Euler (or Navier―Stokes) equations, which is approximated by a pseudo-spectral operator, by complex periodicity conditions along the periodic boundary of each blade row's computational domain, and by the interface boundary conditions between the vane and rotor. Casting the governing equations in harmonic balance form removes the explicit dependence on time. Mathematically, the equations to be solved are similar in form to the steady Euler (or Navier―Stokes) equations with an additional source term proportional to the fundamental frequency of the unsteadiness. Thus, conventional steady-state computational fluid dynamics techniques, including local time stepping and multigrid acceleration, are used to accelerate convergence, resulting in a very efficient unsteady flow solver.

Towards a Quieter Low Pressure Turbine: Design Characteristics and Prediction Needs

This article examines Low Pressure Turbine (LPT) noise in modern aircraft and shows how difficult it is to design a quiet LPT. LPT noise has long been neglected in terms of research and prediction capability due to it being considered of lesser importance. Recent developments in LPT design have led to what is termed a “turbine noise storm,” in which the LPT is designed to be cut-off but in the end there is significant LPT noise and little or no ability to reduce it. These developments include the trend toward lower blade counts and solidities. The situation is further hurt due to the inability to predict LPT noise, particularly for the new type of designs. The inability to reduce the LPT noise once the engine and airplane is designed is due to there being no available light-weight high-temperature acoustic-lining technology that can be added to fix a LPT noise problem without a significant penalty. In an attempt to understand what makes some engines' LPT quiet while others are very loud, modern turbine designs are characterized into several LPT noise “classes” and important aspects of each class are shown. Finally, the need for a semi-empirical prediction is discussed including what essential elements this prediction needs to have to be relevant to modern aircraft preliminary design.

J0501-3-2 スプリッター付き遠心圧縮機インペラの空力特性に及ぼす翼枚数の影響([J0501-3]流体機械の研究開発におけるEFD/CFD(3))

J0501-3-2 スプリッター付き遠心圧縮機インペラの空力特性に及ぼす翼枚数の影響([J0501-3]流体機械の研究開発におけるEFD/CFD(3))

On the Response of a Strongly Diffusing Flow to Propagating Wakes

Further evidence on the similarities between transition and separation phenomena occurring in turbomachinery and wind tunnel flows is provided by measurements on a large scale flat plate under a strong adverse pressure gradient. The flat plate has a long laminar separation bubble and is subjected to a range of disturbances with triggering caused by injection of a transverse jet and subsequently by wakes generated by rods moving transversely upstream of the leading edge. Wakes were originally presented individually. Each individual wake provoked a vigorous turbulent patch, resulting in the instantaneous collapse of the separation bubble. This was followed by a very strong, and stable, calmed region. Following the lead given by the experiments of Gutmark and Blackwelder (1987, “On the Structure of Turbulent Spot in a Heated Laminar Boundary Layer,” Exp. Fluids, 5, pp. 207–229.) on triggered turbulent spots, wakes were then presented in pairs at different wake spacing intervals. In this way wake interaction effects could be investigated in more detail. As in the work on triggered turbulent spots the spacing between impinging wakes was systematically varied; it was found that for close wake spacings the calmed region acted to suppress the turbulence in the following turbulent patch. To investigate whether this phenomenon was a recurring one or whether the flow then reverted back to its unperturbed state, the experiments were repeated with three and four rods instead of two. This has the potential for making available a wide range of variables including direction and speed of rod rotation. It was found that the subsequent wakes were also suppressed by the calming effect. It may be anticipated that this repeating situation is present in a turbomachine, resulting in hidden benefits for blade count and efficiency. There may also conceivably be blade loading advantages while retaining favorable heat transfer conditions in high pressure turbines or stall margin in axial compressors. The inherent and prospective benefits of the calming effect therefore need to be understood thoroughly and new opportunities exploited where this is feasible.a

Two-dimensional RANS simulations of the flow through a compressor cascade with jet flaps

With the aim to reduce the blade count in a stator row, in this paper the application of active blowing is discussed. Based on a high-speed compressor stage blade geometries for a stator cascade with jet flap implementation are developed. Two-dimensional numerical simulations of the cascade flow clarify that a reasonable appliance of the jet is particularly possible at high inlet flow angles, where the invested momentum can delay separation effects. At the compressor reference pitch the entrainment effect due to the jet energy broadens the operation range. The blowing leads to a pronounced increase of the static pressure rise of about 9% in the design point. A stepwise increase of the pitch up to 20%, which is equivalent to a blade count reduction of about 16% in the present case, is considered. The worsen flow guidance through the cascade causes a reduction of the operation range, which is regained by the jet flap. The required mass flow of the jet, chosen in pre-studies, is about 1% which is an adequate amount for practical use in an engine.

Computationally fast harmonic balance methods for unsteady aerodynamic predictions of helicopter rotors

A harmonic balance technique for the analysis of unsteady flows about helicopter rotors in forward flight and hover is presented in this paper. The aerodynamics of forward flight are highly nonlinear, with transonic flow on the advancing blade, subsonic flow on the retreating blade, and stalled flow over the inner portion of the rotor. Nevertheless, the unsteady flow is essentially periodic in time making it well suited for frequency domain analysis. The present method uses periodic boundary conditions that allows one to model the flow field on a computational grid around a single helicopter blade, no matter the actual blade count. Using this approach, we compute several solutions, each one corresponding to one of several instants in time over one period. These time levels are coupled to each other through a spectral time derivative operator in the interior of the computational domain and through the far-field and periodic boundary conditions around the boundary of the domain. In this paper, we apply the method to the three-dimensional Euler equations (although the method can also be applied to three-dimensional viscous flows), and examine the steady and unsteady aerodynamics about wings and rotors.

A flow adaptive aerodynamic probe concept for turbomachinery

A flow adaptive 2D traversing algorithm is developed and demonstrated with measurements in a large axial turbine facility. This novel approach is suited for pneumatic probe and fast response aerodynamic probe measurements. The implementation of the algorithm is fully automated and requires a minimal input, such as blade count and hub and tip diameters, at set-up. The algorithm automatically selects measurement points, such as shear flows, secondary flows, wakes through user-defined detection functions, and adds additional measurement points; therefore higher measurement fidelity in these regions is obtained compared to a traditional measurement method. The flow adaptive 2D traversing algorithm can resolve the overall flow field with 75% fewer measurement points compared to a uniform a measurement grid. This reduction in measurement points results in a measurement time using the flow adaptive algorithm that is 81% quicker than on a uniform measurement grid, without loss of measurement accuracy.

Influence of Impeller Geometry on the Unsteady Flow in a Centrifugal Fan: Numerical and Experimental Analyses

The aim of this study is to evaluate the influence of design parameters on the unsteady flow in a forward-curved centrifugal fan and their impact on the aeroacoustic behavior. To do so, numerical and experimental studies have been carried out on four centrifugal impellers designed with various geometrical parameters. The same volute casing has been used to study these impellers. The effects on the unsteady flow behavior related to irregular blade spacing, blade count and radial distance between the impeller periphery and the volute tongue have been studied. The numerical simulations of the unsteady flow have been carried out using computational fluid dynamics (CFD) tools based on the unsteady Reynolds averaged Navier Stokes (URANS) approach. The study is focused on the unsteadiness induced by the aerodynamic interaction between the volute and the rotating impeller blades. In order to predict the acoustic pressure at far field, the unsteady flow variables provided by the CFD calculations have been used as inputs in the Ffowcs Williams-Hawkings equations (FW-H). The experimental part of this work concerns measurement of aerodynamic performance of the fans using a test bench built according to ISO 5801 (1997) standard. In addition to this, pressure microphones have been flush mounted on the volute tongue surface in order to measure the wall pressure fluctuations. The sound pressure level (SPL) measurements have been carried out in an anechoic room in order to remove undesired noise reflections. Finally, the numerical results have been compared with the experimental measurements and a correlation between the wall pressure fluctuations and the far field noise signals has been found.

Rotor-Stator Interaction Noise in Swirling Flow: Stator Sweep and Lean Effects

An asymptotic technique in the limit of large blade and vane number is employed to study the interaction in mean swirling flow of an unsteady rotor wake with a stator row downstream. The effect of various stator characteristics, including stator sweep, lean and number of vanes, on the tone noise radiated upstream is investigated. The important effects of mean swirl are included in both the rotor wake evolution and the construction of the upstream-radiated sound field. Specifically, we construct the first term in the series expansion of the upstream unsteady pressure in inverse powers of the blade count. We show that sweep and lean in suitable combination (positive sweep and positive lean) can lead to this first term becoming zero, so that the upstream radiation is then approximated by the next (smaller) term in the series, and is therefore necessarily reduced. A simple criterion, based on the form of the incident wake at the stator leading edge, which leads to this first term becoming zero, is identified. This provides a very efficient method for assessing the impact of sweep and lean over a wide range of operating conditions. The analytical nature of the model allows the physical mechanism behind the effectiveness of sweep and lean to be identified.

Numerical Study of Active Flow Control for a Transitional Highly Loaded Low-Pressure Turbine

Active control was simulated numerically for the subsonic flow through a highly loaded low-pressure turbine. The configuration approximated cascade experiments that were conducted to investigate a reduction in turbine stage blade count, which can decrease both weight and mechanical complexity. At a nominal Reynolds number of 25,000 based upon axial chord and inlet conditions, massive separation occurred on the suction surface of each blade due to uncovered turning. Vortex generating jets were then used to help mitigate separation, thereby reducing wake losses. Computations were performed using both steady blowing and pulsed mass injection to study the effects of active flow control on the transitional flow occurring in the aft-blade and near-wake regions. The numerical method utilized a centered compact finite-difference scheme to represent spatial derivatives, that was used in conjunction with a low-pass Pade-type nondispersive filter operator to maintain stability. An implicit approximately factored time-marching algorithm was employed, and Newton-like subiterations were applied to achieve second-order temporal accuracy. Calculations were carried out on a massively parallel computing platform, using domain decomposition to distribute subzones on individual processors. A high-order overset grid approach preserved spatial accuracy in locally refined embedded regions. Features of the flowfields are described, and simulations are compared with each other, with available experimental data, and with a previously obtained baseline case for the noncontrolled flow. It was found that active flow control was able to maintain attached flow over an additional distance of 19–21% of the blade chord, relative to the baseline case, which resulted in a reduction of the wake total pressure loss coefficient of 53–56%.

Numerical Simulation of Separation Control for Transitional Highly Loaded Low-Pressure Turbines

The subsonic flow through highly loaded low-pressure turbines is simulated numerically using a high-order method. The configuration approximates cascade experiments that were conducted to investigate a reduction in turbine stage blade count, which can decrease both weight and mechanical complexity. At a nominal Reynolds number of 25 × 10 3 based upon axial chord and inlet conditions, massive separation occurs on the suction surface of each blade as a result of uncovered turning. Pulsed injection vortex generator jets were then used to help mitigate separation, thereby reducing wake losses. Computations were performed for both uncontrolled and controlled cases and reproduced the transitional flow occurring in the aft-blade and near-wake regions. The numerical method utilizes a centered compact finite difference scheme to represent spatial derivatives, which is used in conjunction with a low-pass Pade-type nondispersive filter operator to maintain stability. An implicit approximately factored time-marching algorithm is employed, and Newton-like subiterations are applied to achieve second-order temporal accuracy. Calculations were carried out on a massively parallel computing platform, using domain decomposition to distribute subzones on individual processors. A high-order overset grid approach preserved spatial accuracy in locally refined embedded regions. Features of the flowfields are elucidated, and simulations are compared with each other and with available experimental data. Relative to the uncontrolled case, it was found that pulsed injection maintained attached flow over an additional 15% of the blade chord, resulting in a 22% decrease of the wake total pressure loss coefficient.

Toward Intra-Row Gap Optimization for One and Half Stage Transonic Compressor

Multistage effects on both aerodynamics and aeromechanics have been identified as significant. Thus, design optimizations for both aerodynamic performance and aeromechanical stability should be done in the unsteady multistage environment. The key issue preventing such a procedure to be carried out is the enormous computing time cost of multistage unsteady simulations. In this paper, a methodology based on the single-passage shape-correction method integrated with an interface disturbance truncation technique has been developed. The capability, efficiency, and accuracy of the developed methodology have been demonstrated for a one and a half stage quasi-three-dimensional transonic compressor with realistic blade counts. Furthermore, the interface disturbance truncation technique enables us to separate multirow interaction effects from the upstream and the downstream, which makes it possible to superimpose different rotor upstream gap effects and rotor downstream gap effects on the middle row rotor aerodynamic damping. In addition, a gap influence coefficient approach has been developed for investigation of all the possible gap spacing combinations of M upstream stator-rotor gaps and N downstream rotor-stator gaps. Then the number of cases that need to be computed has been reduced from M×N to M+N, which saved substantial computing time. The optimization analysis shows significant damping variation (∼300%) within the chosen intrarow gap design space. The intrarow gap spacing could have either stabilizing or destabilizing effects so that the stabilizing axial spacing could be utilized to increase flutter-free margin in aeromechanical designs. The current approach also can be used for setting aeromechanical constraints for aerodynamic performance optimizations.

Blade Count and Clocking Effects on Three-Bladerow Interaction in a Transonic Turbine

A computational study of the multirow interaction mechanisms has been carried out for a one-and-a-half stage (NGV-rotor-stator) transonic turbine. In addition to measurable subharmonic unsteadiness on the rotor blades induced by two fundamental stator passing frequencies, a significant aperiodic (“mistuned”) circumferential variation of unsteady forcing magnitude by about three times has been observed in the downstream stator blades. Further parametric studies with various stator blade counts illustrate that the circumferential variation pattern of the unsteady forcing is dictated by the NGV-stator blade count difference, while the local stator forcing magnitude is affected by its circumferential clocking position relative to the upstream NGV blades. The present work suggests that the circumferential clocking together with the choice of blade count should be considered as an aeromechanical design parameter. For cases with stator-stator (or rotor-rotor) blade counts resulting in a tuned (or nearly tuned) unsteady forcing pattern, the clocking position should be chosen to minimize the unsteady forcing. On the other hand, if the choice of blade counts leads to a significant aerodynamic aperiodicity (mistuning), the clocking-forcing analysis can be used to identify the most vulnerable blade that is subject to the maximum forcing.

Effects of Blade Count on Linearized and Nonlinear Hot Streak Clocking Simulations

The temperature e eld exiting gas turbine combustors is highly nonuniform due to streaks of hot e uid directly downstream of combustor fuel nozzles. These hot streaks have been shown to limit the life of turbine blades. Adjusting the positions of hot streaks with respect to nozzle guide vanes, known as hot streak clocking, can be used to help control blade temperatures in gas turbines. Because hot streak clocking predictions require unsteady, three-dimensional simulations, they are very expensive, and various techniques are used to reduce their cost. Two of these techiques are examined, linearized Navier ‐Stokes solvers and reduced-blade-count simulations. Hot streak clocking simulations have been performed using nonlinear and linearized Navier ‐Stokes solvers for 1 ‐1‐1 and 3‐4‐3 blade-count (blades per row ) cone gurations. The blade-count effects were examined for each solution technique, and the two solution techniques were compared. It is shown that the linearized technique can be used to capture qualitatively hot streak clocking effects. It is also shown that the reduced-blade-count approximation has a signie cant impact on predicted surface temperatures.

Massively Parallel Simulation of the Unsteady Flow in an Axial Turbine Stage

The results from two numerical simulations of the unsteady flow in a 11 stage axial-flow turbine are presented and compared with experimental data to show both the effect of blade count on the solution accuracy and the time-averaged and unsteady flow physics present. TFLO, the three-dimensional, multiblock, massively parallel turbomachinery flow solution procedure is used to simulate the flow through the Aachen 36-vane/41-blade/36-vane 1 ½-stage turbine rig. Comparisons of the time-averaged and unsteady flow solutions of 1-vane/1-blade/1-vane and 6-vane/7-blade/6-vane configurations with the available experimental data are used to show the importance of matching actual blade counts in unsteady flow simulations as closely as possible. In addition, these comparisons are used to quantify the predicted aerodynamic performance differences and highlight the different unsteady flow physics in the two simulations.

Three-Dimensional Simulations of Airfoil Clocking in a 1-1/2 Stage Turbine

Axial turbines have inherently unsteady flow fields because of the relative motion between rotor and stator airfoils. This relative motion leads to viscous and inviscid interactions between blade rows. As the number of stages increases in a turbomachine, the buildup of convected wakes can lead to progressively more complex wake/wake and wake/airfoil interactions. Variations in the relative circumferential positions of airfoils in adjacent rotating or non-rotating blade rows can change these interactions, leading to different unsteady forcing functions on airfoils and different turbine efficiencies. In addition, as the Mach number increases the interaction between blade rows can be intensified due to potential effects. It has been shown, both experimentally and computationally, that airfoil clocking can be used to improve the efficiency and reduce the unsteadiness in multiple-stage axial turbomachines with equal blade counts. While previous investigations have provided an improved understanding of the physics associated with airfoil clocking, most of the work focused on two-dimensional effects. More research is needed to determine how airfoil clocking can be incorporated in the design of three-dimensional airfoils. In the current study three-dimensional simulations of airfoil clocking have been performed for a 1-1/2 stage high-pressure turbine operating at subsonic flow conditions. Comparisons have been made with the available time-averaged experimental data at a single clocking position. The predicted results indicate approximately a 1% efficiency variation as the secondstage stator airfoils are clocked with respect to the first-stage stator airfoils. / cv Cp P P ps Pt f S ss t U a /? 7 TJ in min max ms out st tt oo 1/2 1 2 3 4 NOMENCLATURE Frequency Specific heat at constant volume (P-Pt)/(l/2pUms) Static pressure Pressure surface Total pressure Entropy Suction surface Time Rotor velocity Absolute frame flow angle (from axial) Relative frame flow angle Ratio of specific heats Efficiency Flow coefficient Density