Multistage Turbomachinery Research Articles

Development and Assessment of Hot‐Film Sensor Technique for the Multistage Turbomachinery Environment

The slanted hot-film technique has been successfully modified and adapted for flow measurement in the complex and hostile environment encountered in multistage turbomachinery. The sources of probe and prong vibrations and methods of eliminating them are discussed. The incorporation of compressibility corrections and improvements in calibration techniques developed are discussed. The procedure is validated against the data from a five-hole probe. Suggestions are made for improving the accuracy of measurement. The ensemble-averaged data is resolved into time average, blade periodic, blade aperiodic, and non-deterministic velocity components. This is used to model the turbomachinery flow field. The flow is found to be highly complex with appreciable secondary flow in the casing region, and large flow unsteadiness in the suction-casing corner region.

A Parallel Algorithm for Simulation of Unsteady Flows in Multistage Turbomachinery

The numerical simulation of unsteady flow in multi-stage turbomachinery is computationally expensive. A parallel code using message-passing interface libraries, which was developed to reduce the turnaround time and the cost of computation, is presented in this paper. The paper describes the details of the parallel algorithm, including the parallelization rationale, domain decomposition and processor allocation, and communication patterns. The numerical algorithm was used to simulate the unsteady flow in a six-row test turbine. The numerical results present the salient features of the turbine flow, such as the temporal and spatial vari1 2 ation of velocity, pressure, temperature and blade force. To illustrate the advantages of parallel processing, the turnaround time of the turbine flow simulation was compared on several computers where the code was run in parallel and sequential mode. Nomenclature Cp Pressure coefficient F Force f Frequency p Pressure r Radius T Temperature γ Ratio of specific heats of a gas μ Viscosity ρ Density τ Skin friction ω Angular velocity Subscripts F Half-amplitude Fourier transform hub Hub location n− d Non-dimensional tip Tip location −∞ Upstream infinity Superscripts ∗ Total (or stagnation)

A Transport Model for the Deterministic Stresses Associated With Turbomachinery Blade Row Interactions

The unsteady process resulting from the interaction of upstream vortical structures with a downstream blade row in turbomachines can have a significant impact on the machine efficiency. The upstream vortical structures or disturbances are transported by the mean flow of the downstream blade row, redistributing the time-average unsteady kinetic energy (K) associated with the incoming disturbance. A transport model was developed to take this process into account in the computation of time-averaged multistage turbomachinery flows. The model was applied to compressor and turbine geometry. For compressors, the K associated with upstream two-dimensional wakes and three-dimensional tip clearance flows is reduced as a result of their interaction with a downstream blade row. This reduction results from inviscid effects as well as viscous effects and reduces the loss associated with the upstream disturbance. Any disturbance passing through a compressor blade row results in a smaller loss than if the disturbance was mixed-out prior to entering the blade row. For turbines, the K associated with upstream two-dimensional wakes and three-dimensional tip clearance flows are significantly amplified by inviscid effects as a result of the interaction with a downstream turbine blade row. Viscous effects act to reduce the amplification of the K by inviscid effects but result in a substantial loss. Two-dimensional wakes and three-dimensional tip clearance flows passing through a turbine blade row result in a larger loss than if these disturbances were mixed-out prior to entering the blade row. [S0889-504X(00)01804-3]

Quantitative Visualization of the Flow in a Centrifugal Pump With Diffuser Vanes—II: Addressing Passage-Averaged and Large-Eddy Simulation Modeling Issues in Turbomachinery Flows

The present paper addresses two basic modeling problems of the flow in turbomachines. For simulation of flows within multistage turbomachinery, unsteady Reynolds-averaged Navier–Stokes (RANS) of an entire series of blade rows is typically impractical. On the other hand, when performing RANS of each blade row separately one is faced with major difficulties in matching boundary conditions. A popular remedy is the “passage-averaged” approach. Unsteady effects caused by neighboring rows are averaged out over all blade orientations, but are accounted for through “deterministic” stresses, which must be modeled. To experimentally study modeling issues for deterministic stresses we use particle image velocimetry data of the flow in a centrifugal pump with a vaned diffuser that includes the flow in the impeller, the gap between the impeller and diffuser, between the diffuser vanes and within the volute downstream. The data have been presented in part A of this paper (Sinha and Katz, 1998, “Flow Structure and Turbulence of a Centrifugal Pump with a Vaned Diffuser,” Proceedings of the ASME Fluids Engineering Division, Washington, DC). Deterministic stresses are obtained from the difference between the phase-averaged and passage-averaged data, whereas the Reynolds stresses are determined from the difference between the instantaneous and phase averaged data. In agreement with previous findings, the deterministic stresses are larger than the Reynolds stresses in regions close to the interface between blade rows, and thus must be carefully accounted for in passage-averaged simulations. The Reynolds stresses are larger in regions located far from the transition region. The second series of issues involves modeling for large-eddy simulation. The measured subgrid stresses determined by spatially filtering the data are compared to eddy viscosity models and show significant discrepancies, especially in regions with separating shear layers. Backscatter of energy that persists during phase averaging is also observed. [S0098-2202(00)00901-9]

Multistage three-dimensional Navier-Stokes computation of off-design operation of a four-stage turbine

In this paper a three-dimensional computational methodology for multistage turbomachinery flows is developed, validated and applied in analysing the reduced mass flowrate operation of a four-stage turbine. The flow is modelled by the three-dimensional Favre-Reynolds-averaged Navier-Stokes equations using the Launder-Sharma near-wall k-ɛ turbulence closure, which are integrated using an implicit third-order upwind solver. The computation models the eight blade rows, including the tip clearance gaps of the rotors, using a 6 × 106 points grid, and computes the time-averaged flow using mixing planes between rows. Computational results are compared with measurements for various operating points. After this validation the method is used in a first attempt to analyse the three-dimensional flow at reduced-mass-flow operation.

Aerodynamic Analysis of Multistage Turbomachinery Flows in Support of Aerodynamic Design

This paper summarizes the state of 3D CFD based models of the time-averaged flow field within axial flow multistage turbomachines. Emphasis is placed on models that are compatible with the industrial design environment and those models that offer the potential of providing credible results at both design and off-design operating conditions. The need to develop models free of aerodynamic input from semiempirical design systems is stressed. The accuracy of such models is shown to be dependent upon their ability to account for the unsteady flow environment in multistage turbomachinery. The relevant flow physics associated with some of the unsteady flow processes present in axial flow multistage machinery are presented along with procedures that can be used to account for them in 3D CFD simulations. Sample results are presented for both axial flow compressors and axial flow turbines that help to illustrate the enhanced predictive capabilities afforded by including these procedures in 3D CFD simulations. Finally, suggestions are given for future work on the development of time-averaged flow models. [S0889-504X(00)02002-X]

Boundary Layer Development in Axial Compressors and Turbines: Part 2 of 4—Compressors

This is Part Two of a four-part paper. It begins with Section 6.0 and continues to describe the comprehensive experiments and computational analyses that have led to a detailed picture of boundary layer development on airfoil surfaces in multistage turbomachinery. In this part, we present the experimental evidence used to construct the composite picture for compressors given in the discussion in Section 5.0 of Part 1. We show the data from the surface hot-film gages and the boundary layer surveys, give a thorough interpretation for the baseline operating condition, and then show how this picture changes with variations in Reynolds number, airfoil loading, frequency of occurrence of wakes and wake turbulence intensity. Detailed flow features are described using raw time traces. The use of rods to simulate airfoil wakes is also evaluated.

Boundary Layer Development in Axial Compressors and Turbines: Part 3 of 4— LP Turbines

This is Part Three of a four-part paper. It begins with Section 11.0 and continues to describe the comprehensive experiments and computational analyses that have led to a detailed picture of boundary layer development on airfoil surfaces in multistage turbomachinery. In this part, we present the experimental evidence that we used to construct the composite picture for LP turbines that was given in the discussion in Section 5.0 of Part 1. We present and interpret the data from the surface hot-film gages and the boundary layer surveys for the baseline operating condition. We then show how this picture changes with variations in Reynolds number, airfoil loading, and nozzle–nozzle clocking.

Boundary Layer Development in Axial Compressors and Turbines: Part 4 of 4—Computations and Analyses

This is Part Four of a four-part paper. It begins with Section 16.0 and concludes the description of the comprehensive experiments and computational analyses that have led to a detailed picture of boundary layer development on airfoil surfaces in multistage turbomachinery. In this part, the computational predictions made using several modern boundary layer codes are presented. Both steady codes and an unsteady code were evaluated. The results are compared with time-averaged and unsteady integral parameters measured for the boundary layers. Assessments are made to provide guidance in using the predictive codes to locate transition and predict loss. Conclusions from the computational analyses are then presented.

Wake-Induced Unsteady Flows: Their Impact on Rotor Performance and Wake Rectification

The impact of wake-induced unsteady flows on blade row performance and the wake rectification process is examined by means of numerical simulation. The passage of a stator wake through a downstream rotor is first simulated using a three-dimensional unsteady viscous flow code. The results from this simulation are used to define two steady-state inlet conditions for a three-dimensional viscous flow simulation of a rotor operating in isolation. The results obtained from these numerical simulations are then compared to those obtained from the unsteady simulation both to quantify the impact of the wake-induced unsteady flow field on rotor performance and to identify the flow processes which impact wake rectification. Finally, the results from this comparison study are related to an existing model, which attempts to account for the impact of wake-induced unsteady flows on the performance of multistage turbomachinery.

Solution-adaptive structured-unstructured grid method for unsteady turbomachinery analysis. II - Results

A solution-adapt ive method for the time-accurate analysis of two-dimension al flows in multistage turboma- chinery is presented. The method employs a hybrid structured-unstructured zonal grid topology in conjunction with appropriate modeling equations and solution techniques in each zone, thus combining the advantages of both structured and unstructured grid methods. An efficient and robust grid adaptation strategy is also used for the unstructured grid regions. The numerical methodology is presented in detail in Part I of this article. Results obtained using this method for different turbomachine flow configurations are presented in this article. The numerical results compare well with available experimental data and other structured grid based simulations. ART I of this article describes a hybrid-grid procedure for the analysis of unsteady turbomachinery flows that combines the advantages of both unstructured and structured grid methodologies. The method is implemented within the zonal framework of Ref. 1, which is generalized to include both structured and unstructured grid domains. The region in the immediate vicinity of the airfoils (inner region) is dis- cretized using structured grids, while the rest of the domain (outer region) is discretized using an unstructured triangular grid. In the viscous inner regions, the Navier-Stokes equations are solved using an implicit, third-order accurate, upwind- biased scheme. In the inviscid outer region, the Euler equa- tions are solved using either a central difference scheme or an upwind scheme that incorporates a linear reconstruction procedure. The solution in the outer unstructured region is advanced in time explicitly using the same time-step values as for the structured regions which are time-advanced in an implicit manner. An efficient and robust grid adaptation strat- egy with both grid refinement and coarsening capabilities is also used for the unstructured grid. For generality, three- dimensional effects of stream-tube contraction are also mod- eled. The present method is capable of treating multistage turbomachinery configurations. Part I of this article described the domain decomposition, grid generation, inner and outer grid solution procedures, grid adaptation strategy, and the various boundary conditions used. Results obtained using this method for different turbomachine flow configurations are presented in this article.

Turbomachinery CFD on parallel computers

The role of multistage turbomachinery simulation in the development of propulsion system models is discussed. Particularly, the need for simulations with higher fidelity and faster turnaround time is highlighted. It is shown how such fast simulations can be used in engineering-oriented environments. The use of parallel processing to achieve the required turnaround times is discussed. Current work by several researchers in this area is summarized, as well as efforts at the NASA Lewis Research Center. The latter efforts are focused on implementing the average-passage turbomachinery model on MIMD, distributed memory parallel computers. Performance results are given for inviscid, single blade row and viscous, multistage applications on several parallel computers, including networked workstations.

The Numerical Simulation of a High-Speed Axial Flow Compressor

The advancement of high-speed axial flow multistage compressors is impeded by a lack of detailed flow field information. Recent developments in compressor flow modeling and numerical simulation have the potential to provide needed information in a timely manner. This paper, which consists of two parts, will explore this topic. The first part will address the development of a computer program to solve the viscous form of the average-passage equation system for multistage turbomachinery. Programming issues such as in-core versus out-of-core data storage and CPU utilization (parallelization, vectorization, and chaining) will be addressed. Code performance will be evaluated through the simulation of the first four stages of a five-stage, high-speed, axial flow compressor on a CRAY Y-MP8/8128 computer. The second part will address the flow physics, which can be obtained from the numerical simulation. In particular, an examination of the end-wall flow structure will be made, and its impact on blockage distribution assessed.

Unsteady two- and three-dimensional Navier-Stokes simulations of multistage turbomachinery flows

The flow-field within an axial flow turbomachine, such as a turbine or compressor, is extremely complex because of three-dimensional features such as hub-corner stall, tip-leakage flows, and airfoil wakes. These flow features interact with each other and with rotor and stator airfoils inducing time-varying forces on the airfoils. These complicated rotor-stator interactions must be understood in order to design turbomachines that are light and compact as well as reliable and efficient. Two codes, STAGE-2 and STAGE-3, have been developed to compute these unsteady rotor-stator interaction flows in multistage turbomachines. An implicit, thin-layer Euler/Navier-Stokes zonal algorithm is used to compute the unsteady flow-field within both turbine and compressor configurations. Results include surface pressures and wake profiles for two-dimensional turbine and compressor configurations and surface pressures for a three-dimensional single-stage turbine configuration. The results compare well with experimental data and other unsteady computations.

An Assessment of Computational Fluid Dynamic Techniques in the Analysis and Design of Turbomachinery—The 1990 Freeman Scholar Lecture

The objective of this paper is to review and assess various computational fluid dynamic techniques used for the analysis and design of turbomachinery. Assessments of accuracy, efficiency, range of applicability, effect of physical approximations, and turbulence models are carried out. Suggestions are made as to the most appropriate technique to be used in a given situation. The emphasis of the paper is on the Euler and Navier-Stokes solvers with a brief assessment of boundary layer solutions, quasi three-dimensional and quasi-viscous techniques. A brief review of the techniques and assessment of the following methods are carried out: pressure-based method, explicit and implicit time marching techniques, pseudo-compressibility technique for incompressible flow, and zonal techniques. Recommendations are made with regard to the most appropriate technique for various flow regimes and types of turbomachinery, incompressible and compressible flows, cascades, rotors, stators, liquid-handling and gas-handling turbomachinery. Computational fluid dynamics has reached a high level of maturity; Euler codes are routinely used in design and analysis, and the Navier-Stokes codes will also be commonplace before the end of this decade. But to capture the realism in turbomachinery rotors and multi-stage turbomachinery, it is necessary to integrate the physical models along with the computational techniques. Turbulence and transition modeling, grid generation, and numerical techniques play a key role. Finally, recommendations are made for future research, including the need for validation data, improved acceleration schemes, techniques for two-phase flow, improved turbulence and transition models, development of zonal techniques, and grid generation techniques to handle complex geometries.

Simulation of Three-Dimensional Viscous Flow Within a Multistage Turbine

This work outlines a procedure for simulating the flow field within multistage turbomachinery, which includes the effects of unsteadiness, compressibility, and viscosity. The associated modeling equations are the average passage equation system, which governs the time-averaged flow field within a typical passage of a blade row embedded within a multistage configuration. The results from a simulation of a low aspect ratio stage and one-half turbine will be presented and compared with experimental measurements. It will be shown that the secondary flow field generated by the rotor causes the aerodynamic performance of the downstream vane to be significantly different from that of an isolated blade row.

A Model for Closing the Inviscid Form of the Average-Passage Equation System

A mathematical model is proposed for closing or mathematically completing the system of equations which describes the time-averaged flow field through the blade passages of multistage turbomachinery. These equations, referred to as the average-passage equation system, govern a conceptual model which has proven useful in turbomachinery aerodynamic design and analysis. The closure model is developed so as to insure a consistency between these equations and the axisymmetric through-flow equations. The closure model was incorporated into a computer code for use in simulating the flow field about a high-speed counterrotating propeller and a high-speed fan stage. Results from these simulations are presented.

Axial Compressor Stator Aerodynamics

Axisymmetric, through-flow calculations, currently the “backbone” of most multistage turbomachinery design systems, are being pushed to their limit. This is due to the difference between the complex, three-dimensional flows that actually occur in turbomachinery and the two-dimensional flow assumed in this type of analysis. To foster the development of design analyses that account more accurately for these three-dimensional effects, there is a need for detailed flow field data in a multistage environment. This paper presents a survey of the initial results from a detailed experimental study of the aerodynamics of the second stage of a large scale, two-stage axial compressor. Data were acquired over a range of flow coefficients. The data presented here are for the second stator and include airfoil and endwall flow visualization, and radial-circumferential traverse measurements presented in the form of fullspan contour plots of total pressure. Also presented are the spanwise distributions of total and static pressures, axial velocity, air angles, and blockage. The effect of increased loading on the growth of the hub corner stall and its impact on these parameters is discussed.

On the incompressible flow in axial turbomachinery with several closely packed blade rows

The velocity field in an axial multistage turbomachinery is easily investigated, ignoring the fluid viscosity and compressibility, by means of the actuator disc theory. By developing this theory, general expressions of fluid velocity in such a machine are obtained, these in turn yield the approximate expressions usually given in the literature.