Turbomachine Stage Research Articles

On setting boundary conditions for calculations of turbomachines in 3-d computer packages

Unfortunately, in a growing number of publications of the results of the study of turbomachines in ANSYS and other packages, insufficient attention is paid to the choice of the flow model (absolute or relative) and the setting of boundary conditions in the areas adjacent to the rotating impeller: shroud seal, diaphragm (end) seals, gap over shroudless blades, steam balance holes, etc.As a rule, these areas contain such elements of geometry as a “ledge”, “projection”, “crest”, the flow around which, and, consequently, their resistance depends on the angle of incoming flow relative to the barrier. In the stages of turbomachines in sections behind fixed blade units, angle α1 between the flow velocity vector (in absolute motion) and the circumferential velocity is 10°÷25° (for steam turbine stages), 20°÷35° (for gas turbine stages) and 40°÷75° ((for axial compressor stages). It is shown (using the example of viscous flow around the “ledge + projection” system) that a change in the angle in the range of 15°÷90° changes the resistance by almost 2 times.The choice of a particular type of flow model (absolute or relative) for the domain primarily determines the value of the angle of interaction of the flow lines with an obstacle: it remains close to α1, or close to β1 = α1 + 20°÷70°, which significantly changes the qualitative picture of the viscous flow around the characteristic elements of geometry (“ledge”, “projection”, “crest”), and the values of integral parameters (flow rate, power, efficiency). The lack of recommendations on the correct choice of boundary conditions will inevitably lead not only to the inability to compare the results of various published studies, but also to their objective value. Therefore, the need to substantiate proposals for the choice of flow models in the areas of labyrinth seals, steam balance holes, gaps over shroudless blades, etc. is very relevant.

Prediction of Low-Engine-Order Excitation Due to a Nonsymmetrical Nozzle Ring in a Radial Turbine by Means of the Nonlinear Harmonic Approach

Abstract Low engine order (LEO) excitation in a turbomachine stage can be induced by nonuniform inflow conditions, manufacturing tolerances, or in-service wear. LEOs are known to excite significant forced response vibration amplitudes that can easily cause high cycle fatigue failure of blades. The accurate prediction of LEO excitation usually requires high-fidelity computational fluid dynamics (CFD) models of the full annulus of the machine due to the loss of symmetry leading to excessive computational cost. Previous investigation showed that the aerodynamic excitation stemming from the blade-passing-frequency in a vaned radial inflow turbine can be accurately predicted by using the nonlinear harmonic (NLH) method at highly reduced computational costs. In the current paper, the feasibility of the NLH method for the prediction of LEO excitation due to geometrical asymmetries is investigated for the same test object. An exact digital replica of the nozzle guide ring is created using measured throat width data. NLH simulations resolving different combinations of frequencies and a time-marching calculation are conducted with the new model involving this digital replica. The results show that a NLH model including small number of certain frequencies is able to predict the occurring LEO excitation sufficiently accurate. By comparing results from subsequent forced response analysis with measured vibration amplitudes, a satisfactory agreement was found confirming this conclusion.

Influence of Acoustic Blockage on Flutter Instability in a Transonic Nozzle

In order to predict oscillating loads on a structure, time-linearized methods are fast enough to be routinely used in design and optimization steps of a turbomachine stage. In this work, frequency-domain time-linearized Navier–Stokes computations are proposed to predict the unsteady separated flow generated by an oscillating bump in a transonic nozzle. The influence of regressive pressure waves on the aeroelastic stability is investigated. This case is representative of flutter of a compressor blade submitted to downstream stator potential effects. The influence of frequency is first investigated on a generic oscillating bump to identify the most unstable configuration. Introducing backward traveling pressure waves, it is then showed that aeroelastic stability of the system depends on the phase shift between the wave's source and the bump motion. Finally, feasibility of active control through backward traveling pressure waves is evaluated. The results show a high stabilizing effect even for low amplitude, opening new perspectives for the active control of choke flutter.

Influence of Adjusting Control Accuracy on Pressure Probe Measurements in Turbo Machines

This paper presents a new design of a probe adjusting device intended to position pressure and temperature probes in a flow field. 5-hole, 3-hole and temperature probes can be moved in radial direction and freely rotated about their axis. The high actuation accuracy of 3.9µm in radial direction and 0.09° in angular position is validated in a 2-stage-turbine test rig which is installed at the Institute of Power Plant Technology, Steam and Gas Turbines, RWTH Aachen University. To meet the challenge to calculate the efficiency of a turbo machine which is mainly influenced by the temperature, all probe adjusting devices are positioned simultaneously and controlled by the measuring acquisition system (MAS) so that the same radial position in each stage is measured at the same time. For this purpose a new program has been developed to synchronize actuation and measurement. The slim design of 60mm width allows measurement between the stages of turbomachines with small axial distances between vane and blade. In addition a CFD/FEA shows how the design and combination of materials compensate the thermal expansion of the engine during operation. This allows a minimal safety distance of 0.2mm between rotor and probe to enable measurement as close to the physical boundary as possible. The actuation accuracy is demonstrated with pressure, temperature and angle distribution plots. It is also shown that the resolution of the measuring points, and therefore the actuation distances, has a large impact on the flow field analysis and should be set as high as possible. However the measuring time has to be taken into account.

Using synthetic jet generators in the systems of control of separated flows of turbomachines

The article presents the features of the use of pulse-periodic systems, which are presented in the form of generators of synthetic jets and can be used to eliminate flow separation of blade rows of turbomachines. It is known that the control system of separated flows characterized by greater efficiency in the modes of operation of stages of turbomachines that are close to resonance. Therefore, ensuring the flexibility of the control system of separated flows and the possibility of its adjustment under certain operating conditions of stages of turbomachines, greatly expand the range of effectiveness of the latter. Such flexibility of the control system of separated flow can be achieved by using a moving element, which is used as a vibration exciter. With moving parts can configure the system to the required frequency range, the moving element can also play the role of a diagnostic element of the system. It should be noted that the integral separated wake on the stages of axial turbomachines has a requirement for control method for the presence of feedback, which in turn needs a shaping procedure and settings. Adjusting the natural frequency of the control system of separated flows will provide expansion of its range of effective work, as well as increase the efficiency of stages of turbomachines in general. The study results can be used for the selection of configuration of moving parts of repetitively pulsed systems.

An experimental investigation of unsteady turbulent-wake/boundary-layer interaction

An experimental investigation of unsteady-wake/boundary-layer interaction, similar to that occurring in turbomachinery, has been conducted in a specially modified wind tunnel. Unsteadiness in a turbomachine is periodic in nature, due to the relative motion of rotor and stator blades, resulting in travelling-wave disturbances that affect the blade boundary layers. In the experimental rig, travelling-wave disturbances were generated by a moving airfoil apparatus installed upstream of a flat plate to provide a two-dimensional model of a turbomachine stage. The boundary layer on the flat plate was tripped near the leading edge to generate a turbulent flow prior to interaction with the wakes, and measurements of velocity throughout the boundary layer were taken with a hot-wire probe. The Reynolds number, based on distance along the plate, ranged from 0.144×10 5 to 1.44×10 5, and all data were reduced through a process of ensemble averaging. Due to the nonlinear interactions with the boundary layer, the travelling discrete frequency wakes were found to decrease the shape factor of the velocity profile and to increase the level of turbulent fluctuations. Unlike the phase advance found with stationary-wave external disturbances, velocity profiles subject to the travelling wake fluctuations exhibited increasingly negative phase shifts from the free-stream towards the wall.

Etude expérimentale de l'écoulement instationnaire dans le diffuseur aubé d'une turbomachine centrifuge

The unsteady effects caused by the interaction of the rotor wakes with the flow within the vaned diffuser of a centrifugal turbomachine stage have been experimentally investigated. The experiment has been carried out on the simplified model of a centrifugal pump operating in air at the nominal point. The diffuser flow has been surveyed by means of a two-colour four-beam laser Doppler velocimeter on a blade-to-blade surface at midspan and over two cylindrical measuring surfaces located just upstream of the diffuser vanes leading edges and in proximity of the inlet throat of the diffuser. Instantaneous data taken in phase with the shaft rotation have been averaged and used to construct images of the flow field for different relative positions of impeller blades and diffuser vanes. The sequences are detailed enough to show the evolution of the rotor wakes within the forward part of the diffuser. The rotor blade wakes entering the diffuser are chopped by the vanes leading edges into individual segments which are convected through the diffuser passages where they mix out rapidly.

Time-Accurate Euler Simulation of Interaction of Nozzle Wake and Secondary Flow With Rotor Blade in an Axial Turbine Stage Using Nonreflecting Boundary Conditions

The objective of this paper is to investigate the three-dimensional unsteady flow interactions in a turbomachine stage. A three-dimensional time-accurate Euler code has been developed using an explicit four-stage Runge–Kutta scheme. Three-dimensional unsteady nonreflecting boundary conditions are formulated at the inlet and the outlet of the computational domain to remove the spurious numerical reflections. The three-dimensional code is first validated for two-dimensional and three-dimensional cascades with harmonic vortical inlet distortions. The effectiveness of the nonreflecting boundary conditions is demonstrated. The unsteady Euler solver is then used to simulate the propagation of nozzle wake and secondary flow through the rotor and the resulting unsteady pressure field in an axial turbine stage. The three-dimensional and time-dependent propagation of nozzle wakes in the rotor blade row and the effects of nozzle secondary flow on the rotor unsteady surface pressure and passage flow field are studied. It was found that the unsteady flow field in the rotor is highly three dimensional and the nozzle secondary flow has significant contribution to the unsteady pressure on the blade surfaces. Even though the steady flow at the midspan is nearly two dimensional, the unsteady flow is three dimensional and the unsteady pressure distribution cannot be predicted by a two-dimensional analysis.

Investigation of the Laminar-Turbulent Transition of Boundary Layers Disturbed by Wakes

The boundary layer transition under instationary afflux conditions as present in the stages of turbomachines is investigated. A model for the transition process is introduced by means of time-space distributions of the turbulent spots during transition and schematic drawings of the instantaneous boundary layer thicknesses. To confirm this model, measurements of the transition with zero and favorable pressure gradient are performed.