Axial Flow Turbomachines Research Articles

Conventional rotational molding process and aerodynamic characteristics of an axial-flow hollow blades rotor

In this work, the rotational molding process is developed to manufacture in one piece an axial-flow turbomachine rotor with hollow blades. Giving to our knowledge, this process has never been employed in the making of these turbomachine components. Indeed, the blades of these rotors are typically solid blades and are making by injection molding, machining, or thermoforming. The effects of three relevant factors of the rotational molding process are studied here: oven temperature, time of heating, and cooling rate. The cooling of the moving mold is managed by simple convection-of-air, or by convection-of-air charged with water particles. For the oven temperature of 285 °C, hollow-blades rotors of good quality are gotten in 12 min per cycle. In addition, aerodynamic characteristics of one rotational molded rotor are compared to those of another part machined in aluminum piece. Characteristics of this aluminum rigid-rotor are assumed as reference.

Numerical Study of a Turbulent Single-Stage Axial Flow

This paper studies the flow in an axial fan with a rotor in front and stators (vanes) in the rear. This configuration is typical of a single-stage axial flow turbo machine. The standard  model with enhanced wall treatment. Interaction between these components is determined considering the rotor and stator together in a single calculation. Numerical calculations have been performed to investigate the effects of different angular velocities on pressure contours, mass flow rates and total pressure and the results have been shown and discussed through figures. The results show that the increase of angular velocity has a significant effect on the governing physical parameters.

Prediction of Cascade Performance of Circular-Arc Blades with CFD

Thin circular-arc blade is often used as a guide vane, a deflecting vane, or a rotating blade of low pressure axial-flow turbomachine because of its easy manufacture. Ordinary design of the blade elements of these machines is done by use of the carpet diagrams for a cascade of circular-arc blades. However, the application of the carpet diagrams is limited to relatively low cambered blade operating under optimum inlet flow conditions. In order to extend the applicable range, additional design data is necessary. Computational fluid dynamics (CFD) is a promising method to get these data. In this paper, two-dimensonal cascade performances of circular-arc blade are widely analyzed with CFD. The results have been compared with the results of experiment and potential theory, and useful information has been obtained. Turning angle and total pressure loss coefficients are satisfactorily predicted for lowly cambered blade. For high camber angle of <TEX>$67^{\circ}$</TEX>, the CFD results agree with experiment for the angle of attack less than that for shockless inlet condition.

Decomposition of modal acoustic power due to cascade–turbulence interaction

This paper investigates the modal acoustic power (MAP) generated by a cascade of flat-plate airfoils interacting with homogeneous, isotropic turbulence. The basic formulation for the upstream and downstream acoustic power based on the analytical theory of Smith [Discrete frequency sound generation in axial flow turbomachines, Reports and Memoranda no. 3709, Aeronautical Research Council, 1972] and its generalization to broadband noise due to Cheong et al. [High frequency formulation for the acoustic power spectrum due to cascade–turbulence interaction. Journal of the Acoustical Society of America 119 (2006) 108–122]. The MAP has been expressed as the sum of cut-on acoustic modes, whose modal power is the product of three terms: a term that specifies the wavenumber distribution of mean square velocity, a sound power factor that specifies the efficiency of radiation, and an acoustic blade response function. The effect of these terms on the MAP is discussed in detail in this paper. The acoustic blade response functions are found to determine the modal lines of minimum sound power in mode-frequency maps of the MAP. The upstream sound power factor is less than the downstream power factor, which is generally large, especially away from the cut-off frequency. Both power factors are small for modes close to cut-off. Modes close to cut-off, therefore, do not contribute significantly to the radiated acoustic power in the downstream direction, even though the modal pressure amplitude for these modes is high since they are excited close to resonance. For an isotropic turbulent gust, the mean square velocity wave number spectrum reduces only the magnitude of the MAP without altering the distribution of power in the frequency-mode map.

Analytical and numerical investigation of the optimum pressure distribution along a low-pressure axial fan blade

In the field of axial flow turbomachines, the two-dimensional cascade model is often used experimentally or numerically to investigate fundamental flow characteristics and overall performance of the impeller. The core of the present work is a design method for axial fan cascades aiming to derive inversely the optimum blade shape based on the requirements of the impeller and not using any predefined aerofoil profiles. While most design strategies based on the aerofoil theory assume constant total pressure at all streamlines, i.e. free-vortex flow, this paper investigates the possibility of varying the total pressure along the blade and based on that, an analytical expression of the outlet blade angle is determined. When computing the blade profile at a specified radius, critical parameters reflecting on the flow characteristics are observed and adjusted (i.e. sufficient lift and controlled deceleration of the flow on the contour) so that the resulting profile is derived for minimum losses. The validation of this design strategy is given by the numerical results obtained when employed as an optimization tool for an industrial fan: 10–20 per cent absolute increase in the static efficiency of the optimized impeller.

Measurement and Prediction of Aerodynamic Stability of an Axial-Flow Ventilation Fan near the Stall Condition

This paper attempts to look at the unsteady flow pressure on the blade tip in order to quantify the inception of instability of an axial-flow ventilation fan. A test rig was set up to measure the unsteady static pressure near the rotor blade tip region by mounting several transducers on the outer casing. The data sampling system was configured for acquisition and post analysis. The measurement results showed that, as the throttle was closed, the flow disturbance appeared firstly at the rotor inlet. The subsequent aerodynamic instability of the fan was dependent upon the time evolution of such disturbances, which will notably be attenuated due to the viscous flow effect passing through the blade passage. The disturbance at the rotor inlet was shown to be enhanced as the throttle was progressively closed, and the unstable behaviour was triggered by the large disturbance at the rotor inlet. Using a 2D aerodynamic stability model for an axial-flow turbomachine, the off-design points and the stall margin for the axial-flow ventilation fan were numerically predicted and compared with experimental results.

Effects of Wetness in Steam Turbines

Courses in mechanical engineering usually introduce the theory of axial-flow turbo-machines in terms of simple velocity triangles representing the bulk flow of ideal compressible fluid through the blade passages. A distinctive practical difference, peculiar to steam turbines (ST), is the presence of liquid-water in the flow field. The steam wetness in such turbines is widely known to be doubly-damaging, leading to both loss of efficiency and to mechanical damage (erosion, etc.) of the machine components. Over recent decades, a whole new field of mechanical engineering science has evolved on the subject of wetness in steam turbines, and general practices have been established within the industry. This article reviews the general effects that are of major importance to the turbine designer/engineer, power plant operator, and especially to researchers in this field.

Viscous and Inviscid Linear/Nonlinear Calculations Versus Quasi-Three-Dimensional Experimental Cascade Data for a New Aeroelastic Turbine Standard Configuration

This paper presents a new International Standard Configuration to be added to an already existing set of 10 configurations for unsteady flow through vibrating axial-flow turbomachine cascades. This 11th configuration represents a turbine blade geometry with transonic design flow conditions with a normal shock positioned at 75 percent real chord on the suction side. Out of a set of test cases covering all relevant flow regimes two cases were selected for publication: A subsonic, attached flow case, and an off-design transonic case showing a separation bubble at 30 percent real chord on the suction side. The performed tests are shown to be repeatable and suitable for code validations of numerical models predicting flutter in viscous flows. The validity of the measured data of the two public cases was examined and comparisons with other tests were conducted. Sometimes a large difference in aerodynamic damping was observed on cases with similar flow conditions. This was investigated at three transonic cases with almost identical inlet flow conditions and only small variations in outlet Mach number. It was found that the differences in the global damping are due to very local changes on the blade surface in the shock region, which obtain a large influence by the integration because of the discrete measuring points. Hence it is recommended not to look at the global damping for code validations but more precisely to the local values. These show a common tendency, which is reproducible with different numerical methods. This was demonstrated with a potential model, a linear Euler model, a nonlinear Euler model, and a Navier–Stokes solver, all applied to predict flutter of each test case with a 2D/Q3D approach. This paper demonstrates both the limitations of inviscid codes to predict flutter in viscous flow regimes, and their cost advantage in attached flow calculations. The need for viscous code development and validation is pointed out. This should justify and encourage the publication of thoroughly measured test cases with viscous effects.

THE EFFECTS OF PERIODIC WAKE STRUCTURES ON TURBULENT BOUNDARY LAYERS

Compressor and turbine blade boundary layers in axial-flow turbomachines are subject to periodically disturbed flow. This study modelled these conditions in a wind tunnel with circular cylinders traversing in front of a flat plate. Turbulent boundary layer velocity profiles on the flat plate were measured with a hot-wire anemometer. The turbulence intensity in the boundary layer was found to be significantly increased by the unsteady freestream flow, resulting in increased characteristic boundary layer thicknesses and skin friction coefficient. The vortex structure within the unsteady cylinder wakes was found to explain the behaviour of the velocity and phase angle profiles in the boundary layer.

Aeroelastic stability of cascades in turbomachinery

State-of-the-art prediction of the aeroelastic stability of cascades in axial-flow turbomachines is reviewed. The first main chapter of the article presents a comprehensive formulation of the two- and three-dimensional classical (unstalled) flutter problem of tuned and mistuned rotor blade rows and bladed disc assemblies. Within the framework of linearized analysis, a complete and generalized theory in modal form is outlined, comprising the various formulations of the cascade flutter problem distributed in fragments throughout the literature. Brief outlines are also made of recent advances in unsteady aero-dynamic methods for turbomachinery aeroelastic applications. The second main chapter contains a parametric study of the classical flutter stability characteristics of compressor and turbine cascades in subsonic and supersonic flow. Stability boundaries and dominant trends in flutter behaviour are outlined, and the significant effects of blade mistuning on the aeroelastic stability of turbomachine bladings are highlighted.

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.

Families of variational principles for inverse and H A hybrid problems of an S 2 stream sheet in mixed flow turbomachines

This paper deals with the semi-inverse, H A, and inverse problems of compressible flow along an S 2 stream sheet in mixed flow turbomachines, for which families of variational principles (VPs) are derived. In the VPs for the semi-inverse problem, the distributed gas injection and/or suction along the hub walls and casing walls are accounted for. VP families for axial and radial flow turbomachines can be obtained directly from this paper as special cases. Great attention is paid to taking full advantage of natural boundary conditions and “artificial interfaces” in order to offer a perfect theoretical basis for the finite element method or other direct variational methods in computational aerodynamics of turbomachinery. This theory also provides broader and versatile ways for blading design.

Predictions of Endwall Losses and Secondary Flows in Axial Flow Turbine Cascades

This paper describes the development of a semi-empirical model for estimating end-wall losses. The model has been developed from improved understanding of complex endwall secondary flows, acquired through review of flow visualization and pressure loss data for axial flow turbomachine cascades. The flow visualization data together with detailed measurements of viscous flow development through cascades have permitted more realistic interpretation of the classical secondary flow theories for axial turbomachine cascades. The re-interpreted secondary flow theories together with integral boundary layer concepts are used to formulate a calculation procedure for predicting losses due to the endwall secondary flows. The proposed model is evaluated against data from published literature and improved agreement between the data and predictions is demonstrated.

Device for vibration damping in a guide vane ring

Vibration damping guide vane ring assembly of an axial flow turbomachine, including mutually adjacent guide vanes, individual cover plates each being rigidly connected to a respective guide vane forming a cover band of the guide vane ring, the cover plates being frictionally connected to each other and the cover plates of the adjacent guide vanes having wedge-shaped recesses formed therein between the cover plates, and wedge-shaped damping elements having surfaces and being movably inserted in the recesses, each of the wedge-shaped damping elements exerting a vibration damping pressure on the respective cover plates through the surfaces of the damping elements in response to an axial pressure difference across the guide vane ring.

Experimental Cascade Analysis of a Transonic Compressor Rotor Blade Section

A transonic compressor rotor blade cascade was tested in order to elucidate the flow behavior in the transonic regime and to determine the performance characteristic in the whole operating range of a rotor blade section. The experiments have been conducted in a transonic cascade wind tunnel, which enables tests even at sonic inlet velocities. The influence of the upstream Mach number between 0.8 and 1.1 and the inlet flow angle between choking and stalling of the blade row was investigated. The effect of the axial velocity density ratio (AVDR) could be studied by applying an endwall suction device. Furthermore, the level of the shock losses was determined from a wake analysis. A final comparison of cascade losses and those of the corresponding rotor blade element shows good agreement which underlines the applicability of the cascade model in the design of axial flow turbomachines.

Discharge Characteristics of Axial Flow Turbomachines with Plane-Symmetric Rotor and Stator

Flow pattern in the rotor of turbomachines changes discontinuously when flow rate decreases beyond a critical limit. Typical examples are seen in reverse flow at the suction flow at the suction of axial and centrifugal impellers, in the reversal of discharge direction of centripetal impellers, etc. The purpose of this study is to find a clue to the criteria which initiate the transition from one pattern to the other. Non-staggered rotor was installed in an annulus between two stators with plane-symmetric configurations, so that the flow condition became perfectly symmetric. The flow pattern took either discharge or recirculation mode according to the geometry of rotor and stator. Transition criteria were discussed based on the measured driving torque and discharge flow.

Numerical Analysis of 3-D Potential Flow in Axial Flow Turbomachines

A numerical method is developed for analysing a three-dimensional steady incompressible potential flow through a cascade in axial flow turbomachines. In this method, the velocity potential of the absolute flow is determined by the functional minimization finite element method using hexahedral trilinear elements, and the velocity is calculated by a newly developed finite-difference method based on a quadratic polynomial approximation. A significant feature of the present method is to consider reasonably vortex sheets shedding from the trailing edge of blades in the finite element procedure. Therefore, the present method is applicable to more general design conditions including nonuniform blade loadings. As a numerical example, the flow through a moving compressor cascade is computed, and the calculated velocity and pressure distributions are shown. The solution is obtained stably and the computer time is relatively small. But the required computer memory is rather large.

Numerical Analysis of 3-D Potential Flow in Centrifugal Turbomachines

A numerical method is developed for analysing a three-dimensional steady incompressible potential flow through an impeller in centrifugal turbomachines. The method is the same as the previous method which was developed for the axial flow turbomachines, except for some treatments in the downstream region. In order to clarify the validity and limitation of the method, a comparison with the existing experimental data and numerical results is made for radial flow compressor impellers. The calculated blade surface pressure distributions almost coincide with the quasi-3-D calculation by Krimerman and Adler, but are different partly from the quasi-3-D calculation using one meridional flow analysis. It is suggested from this comparison that the flow through an impeller with high efficiency near the design point can be predicted by this fully 3-D numerical method.

Effect of phase variation on total acoustic power of axial-flow turbomachine noise.

This paper deals with the effect of linear phase variation on total acoustic power generated from an axial-flow turbomachinery. In a previous paper (J. Acoust. Soc. Jpn.(E) 2, 203-211), the authors derived a simple expression on the power when all the fluctuating forces on rotor (or stator) blades are in phase. The expression is extended to the case where the phase of the fluctuations varies linearly with the circumferential distance. This is a model for turbomachines with differentn umber of stator and rotor blades. It is shown that the Fourier components of the fluctuation contributing to the power are related to the phase variation and that the dependence of some parameters on the power has the same form as that where the fluctuationsa re in phase. Furthermore, it is suggested that the noise with discrete frequencies is emitted mainly from rotor blades not from stator ones. To obtain a further understanding an estimation of the fluctuationa re made using a lineart heory and important effectos f blade geometry and flow configuration are pointed out.

The Influence of Blade Number Ratio and Blade Row Spacing on Axial-Flow Compressor Stator Blade Dynamic Load and Stage Sound Pressure Level

In axial-flow turbomachines considerable dynamic blade loads and noise production occur as a result of the unsteady blade row interaction between rotor and stator blades. This paper presents results of midspan measurements of the dynamic pressure distribution on the stator blade surface (fixed number of blades) for various rotor-blade numbers and various axial clearances between rotor and stator. For this purpose, one stator blade had been provided with eleven semiconductor pressure transducers in the midspan section. Simultaneously, the sound pressure level was measured at two axial distances downstream of the stator by four condensator microphones distributed along the circumference in each of the two sections and mounted flush with the wall surface. The wake-flow distribution downstream of the rotor could be obtained by a rotating three-hole pressure probe. The results of the corresponding dynamic pressure-measurements and noise measurements are discussed and compared with results from theory.