Showing papers in "Journal of Turbomachinery-transactions of The Asme in 1991"

Film-cooling effectiveness downstream of a single row of holes with variable density ratio

Abstract: Film-cooling effectiveness was studied using a row of inclined holes that injected cryogenically cooled air across a flat, adiabatic test plate. The density ratio of the coolant to mainstream varied from 1.2 to 2.0. Surface temperatures were measured using a unique surface thermocouple arrangement free of conduction errors. Temperatures were obtained along the jet centerline and across a number of lateral locations. By independently varying density ratio and blowing rate, scaling of adiabatic effectiveness with mass flux ratio, velocity ratio, and momentum ratio was determined. Depending on the momentum flux ratio, the jet either remains attached to the surface, detaches and then reattaches, or is fully detached. For attached jets, the centerline effectiveness scaled with the mass flux ratio. However, for detached-reattached jets, a consistent scaling was not found although the general distribution of the centerline effectiveness scaled with momentum flux ratio. Laterally averaged effectiveness was found to be dependent on density ratio and momentum flux ratio. Decreases in density ratio and increases in momentum flux ratio were found to reduce the spreading of the film cooling jet significantly and thereby reduce laterally averaged effectiveness.

Tip Leakage Flow in Axial Compressors

Abstract: Experimental measurements in a linear cascade with tip clearance are complemented by numerical solutions of the three-dimensional Navier–Stokes equations in an investigation of tip leakage flow. Measurements reveal that the clearance flow, which separates near the entry of the tip gap, remains unattached for the majority of the blade chord when the tip clearance is similar to that typical of a machine. The numerical predictions of leakage flow rate agree very well with measurements, and detailed comparisons show that the mechanism of tip leakage is primarily inviscid. It is demonstrated by simple calculation that it is the static pressure field near the end of the blade that controls chordwise distribution of the flow across the tip. Although the presence of a vortex caused by the roll-up of the leakage flow may affect the local pressure field, the overall magnitude of the tip leakage flow remains strongly related to the aerodynamic loading of the blades.

Rotating Waves as a Stall inception Indication in Axial Compressors

Abstract: Stall inception has been studied in two low speed compressors (a single-stage and a three-stage) and in a high speed three-stage compressor, using temporally and spatially resolved measurements. In all three machines, rotating stall was preceded by a period in which small amplitude waves were observed travelling around the circumference of the machine at a speed slightly less than the fully developed rotating stall cell speed. The waves evolved smoothly into rotating stall without sharp changes in phase or amplitude, implying that, in the machines tested, the prestall waves and the fully developed rotating stall are two stages of the same phenomenon. The growth rate of these disturbances was in accord with that predicted by current analytical models. The prestall waves were observed both with uniform and with distorted inflow, but were most readily discerned with uniform inflow. Engineering uses and limitations of these waves are discussed.Copyright © 1990 by ASME

A New Hue Capturing Technique for the Quantitative Interpretation of Liquid Crystal Images Used in Convective Heat Transfer Studies

Abstract: A new image processing based color capturing technique for the quantitative interpretation of liquid crystal images used in convective heat transfer studies is presented. This method is highly applicable to the surfaces exposed to convective heating in gas turbine engines. It is shown that, in the single-crystal mode, many of the colors appearing on the heat transfer surface correlate strongly with the local temperature. A very accurate quantitative approach using an experimentally determined linear hue vs temperature relation is found to be possible. The new hue-capturing process is discussed in terms of the strength of the light source illuminating the heat transfer surface, the effect of the orientation of the illuminating source with respect to the surface, crystal layer uniformity, and the repeatability of the process. The present method is more advantageous than the multiple filter method because of its ability to generate many isotherms simultaneously from a single-crystal image at a high resolution in a very time-efficient manner.

Film Cooling on a Gas Turbine Rotor Blade

Abstract: The film cooling effectiveness on a low-speed stationary cascade and the rotating blade has been measured by using a heat-mass transfer analogy. The film cooling effectiveness on the suction surface of the rotating blade fits well with that on the stationary blade, but a low level of effectiveness appears on the pressure surface of the rotating blade. In this paper, typical film cooling data will be presented and film cooling on a rotating blade is discussed

Similarity analysis of compressor tip clearance flow structure

Abstract: A new approach is presented for analyzing compressor tip clearance flow. The basic idea is that the clearance velocity field can be (approximately) decomposed into independent throughflow and crossflow, since chordwise pressure gradients are much smaller than normal pressure gradients in the clearance region. As in the slender body approximation in external aerodynamics, this description implies that the three-dimensional steady clearance flow can be viewed as a two-dimensional, unsteady flow. Using this approach, a similarity scaling for the crossflow in the clearance region is developed and a generalized description of the clearance vortex is derived. Calculations based on the similarity scaling agree well with a wide range of experimental data in regard to flow features such as crossflow velocity field, static pressure field, and tip clearance vortex trajectory.

Active Stabilization of Centrifugal Compressor Surge

Abstract: Active suppression of centrifugal compressor surge has been demonstrated on a centrifugal compressor equipped with a servo-actuated plenum exit throttle controller. The control scheme is fundamentally different from conventional surge control techniques in that it addresses directly the dynamic behavior of the compression system to displace the surge line to lower mass flows. The method used is to feed back perturbations in plenum pressure rise, in real time, to a fast-acting control valve. The increased aerodynamic damping of incipient oscillations due to the resulting valve motion allows stable operation past the normal surge line. For the compressor used, a 25 percent reduction in the surge point mass flow was achieved over a range of speeds and pressure ratios. Time-resolved measurements during controlled operation revealed that the throttle required relatively little power to suppress the surge oscillations, because the disturbances are attacked in their initial stages. Although designed for operation with small disturbances, the controller was also able to eliminate existing, large-amplitude, surge oscillations. Comparison of experimental results with theoretical predictions showed that a lumped parameter model appeared adequate to represent the behavior of the compression system with the throttle controller and, perhaps more importantly, to be used in the design of more sophisticated control strategies.

Influence of Surface Heat Flux Ratio on Heat Transfer Augmentation in Square Channels With Parallel, Crossed, and V-Shaped Angled Ribs

Abstract: The effect of wall heat flux ratio on the local heat transfer augmentation in a square channel with two opposite in-line ribbed walls was investigated for Reynolds numbers from 15,000 to 80,000. The square channel composed of ten isolated copper sections has a length-to-hydraulic diameter ratio (L/D) of 20. The rib height-to-hydraulic diameter ratio (e/D) is 0.0625 and the rib pitch-to-height ratio (P/e) equals 10

Dynamic Control of Centrifugal Compressor Surge Using Tailored Structures

Abstract: A new method for dynamic control of centrifugal compressor surge is presented. The approach taken is to suppress surge by modifying the compression system dynamic behavior using structural feedback. More specifically, one wall of a downstream volume, or plenum, is constructed so as to move in response to small perturbations in pressure. This structural motion provides a means for absorbing the unsteady energy perturbations produced by the compressor, thus extending the stable operating range of the compression system. In the paper, a lumped parameter analysis is carried out to define the coupled aerodynamic and structural system behavior and the potential for stabilization. First-of-a-kind experiments are then conducted to examine the conclusions of the analysis. As predicted by the model and demonstrated by experiment, a movable plenum wall lowered the mass flow at which surge occurred in a centrifugal compression system by roughly 25 percent for a range of operating conditions. In addition, because the tailored dynamics of the structure acts to suppress instabilities in their initial stages, this control was achievable with relatively little power being dissipated by the movable wall system, and with no noticeable decrease in steady-state performance. Although designed on the basis of linear system considerations, the structural control is shown to be capable of suppressing existing large-amplitude limit cycle surge oscillations.

Comparison of Time-Resolved Turbine Rotor Blade Heat Transfer Measurements and Numerical Calculations

Abstract: Time-resolved turbine rotor blade heat transfer data are compared with ab initio numerical calculations. The data were taken on a transonic, 4-to-1 pressure ratio, uncooled, single-stage turbine in a short-duration turbine test facility. The data consist of the time history of the heat transfer distribution about the rotor chord at midspan. The numerical calculation is a time accurate, two-dimensional, thin shear layer, multiblade row code known as UNSFLO

Simulating Boundary Layer Transition With Low-Reynolds-Number k–ε Turbulence Models: Part 1—An Evaluation of Prediction Characteristics

Abstract: An analysis and evaluation of the capability of k –e low-Reynolds-number turbulence models to predict transition in external boundary-layer flows subject to free-stream turbulence is presented. The similarities between the near-wall cross-stream regions in a fully turbulent boundary layer and the progressive stages through which developing boundary layers pass in the streamwise direction are used to describe the mechanisms by which the models simulate the transition process. Two representative models (Jones and Launder, 1972; Lam and Bremhorst, 1981) are employed in a series of computational tests designed to answer some specific practical questions about the ability of these models to yield accurate, reliable answers over a range of free-stream turbulence conditions.

Detection of a Rotating Stall Precursor in Isolated Axial Flow Compressor Rotors

Abstract: Statistical characteristics of pressure fluctuation on the casing wall of two axial flow compressor rotors have been investigated experimentally to find a precursor of rotating stall. Near stall, the casing wall pressure across a flow passage near the leading edge is characterized by a highly unsteady region where low-momentum fluid accumulates. The periodicity of the pressure fluctuation with blade spacing disappears and an alternative phenomenon comes into existence, which supports the disturbance propagating at a different speed from the rotor revolution. The precursor of rotating stall can be detected by monitoring collapse of the periodicity in the pressure fluctuation. To represent the periodicity qualitatively, a practical detection parameter has been proposed, which is easily obtained from signals of a single pressure sensor installed at an appropriate position on the casing wall during operation of a compressor.

Three-dimensional flowfields inside a transonic compressor with swept blades

Abstract: The concept of swept blades for a transonic or supersonic compressor was reconsidered by Wennerstrom in the early 1980s. Several transonic rotors designed with swept blades have shown very good aerodynamic efficiency. The improved performance of the rotor is believed to be due to reduced shock strength near the shroud and better distribution of secondary flows.A three-dimensional flowfield inside a transonic rotor with swept blades is analyzed in detail experimentally and numerically. A Reynolds-averaged Navier-Stokes equation is solved for the flow inside the rotor. The numerical solution is based on a high-order upwinding relaxation scheme, and a two-equation turbulence model with a low Reynolds number modification is used for the turbulence modeling. To properly predict flows near the shroud, the tip-clearance flow also must be properly calculated. The numerical results at three different operating conditions agree well with the available experimental data and reveal various interesting aspects of shock structure inside the rotor.Copyright © 1990 by ASME

Film Cooling Research on the Endwall of a Turbine Nozzle Guide Vane in a Short Duration Annular Cascade: Part 1—Experimental Technique and Results

Abstract: Heat transfer and aerodynamic measurements have been made on the endwalls of an annular cascade of turbine nozzle guide vanes in the presence of film cooling. The results indicate that high levels of cooling effectiveness can be achieved on the endwalls of turbine nozzle guide vanes (NGV). The NGV were operated at the correct engine nondimensional conditions of Reynolds number, Mach number, gas-to-wall temperature ratio, and gas-to-coolant density ratio. The results show that the secondary flow and horseshoe vortex act on the coolant, which is convected toward the suction side of the NGV endwall passage

Design and Testing of a Controlled Diffusion Airfoil Cascade for Industrial Axial Flow Compressor Application

Abstract: Similar to jet engine development, modern design methods are used today to improve the performance of industrial compressors. In order to verify the loading limits, a cascade profile representative for the first rotor hub section of an industrial compressor has been designed by optimizing the suction surface velocity distribution using a direct boundary layer calculation method. The blade shape was computed with an inverse full potential code and the resulting cascade was tested in a cascade wind tunnel. The experimental results confirmed the design intent and resulted in a low loss coefficient of 1.8 percent at design condition and an incidence range of nearly 12 deg (4 percent loss level) at an inlet Mach number of 0.62.

Comparison of Heat Transfer Measurements With Computations for Turbulent Flow Around a 180 Degree Bend

Abstract: Results of detailed heat transfer measurements are presented for all four walls of a 180 deg 1:1 aspect ratio duct. Experiments using a transient heat transfer technique with liquid crystal thermography were conducted for turbulent flow over a Reynolds numbers range of 12,500-50,000. Computational results using a Navier-Stokes code are also presented to complement the experiments. Two near-wall shear-stress treatments (wall functions and the two layer wall integration method) were evaluated in conjunction with k-e formulation of turbulence to assess their ability to predict high local gradients in heat transfer

The Influence of Density Difference Between Hot and Coolant Gas on Film Cooling by a Row of Holes: Predictions and Experiments

Abstract: The two-dimensional boundary-layer procedure of Schonung and Rodi 1] for calculating film cooling by a row of holes was extended to account for density differences between hot gas and injected coolant gas. The extensions concern the injection model for leaping over the immediate blowing region in the boundary-layer calculation and also the dispersion model for taking into account three-dimensional effects. The extended model is tested for a density ratio of ρ j /ρ e ≃ 2 for both flat-plate situations and film cooling on a model turbine blade

Simulating Boundary Layer Transition With Low-Reynolds-Number k–ε Turbulence Models: Part 2—An Approach to Improving the Predictions

Abstract: An approach for improving the prediction of boundary layer transition with k−e type low-Reynolds-number turbulence models is developed and tested. A modification is proposed that limits the production term in the turbulent kinetic energy equation and is based on a simple stability criterion and correlated to the free-stream turbulence level

The Mass Transfer Analogy to Heat Transfer in Fluids With Temperature-Dependent Properties

Abstract: The analogy between heat transfer in a single-component fluid and isothermal mass transfer of a two-component fluid without chemical reaction is presented. The analogy is well established and frequently used for fluids with constant properties. However, in various applications such as in the cooling of hot components in gas turbines, the temperature varies widely, causing significant fluid property variations. The present paper reviews the constant-property situation and considers in detail the conditions necessary to ensure similarity of the two transport processes with temperature and concentration-dependent fluid properties. An application of the variable property analogy to mass transfer in binary mixtures is presented along with specific recommendations for the CO2 –air and Freon-air systems. It is demonstrated that the essential similarity conditions of the analogy are very well fulfilled for film cooling, total coverage film cooling, and impingement cooling when the heat (mass) flux into the wall in the transport process is zero. The heat/mass transfer analogy can, therefore, be used with confidence for these processes.

Assessment of Laminar-Turbulent Transition in Closed Disk Geometries

Abstract: Finite-difference solutions are presented for rotationally induced flows in the closed space between two coaxial disks and an outer cylindrical shroud, in which there is no superimposed flow. The solutions are obtained with an elliptic-flow calculation procedure and an anisotropic low turbulence Reynolds number k-e model for the estimation of turbulent fluxes

A Theoretical Study of Ingress for Shrouded Rotating Disk Systems With Radial Outflow

Abstract: Copyright © 1989 by ASME.Sealing of the cavity formed between a stationary disc and a rotating disc under axisymmetric conditions is considered. A mathematical model of the flow in the cavity based on momentum integral methods is described and this is coupled to a simple model of the seal for the case when no ingress occurs. Predictions of the minimum imposed flow required to prevent ingress are obtained and shown to be in reasonable agreement with the data of Bayley and Owen (1970), Owen and Phadke (1982), Phadke (1982), and Phadke and Owen (1982, 1983, 1988). With an empirical constant in the model chosen to match this data predictions for the minimum sealing flow are shown to be in good agreement with Graber et al's (1987) measurements. The analysis of Phadke's data also indicates the measurements for small seal clearances must be viewed with caution due to errors in setting the seal clearance. These errors are estimated to be twice the minimum clearance considered. Seal behaviour when ingress occurs is also considered and estimates of the amount of ingress are made from the available data.

Discharge Coefficients of Cooling Holes With Radiused and Chamfered Inlets

Abstract: The flow of cooling air within the internal passages of gas turbines is controlled and metered using orifices formed of holes in disks and casings. The effects of inlet radiusing and chamfering of these holes on the discharge coefficients forms the subject of this paper. Experimental results for a range of radiusing and chamfering ratios for holes of different length-to-diameter ratios are presented covering the range of pressure ratios of practical interest. The results indicate that radiusing and chamfering are both beneficial in increasing the discharge coefficient. Increases of 10-30 percent are possible

A Zonal Approach for Navier–Stokes Computations of Compressible Cascade Flow Fields Using a TVD Finite Volume Method

Abstract: A new zonal approach for computation of compressible viscous flows in cascades has been developed. The two-dimensional, Reynolds-averaged Navier–Stokes equations are discretized spatially by a cell-centered finite volume formulation. In order to make the present approach robust, the inviscid fluxes at cell interfaces are evaluated using a highly accurate TVD scheme based on the MUSCL-type approach with the Roe’s approximate Riemann solver. The viscous fluxes are determined in a central differencing manner. To simplify the grid generation, a composite zonal grid system is adopted, in which the computational domain is divided into nonoverlapping zones, and structured grids are generated independently in each zone. The zonal boundary between two zones is uniquely defined by cell interfaces of one zone, which ensures the uniqueness of the zonal boundary. Communication from one zone to the other is accomplished by numerical fluxes across the zonal boundary. It should be noted that the complete conservation of the numerical fluxes across the zonal boundary can be satisfied by directly evaluating the numerical fluxes using the finite volume method and by ensuring the uniqueness of the zonal boundary. In order to demonstrate the versatility of the present zonal approach, numerical examples are presented for viscous flows through a transonic turbine cascade.

Advances in the Numerical Analysis of Linearized Unsteady Cascade Flows

Abstract: This paper describes two new developments in the numerical analysis of linearized unsteady cascade flows, which have been motivated by the need for an accurate analytical procedure for predicting the onset of flutter in highly loaded compressors. In previous work, results were determined using a two-step or single-pass procedure in which a solution was first determined on a rectilinear-type cascade mesh to determine the unsteady flow over an extended blade-passage solution domain and then on a polar-type local mesh to resolve the unsteady flow in high-gradient regions. In the present effort a composite procedure has been developed in which the cascade-and local-mesh equations are solved simultaneously. This allows the detailed features of the flow within the local mesh region to impact the unsteady solution over the entire domain. In addition, a new transfinite local mesh has been introduced to permit a more accurate modeling of unsteady shock phenomena. Numerical results are presented for a two dimensional compressor-type cascade operating at high subsonic inlet Mach number and high mean incidence to demonstrate the impact of the new composite- and local-mesh analyses on unsteady flow predictions.

The Influence of Inlet Swirl Distortions on the Performance of a Jet Propulsion Two-Stage Axial Compressor

Abstract: Aeroengine intakes containing S-shaped diffusers produce different types of inlet swirl distortions and essentially a combination of a twin swirl and a bulk swirl. The main object of this investigation was to assess the influence of inlet swirl distortions on the performance of a transonic two-stage axial compressor installed in a turbo jet bypass engine Larzac 04. A typical inlet swirl distortion was simulated by a delta-wing in front of the engine. An experimental method was investigated to measure the performance map of the installed low pressure compressor for different engine operating lines. The influence of an inlet swirl distortion with different strengths on the performance map of the compressor was investigated experimentally. It is shown that the performance parameters decrease and a temperature distortion is generated behind the compressor. As the basis of the theoretical investigations of the performance map including inlet swirl distortions a computing model considering four compressors working in parallel was established. The model is based on the idea that an inlet swirl distortion can be substituted by two fundamental types of swirl components, i.e. a bulk swirl co-rotating, and a bulk swirl counter-rotating to the revolution of the compressor. Computed performance maps of the compressor will be discussed and compared with the experimental data.Copyright © 1990 by ASME

Temporally and Spatially Resolved Flow in a Two-Stage Axial Compressor: Part 1—Experiment

Abstract: The fluid dynamics of turbomachines are extremely complex, due in part to the aerodynamic interactions between rotors and stators. It is necessary to acquire fluid dynamic data that reflect the interactive nature of a turbomachine to correlate with the fluid dynamics predicted from modern analyses. The temporal and spatial variations in the midspan aerodynamics of the second stage of a two-stage compressor have been studied with a two-component LDV system. Spatial variations were examined by traversing the LDV probe volume through a dense matrix of both axial and circumferential positions, while temporal resolution was achieved by acquiring all data as a function of the instantaneous rotor position. Hence, the data set reveals rotor and stator wake structure and decay in both the stationary and rotating frames of reference. The data also compared very favorably with extensive pneumatic measurements previously acquired in this compressor. In Part 2 of the paper, the data are used in the assessment of a prediction of the flow in the compressor using a time-accurate, thin-layer, two-dimensional Navier–Stokes analysis.