Showing papers on "Axial compressor published in 1996"

Experimental and Computational Investigation of the Tip Clearance Flow in a Transonic Axial Compressor Rotor

Abstract: Experimental and computational techniques are used to investigate tip clearance flows in a transonic axial compressor rotor at design and part speed conditions. Laser anemometer data acquired in the endwall region are presented for operating conditions near peak efficiency and near stall at 100% design speed and at near peak efficiency at 60% design speed. The role of the passage shock/leakage vortex interaction in generating endwall blockage is discussed. As a result of the shock/vortex interaction at design speed, the radial influence of the tip clearance flow extends to 20 times the physical tip clearance height. At part speed, in the absence of the shock, the radial extent is only 5 times the tip clearance height. Both measurements and analysis indicate that under part-speed operating conditions a second vortex, which does not originate from the tip leakage flow, forms in the endwall region within the blade passage and exits the passage near midpitch. Mixing of the leakage vortex with primary flow downstream of the rotor at both design and part speed conditions is also discussed.

Implantable electric axial-flow blood pump with blood-cooled bearing

Abstract: An axial-flow blood pump (10) has a rotor (20) suspended in ball-and-cup bearings (38, 34) which are blood-cooled but not actively blood-lubricated. The ball-and-cup structures are made of highly heat-conductive material and are in heat-transferring contact with heat-conductive stator blades (30) that serve as heat sinks for the bearings. The ball-and-cup structures are radially much smaller than the stator blades. The ball-to-cup interface has so small a gap that the ball-to-cup structures present an essentially continuous surface to the blood flow.

Gas-liquid mixing studies : A comparison of Rushton turbines with some modern impellers

Abstract: Based on the literature, a comparison of the Rushton turbine with other impellers, many of them modern commercial developments, is made. This comparison shows that there are many impellers available, some radial and some axial, which eliminate many of the weaknesses associated with the Rushton though some weaknesses remain or others are introduced. Based on this analysis, a new axial flow hydrofoil impeller, the APV-B2, has been designed and some preliminary results are also reported. It has a low ungassed power number so that a relatively low torque is required to drive it when using it with a large impeller-to-tank diameter ratio. This large diameter also enables greater volumes of gas to be handled without flooding at similar energy dissipation rates. In the downward pumping mode, torque fluctuations found with such a mode of operation are less than with other axial flow hydrofoils, and in the upward pumping mode they are essentially eliminated. This latter mode, as with pitched blade turbines, is also extremely effective at gas dispersion and at a specific ungassed power input of 1 kW/m 3 , a typical industrial value, the power reduction on gassing is negligible.

An LDA study of the turbulent flow field in a baffled vessel agitated by an axial, down-pumping hydrofoil impeller

Abstract: The turbulent flow of water in a cylindrical, baffled vessel with an axial flow hydrofoil impeller, either a Chemineer HE3 (CHE3) or a Prochem Maxflo T (PMT), has been studied using a laser Doppler anemometer. Using ensemble averaging, the mean axial and radial flow and the associated fluctuating components were obtained for the whole of the vessel; plus similar data for the tangential component close to the impeller. Assuming axial symmetry, flow rates were calculated as were flow numbers and circulation flow numbers. Power numbers were also determined. All the data obtained were used to compare the circulation efficiency of the two hydrofoils plus a pitched blade and a Rushton turbine. This comparison showed that the CHE3 required a power input of about 2/3 of that for the PMT and about 1/3 of that for the pitched blade and Rushton turbine to obtain the same axial-radial circulation in the tankz.

Hysteresis in swirling jets

Abstract: This paper explains hysteretic transitions in swirling jets and models external flows of vortex suction devices. Toward this goal, the steady rotationally symmetric motion of a viscous incompressible fluid above an infinite conical stream surface of a half-angle 8, is studied. The flows analysed are generalizations of Long's vortex. They correspond to the conically similar solutions of the Navier-Stokes equations and are characterized by circulation r, given at the surface and axial flow force 4. Asymptotic analysis and numerical calculations show that four (for Oe d 90") or five (for 8, > 90") solutions exist in some range of r, and 4. The solution branches form hysteresis loops which are related to jump transitions between various flow regimes. Four kinds of jump are found : (i) vortex breakdown which transforms a near-axis jet into a two-cell flow with a reverse flow near the axis and an annular jet fanning out along conical surface 8 = 8, < 0,; (ii) vortex consolidation causing a reversal of (i); (iii) jump flow separation from surface 8 = 8,; and (iv) jump attachment of the swirling jet to the surface. As r, and/or J1 decrease, the hysteresis loops disappear through a cusp catastrophe. The physical reasons for the solution non-uniqueness are revealed and the results are discussed in the context of vortex breakdown theories. Vortex breakdown is viewed as a fold catastrophe. Two new striking effects are found: (i) there is a pressure peak of O(TE) inside the annular swirling jet; and (ii) a consolidated swirling jet forms with a reversed ('anti-rocket ') flow force.

Compressor with intercooling

Abstract: A gas turbine has a mainly axial-flow two-stage compressor (1A,1B), with an intercooler (5). It also has at least one combustion chamber (7,8), one or more turbines (2A,2B), and a recuperator (6) in which the compressor air is heated to the inlet temperature in the combustion chamber (7). At least a first compressor component (1A) has water injectors (12), and the intercooler has equipment for water recuperation (13). The latter is connected to the injectors via a delivery pump (14). The injectors may be arranged in the compressor in the plane of the stator blades, and may also be arranged over the entire height of the compressor duct.

Turbofan engine with reduced noise

Abstract: A noise reduction kit for modifying a two (2) spool axial flow turbofan engine with multi-stage compressors and fan driven by multi-stage reaction turbines, and a thrust of at least about 18,000 lbs. at sea level. There is a fan at the upstream end of the core engine for generating axial fan air flow through bypass ducts terminating at a common nozzle, the common nozzle having a mixing plane area for each of the fan air flow and for the exhaust gas in a range between 700 and 800 square inches. A target thrust reverser includes opposing doors rotatable into position to block and divert the flow of exhaust gas for generating reverse thrust. Mixing means for radially diverting fan air and permitting radially outward expansion of exhaust gas is provided coaxially downstream to the core engine. An acoustically dampened light bulb-shaped nose cone is provided for coaxial attachment to an upstream end of the core engine. The axial flow front fan is axially separated from the inlet guide vane is extended relatively forwardly. A bleed air valve, selectively operational to bleed air from the core engine when the valve is open, is ducted for directing bleed air to the vicinity of the common nozzle.

The effect of sheared axial flow on the linear stability of the Z‐pinch

Abstract: A linear analysis of the ideal magnetohydrodynamic (MHD) stability of the Z‐pinch is presented in which plasma flows are included in the equilibrium. With sheared axial flows it is found that substantial stabilization of internal modes is possible for some equilibrium profiles. For this to occur equilibria with a change in fluid velocity across the pinch radius of about Mach 2 are required. However, this ignores the surrounding vacuum and for the more realistic free boundary modes flows of about Mach 4 are required to stabilize all global MHD modes. This stabilization of MHD modes is not observed for all equilibria however. This fact, combined with the supersonic flow speeds required for stability, make it unlikely that a Z‐pinch could in practice be stabilized by the introduction of sheared flow.

Wake Mixing in Axial Flow Compressors

Abstract: Over the years it has been speculated that the performance of multi-stage axial flow compressors is enhanced by the passage of a wake through a blade row prior to being mixed-out by viscous diffusion. The link between wake mixing and performance depends on the ability to recover the total pressure deficit of a wake by a reversible flow process. This paper shows that such a process exists, it is unsteady, and is associated with the kinematics of the wake vorticity field. The analysis shows that the benefits of wake total pressure recovery can be estimated from linear theory and quantified in terms of a volume integral involving the deterministic stress and the mean strain rate. In the limit of large reduced frequency the recovery process is shown to be a direct function of blade circulation. Results are presented which show that the recovery process can reduce the wake mixing loss by as much as seventy percent. Under certain circumstances this can lead to nearly a point improvement in stage efficiency, a nontrivial amount.Copyright © 1996 by ASME

Unsteady Flow in a Rigid 3-D Model of the Carotid Artery Bifurcation

Abstract: In the present study, finite element calculations are performed of blood flow in the carotid artery bifurcation under physiological flow conditions. The numerical results are compared in detail with laser-Doppler velocity measurements carried out in a perspex model. It may be concluded that the numerical model as presented here is well capable in predicting axial and secondary flow of incompressible Newtonian fluids in rigid-walled three-dimensional geometries. With regard to the flow phenomena occurring, a large region with reversed axial flow is found in the carotid sinus opposite to the flow divider. This region starts to grow at peak systole, has its maximal shape at minimal flow rate and totally disappears at the start of the acceleration phase. C-shaped axial velocity contours are formed in the deceleration phase, which are highly influenced by secondary flows. These latter flows are mainly induced by centrifugal forces, flow branching, and tapering of the carotid sinus. Lowering the sinus angle, the angle between the main branch and the carotid sinus, results in a smaller region with reversed axial flow.

Gas flow and lubrication of a scroll compressor

Abstract: The flow, use, interaction and separation of lubricant and gas flowing through the suction pressure portion of a low-side refrigeration scroll compressor is managed by the use of a drive motor mounting sleeve and a multi-ported frame. The mounting sleeve and frame provide for the direction of oil to surfaces within the low side of the compressor shell which require lubrication as well as the conduct of suction gas to the scroll compression mechanism in a manner which cools the compressor drive motor yet which maintains the respective flows of oil and suction gas sufficiently separate to ensure that excessive amounts of oil are not conducted out of the compressor in the gas which is compressed thereby. Lubrication is enhanced by the use of a vent passage which opens into a relatively lower pressure region within the suction pressure portion of the compressor shell. The vent induces lift and assists in the delivery of oil, upward and through a gallery in the compressor's drive shaft, to the various surfaces in the upper portion of the compressor which require lubrication.

Improved Blade Profile Loss and Deviation Angle Models for Advanced Transonic Compressor Bladings: Part II—A Model for Supersonic Flow

Abstract: New blading concepts as used in modern transonic axial-flow compressors require improved loss and deviation angle correlations. The new model presented in this paper incorporates several elements and treats blade-row flows having subsonic and supersonic inlet conditions separately. In the first part of this paper two proved and well-established profile loss correlations for subsonic flows are extended to quasi-two-dimensional conditions and to custom-tailored blade designs. Instead of a deviation angle correlation, a simple method based on singularities is utilized. The comparison between the new model and a recently published model demonstrates the improved accuracy in prediction of cascade performance achieved by the new model.

Effects of Shrouded Stator Cavity Flows on Multistage Axial Compressor Aerodynamic Performance

Abstract: Experiments were performed on a low-speed multistage axial-flow compressor to assess the effects of shrouded stator cavity flows on aerodynamic performance. Five configurations, which involved changes in seal-tooth leakage rates and/or elimination of the shrouded stator cavities, were tested. Data collected enabled differences in overall individual stage and the third stage blade element performance parameters to be compared. The results show conclusively that seal-tooth leakage ran have a large impact on compressor aerodynamic performance while the presence of the shrouded stator cavities alone seemed to have little influence. Overall performance data revealed that for every 1% increase in the seal-tooth clearance to blade-height ratio the pressure rise dropped up to 3% while efficiency was reduced by 1 to 1.5 points. These observed efficiency penalty slopes are comparable to those commonly reported for rotor and cantilevered stator tip clearance variations. Therefore, it appears that in order to correctly predict overall performance it is equally important to account for the effects of seal-tooth leakage as it is to include the influence of tip clearance flows. Third stage blade element performance data suggested that the performance degradation observed when leakage was increased was brought about in two distinct ways. First, increasing seal-tooth leakage directly spoiled the near hub performance of the stator row in which leakage occurred. Second, the altered stator exit now conditions caused by increased leakage impaired the performance of the next downstream stage by decreasing the work input of the downstream rotor and increasing total pressure loss of the downstream stator. These trends caused downstream stages to progressively perform worse. Other measurements were acquired to determine spatial and temporal flow field variations within the up-and-downstream shrouded stator cavities. Flow within the cavities involved low momentum fluid traveling primarily in the circumferential direction at about 40% of the hub wheel speed. Measurements indicated that the flow within both cavities was much more complex than first envisioned. A vortical flow structure in the meridional plane, similar to a driven cavity, existed within the upstream cavity Furthermore, other spatial and temporal variations in Row properties existed. the most prominent being caused by the upstream potential influence of the downstream blade. This influence caused the fluid within cavities near the leading edges of either stator blades in space or rotor blades in time to be driven radially inward relative to fluid near blade mid-pitch. This influence also produced large unsteady velocity fluctuations in the downstream cavity because of the passing of the downstream rotor blade.

Controls and Diagnostics Committee Best 1994 Paper Award: Modeling for Control of Rotating Stall in High-Speed Multistage Axial Compressors

Abstract: Using a two-dimensional compressible flow representation of axial compressor dynamics, a control-theoretic input-output model is derived, which is of general utility in rotating stall/surge active control studies. The derivation presented here begins with a review of the fluid dynamic model, which is a two-dimensional stage stacking technique that accounts for blade row pressure rise, loss, and deviation as well as blade row and interblade row compressible flow. This model is extended to include the effects of the upstream and downstream geometry and boundary conditions, and then manipulated into a transfer function form that dynamically relates actuator motion to sensor measurements. Key relationships in this input-output form are then approximated using rational polynomials. Further manipulation yields an approximate model in standard form for studying active control of rotating stall and surge. As an example of high current relevance, the transfer function from an array of jet actuators to an array of static pressure sensors is derived. Numerical examples are also presented, including a demonstration of the importance of proper choice of sensor and actuator locations, as well as a comparison between sensor types. Under a variety of conditions, it was found that sensor locations near the front of the compressor ormore » in the downstream gap are consistently the best choices, based on a quadratic optimization criterion and a specific three-stage compressor model. The modeling and evaluation procedures presented here are a first step toward a rigorous approach to the design of active control systems for high-speed axial compressors.« less

Rotating Stall in a Single-Stage Axial Flow Compressor

Abstract: This paper describes the results on an experimental investigation of rotating stall flow inside a single-stage axial flow compressor. Tests were carried out in two steps. First, measurements were taken to investigate the transition process into rotating stall. The compressor starts into rotating stall via the modal route with a single rotating stall cell. Further throttling yields to a two-cell shape followed by a significant outlet pressure drop. Both transition processes are discussed in detail. Results from the Moore-Greitzer theory are compared with measured data. In a second step, measurements were taken to determine the three-dimensional unsteady structure of a fully developed rotating-stall cell. Based on unsteady total pressure and three-dimensional hot-wire data, the structure ofa rotating stall cell could be resolved in detail upstream and downstream of the rotor. A typical part-span stall was found. By inserting the measured data into the Euler equations, convective and unsteady effects on the pressure fluctuations can be isolated. A dependence between the radial flow inside the stall cell and the unsteady flow accelerations was found.

On the azimuthal mode structure of rotating blade flow instabilities in axial turbomachines

Abstract: An experimental investigation is described to study the azimuthal structure of the rotating blade flow instability which was recently observed in axial turbomachines. An azimutnal mode analysis is applied to the unsteady pressures at the casing wall immediately upstream of the inlet plane of the rotor. The rotating instability is characterized as an unstable source with a narrow-band frequency and unsteady rotational speed. It is shown that there is only one dominant mode at each peak frequency of the rotating instability components, and subsequent modes are continuously numbered. The peaks due to the rotating instability modes are not tonal in nature but spread over a narrow band. The frequency variation observed can be explained by either a variation of the source frequency, a variation of the angular velocity of the modes, or a combination of both. Similar to rotating stall, which is a frozen flow pattern, the rotating instability source rotates in the same direction as the rotor at about half the shaft speed. The rotating instability components comprise azimuthal modes of much higher order than rotating stall. At highly throttled operation, both flow phenomena are observed at the same time. The mode orders of rotating instability and rotating stall are not harmonically related. In agreement with earlier studies, the tip clearance noise of axial flow machines is found to be generated by the interaction between the rotating instability pattern and the rotor blades.

Variable pressure and variable air flow turbofan engines

Abstract: A variable cycle gas turbine engine has, in axial flow serial relationship, a fan assembly, a core engine, and an exhaust assembly. The core engine includes a compressor, a combustor, a high pressure turbine and a first shaft interconnecting the compressor and the high pressure turbine. The fan assembly is connected to a low pressure turbine by a second shaft which is coaxial with the first shaft. The fan assembly includes a plurality of variable pitch inlet guide vanes, variable pitch stator vanes, and variable pitch outlet guide vanes, the pitch of which are adjusted to regulate the airflow through the gas turbine engine and the pressure ratio across the fan assembly as controlled by the exhaust assembly.

Compressor stall diagnostics and avoidance

Abstract: A pressure sensor (12) is located in the compressor stage of a gas turbine engine (10) to provide a pressure signal (PR1) that shows the compressor flow characteristics. The pressure signal (PR1) is applied to a bandpass filter (16) with roll-offs above and below N2. The difference between the filter output and a stored value for the pressure signal is integrated, and compressor bleed valves (18) are opened if the integral exceeds a stored threshold. The health of a compressor stage is determined by analyzing the magnitude of compressor pressure variations at N2 while accelerating the engine and by comparing the magnitude with values obtained from a compressor with a known stall margin.

An Analysis of Axial Compressors Fouling and a Cleaning Method of Their Blading

Abstract: The paper describes the phenomenon of axial compressor fouling due to aerosols contained in the air Key parameters having effect on the level of fouling are determined A mathematical model of a progressive compressor fouling using the stage-by-stage calculation method is developed Calculation results on the influence of fouling on the compressor performance are presented A new index of sensitivity of axial compressors to fouling is suggested The paper gives information about the Turbotect’s deposit cleaning method of compressor blading and the results of its application on an operating industrial gas turbine Regular on line and off line washings of compressor flow path make it possible to maintain a high level of engine efficiency and outputCopyright © 1996 by ASME

Active Control of an Axial Flow Compressor via Pulsed Air Injection

Abstract: This paper presents the use of pulsed air injection to control the onset of rotating stall in a low-speed, axial flow compressor. By measuring the unsteady pressures near the rotor face, a control algorithm determines the magnitude and phase of the first mode of rotating stall and controls the injection of air in the front of the rotor face. Experimental results show that this technique slightly extends the stall point of the compressor and eliminates the hysteresis loop normally associated with rotating stall. A parametric study is used to determine the optimal control parameters for suppression of stall. Analytic results---using a low-dimensional model developed by Moore and Greitzer combined with an unsteady shift in the compressor characteristic to model the injectors---give further insights into the operation of the controller. Based on this model, we show that the behavior of the experiment can be explained as a change in the bifurcation behavior of the system under nonlinear feedback. A higher fidelity simulation model is then used to further verify some of the specific performance characteristics that are observed in experiments.

Impact of Rotor Wakes on Steady-State Axial Compressor Performance

Abstract: This paper addresses the causal link first described by Smith between the unsteady flow induced by the rotor wakes and the compressor steady-state performance. As an initial step, inviscid flow in a compressor stage is examined. First of a kind numerical simulations are carried out to show that if the rotor wakes are mixed out after (as opposed to before) the stator passage, the time-averaged overall static pressure rise is increased and the mixing loss is reduced. An analytical model is also presented and shown to agree with the numerical results; the model is then used to examine the parametric trends associated with compressor design parameters.Copyright © 1996 by ASME

Application of Computational Fluid Dynamics Techniques to Blood Pumps.

Abstract: Present-day computational fluid dynamics (CFD) techniques can be used to analyze the behavior of fluid flow in a variety of pumps. CFD can be a powerful tool during the design stage for rapid virtual prototyping of different designs, analyzing performance parameters, and making design improvements. Computational flow solutions provide information such as the location and size of stagnation zones and the local shear rate. These parameters can be correlated to the extent of hemolysis and thrombus formation and are critical to the success of a blood pump. CFD-ACE, an advanced commercial CFD code developed by CFD Research Corporation, has been applied to fluid flows in rotary machines, such as axial flow pumps and inducers. Preprocessing and postprocessing tools for efficient grid generation and advanced graphical flow visualization are integrated seamlessly with CFD-ACE. The code has structured multiblock grid capability, non-Newtonian fluid treatment, a variety of turbulence models, and an Eulerian-Langrangian particle tracking model. CFD-ACE has been used successfully to study the flow characteristics in an axial flow blood pump. An unstructured flow solver that greatly automates the process of grid generation and speeds up the flow simulation is under development.

Off-Design Transition and Separation Behavior of a CDA Cascade

Abstract: The design of modern axial flow compressor blade sections as well as the code validation require experimental information about the transition and separation behavior of blade surface boundary layers. The experience has shown in the past that such information has to be obtained on the whole surface and not only by point measurements because both transition and separation may be of a three-dimensional nature even in a straight cascade. Therefore, a new visualization technique based on Liquid Crystals (LC), showing the adiabatic wall temperature, has been developed. With this method, transition, local separation, and complete separation can be detected. Design and off-design data of a subsonic (M{sub 1} = 0.62) Controlled Diffusion Airfoil (CDA) compressor cascade measured in a wind tunnel are presented. The LC results are supplemented by ink-injection tests and overall performance data.

Stall inception measurements in a high-speed multistage compressor

Abstract: This paper presents unsteady measurements taken in a high-speed four-stage aeroengine compressor prior to the onset of aerodynamic flow instabilities. In this experiment, 40 fast-response pressure transducers have been located at various axial and circumferential positions throughout the machine in order to give a very detailed picture of stall inception. At all the compressor speeds investigated, the stall pattern observed is initiated by a very short length-scale finite-amplitude disturbance, which propagates at a fast rate around the annulus. This initial stall cell leads to a large-amplitude system instability in less than five rotor revolutions. Varying the IGV setting angle is found to have a strong influence on the axial location of the first disturbance detected. In particular, transferring the aerodynamic loading from front to downstream stages moves the first disturbance detected from the first to the last stage of the compressor. Other repeatable features of the stall inception pattern in this compressor have been identified using a simple analysis technique particularly appropriate to the study of short length-scale disturbances. It is found that the origins of instabilities are tied to particular tangential positions in both the stationary and rotating frames of reference. These measurements lead to the conclusion that the stall inception process in high-speed multistage compressors can be characterized by some very local and organized flow phenomena. Moreover, there is no evidence of prestall waves in this compressor.

A design improvement strategy for axial blood pumps using computational fluid dynamics.

Abstract: During the initial stages of concept development of non traditional axial flow pumps, numeric simulation offers an attractive advantage. Computational fluid dynamics (CFD) provides the rationale to evolve the design numerically such that undesirable flow features may be significantly mitigated before a physical prototype is fabricated. The initial design of a novel axial flow blood pump is shown through CFD analysis to exhibit large regions of reverse flow. Such fluid dynamic behavior not only decreases the pump's hydrodynamic efficiency, but, more significantly, increases its overall potential for blood trauma and thrombogenesis. The design improvement strategy consists of creating a geometric model of the blood wetted surfaces and changing the associated geometric parameters such that more desirable fluid dynamic behavior is systematically attained with each incremental modification. The fluid flow through each new pump design is analyzed by numerically solving the incompressible Navier-Stokes equations in rotating coordinates. Marked improvements in the major fluid dynamic aspects of the axial flow pump were observed over an evolutionary sequence of four generations of pump design.

Multistage pumps and compressors

Abstract: A multistage pump or compressor includes a series of axial flow stages. Each stage comprises an impeller for imparting whirl to the pumped fluid in one direction and a stator including vanes for imparting whirl to the pumped fluid in the opposite direction. The stator vanes define flow passages configured such that the fluid flows through the passages at substantially constant absolute velocity. The average ratio of stage axial length to impeller diameter for each axial flow stage is less than 0.4.