Showing papers on "Axial compressor published in 2004"

Three-Dimensional Separations in Axial Compressors

Abstract: Flow separations in the corner regions of blade passages are common. The separations are three dimensional and have quite different properties from the two-dimensional separations that are considered in elementary courses of fluid mechanics. In particular the consequences for the flow may be less severe than the two-dimensional separation. This paper describes the nature of three-dimensional separation and addresses the way in which topological rules, based on a linear treatment of the Navier-Stokes equations, can predict properties of the limiting streamlines, including the singularities which form. The paper shows measurements of the flow field in a linear cascade of compressor blades and compares these with the results of 3D CFD. For corners without tip clearance, the presence of three-dimensional separation appears to be universal and the challenge for the designer is to limit the loss and blockage produced. The CFD appears capable of predicting this.Copyright © 2004 by ASME

Flow Mechanism for Stall Margin Improvement due to Circumferential Casing Grooves on Axial Compressors

Abstract: A computational study is carried out to understand the physical mechanism responsible for the improvement in stall margin of an axial flow rotor due to the circumferential casing grooves. Computational fluid dynamics simulations show an increase in operating range of the low speed rotor in the presence of casing grooves. A budget of the axial momentum equation is carried out at the rotor casing in the tip gap in order to understand the physical process behind this stall margin improvement. It is shown that for the smooth casing the net axial pressure force at the rotor casing in the tip gap is balanced by the net axial shear stress force. However, for the grooved casing the net axial shear stress force acting at the casing is augmented by the axial force due to the radial transport of axial momentum, which occurs across the grooves and power stream interface. This additional force adds to the net axial viscous shear force and thus leads to an increase in the stall margin of the rotor.

Interaction of rim seal and annulus flows in an axial flow turbine

Abstract: A combined computational fluid dynamics (CFD) and experimental study of interaction of main gas path and rim sealing flow is reported. The experiments were conducted on a two stage axial turbine and included pressure measurements for the cavity formed between the stage 2 rotor disc and the upstream diaphragm for two values of the diaphragm-to-rotor axial clearance. The pressure measurements indicate that ingestion of the highly swirling annulus flow leads to increased vortex strength within the cavity. This effect is particularly strong for the larger axial clearance. Results from a number of steady and unsteady CFD models have been compared to the measured results. Good agreement between measurement and calculation for time-averaged pressures was obtained using unsteady CFD models, which predicted previously unknown unsteady flow features. This led to fest response pressure transducer measurements being made on the rig, and these confirmed the CED prediction.

Gas Turbine Axial Compressor Fouling And Washing.

Abstract: Engineer, Turbomachinery Group, LNG Product Development Center, with Bechtel Corporation, in Houston, Texas. His 25 years of industry experience encompasses turbomachinery design, engine development, vibration and failure analysis, and the testing of gas turbines and compressors. He has developed several aerothermal and transient analysis techniques for the condition monitoring of gas turbines. In the past he was Chief Engineer of Mee Industries, Gas Turbine Division, and Boyce Engineering International Inc. Mr. Meher-Homji has a B.S. degree (Mechanical Engineering) from Shivaji University, an M.E. degree from Texas A&M University, and an MBA from the University of Houston. He is a member and past Chair of ASME’s International Gas Turbine Institute’s Industrial and Cogeneration Committee. Mr. MeherHomji was named Fellow of the ASME in 1997. He is a registered Professional Engineer in the State of Texas, and has several publications in the area of turbomachinery engineering.

Parametric Study of Tip Clearance—Casing Treatment on Performance and Stability of a Transonic Axial Compressor

Abstract: The control of tip leakage flow (TLF) through the clearance gap between the moving and stationary components of rotating machines is still a high-leverage area for improvement of stability and performance of aircraft engines. Losses in the form of flow separation, stall, and reduced rotor work efficiency are results of the tip leakage vortex (TLV) generated by interaction of the main flow and the tip leakage jet induced by the blade pressure difference. The effects are more detrimental in transonic compressors due to the interaction of shock-TLV. It has been previously shown that the use of slots and grooves in the casing over tip of the compressor blades, known as casing treatment, can substantially increase the stable flow range and therefore the safety of the system but generally with some efficiency penalties. This paper presents a numerical parametric study of tip clearance coupled with casing treatment for a transonic axial-flow compressor NASA Rotor 37. Compressor characteristics have been compared to the experimental results for smooth casing with a 0.356 mm tip clearance and show fairly good agreement. Casing treatments were found to be an effective means of reducing the negative effects of tip gap flow and vortex, resulting in improved performance and stability. The present work provides guidelines for improvement of steady-state performance of the transonic axial-flow compressors and improvement of the stable operating range of the system.Copyright © 2004 by ASME

Influence of Surface Roughness on Three-Dimensional Separation in Axial Compressors

Abstract: Surface roughness on a stator blade was found to have a major effect on the three-dimensional (3D) separation at the hub of a single-stage low-speed axial compressor. The change in the separation with roughness worsened performance of the stage. A preliminary study was carried out to ascertain which part of the stator suction surface and at what operating condition the flow is most sensitive to roughness. The results show that stage performance is extremely sensitive to surface roughness around the leading edge and peak-suction regions, particularly for flow rates corresponding to design and lower values. Surface flow visualization and exit loss measurements show that the size of the separation, in terms of spanwise and chordwise extent, is increased with roughness present. Roughness produced the large 3D separation at design flow coefficient that is found for smooth blades nearer to stall. A simple model to simulate the effect of roughness was developed and, when included in a 3D Navier-Stokes calculation method, was shown to give good qualitative agreement with measurements.Copyright © 2004 by ASME

Unsteady three-dimensional flow phenomena due to breakdown of tip leakage vortex in a transonic axial compressor rotor

Abstract: Unsteady three-dimensional flow fields in a transonic axial compressor rotor (NASA Rotor 37) have been investigated by unsteady Reynolds-averaged Navier-Stokes simulations. The simulations show that the breakdown of the tip leakage vortex occurs in the compressor rotor because of the interaction of the vortex with the shock wave. At near-peak efficiency condition small bubble-type breakdown of the tip leakage vortex happens periodically and causes the loading of the adjacent blade to fluctuate periodically near the leading edge. Since the blade loading near the leading edge is closely linked to the swirl intensity of the tip leakage vortex, the periodic fluctuation of the blade loading leads to the periodic breakdown of the tip leakage vortex, resulting in self-sustained flow oscillation in the tip leakage flow field. However, the tip leakage vortex breakdown is so weak and small that it is not observed in the time-averaged flow field at near-peak efficiency condition. On the other hand, spiral-type breakdown of the tip leakage vortex is caused by the interaction between the vortex and the shock wave at near-stall operating condition. The vortex breakdown is found continuously since the swirl intensity of tip leakage vortex keeps strong at near-stall condition. The spiral-type vortex breakdown has the nature of self-sustained flow oscillation and gives rise to the large fluctuation of the tip leakage flow field, in terms of shock wave location, blockage near the rotor tip and three-dimensional separation structure on the suction surface. It is found that the breakdown of the tip leakage vortex leads to the unsteady flow phenomena near the rotor tip, accompanying large blockage effect in the transonic compressor rotor at the near-stall condition.Copyright © 2004 by ASME

An evaluation of the effects of water injection on compressor performance

Abstract: The injection of water droplets into compressor inlet ducting is now commonly used as a means of boosting the output from industrial gas turbines. The chief mechanisms responsible for the increase in power are the reduction in compressor work per unit flow and the increase in mass flow rate, both of which are achieved by evaporative cooling upstream of and within the compressor. This paper examines the impact of such evaporative processes on compressor operation, focussing particular attention on cases with substantial overspray-i.e., for which significant evaporation takes place within the compressor itself, rather than in the inlet. A simple numerical method is described for the computation of wet compression processes, based on a combination of droplet evaporation and mean-line calculations. The method is applied to a generic compressor geometry in order to investigate the nature of the off-design behavior that results from evaporative cooling. Consideration is also given to the efficiency of the compression process, the implications for choking and stall, and the magnitude of the thermodynamic loss resulting from irreversible phase change.

Effect of tip clearance on stall evolution process in a low-speed axial compressor stage

Abstract: Effect of the tip clearance on the transient process of rotating stall evolution has been studied experimentally in a low-speed axial compressor stage with various stator-rotor gaps. In the previous authors’ experiments for the small tip clearance, the stall evolution process of the rotor was sensitive to the gaps between the blade rows. For the large tip clearance, however, little difference is observed in the evolution processes independently of the blade row gap. In the first half process, it is characterized by gradual reduction of overall pressure-rise with flow rate decreasing, and the number of short length-scale disturbances is increasing with their amplitude increasing. In the latter half a long length-scale disturbance develops rapidly to result in deep stall. Just before the stall inception the spectral power density of the casing wall pressure reveals the existence of rotating disturbances with broadband high frequency near a quarter of the blade passing frequency. This is caused by the short length-scale disturbances occurring intermittently. A flow model is presented to explain mechanisms of the rotating short length-scale disturbance, which includes a tornado-like separation vortex and tip-leakage vortex breakdown. The model is supported by a result of a numerical unsteady flow simulation.Copyright © 2004 by ASME

Gas-liquid mass transfer: a comparison of down- and up-pumping axial flow impellers with radial impellers

Abstract: The performance of a down- and up-pumping pitched blade turbine and A315 for gas–liquid dispersion and mass transfer was evaluated and then compared with that of Rushton and Scaba turbines in a small laboratory-scale vessel. The results show that, when the axial flow impellers are operated in the up-pumping mode, the overall performance is largely improved compared with the down-pumping configuration. Compared with the radial turbines, the up-pumping A315 has a high gas handling capacity, equivalent to the Scaba turbine, and is economically much more efficient in terms of mass transfer than both turbines. On the other hand, the up-pumping pitched blade turbine is not as well adapted to such applications. Finally, the axial flow impellers in the down-pumping mode have the lowest performance of all the impellers studied, although the A315 is preferred of the pitched blade turbine.

Wavelet analysis of rotor-tip disturbances in an axial-flow compressor

Abstract: Dynamic pressure signals taken from a low-speed axial compressor are analyzed with a wavelet transform. Several practical issues of continuous wavelet transforms, within the context of rotor-tip flow analysis, are discussed. The discrete form of continuous wavelet transform is presented with a graphical derivation that uniquely explains a complicated mathematical process with a readable graphical language without losing much mathematical strictness. With this wavelet tool, the data from a low-speed, three-stage axial compressor are analyzed. In these data, the wavelet analysis is able to pinpoint the exact time when spikes were initiated, as early as hundreds of rotor re volutions prior to stall, and track them thereafter. It is found that the emergence of the spikes is strongly related to two structural asymmetries of this compressor, one in the rotating frame and the other in the stationary frame. The spike development is not continuous. The ability to track the prestall process in both time and space, as demonstrated in this paper, makes wavelet analysis an effective tool for investigating the complicated instability phenomena of axial compressors.

Variable capacity scroll compressor

Abstract: Disclosed is a variable capacity scroll compressor in which a high-pressure fluid within a thermodynamic cycle is introduced into the inside of the compressor to increase the compression volume and also the fluid inhaled/exhausted from the compressor allows the fluid being compressed to be bypassed in multi-stages, thereby varying the capacity of the compression fluid.

Compressor blade with dovetail slotted to reduce stress on the airfoil leading edge

Abstract: A blade of an axial compressor comprising: an airfoil is disclosed that has a leading edge and a root; a platform attached to the root of the airfoil; a dovetail attached to a side of the platform opposite to the airfoil; a neck of the dovetail adjacent the platform, and a slot in the neck and generally parallel to the platform, and the slot extends from a front of the neck to position in the neck beyond a line formed by the leading edge of the blade.

Simulation of flow generated by an axial-flow impeller: Batch and continuous operation

Abstract: It is important to extend and to validate computational flow models to simulate continuous operation of stirred vessels and to capture possible interaction of feed inlet/outlet with the flow generated by impellers. In the present work, we have developed and used a computational model to understand the flow generated by an axial flow impeller in a batch and a continuously operated baffled vessel. A multiple reference frames approach was used to simulate flow generated by the Mixel TT impeller in stirred vessel. The predicted velocity results show reasonably good agreement (qualitative as well as quantitative) with the experimental data. Characteristics of flow around blades of Mixel TT were studied using the computational model. The computational model was extended to simulate flow and mixing in a continuous operation. Simulations were carried out to understand the interaction of the jet emanating from the feed pipe and the flow generated by the impeller. Model predictions were compared with published experimental data, obtained by laser Doppler velocimetry. The differences and similarities between batch and continuous operation are highlighted. Mixing simulations were carried out to examine possible short-circuiting and non-ideal behaviour of the continuous operation of the stirred vessel. Influence of the impeller speed, feed rate and location of inlet/outlet on mixing and on the extent of non-ideality of flow was studied. The computational model and results discussed in this work will be useful for understanding the mixing process in continuous-flow stirred vessels.

The Impact of Forward Swept Rotors on Tip Clearance Flows in Subsonic Axial Compressors

Abstract: This paper presents an experimental and analytical study of the impact of forward swept rotors on tip-limited, low-speed, multistage axial compressors. Two different configurations were examined, one with strong tip-clearance flows and the other with more moderate levels. Evaluations were done at multiple rotor tip clearances to assess differences in clearance sensitivity. Compared to conventionally stacked radial rotors, the forward swept blades demonstrated improvements in stall margin, efficiency and clearance sensitivity. The benefits were more pronounced for the configuration with stronger tip-clearance flows. Detailed flow measurements and three-dimensional viscous CFD analyses were used to investigate the responsible flow mechanisms. Forward sweep causes a spanwise redistribution of flow toward the blade tip and reduces the tip loading in terms of static pressure coefficient. This results in reduced tip-clearance flow blockage, a shallower (more axial) vortex trajectory and a smaller region of reversed flow in the clearance gap.

Compressor with controllable recirculation and method therefor

Abstract: There is provided a compressor (10) and an associated method for controlling a recirculation flow to control surging in the compressor. The compressor includes a housing (12) and a compressor wheel (16) mounted therein. A recirculation passage (41) receives compressed air from the compressor and recirculates the compressed air to an inlet passage (20) of the housing and, in particular, to leading edges (32) of blades (18) of the compressor wheel. An adjustable flow control device (60) is configured to control the flow of the compressed air through the recirculation passage to control a surge characteristic of the compressor. For example, the flow control device can include one or more valves (Vl, V2, V3), each of which can be adjusted by an actuator (64).

Investigation on a type of flow control to weaken unsteady separated flows by unsteady excitation in axial flow compressors

Abstract: By solving unsteady Reynolds-averaged Navier-Stokes equations discretized by a high-order scheme, the results showed that the disordered unsteady separated flow could be effectively controlled by periodic suction and blowing in a wide range of incidences, resulting in enhancement of time-averaged aerodynamic performances of an axial compressor cascade. The effects of unsteady excitation frequency, amplitude, and excitation location were investigated in detail. The effective excitation frequency spans a wide spectrum, and there is an optimal excitation frequency that is nearly equal to the characteristic frequency of vortex shedding. Excitation amplitude exhibits a threshold value (nearly 10% in terms of the ratio of maximum velocity of periodic suction and blowing to the velocity of free flow) and an optimal value (nearly 35%). The optimal excitation location is just upstream of the separation point. We also explored feasible unsteady actuators by utilizing the upstream wake for constraining unsteady separation in axial flow compressors.

Early Stall Warning Technique for Axial-Flow Compressors

Abstract: This paper proposes a unique stall warning index based on pressure signals by high response transducers on the casing wall at the rotor leading edge location. The aim of the research is to explore the possibility of reducing current excessive stall margin requirement for compressor design based on the worst case scenario. The index is generated by computing correlation degradation of pressure time histories of current and one revolution before over each blade pitch. Tests conducted on a research compressor exhibits that the correlation diminishes significantly with proximity to stall and the proposed technique might have the capability of generating a stall warning signal sufficiently in advance of spike inception. Extensive experiments on a research compressor show that the degree of the index degradation depends on various factors such as flow coefficient, tip clearance and rotor blade incidence. In order to obtain a reliable stall warning signal in practical use, these effects must be carefully examined.Copyright © 2004 by ASME

Design and transient computational fluid dynamics study of a continuous axial flow ventricular assist device.

Abstract: A ventricular assist device (VAD), which is a miniaturized axial flow pump from the point of view of mechanism, has been designed and studied in this report. It consists of an inducer, an impeller, and a diffuser. The main design objective of this VAD is to produce an axial pump with a streamlined, idealized, and nonobstructing blood flow path. The magnetic bearings are adapted so that the impeller is completely magnetically levitated. The VAD operates under transient conditions because of the spinning movement of the impeller and the pulsatile inlet flow rate. The design method, procedure, and iterations are presented. The VAD's performance under transient conditions is investigated by means of computational fluid dynamics (CFD). Two reference frames, rotational and stationary, are implemented in the CFD simulations. The inlet and outlet surfaces of the impeller, which are connected to the inducer and diffuser respectively, are allowed to rotate and slide during the calculation to simulate the realistic spinning motion of the impeller. The flow head curves are determined, and the variation of pressure distribution during a cardiac cycle (including systole and diastole) is given. The axial oscillation of impeller is also estimated for the magnetic bearing design. The transient CFD simulation, which requires more computer resources and calculation efforts than the steady simulation, provides a range rather than only a point for the VAD's performance. Because of pulsatile flow phenomena and virtual spinning movement of the impeller, the transient simulation, which is realistically correlated with the in vivo implant scenarios of a VAD, is essential to ensure an effective and reliable VAD design.

Performance Degradation Due to Blade Surface Roughness in a Single-Stage Axial Turbine

Abstract: Turbine blades experience significant surface degradation with service. Previous studies indicate that an order of magnitude or greater increase in roughness height is typical, and these elevated levels of surface roughness significantly influence turbine efficiency and heat transfer. This paper presents measurement and a mean line analysis of turbine efficiency reduction due to blade surface roughness. Performance tests have been conducted in a low speed, single-stage, axial flow turbine with roughened blades. Sheets of sandpaper with equivalent sandgrain roughnesses of 106 and 400 μm have been used to roughen the blades. The roughness heights correspond to foreign deposits on real turbine blades measured by Bons et al. [1]. In the transitionally rough regime (106 μm), normalized efficiency decreases by approximately 4 percent with either roughened stator or roughened rotor and 8 percent with roughness on both the stator and rotor blades. In the fully rough regime (400 μm), normalized efficiency decreases by 2 percent with roughness on the pressure side and by 6 percent with roughness on the suction side. Also, the normalized efficiency decreases by 11 percent with roughness only on stator vanes; 8 percent with roughness only on rotor blades; and 19 percent with roughness on both the stator and rotor blades.Copyright © 2004 by ASME

Design and Test of an Ultra-Low Solidity Flow-Controlled Compressor Stator

Abstract: A full annulus fluidic flow-controlled compressor stator ring was designed and tested in the third stage of a four-stage low-speed research compressor. The solidity of the flow-controlled stator was near unity and significantly below design practice with a commensurately high diffusion factor. The design intent was to reduce the vane count by 30% and load the stator to the point of stall at the design point, then employ flow control to restore attached boundary layers and regain design-point stage matching. The flow control applied, which maintained attached flow, was 1% of the compressor mass flow and was introduced via discrete steady jets on the suction side of the stator. The design method used steady Computational Fluid Dynamics (CFD) with the flow control jets simulated to drive stator exit angles, velocities, and blockage to match the baseline machine. The experiment verified the pretest predictions and demonstrated degraded compressor performance without flow control and restoration of the pumping characteristics of the baseline high solidity compressor when flow control was applied. An assessment of the engine cvcle impact of the flow-controlled compressor shows a 2.1 point stage efficiency reduction for the increased loading. Extrapolation of the data and analysis to a high-speed compressor shows a more modest 0.5 point stage efficiency trade.