Modern Compressor Research Articles

Compressor Leading Edge Spikes: A New Performance Criterion

Compressor blades often have a small “spike” in the surface pressure distribution at the leading edge. This may result from blade erosion, manufacture defects, or compromises made in the original design process. This paper investigates the effect of these spikes on profile loss, and presents a criterion to ensure they are not detrimental to compressor performance. In the first part of the paper, two geometries of leading edge are tested. One has a small spike, typical of those found on modern compressors; the other has no spike, characteristic of an idealized leading edge. Testing was undertaken on the stator of a single-stage low speed compressor. The time resolved boundary layer was measured using a hot-wire microtraversing system. It is shown that the presence of the small spike changes the time resolved transition process on the suction surface, but that this results in no net increase in loss. In the second part of the paper, spike height is systematically changed using a range of leading edge geometries. It is shown that below a critical spike height, the profile loss is constant. If the critical spike height is exceeded, the leading edge separates and profile loss rises by 30%. Finally, a criterion is developed, based on the total diffusion across the spike. Three different leading edge design philosophies are investigated. It is shown that if the spike diffusion factor is kept below 0.1 over the blade’s incidence range, performance is unaffected by leading edge geometry.

Numerical investigation of a high-subsonic axial-flow compressor rotor with non-axisymmetric hub endwall

The major source of loss in modern compressors is the secondary loss. Non-axisymmetric endwall profile contouring is now a well established design methodology in axial flow turbines. However, flow development in axial compressors is differ from turbines, the effects of non-axisymmetric endwall to axial compressors requires flow analysis in detail. This paper presents both experimental and numerical data to deal with the application of a non-axisymmetric hub endwall in a high-subsonic axial-flow compressor. The aims of the experiment here were to make sure the numerically obtained flow fields is the physical mechanism responsible for the improvement in efficiency, due to the non-axisymmetric hub endwall. The computational results were first compared with available measured data of axisymmetric hub endwall. The results agreed well with the experimental data for estimation of the global performance. The coupled flow of the compressor rotor with non-axisymmetric hub endwall was simulated by a state-of-the-art multi-block flow solver. The non-axisymmetric hub endwall was designed for a subsonic compressor rotor with the help of sine and cosine functions. This type of non-axisymmetric hub endwall was found to have a significant improvement in efficiency of 0.45% approximately and a slightly increase for the total pressure ratio. The fundamental mechanisms of non-axisymmetric hub endwall and their effects on the subsonic axial-flow compressor endwall flow field were analyzed in detail. It is concluded that the non-axisymmetric endwall profiling, though not optimum, can mitigate the secondary flow in the vicinity of the hub endwall, resulting in the improvement of aerodynamic performance of the compressor rotor.

Particle Image Velocity Investigation of a High Speed Centrifugal Compressor Diffuser: Spanwise and Loading Variations

An efficient diffuser is essential to a modern compressor stage due to its significance in stage performance, durability, and operability. To address the need for data that describe the complex, unsteady flow field in a vaned diffuser, particle image velocity is utilized to characterize the spanwise and circumferential variations in the flow features in the vaned diffuser passage of a transonic centrifugal compressor. The spanwise variation in the diffuser flow field is further investigated by the comparison of three different operating conditions representative of low, nominal, and high loading. These data demonstrate that not only the diffuser flow field is highly dependent on the operation conditions, e.g., hub-to-shroud variation increases with loading, but also the circumferential periodicity, created by the highly three dimensional impeller discharge flow, generates a larger unsteadiness toward the hub region of the vaned diffuser.

Spike-Type Compressor Stall Inception, Detection, and Control

An aerodynamic instability known as stall occurs in axial compressors as the mass flow rate is reduced and the blade loading reaches its limit. At this limiting condition, an easily recognizable flow breakdown process, known as spike-type stall inception, is observed in most modern compressors. This article begins by examining measurements from both low- and high-speed compressors to explain the characteristic features of spike-type stall. This is followed by a review of past work on compressor stability and an assessment of recent advances in this field. Included here is a study of the three-dimensional flow features that typify spike formation and its eventual growth into a mature stall cell. We also consider the formation criteria for spike-type stall and the means for early detection and possible control. On the computational side, a possible mechanism for spike formation is identified from three-dimensional studies of the flow in the rotor tip region. This mechanism involves tip-clearance backflow at the blade's trailing edge in combination with forward spillage of tip-leakage flow at the leading edge. This flow pattern implies that a successful stall-control technology will have to rely on an effective means of suppressing tip-clearance backflow and forward spillage.

Modern compressor equipment

Modern compressor equipment

PISTON EXPANDER-COMPRESSOR UNIT HAVING SELF-ACTING GAS DISTRIBUTION SYSTEMS

plants [1]. Operation of such expanders at a low initial air pressure (as low as 0.8 MPa) to produce moderate temperatures is inefficient due to impairment of specific energy parameters. This is associated primarily with forced gas distribution systems used in piston (reciprocating) expanders. In a forced valve drive (external and internal), there is a large number of highly passive (sluggish) moving parts, because of which the rotation speed of piston expanders is low (as low as 500 rpm with an external drive and 1000 rpm with an internal drive) [1]. One way of refining designs of piston expanding machines is to replace forced gas distribution systems by self-acting valves [2–4]. In the piston expander cylinder, the gas may be made to move by a concurrent flow or by a nonconcurrent flow scheme: the concurrent flow scheme provides for a normally open inlet valve and an exit port at the end of the piston stroke (at the lower dead spot); the nonconcurrent flow scheme provides for a normally open inlet and a closed outlet valve. Self-acting valves, by virtue of their low inertia, allow one to raise the rotation speed of the expander crankshaft to the level of the rotation speed of modern high-speed piston compressors, which makes it possible to combine a compressor and an expander into an expander-compressor unit (ECU) with a common crannkshaft. Expander-compressor units designed to obtain moderate temperatures and operating at a low pressure can be built on unified compressor bases with a rated piston force of the order of 2.5–16 kN. In recent years, several designs have been developed and investigations have been carried out into the processes that occur in piston expanders having self-acting valves [5]. Let us examine a piston ECU having an expander operating in accordance with the concurrent flow [6] and nonconcurrent flow [7] gas distribution schemes. The experimental investigations of the ECU were carried out on a bench built on the basis of the vertical two-stage two-set ship compressor 20K1 which has one-way operating differential pistons: cylinder diameters 0.1 and 0.035 m respectively, piston stroke 0.1 m, rated rotation speed 500 rpm, air suction (intake) pressure of the compressor atmospheric, and compression ratio 4–8. The compressor 20K1 was converted to an ECU as follows. The valve heads of the cylinders with a diameter 0.035 m of the second stage of the compressor were replaced by valve heads with self-acting valves that ensure gas distribution of the function of the expander that operates at a low initial pressure (as low as 1 MPa). In the lower part of one of the cylinders (diameter 0.035 m), a circular slit was made on the perimeter for the exit of the expanded cooled air. Chemical and Petroleum Engineering, Vol. 37, Nos. 9–10, 2001

Stator well flows in axial compressors

Abstract This paper reports a study of the flow conditions in spaces often found beneath the stationary blades of modern compressors – stator wells. The design of the stator wells in a modern high-pressure turbofan can be a limiting factor. This study shows good comparison with existing experimental data upon the pressures and velocities in related geometries. The deleterious effects upon machine performance of the flows between the stator well and the mainstream are considered. The purpose of this paper is to show that these effects, especially seal leakage, can be reduced by the incorporation of fins on the rotating component of the downstream well. This works by increasing the tangential velocity in this region, so reducing the pressure there and thus the inter-well leakage for a given radial clearance in the intermediate labyrinth seal and hence the outflow into the mainstream from the upstream well.

Unsteady Flow and Whirl-Inducing Forces in Axial-Flow Compressors: Part I—Experiment

An experimental and theoretical investigation has been conducted to evaluate the effects seen in axial-flow compressors when the centerline of the rotor is displaced from the centerline of the static structure of the engine. This creates circumferentially nonuniform rotor-tip clearances, unsteady flow, and potentially increased clearances if the rotating and stationary parts come in contact. The result not only adversely affects compressor stall margin, pressure rise capability, and efficiency, but also generates an unsteady, destabilizing, aerodynamic force, called the Thomas/Alford force, which contributes significantly to rotor whirl instabilities in turbomachinery. Determining both the direction and magnitude of this force in compressors, relative to those in turbines, is especially important for the design of mechanically stable turbomachinery components. Part I of this two-part paper addresses these issues experimentally and Part II presents analyses from relevant computational models. Our results clearly show that the Thomas/Alford force can promote significant backward rotor whirl over much of the operating range of modern compressors, although some regions of zero and forward whirl were found near the design point. This is the first time that definitive measurements, coupled with compelling analyses, have been reported in the literature to resolve the long-standing disparity in findings concerning the direction and magnitude of whirl-inducing forces important in the design of modern axial-flow compressors.

Full-Annulus Simulations of Airfoil Clocking in a 1-1/2 Stage Axial Compressor

Axial compressors have inherently unsteady flow fields because of relative motion between rotor and stator airfoils. This relative motion leads to viscous and inviscid (potential) interactions between blade rows. As the number of stages increases in a turbomachine, the buildup of convected wakes can lead to progressively more complex wake/wake and wake/airfoil interactions. Variations in the relative circumferential positions of stators or rotors can change these interactions, leading to different unsteady forcing functions on airfoils and different compressor efficiencies. In addition, as the Mach number increases the interaction between blade rows can be intensified due to potential effects.It has been shown, both experimentally and computationally, that airfoil clocking can be used to improve the efficiency and reduce the unsteadiness in multiple-stage axial turbomachines with equal blade counts in alternate blade rows. While previous investigations have provided an improved understanding of the physics associated with airfoil clocking, more research is needed to determine if airfoil clocking is viable for use in modern gas-turbine compressors. This paper presents the results of a combined experimental/computational research effort to study the physics of airfoil clocking in a high-speed axial compressor. Computational simulations have been performed for eight different clocking positions of the stator airfoils in a 1-1/2 stage high-speed compressor. To accurately model the experimental compressor, full-annulus simulations were conducted using 34 IGV, 35 rotor and 34 stator airfoils. It is common practice to modify blade counts to reduce the computational work required to perform turbomachinery simulations, and this approximation has been made in all computational clocking studies performed to date. A simulation was also performed in the present study with 1 inlet guide vane, 1 rotor airfoil, and 1 stator airfoil to model blade rows with 34 airfoils each in order to examine the effects of this approximation. Time-averaged and unsteady data (including performance and boundary layer quantities) were examined. The predicted results indicate that simulating the full annulus gives better qualitative agreement with the experimental data, as well as more accurately modeling the interaction between adjacent blade rows.Copyright © 1999 by ASME

Delta algorithms

Delta algorithms compress data by encoding one file in terms of another. This type of compression is useful in a number of situations: strong multiple versions of data, displaying differences, merging changes, distributing updates, storing backups, transmitting video sequences, and others. This article studies the performance parameters of several delta algorithms, using a benchmark of over 1,300 pairs of files taken from two successive releases of GNU software. Results indicate that modern delta compression algorithms based on Ziv-Lempel techniques significantly outperform diff , a popular but older delta compressor, in terms of compression ratio. The modern compressors also correlate better with the actual difference between files without sacrificing performance.

IGV–Rotor Interaction Analysis in a Transonic Compressor Using the Navier–Stokes Equations

A recently developed, time-accurate multigrid viscous solver has been extended to handle quasi-three-dimensional effects and applied to the first stage of a modern transonic compressor. Interest is focused on the inlet guide vane (IGV)-rotor interaction where strong sources of unsteadiness are to be expected. Several calculations have been performed to predict the stage operating characteristics. Flow structures at various mass flow rates, from choke to near stall, are presented and discussed. Comparisons between unsteady and steady pitch-averaged results are also included in order to obtain indications about the capabilities of steady, multi-row analyses.

A High-Speed Capacitance-Based System for Gaging Turbomachinery Blading Radius During the Tip Grind Process

During the manufacture and overhaul of gas turbines, it is necessary to ensure that all blades in a stage are of an equal and known length to minimize the loss in performance that arises as a consequence of the clearance between rotor tip and engine casing. Modern compressor and turbine blades are generally loose fitting in their root fixings and only adopt their true working position when running. In this paper, a new technique for measuring the rotor radius over individual blades is described. The measurement technique utilizes a capacitance-based clearance measurement system, which enables rotor radius to be measured over each blade while spinning fast enough to ensure that the blades are centrifugally loaded into their true working position. The measurement technique is described, as is the system utilized to calibrate and reduce output into engineering units. The mechanical design of the “measurement head” is presented and the CNC lathe to which it interfaces described. Finally, the results of an experimental program, utilizing a fully bladed compressor disk undertaken to ascertain system performance, are presented.

Numerical integration of the blade-to-blade surface Euler equations in vibrating cascades

The unsteady aerodynamics of fluttering cascades is a demanding problem in the analysis of modern compressors. An algorithm has been developed for numerically integrating the Euler equations in biade-to-biade surface formulation. The method simulates all the interblade channels of an annular cascade. The equations are discretized in a grid that moves in order to follow the vibration of the blades. The equations are integrated using the explicit MacCormack scheme in finite-differ ence formulation. Mistuned vibration as well as standing-, traveling-, or influence-wav e modes may be readily simulated. Also, two other faster methods have been developed, simulating traveling and influence waves, respectively. A number of numerical results show the aptitude of the method to simulate both started and unstarted supersonic flow in vibrating cascades. A first comparison with available wind-tunnel data corroborates the validity of this approach to predict supersonic flutter of fans and compressors.