Performance Of Centrifugal Compressor Research Articles

Application of particle image velocimetry to a transonic centrifugal compressor

As part of an ongoing research project the performance and internal flow field of a high-pressure ratio centrifugal compressor is being investigated. Based on previous, primarily, point-wise laser-optical measurements the compressor was redesigned and resulted in an improved impeller and diffuser with a single-stage pressure ratio of 6:1 at 50,000 rpm. Current research activities involve the use of particle image velocimetry (PIV) to analyze and further improve the understanding of the complex flow phenomena inside the vaned diffuser given the capability of PIV of capturing spatial structures. The study includes phase-resolved measurements of the flow inside a diffuser vane passage with respect to the impeller blade position. Both, instantaneous and phase-averaged velocity fields are presented. The flow field results obtained by PIV are to be used for future validation of the related CFD calculations, which in turn are expected to lead to further improvements in compressor performance. In addition, the potential of stereo PIV for this type of turbomachinery application could be successfully demonstrated.

Improving a one-dimensional centrifugal compressor performance prediction method

A one-dimensional centrifugal compressor performance prediction technique that has been available for some time is updated as a result of extracting the component performance from three-dimensional computational fluid dynamic (CFD) analyses. Confidence in the CFD results is provided by comparison of overall performance for one of the compressor examples. The extracted impeller characteristic is compared with the original impeller loss model, and this indicated that some improvement was desirable. The position of least impeller loss was determined using a traditional axial compressor cascade method, and suitable algebraic expressions were derived to match the CFD data. The merit of the approach lies with the relative ease that CFD component performance currently can be achieved and adjusting one-dimensional methods to agree with the CFD-derived models.

운전 익단간극을 고려한 마이크로터빈 코어용 원심압축기의 성능특성 연구

Tip clearance of centrifugal compressor affects the performance. Larger tip clearance results in lower efficiency. What really affects the performance is the running tip clearance, not the cold tip clearance. When the compressor is operating, blade strain and the pressure difference between impeller backplate and hub affects the running tip clearance. This paper describes measured running tip clearance and its effects on the performance of centrifugal compressor. Cold tip clearance before operation was 0.4 ㎜ and running tip clearance varied from 0.86 ㎜ to 0.25 ㎜ with impeller exit pressure. As the pressure at impeller exit increases, the running tip clearance tends to decreases. The target running tip clearance for compressor at 100 % speed was 0.3 ㎜, and it turned out to be exactly 0.30 ㎜ from experiment.

Effect of diffuser vane height and position on the performance of a centrifugal compressor

The present paper reports experimental investigations on the effect of diffuser vane height and position on the performance of a low-speed centrifugal compressor. The diffuser vane height is systematically varied from 0.2 to 0.9 times the diffuser width. In addition, the effect of vane position is examined by fixing the partial vanes to the hub, shroud, or hub and shroud. The compressor performance is determined with these vanes in the vane and low solidity vane diffuser configurations. It is found that there is an optimum height for the diffuser vane height. In the present investigation, it is found to be 0.3 times the diffuser width. The effect of position of the partial vanes on the hub or shroud on the compressor performance is found to be negligible. However, when partial vanes are fixed on the hub and shroud staggered at half spacing, the compressor performance is improved substantially.

R134a 터보냉동기용 원심압축기의 익단간극이 성능특성에 미치는 영향에 관한 수치해석적 연구

In this study, the overall performance and the effect of the tip leakage flow of the centrifugal compressor with a refrigerant HFC-134a were numerically studied using CFX-TASCflow. To study the effect of the tip leakage flow, the numerical study of the overall performance of HFC-134a centrifugal compressor with a cascade diffuser was preceded and compared with the experimental result. Six different tip clearances were used to consider the influence of the tip clearance on the performance. The tip leakage flow was illustrated for qualitative discussion. The results obtained in this study can be applied to the determination of the cold clearance.

Numerical investigation of a high-speed centrifugal compressor with hub vane diffusers

Hub vane diffusers can obtain higher pressure recovery and efficiency compared with the vaneless diffuser and a wider operating range compared with the vane diffuser. To improve the performance of high pressure ratio centrifugal compressors it is necessary to understand the flow phenomena in the hub vane diffusers. Although some research work on hub vane diffuser flow has been published, many unknown flow physics and disputed questions still remain. A computational analysis of the flowfield in a high-speed centrifugal compressor with a backswept unshrouded impeller and wedge vane channel diffuser is presented. In order to obtain the optimum hub vane height, the pressure and velocity fields were numerically simulated for eight kinds of hub vane, which are varied as follows: h/b = 0 (vaneless), 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 and 1 (vane). The advanced Navier-Stokes solver EURANUS/TURBO is applied with a Baldwin-Lomax model for closure. The calculations are performed with a central space discretization scheme, with second- and fourth-order artificial dissipation terms. The numerical procedure applied a four-stage explicit Runge-Kutta scheme, coupled to local time stepping. The good agreement of the computed stage performance and the experimental data of the stage with the vane diffuser shows that the solver is effective and reasonable. Numerical results show that the maximum flowrate attained by the stage with the vane diffuser is about 12.5 per cent less than with the hub vane diffusers. The stage efficiency reaches its maximum value for the hub vane diffuser with an h/b ratio of 0.5 at lower flowrate and for the hub vane diffuser with an h/b ratio of 0.4 at higher flowrate. The hub vane diffusers have a wider operating range than the vane diffuser. The hub vane diffuser with an h/b ratio of 0.3 shows a higher mass-averaged static pressure coefficient up to a flow coefficient φ = 0.16. The hub vane diffuser with an h/b ratio of 0.4 shows a slightly higher mass-averaged static pressure coefficient over an operating range of φ = 0.18–0.28, but it falls beyond φ = 0.3 below that of the vaneless diffuser. The performance of the centrifugal compressor with the shroud vane diffuser is better than that with the hub vane diffuser for the same height of the vane.

The Inlet Flow Structure of a Centrifugal Compressor Stage and Its Influence on the Compressor Performance

The performance of centrifugal compressors can be seriously degraded by inlet flow distortions that result from an unsatisfactory inlet configuration. In this present work, the flow is numerically simulated and the flow details are analyzed and discussed in order to understand the performance behavior of the compressor exposed to different inlet configurations. In a previous work, complementary to this present work, experimental tests were carried out for the comparison of a centrifugal compressor stage performance with two different inlet configurations: one of which was a straight pipe with constant cross-sectional area and the other a 90-deg curved pipe with nozzle shape. The comparative test results indicated significant compressor stage performance difference between the two different inlet configurations. Steady-state compressor stage simulation including the impeller and diffuser with three different inlets has been carried out to investigate the influence of each inlet type on the compressor performance. The three different inlet systems included a proposed and improved inlet model. The flow from the bend inlet is not axisymmetric in the circumferential and radial distortion, thus the diffuser and the impeller are modeled with fully 360-deg passages.

Aerodynamics of a Transonic Centrifugal Compressor Impeller

High-pressure ratio centrifugal compressors are applied to turbochargers and turboshaft engines because of their small dimensions, high efficiency, and wide operating range. Such a high-pressure ratio centrifugal compressor has a transonic inlet condition accompanied with a shock wave in the inducer portion. It is generally said that extra losses are generated by interaction of the shock wave and the boundary layers on the blade surface. To improve the performance of high-pressure ratio centrifugal compressor, it is necessary to understand the flow phenomena. Although some research works on transonic impeller flow have been published, some unknown flow physics are still remaining. The authors designed a transonic impeller, with an inlet Mach number about 1.3, and conducted detailed flow measurements by using laser doppler velocimetry (LDV). In the result, the interaction between the shock wave and tip leakage vortex at the inducer and flow distortion at the downstream of inducer were observed. The interaction of the boundary layer and the shock wave was not observed. Also, computational flow analysis was conducted and compared with experimental results.

Effect of Prewhirl on the Performance of Centrifugal Compressors

Effect of Prewhirl on the Performance of Centrifugal Compressors

Influence of Tip Clearance on the Flow Field and Aerodynamic Performance of the Centrifugal Impeller

AbstractThis paper studies numerically the influence of the tip clearance on the three-dimensional viscous flowfield and performance of the NASA Low-Speed Centrifugal Compressor (LSCC) impeller with a vaneless diffuser. A three-dimensional viscous code developed by the authors is applied with several acceleration methods: local time-step, multigrid and residual smoothing. The computations were performed under several operating conditions with four different tip clearance sizes (0.0%, 50%, 100% and 200% design tip-clearance). The numerical results show that: (1) the throughflow wake is considerably influenced by the tip-clearance. The low-kinetic-energy fluid accumulated in the shroud/pressure side of the unshrouded impeller, but in the shroud/suction region for 0.0 tip clearance; (2) there possibly exists an optimum size of the tip-clearance which is not the zero tip clearance to make the flow loss minimized.

Effect of Prewhirl on the Performance of Centrifugal Compressors

This article has no abstract.

Editorial

This volume contains eight papers on turbomachinery covering CFD applications, detailed flow measurements on unsteady flow, new design methods and control devices, and conventional empirical approaches. Shi and Tsukamoto succeeded in predicting pressure fluctuations caused by the interaction between a centrifugal impeller and diffuser vanes, using a Navier-Stokes code with standard k−ε turbulence model. Using a commercial code, Gu et al. succeeded in predicting the performance of a single-stage centrifugal compressor and studied the volute/diffuser flow in detail at design and off design conditions. By combining a scheme called the Single-Sweep Method with an algebraic eddy viscosity model, Kochevsky made a parametric study of the energy loss in annular diffusers with a rotating hub and inlet flow swirl, with the purpose of minimizing the loss in annular diffusers. These papers suggest that CFD is now becoming capable of predicting not only steady flows under design flow conditions, but also unsteady flows and off-design flows. Sinha, Pinarbasi and Katz, using PIV and pressure fluctuation measurements, made detailed observations of unsteady flow process during the onset and developed stages of rotating stall within a vaned diffuser of a centrifugal pump. This work is useful for refining CFDs so that they can predict local flow process, as well as for obtaining a physical understanding of the stalling process. The combination of those efforts in CFD and detailed flow measurements will lead CFDs to more useful design tools that require less experience and fewer empirical factors, and also to methods for clarifying the flow physics in turbomachinery. New production technologies can expand the applications of turbomachinery. Liu, Nishi and Yoshida have shown that a high efficiency mini turbopump is possible by employing a larger outlet blade angle and a larger number of blades with a smaller outlet/inlet area ratio, combined with high precision manufacturing technology. Two papers propose new control methods. Saha et al. propose the application of radial grooves on the casing walls between impeller exit and vaned diffuser throat, to control vaned diffuser rotating stall in radial centrifugal impellers. In many cases the suppression was realized at the cost of head decrease, but it was shown that shallow grooves can decrease the rotating stall onset flow rate with acceptable head decrease. A pitch-flap coupling mechanism to control the output power of a horizontal axis wind turbine was studied by Shimizu and Kamada to determine the optimum pitch angle to flap angle ratio and also the flow physics of control. Despite the developments of CFD as mentioned before, we still need to depend largely on empirical relations in various aspects of turbomachinery design. Lazzarotto et al. studied the influence of Reynolds number on the performance of cross flow fans with various casing geometry. Engin and Gur studied the effects of solid particles on the performance of open type centrifugal pump impellers with various tip clearance. Such empirical studies will remain a requirement for the practical design of various types of turbomachinery.

The influence of inlet flow distortion on the performance of a centrifugal compressor and the development of an improved inlet using numerical simulations

The performance of centrifugal compressors can be seriously affected by inlet flow distortions due to the unsatisfactory nature of the inlet configuration and the resulting inlet flow structure. Experimental tests have been carried out for the comparison of centrifugal compressor stage efficiency with two different inlet configurations, one of which is straight with constant cross-sectional area and the other a 90° curved pipe with nozzle shape. The comparative test results indicated significant compressor stage performance difference between the two different inlet configurations and the details are discussed to understand the performance behaviour of the compressor exposed to the distorted flow from the bend inlet configuration. The experimental investigation motivated the need for a new inlet design as well as a clear picture of the detailed flow field in the existing inlet design using numerical simulations. Two design approaches are reported in this paper, one of which is the location of vanes and the other the length of the curvature radius. For a more effective design method, a generalized formula is developed for the optimum position and number of vanes in such a way that each divided flow passage with vanes shares the same pressure gradient in radial direction. Numerical simulation results are presented and discussed in terms of mass-averaged parameters and flow structures, based on the comparison of flow properties at the pipe exit cross-sectional area of each design. Finally, new designs of different inlet systems are proposed to reduce the secondary flow and to provide flow as uniform as possible for a compressor.

The design and analysis of pipe diffusers for centrifugal compressors

The geometry of the region between the impeller tip and diffuser throat is accepted as being very influential, little understood and crucial to the performance of centrifugal compressors. This paper presents an experimental investigation of the flow in this region where the diffusing section of the compressor is of the ‘pipe diffuser’ configuration. The results are presented with an emphasis on design-type correlations. The principal features under investigation were the size of the diffuser throat and the influence of the number of diffuser passages.

The Performance of a Centrifugal Compressor With High Inlet Prewhirl

The performance requirements of centrifugal compressors usually include a broad operating range between surge and choke. This becomes increasingly difficult to achieve as increased pressure ratio is demanded. In order to suppress the tendency to surge and extend the operating range at low flow rates, inlet swirl is often considered through the application of inlet guide vanes. To generate high inlet swirl angles efficiently, an inlet volute has been applied as the swirl generator, and a variable geometry design developed in order to provide zero swirl. The variable geometry approach can be applied to increase the swirl progressively or to switch rapidly from zero swirl to maximum swirl. The variable geometry volute and the swirl conditions generated are described. The performance of a small centrifugal compressor is presented for a wide range of inlet swirl angles. In addition to the basic performance characteristics of the compressor, the onsets of flow reversals at impeller inlet are presented, together with the development of pressure pulsations, in the inlet and discharge ducts, through to full surge. The flow rate at which surge occurred was shown, by the shift of the peak pressure condition and by the measurement of the pressure pulsations, to be reduced by over 40 percent.

Aerodynamic Performance and Instability Initiation of a High Speed Research Centrifugal Compressor

Aerodynamic Performance and Instability Initiation of a High Speed Research Centrifugal Compressor

The effect of pinched diffuser on aerodynamic performance in a centrifugal compressor

The effect of 15% pinched diffuser in a centrifugal air compressor with a cascade airfoil diffuser on the aerodynamic performance is investigated using a numerical approach. The commercial CFD code for three-dimensional, turbulent, compressible flow fields is executed for various mass flow rates at a design speed which can be obtained as long as the calculation succeeds. The pinched diffuser is found to help improve the instability of flow within vaneless diffuser space, especially the reverse flow near shroud, and to change both stall/surge line and choking line to increase the surge margin. It is also found to generate more favorable increase of static pressure in diffuser region, and to increase the resulting pressure ratio and efficiency.

Numerical Analysis of the Three-Dimensional Swirling Flow in Centrifugal Compressor Volutes

The improvement of centrifugal compressor performance and the control of the radial forces acting on the impeller due to the circumferential variation of the static pressure caused by the volute require a good understanding of the flow mechanisms and an accurate prediction of the flow pattern inside the volute. A three-dimensional volute calculation method has been developed for this purpose. The volute is discretized by means of hexahedral elements. A cell vertex finite volume approach is used in combination with a time-marching procedure. The numerical procedure makes use of a central space discretization and a four-step Runge–Kutta time-stepping scheme. The artificial dissipation used in the solver is based on the fourth-order differences of the conservative variables. Implicit residual smoothing improves the convergence rate. The loss model implemented in the code accounts for the losses due to internal shear and friction losses on the walls. A comparison of the calculated and measured results inside a volute with elliptical cross section reveals that the modified Euler solver accurately predicts the velocity and pressure distribution inside and upstream of the volute.

Performance of a high-pressure-ratio centrifugal compressor influenced by distribution of tip clearance of the mixed-flow impeller.

To investigate influences of the distribution of tip clearance on the performance of centrifugal compressors, the mixed-flow impeller of a centrifugal compressor with l0:1 pressure ratio was tested with various distributions of the tip clearance varied by not only the axial movement but also the radial reforming of the shroud casing relative to the impeller. Measured data on the decrements of the input head and of the impeller efficiency due to the distribution of tip clearance are compared with the predicted values. The change in the available flow range due to the distribution of tip clearance is also discussed.

Dynamic Similitude Theory: Key to Understanding the ASME Compressors Performance Test

Summary Centrifugal compressors for gas-lift or injection stations in remote or inaccessible locations should be performance tested by the manufacturer before installation. Because the gas available to the manufacturer is quite different from that at the site, equations correlating test conditions to actual conditions are required. Developed in the 1960's, the American Soc. of Mechanical Engineers' (ASME) Power Test Code-10 (PTC- 10) is the acknowledged industry standard defining these correlations. The physics and mathematics of these correlations are not contained in the code, which is typical of all industry standards. All fluid flow is governed by the conservation equations of mass, energy, and momentum. The original authors of the code took this basic fact and developed the correlations that are essential for proper testing. Introduction The task of verifying a centrifugal compressor's performance at a manufacturer's facility is quite tedious and complicated. Engineers not normally involved in the process can be overwhelmed and confused by the ASME PTC-10, the document that establishes the test rules. Because the objective of the test code is to set forth the test rules, it does not provide the user with any theoretical explanation of why the test conditions are identical to operating conditions. Without this background, it is difficult to understand the test procedure and even more difficult to interpret the test results. This paper explains the theory behind the PTC-10 and presents a practical application. PTC-10 Test Objective The objective of a PTC-10 test is to allow the compressor operator to verify that the machine will perform as specified: it will deliver the specified flow, at the specified inlet conditions, to the specified pressure rise, with the specified efficiency and speed (RPM). Hence, the four parameters that must be verified during testing are flow rate, pressure rise, efficiency, and speed. Three different testing techniques (classes) are defined by the code. In Class 1, the test gas and specified gas, as well as inlet and outlet conditions, are identical. In Class 2, the test gas is not the same as the operating gas. The compressor test speed, pressure, temperature, and flow rates are all adjusted so that the test condition is dynamically equivalent to the specified condition. The test in Class 3 is identical to a Class 2 test, except that certain thermodynamic properties of either gas exceed the limits of Table 4 of the Code. Since a Class 1 test is straightforward, a detailed explanation is not required. For Classes 2 and 3, PTC-10 stipulates that the following values, calculated from the test and specified conditions, be identical to within a few percentage points:volumetric flow ratio (dimensionless),capacity/speed ratio,Mach number (dimensionless), andReynolds number (dimensionless). When these numbers are equal, dynamic similitude between the test and specified conditions is obtained. A Class 3 test is performed when the ratio of specific heats, for either gas varies excessively from compressor inlet to outlet. Additionally, when either of two thermodynamic functions, X and Y (defined in later sections), exceeds certain values, a Class 3 test is required. The only actual differences that occur between these two classes are the equations used for calculating the compressor performance. In all other aspects, these classes are identical. The process used to verify that the test and specified parameters are identical is as follows.Test conditions are selected so that dynamic similitude is achieved.The specified and test parameters of flow, pressure rise, efficiency, and speed are transformed to dimensionless numbers with the equations in PTC-10.The machine is then run with the test gas at the test flow rate, and the corresponding pressure rise is then measured. Compressor efficiency is calculated from the measured temperature rise.Comparisons between the test parameters and specified parameters are then made. If the test values do not equal or exceed the specified values within limits agreed to by the purchaser, the manufacturer must take corrective action.