Effects of Vaneiess Diffuser Geometry on Flow Instability in Centrifugal Compression Systems

1981 ◽  
Author(s):  
A. N. Abdelhamid
Keyword(s):  
Critical Angle ◽  
Previous Investigation ◽  
System Analysis ◽  
Flow Instability ◽  
Rate Of Change ◽  
Radius Ratio ◽  
Vaneless Diffuser ◽  
Flow Angle ◽  
Flow Oscillations ◽  
Initial Pattern

Experiments were conducted to determine the effects of vaneless diffuser radius ratio on the occurrence of self-excited flow oscillations in a centrifugal compression system. Analysis of the results indicated the successive occurrence of two types of diffuser rotating pressure patterns as the flow rate in the system was gradually decreased. The rotational speed of the latter pattern was higher than that of the initial pattern and both speeds varied inversely with diffuser radius ratio. The critical flow angle at which each pattern was first observed increased with diffuser radius ratio. However, for diffuser radius ratio equal to and larger than 1.75 the rate of change of the critical angle with radius ratio decreased significantly. The results also showed that the minimum diffuser radius ratio necessary for self excitation of each pattern were different. Occurrence of two rotating pressure patterns confirms analytical results presented in a previous investigation.

Study of Vaneless Diffuser Rotating Stall Based on Two-Dimensional Inviscid Flow Analysis

1996 ◽  
Vol 118 (1) ◽  
pp. 123-127 ◽  
Author(s):  
Yoshinobu Tsujimoto ◽  
Yoshiki Yoshida ◽  
Yasumasa Mori
Keyword(s):  
Propagation Velocity ◽  
Flow Instability ◽  
Present Report ◽  
Inviscid Flow ◽  
Rotating Stall ◽  
Critical Flow ◽  
Two Dimensional ◽  
Vaneless Diffuser ◽  
Flow Angle ◽  
Pressure Disturbance

Rotating stalls in vaneless diffusers are studied from the viewpoint that they are basically two-dimensional inviscid flow instability under the boundary conditions of vanishing velocity disturbance at the diffuser inlet and of vanishing pressure disturbance at the diffuser outlet. The linear analysis in the present report shows that the critical flow angle and the propagation velocity are functions of only the diffuser radius ratio. It is shown that the present analysis can reproduce most of the general characteristics observed in experiments: critical flow angle, propagation velocity, velocity, and pressure disturbance fields. It is shown that the vanishing velocity disturbance at the diffuser inlet is caused by the nature of impellers as a “resistance” and an “inertial resistance,” which is generally strong enough to suppress the velocity disturbance at the diffuser inlet. This explains the general experimental observations that vaneless diffuser rotating stalls are not largely affected by the impeller.

Rotating Stall Characteristics in a Wide Vaneless Diffuser

2005 ◽  
Author(s):  
S. Ljevar ◽  
H. C. de Lange ◽  
A. A. van Steenhoven
Keyword(s):  
Boundary Layers ◽  
Flow Instability ◽  
Stability Limit ◽  
Rotating Stall ◽  
Two Dimensional ◽  
Vaneless Diffuser ◽  
Flow Angle ◽  
Wall Boundary ◽  
The Stability ◽  
Rotating Instability

Paper reports a numerical study on vaneless diffuser flow instability performed for the purpose of better understanding of rotating stall mechanism in radial vaneless diffusers. This analysis is restricted to the two-dimensional flow where effect of wall boundary layers is neglected. Numerical results reveal that a two-dimensional rotating flow instability similar to rotating stall occurs when critical flow angle is exceeded. They also show that the stability limit and the structure of a two dimensional rotating instability are influenced by the configuration geometry and inlet and outlet flow conditions. Good agreement with data from the literature is found for the stability limit and number and speed of propagating cells. Number of cells and their speed is somewhat higher than observed in experiments from literature. This might imply that inception point is caused by the core flow instability and that wall boundary layers are more determinative for the structure of rotating instability.

A New Technique for Stabilizing the Flow and Improving the Performance of Vaneless Radial Diffusers

1987 ◽  
Vol 109 (1) ◽  
pp. 36-40 ◽  
Author(s):  
A. N. Abdel-Hamid
Keyword(s):  
Flow Instability ◽  
Pressure Rise ◽  
Outer Radius ◽  
Operating Conditions ◽  
Flow Angle ◽  
Diffuser Flow ◽  
Flow Oscillations ◽  
Inlet Conditions ◽  
A New Technique ◽  
Unsteady Characteristics

Experiments were conducted to investigate the effects of using small exit vanes on the characteristics of an otherwise vaneless radial diffuser of outer radius and width-to-inlet radius ratios of 1.75 and 0.116, respectively. The steady and unsteady characteristics of the diffuser were evaluated as the angle of the vanes was varied continuously at several diffuser inlet conditions. The measurements showed that the influence of the adjustable exit vanes on the diffuser flow field increased as the diffuser inlet flow angle was decreased. Elimination of the self-excited flow oscillations in the diffuser was possible for all operating conditions by appropriate setting of the exit vane angle. Moreover for the diffuser investigated here and with optimum setting of the exit vanes angle the static pressure rise coefficient was significantly improved compared to the case of a pure vaneless diffuser and was found to remain almost constant at low values of diffuser inlet angles. Relative to other techniques for controlling flow instability in vaneless diffusers the proposed method offers mechanical simplicity and improved overall performance.

Experimental Characterization of Vaneless Diffuser Rotating Stall: Part VI — Reduction of Three Impeller Results

2006 ◽  
Author(s):  
E. A. Carnevale ◽  
G. Ferrara ◽  
L. Ferrari ◽  
L. Baldassarre
Keyword(s):  
Rotating Stall ◽  
Radius Ratio ◽  
Vaneless Diffuser ◽  
Cross Sectional ◽  
Stall Inception ◽  
Return Channel ◽  
Geometrical Scale ◽  
And Diffusion ◽  
Basic Configuration

Vaneless diffuser rotating stall is a major problem for centrifugal compressors since it is a limit to their working range. In the literature some good correlations for predicting stall inception can be found but they do not adequately cover the case of the last stage configuration, especially for very low blade-outlet-width-to-impeller-radius-ratio impellers typically used in high-pressure applications. Extensive research has been performed to define diffuser stall limits for this family of stages: three impellers characterized by different blade-outlet-width-to-impeller-radius-ratios have been tested with different diffuser configurations (different pinch shapes, diffuser widths and diffusion ratios). The basic configuration comprises a 1:1 geometrical scale stage with a return channel upstream, a 2D impeller with a vaneless diffuser and a volute with a constant cross sectional area downstream. Several diffuser types with different widths and diffusion ratios were tested. Detailed experimental results have been reported in previous works [1, 2, 3 and 4]. In this paper experimental data are reviewed in order to analyze impeller influence on diffuser stability and to develop some summarizing consideration on stall behavior of vaneless diffuser for impeller with low blade-outlet-width-to-impeller-radius-ratio.

The Features of Flow in a Parallel-Walled Radial Vaneless Diffuser

2000 ◽  
Author(s):  
K. B. Abidogun ◽  
S. A. Ahmed
Keyword(s):  
Tangential Velocity ◽  
Maximum Flow ◽  
Calculation Model ◽  
Angle Distribution ◽  
Limited Data ◽  
Vaneless Diffuser ◽  
Centrifugal Blower ◽  
Flow Angle ◽  
Flow Features ◽  
Measurement Path

Abstract Detailed experimental investigation of flow features in a parallel-walled radial vaneless diffuser of a centrifugal blower was carried out. Maximum flow rate through the blower, at a constant impeller speed of 1500 rpm, was maintained throughout the experiment to ensure that no self-exited flow oscillation occurs in the diffuser. The symmetrical flowfield in the diffuser was measured along a radial path using an X-wire probe. The radial and tangential velocity distributions and their statistics, as well as flow angle distribution, are reported. The results presented in this paper agree well with earlier work on this subject. For instance, the flow angle at half the diffuser width, which is the position at which critical flow angle (measured from the radial direction) have been generally reported to be about 78°, did not exceed 65° from the diffuser inlet to its outlet along the measurement path. The result also showed that the flow exiting the impeller is skewed as revealed by the triple velocity product correlations. This data is a useful tool for vaneless diffuser calculation model developers as there are very limited data that paralleled the current one as expounded in the text of this paper.

Numerical Analysis of the Flow Inside a Centrifugal Compressor’s Vaneless Diffuser and Volute

2001 ◽  
Author(s):  
Hooman Rezaei ◽  
Abraham Engeda ◽  
Paul Haley
Keyword(s):  
Experimental Data ◽  
Numerical Analysis ◽  
Mass Flow ◽  
Operating Conditions ◽  
Vaneless Diffuser ◽  
Flow Angle ◽  
Flow Rates ◽  
Mass Flow Rates ◽  
Minimum Medium ◽  
Good Agreement

Abstract The objective of this work was to perform numerical analysis of the flow inside a modified single stage CVHF 1280 Trane centrifugal compressor’s vaneless diffuser and volute. Gambit was utilized to read the casing geometry and generating the vaneless diffuser. An unstructured mesh was generated for the path from vaneless diffuser inlet to conic diffuser outlet. At the same time a meanline analysis was performed corresponding to speeds and mass flow rates of the experimental data in order to obtain the absolute velocity and flow angle leaving the impeller for those operating conditions. These values and experimental data were used as inlet and outlet boundary conditions for the simulations. Simulations were performed in Fluent 5.0 for three speeds of 2000, 3000 and 3497 RPM and mass flow rates of minimum, medium and maximum. Results are in good agreement with the experimental ones and present the flow structures inside the vaneless diffuser and volute.

Determination of Instability in Zr-2.5Nb-0.5Cu Using Lyapunov Function

2015 ◽  
Vol 830-831 ◽  
pp. 329-332 ◽  
Author(s):  
Kuldeep Kumar Saxena ◽  
Vivek Pancholi ◽  
Dinesh Srivastava ◽  
G.K. Dey ◽  
Sanjay K. Jha ◽  
...  
Keyword(s):  
Lyapunov Function ◽  
Strain Rate ◽  
Flow Instability ◽  
Peak Stress ◽  
Strain Rate Range ◽  
Rate Of Change ◽  
Deformation Condition ◽  
Hot Workability ◽  
Lower Temperature

Hot workability of Zr-2.5Nb-0.5Cu alloy has been investigated by means of hot compression test using Gleeble-3800®, in the temperature and strain rate range of 700 to 925°C and 0.01-10s-1, respectively. Deformation behavior was characterized in terms of flow instability using peak stress with the help of Lyapunov Function. The true stress-strain curves shows that softening occurs at all lower temperature and for entire strain rates of deformation. The instable flow was suggested by negative m value at deformation condition of 700°C (5 and 10 s-1), while s value at 925°C (10 s-1). The combined result of rate of change of m and s with respect to log strain rate suggest that the deformation condition ranges from 725-780°C (10-2- 10-1 s-1) and 700°C (1-10 s-1) representing safe domain for stable flow.

The Unsteady Behavior of Diffuser Stall in a Centrifugal Compressor With Vaneless Diffuser

2020 ◽  
Author(s):  
Hiroshi Miida ◽  
Kenta Tajima ◽  
Nobumichi Fujisawa ◽  
Yutaka Ohta
Keyword(s):  
Mass Flow ◽  
Centrifugal Compressor ◽  
Rotational Speed ◽  
Flow Region ◽  
Cfd Analysis ◽  
Vaneless Diffuser ◽  
Low Velocity ◽  
Flow Angle ◽  
Boundary Separation ◽  
Low Mass

Abstract The unsteady diffuser stall behavior in a centrifugal compressor with a vaneless diffuser was investigated by experimental and computational analyses. The diffuser stall generated as the mass flow rate decreased. The diffuser stall cell rotated at 25–30% of the impeller rotational speed, with diffuser stall fluctuations observed at 180° from the cutoff. The diffuser stall fluctuation magnitude gradually increased near the cutoff. Based on diffuser inlet velocity measurements, the diffuser stall fluctuations generated near both the shroud and hub sides, and the diffuser stall appeared at 180° and 240° from the cutoff. According to the CFD analysis, the mass flow fluctuations at the diffuser exit showed a low mass flow region, rotating at approximately 25% of the impeller rotational speed. They began at 180° from the cutoff and developed as this region approached the cutoff. Therefore, the diffuser stall could be simulated by CFD analysis. First, the diffuser stall cell originated at 180° from the cutoff by interaction with boundary separation and impeller discharge vortex. Then, the diffuser stall cell further developed by boundary separation accumulation and the induced low velocity area, located at the stall cell center. The low velocity region formed a blockage across the diffuser passage span. The diffuser stall cell expanded in the impeller rotational direction due to boundary separation caused by a positive flow angle. Finally, the diffuser stall cell vanished when it passed the cutoff, because mass flow recovery occurred.

Experimental Investigation of Rotating Stall Characteristics in a Radial Vaneless Diffuser

2000 ◽  
Author(s):  
K. B. Abidogun ◽  
S. A. Ahmed
Keyword(s):  
Flow Instability ◽  
Cell Structure ◽  
Rotating Stall ◽  
Flow Coefficient ◽  
Critical Flow ◽  
Vaneless Diffuser ◽  
Pressure Transducers ◽  
Mass Flowrate ◽  
Hot Wires ◽  
Good Agreement

Rotating stall characteristics in a radial vaneless diffuser model was investigated experimentally. Measurements were made using hot-wires and pressure transducers (static and dynamic). The mass flowrate through the blower, at constant impeller speed, was gradually reduced until flow instability occurred in the diffuser. This enabled the onset and propagation of rotating stall to be fully described. The blower was operated without the vaneless diffuser in order to ascertain the cause of the flow instability. It was discovered that the impeller did not stall at the flow rates at which the blower was operated with the diffuser. The critical flow angles measured at the diffuser inlet, and midway between the diffuser walls, were in good agreement with earlier reported values in the open literature. The maximum number of rotating stall cells found in this study was two. The single-stall cell structure was found to be dominant over the two-stall cell structure at flow coefficients much lower than the critical flow coefficient.

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