A theory for rotating stall in contra-rotating fans

2020 ◽  
pp. 095440622096253
Author(s):  
Hossein Khaleghi ◽  
Mohammad Javad Shahriyari ◽  
Martin Heinrich
Keyword(s):  
Single Cell ◽  
Critical Role ◽  
Pressure Rise ◽  
Transient Behavior ◽  
Rotating Stall ◽  
Speed Ratio ◽  
Rotor Speed ◽  
Modified Model ◽  
Stall Cells ◽  
Stagger Angle

This paper reports on a theory of rotating stall in contra-rotating fans and compressors. The theory is developed from Moore’s theory. A second-order hysteresis is used in the current study for the pressure rise of the counter-rotating rows. This enables the model to predict the transient behavior of the stall cell. Comparing the experimental results with the theory shows that the modified model can predict the speed of the stall cells fairly accurately. Results show that the rotor speed ratio plays a critical role in the stall cell speed and its direction of rotation. Furthermore, the developed model makes it possible to study the effects of stagger angle and number of stall cells. The conditions under which pure rotating stall can occur in contra-rotating fans are also discussed in this paper. It is shown that the stall cells merge to form a single cell before a stable fully-developed rotating stall is established.

A Model for Stall and Surge in Low-Speed Contra-Rotating Fans

2019 ◽  
Vol 141 (8) ◽  
Author(s):  
Mohammad Javad Shahriyari ◽  
Hossein Khaleghi ◽  
Martin Heinrich
Keyword(s):  
Rotational Speed ◽  
Current Model ◽  
Characteristic Curve ◽  
Pressure Rise ◽  
Rotating Stall ◽  
Negative Slope ◽  
Initial Slope ◽  
Speed Ratio ◽  
Low Speed ◽  
Fan Performance

This paper reports on a theory for poststall transients in contra-rotating fans, which is developed from the basic Moore–Greitzer theory. A second-order hysteresis term is assumed for the fan pressure rise, which gives the theory more capabilities in predicting the fan instabilities. The effect of the rotational speed ratio of the two counter rotating rotors on the fan performance during the occurrence of surge and rotating stall are studied (the rotational speed of the front rotor is assumed to be kept constant whereas the speed of the rear rotor is variable). One of the new capabilities of the current model is the possibility of investigating the effect of the initial slope on the fan characteristic. Results reveal that unlike the conventional fans and compressors, in the current contra-rotating fan stall cannot be initiated from the negative slope portion of the fan pressure rise characteristic curve. One of the important advantages of the developed model is that it enables investigation of the effect of the rate of throttling on the instabilities. Results show that more the rotational speed of the rear rotor, the more robust to surge (caused by throttling) the fan is.

Effects of Rotor-Speed-Ratio and Crosswind Inlet Distortion on Off-Design Performance of Contra-Rotating Propelling Unit

2016 ◽  
Author(s):  
Stavros V. Vouros ◽  
Alexandros C. Chasoglou ◽  
Theofilos G. Efstathiadis ◽  
Anestis I. Kalfas
Keyword(s):  
Wind Tunnel ◽  
Mass Flow ◽  
Total Pressure ◽  
Static Pressure ◽  
Pressure Rise ◽  
Operating Conditions ◽  
Speed Ratio ◽  
Rotor Speed ◽  
Design Performance ◽  
Inlet Distortion

This paper presents an investigation of the effects of rotor-speed-ratio (RSR) and inlet crosswind distortion on the off-design performance of a contra-rotating propelling unit. A dedicated wind tunnel has been built in order to study the off-design performance in distorted and undistorted conditions. A miniature 8-hole pressure rake has been calibrated to measure the total and static pressure field across the blade span, upstream and downstream of the propellers. A data analysis algorithm has been developed for the rapid exportation of desensitized and reliable mass-averaged characteristics, while wall blockage effects are accounted for incorporating a correction factor. Investigation of rotor-speed-ratio effects at undistorted inlet conditions show that, efficiency is the highest when the front rotor operates at rotational speed equal to 90% of the aft rotor’s speed. For the optimum RSR, the effects of crosswind inlet distortion are investigated. Crosswind mass-flow is generated by a 16-fan array, accommodated in a secondary square wind-tunnel, with its center axis perpendicular to propeller main flow direction. In order to achieve realistic crosswind flow characteristics, the pressure side of the secondary wind tunnel is being used, after proper flow conditioning. Total and static pressure distribution highlights the location of a primary and a secondary stall cell, both upstream and downstream of the propellers. The results indicate a spanwise shift of the inlet pressure profile towards the direction of crosswind. A rotation of the wake is also observed due to propellers’ remaining swirl. Finally, the effect of distortion on the overall performance is being investigated, by the exportation of averaged total pressure rise for the various examined inlet distortion cases. Mass-flow is slightly increased; however, total pressure rise is dramatically reduced due to crosswind flow effects. Especially for the case of landing under extremely strong crosswind, the ratio between total pressure rise and inlet dynamic head is 4.2 times lower, compared to undistorted inlet operating conditions.

A Theory of Rotating Stall of Multistage Axial Compressors: Part I—Small Disturbances

1984 ◽  
Vol 106 (2) ◽  
pp. 313-320 ◽  
Author(s):  
F. K. Moore
Keyword(s):  
Pressure Rise ◽  
Propagation Speed ◽  
Rotating Stall ◽  
Flow Coefficient ◽  
Fluid Inertia ◽  
Flow Distortion ◽  
Axial Compressors ◽  
Experimental Values ◽  
Stagger Angle ◽  
Small Disturbances

An analysis is made of rotating stall in compressors of many stages, finding conditions under which a flow distortion can occur which is steady in a traveling reference frame, even though upstream total and downstream static pressure are constant. In the compressor, a pressure-rise hysteresis is assumed. Flow in entrance and exit ducts yield additional lags. These lags balance to give a formula for stall propagation speed. For small disturbances, it is required that the compressor characteristics be flat in the neighborhood of average flow coefficient. Results are compared with the experiments of Day and Cumpsty. If a compressor lag of about twice that due only to fluid inertia is used, predicted propagation speeds agree almost exactly with experimental values, taking into account changes of number of stages, stagger angle, row spacing, and number of stall zones. The agreement obtained gives encouragement for the extension of the theory to account for large amplitudes.

scholarly journals Stalled Flow Performance for Axial Compressors: I — Axisymmetric Characteristic

1984 ◽  
Author(s):  
S. G. Koff ◽  
E. M. Greitzer
Keyword(s):  
Flow Model ◽  
Flow Characteristic ◽  
Pressure Rise ◽  
Rotating Stall ◽  
Reversed Flow ◽  
Axial Compressors ◽  
Point Pressure ◽  
Multistage Compressors ◽  
Entire Flow ◽  
Stagger Angle

A study of stalled flow performance of multistage compressors is presented. A new compressor characteristic is developed, describing the axisymmetric pumping performance over the entire compressor flow range, including reversed flow. This axisymmetric characteristic is required in any current rotating stall model. It is possible for the axisymmetric performance to rise above the measured stall point pressure rise, thus indicating greater unstalled pressure rise potential. In this context, the axisymmetric characteristic in forward flow is viewed as paralleling diffuser performance. A simple two-dimensional reversed flow model is presented, and is shown to be in reasonable agreement with available high backflow compressor data. The model predicts that the blade stagger angle greatly influences the reversed flow characteristic. Calculations are carried out using the rotating stall model of Moore and the axisymmetric characteristic developed herein, and a technique is suggested for estimating the axisymmetric curve over the entire flow range.

Influence of Rotating Instability on Stall Inception Patterns in a Variable-Pitch Axial-Flow Fan

2006 ◽  
Author(s):  
Takahiro Nishioka ◽  
Shuuji Kuroda ◽  
Tsukasa Nagano ◽  
Hiroshi Hayami
Keyword(s):  
Rotor Blade ◽  
Axial Flow ◽  
Pressure Rise ◽  
Leading Edge ◽  
Rotating Stall ◽  
Axial Flow Fan ◽  
Tip Leakage Vortex ◽  
Stall Inception ◽  
Tip Leakage ◽  
Stagger Angle

An experimental study was conducted to investigate the inception patterns of rotating stall at different rotor blade stagger-angle settings with the aim of extending the stable operating range for a variable-pitch axial-flow fan. Pressure and velocity fluctuations were measured for a low-speed axial-flow fan with a relatively large tip clearance. Two stagger-angle settings were tested, the design setting, and a high setting which was 10 degrees greater than the design setting. Rotating instability (RI) was first observed near the peak pressure-rise point at both settings. It propagated in the rotation direction at about 40 to 50% of the rotor rotation speed, and its wavelength was about one rotor-blade pitch. However, the stall-inception patterns differed between the two settings. At the design stagger-angle setting, leading edge separation occurred near the stall-inception point, and this separation induced a strong tip leakage vortex that moved upstream of the rotor. This leakage vortex simultaneously induced a spike and a RI. The conditions for stall inception were consistent with the simple model of the spike-type proposed by Camp and Day. At the high stagger-angle setting, leading edge separation did not occur, and the tip leakage vortex did not move upstream of the rotor. Therefore, a spike did not appear although RI developed at the maximum pressure-rise point. This RI induced a large end-wall blockage that extended into the entire blade passage downstream of the rotor. This large blockage rapidly increased the rotor blade loading and directly induced a long length-scale stall cell before a spike or modal disturbance appeared. The conditions for stall inception were not consistent with the simple models of the spike or modal-type. These findings indicate that the movement of the tip leakage vortex associated with the rotor blade loading affects the development of a spike and RI and that the inception pattern of a rotating stall depends on the stagger-angle setting of the rotor blades.

Influence of Rotor Stagger Angle on Rotating Stall Inception in an Axial-Flow Fan

2003 ◽  
Author(s):  
Takahiro Nishioka ◽  
Shuuji Kuroda ◽  
Tadashi Kozu
Keyword(s):  
Flow Rate ◽  
Axial Flow ◽  
Rotating Stall ◽  
Short Length ◽  
Rotor Blades ◽  
Length Scale ◽  
Axial Flow Fan ◽  
Stall Inception ◽  
Stall Cells ◽  
Stagger Angle

Inception patterns of rotating stall in a low-speed axial flow fan have been investigated experimentally. Experiments have been carried out at two different stagger angle settings for rotor blades. Pressure and velocity fluctuations were measured to elucidate the features of the stall cells and the stall inception patterns. At the design stagger angle setting for the rotor blades, a short length-scale stall cell known as a “spike” and multiple short length-scale stall cells appear when the slope of pressure-rise characteristic is almost zero. These stall cells grow into a long length-scale stall cell as flow rate decreases. The spike and the multiple short length-scale stall cells do not make the slope of the characteristic positive. However, the long length-scale stall cell induces a full-span stall, and makes the slope of the characteristic positive. At the small stagger angle, a long length-scale disturbance known as a “modal oscillation” is observed first, when the slope of the characteristic is positive. Then the spikes appear together with the modal oscillation as flow rate decreases. The long length-scale stall cell is generated by the spikes without change in the size of the modal oscillation. Suction-tip corner stall occurs in the stator passage near the peak of the characteristic at both the design and the small stagger angle settings. At the design stagger angle, however, the corner stall does not induce the modal oscillation and does not make the characteristic positive. In contrast, the corner stall at the small stagger angle induces the modal oscillation and makes the characteristic positive because it is larger than that at the design stagger angle. It is concluded from these results that the rotating stall inception patterns depend on the rotor stagger angle, which influences blade loading and rotor-stator matching.

A Theory of Post-Stall Transients in Axial Compression Systems: Part II—Application

1986 ◽  
Vol 108 (2) ◽  
pp. 231-239 ◽  
Author(s):  
E. M. Greitzer ◽  
F. K. Moore
Keyword(s):  
Axial Compression ◽  
Parametric Study ◽  
Pressure Rise ◽  
Transient Behavior ◽  
Rotating Stall ◽  
Future Research ◽  
System Response ◽  
System Behavior ◽  
Flow Pressure ◽  
The Impact

Using the theory developed in Part I, calculations have been carried out to show the evolution of the mass flow, pressure rise, and rotating-stall cell amplitude during compression system post-stall transients. In particular, it is shown that the unsteady growth or decay of the stall cell can have a significant effect on the instantaneous compressor pumping characteristic and hence on the overall system behavior. A limited parametric study is carried out to illustrate the impact of different system features on transient behavior. It is shown, for example, that the ultimate mode of system response, surge or stable rotating stall, depends not only on the B parameter, but also on the compressor length-to-radius ratio. Small values of the latter quantity tend to favor the occurrence of surge, as do large values of B. Based on the analytical and numerical results, some specific topics are suggested for future research on post-stall transients.

Stall Inception in Axial Flow Compressors

1993 ◽  
Vol 115 (1) ◽  
pp. 1-9 ◽  
Author(s):  
I. J. Day
Keyword(s):  
Axial Flow ◽  
Pressure Rise ◽  
Finite Amplitude ◽  
Rotating Stall ◽  
Short Length ◽  
Stall Inception ◽  
Current Test ◽  
The World ◽  
Hot Wires ◽  
Stall Cells

Studies have been conducted on two laboratory test compressors to investigate the process leading to the formation of finite amplitude rotating stall cells. The measurements were obtained from circumferential arrays of hot wires and were spatially and temporarily analyzed to show that modal perturbations are not always present prior to stall, and when present, sometimes have little direct effect on the formation of the stall cells. The measurements lead to the conclusion that the occurrence of modal perturbations, and the formation of finite amplitude stall cells, are two separate phenomena, both occurring under roughly the same conditions at the peak of the pressure rise characteristic. The measurements also underline the hitherto unsuspected importance of short length scale disturbances in the process of stall inception. Examples are given of different ways in which stall cells can develop and the conclusions are backed up with a summary of current test data from various machines around the world.

Stall Inception in Axial Flow Compressors

1991 ◽  
Author(s):  
I. J. Day
Keyword(s):  
Axial Flow ◽  
Pressure Rise ◽  
Finite Amplitude ◽  
Rotating Stall ◽  
Short Length ◽  
Stall Inception ◽  
Current Test ◽  
The World ◽  
Hot Wires ◽  
Stall Cells

Studies have been conducted on two laboratory test compressors to investigate the process leading to the formation of finite amplitude rotating stall cells. The measurements were obtained from circumferential arrays of hot-wires and were spatially and temporarily analysed to show that modal perturbations are not always present prior to stall, and when present, sometimes have little direct effect on the formation of the stall cells. The measurements lead to the conclusion that the occurrence of modal perturbations, and the formation of finite amplitude stall cells, are two separate phenomena; both occurring under roughly the same conditions at the peak of the pressure rise characteristic. The measurements also underline the hitherto unsuspected importance of short length scale disturbances in the process of stall inception. Examples are given of different ways in which stall cells can develop and the conclusions are backed up with a summary of current test data from various machines around the world.

Axisymmetrically Stalled Flow Performance for Multistage Axial Compressors

1986 ◽  
Vol 108 (2) ◽  
pp. 216-223 ◽  
Author(s):  
S. G. Koff ◽  
E. M. Greitzer
Keyword(s):  
Reasonable Agreement ◽  
Flow Model ◽  
Flow Characteristic ◽  
Pressure Rise ◽  
Rotating Stall ◽  
Two Dimensional ◽  
Reversed Flow ◽  
Axial Compressors ◽  
Point Pressure ◽  
Stagger Angle

A study of the stalled flow performance of multistage axial compressors is presented. A proposal is made regarding the form of axisymmetric pumping performance in stall (which is a requisite of current rotating stall models) over the entire compressor flow range, including reversed flow. It is also shown that the axisymmetric performance can rise above the measured stall point pressure rise, thus indicating greater unstalled pressure rise potential. A simple two-dimensional reversed flow model is presented, and is shown to be in reasonable agreement with available high backflow compressor data. The model predicts that the blade stagger angle greatly influences the reversed flow characteristic. Calculations are also carried out applying this axisymmetric characteristic to the rotating stall model of Moore.

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