scholarly journals Multifrequency Instability of Cavitating Inducers

2006 ◽  
Vol 129 (6) ◽  
pp. 731-736 ◽  
Author(s):  
Christopher E. Brennen
Keyword(s):  
High Speed ◽  
Low Frequency ◽  
Performance Degradation ◽  
Cavitation Number ◽  
Pressure Measurements ◽  
Higher Harmonics ◽  
Unsteady Pressure ◽  
Arbitrary Frequency ◽  
Unsteady Pressure Measurements

Recent testing of high-speed cavitating turbopump inducers has revealed the existence of more complex instabilities than the previously recognized cavitating surge and rotating cavitation. This paper explores one such instability that is uncovered by considering the effect of a downstream asymmetry, such as a volute on a rotating disturbance similar to (but not identical to) that which occurs in rotating cavitation. The analysis uncovers a new instability that may be of particular concern because it occurs at cavitation numbers well above those at which conventional surge and rotating cavitation occur. This means that it will not necessarily be avoided by the conventional strategy of maintaining a cavitation number well above the performance degradation level. The analysis considers a general surge component at an arbitrary frequency ω present in a pump rotating at frequency Ω and shows that the existence of a discharge asymmetry gives rise not only to beat components at frequencies, Ω−ω and Ω+ω (as well as higher harmonics), but also to rotating as well as surge components at all these frequencies. In addition, these interactions between the frequencies and the surge and rotating modes lead to “coupling impedances” that effect the dynamics of each of the basic frequencies. We evaluate these coupling impedances and show not only that they can be negative (and thus promote instability) but also are most negative for surge frequencies just a little below Ω. This implies potential for an instability involving the coupling of a surge mode with a frequency around 0.9Ω and a low-frequency rotating mode about 0.1Ω. We also examine how such an instability would be manifest in unsteady pressure measurements at the inlet to and discharge from a cavitating pump and establish a “footprint” for the recognition of such an instability.

scholarly journals Transient Behavior of a Radial Vaneless Diffuser

2011 ◽  
Author(s):  
Antoine Dazin ◽  
Patrick Dupont ◽  
Guy Caignaert ◽  
Ge´rard Bois
Keyword(s):  
Experimental Data ◽  
High Speed ◽  
Laser Sheet ◽  
High Repetition Rate ◽  
Optical Methods ◽  
Pressure Measurements ◽  
Vaneless Diffuser ◽  
Test Rig ◽  
Unsteady Pressure ◽  
Unsteady Pressure Measurements

The paper refers to the behavior of a radial flow pump vaneless diffuser during a starting period. Results obtained with a 1D numerical model are compared with some new experimental data which have been obtained using 2D/3C High repetition rate PIV within the diffuser coupled with unsteady pressure measurements. These tests have been performed on a test rig with a radial impeller matched with a vaneless diffuser. They have been made in air, on a test rig well adapted for studies on interactions between impeller and diffuser, as well as for the use of optical methods and especially Particle Image Velocimetry (PIV) as there is no volute downstream of the diffuser. The present study refers to new experiments combining pressure measurements and 2D/3C High Speed PIV at partial flow rates within a vaneless diffuser with a large outlet radius. Four Bru¨el & Kjaer condenser microphones are used for the unsteady pressure measurements. They were flush mounted on the shroud side of the diffuser wall and on the suction pipe of the pump. The sampling frequency was 2048 Hz. For PIV measurements, the laser sheet was generated by a Darwin PIV ND:YLF Laser at three heights within the diffuser. PIV snapshots have been recorded by two identical CMOS cameras. A home made software has been used for the images treatment. The results consist in fields of 80 × 120 mm2 and 81 × 125 velocity vectors with a temporal resolution of 250 velocity maps per second. For each flow rate and each laser sheet height in the diffuser, two acquisitions of about 1500 velocity maps have been realised. The experimental data are compared with the ones provided by a 1D transient model of the flow within the diffuser.

scholarly journals Time-resolved flow field investigation in an industrial centrifugal compressor application involving TR-PIV synchronized with unsteady pressure measurements

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Joachim Klinner ◽  
Melanie Voges ◽  
Michael Schroll ◽  
Alessandro Bassetti ◽  
Christian Willert
Keyword(s):  
Flow Field ◽  
High Speed ◽  
Laser System ◽  
Power Spectra ◽  
Field Investigation ◽  
High Repetition Rate ◽  
Pressure Measurements ◽  
Time Resolved ◽  
Unsteady Pressure ◽  
Unsteady Pressure Measurements

We report on combined velocity and unsteady pressure measurements obtained on an radial compressor with vaneless diffuser and asymmetric volute. Time-resolved PIV recordings were acquired at 26 kHz both upstream of the impeller as well as within the vaneless diffusor at several rotation speeds at clean conditions and prior to the onset of instabilities within the rotor. The velocity data was acquired with a high-repetition rate, double-pulse laser system consisting of two combined DPSS lasers and a high-speed CMOS camera that was synchronized with multi-point unsteady pressure measurements. Details on the facility, the utilized instrumentation and data processing are provided with particular focus on the spectral and coherence analysis. Power spectra obtained from time records of the inlet velocity and unsteady pressure reveal an increase of low-frequency fluctuations below the blade passing frequency and the occurrence of a mode-locked behaviour indicating the presence of rotating instabilities. High levels of correlation between velocity and unsteady pressure signals not only confirm the temporal coherence of the acquired data but also reveal a direct coupling between flow field and pressure signature that is more prominent upstream of the rotor rather than in the diffusor.

Effect of back pressure and freestream dynamic pressure on a typical Ramjet engine duct under realistic supersonic inlet condition

2017 ◽  
Vol 122 (1247) ◽  
pp. 83-103 ◽  
Author(s):  
R. Saravanan ◽  
S.L.N. Desikan ◽  
T.M. Muruganandam
Keyword(s):  
Dynamic Pressure ◽  
Back Pressure ◽  
Flow Visualisation ◽  
Pressure Measurements ◽  
Shock Train ◽  
Unsteady Pressure ◽  
Expansion Waves ◽  
Ramjet Engine ◽  
Flap Deflection ◽  
Unsteady Pressure Measurements

ABSTRACTThe present study investigates the behaviour of the shock train in a typical Ramjet engine under the influence of shock and expansion waves at the entry of a low aspect ratio (1:0.75) rectangular duct/isolator at supersonic Mach number (M = 1.7). The start/unstart characteristics are investigated through steady/unsteady pressure measurements under different back and dynamic pressures while the shock train dynamics are captured through instantaneous Schlieren flow visualisation. Two parameters, namely pressure recovery and the pressure gradient, is derived to assess the duct/isolator performance. For a given back pressure, with maximum blockage (9% above nominal), the duct/isolator flow is established when the dynamic pressure is increased by 23.5%. The unsteady pressure measurements indicate different scales of eddies above 80 Hz (with and without flap deflection). Under the no flap deflection (no back pressure) condition, the maximum fluctuating pressure component is 0.01% and 0.1% of the stagnation pressure at X/L = 0.03 (close to the entry of the duct) and X/L = 0.53 (middle of the duct), respectively. Once the flap is deflected (δ = 8°), decay in eddies by one order is noticed. Further increase in back pressure (δ ≥ 11°) leads the flow to unstart where eddies are observed to be disappeared.

Unsteady Pressure Measurements in a Heated Rotating Cavity

2021 ◽  
Author(s):  
Richard W. Jackson ◽  
Hui Tang ◽  
James A. Scobie ◽  
Oliver J. Pountney ◽  
Carl M. Sangan ◽  
...  
Keyword(s):  
Vortex Pair ◽  
Practical Significance ◽  
Pressure Measurements ◽  
Rotating Disc ◽  
Frequency Spectra ◽  
Paper Properties ◽  
Unsteady Pressure ◽  
Rotating Cavity ◽  
Buoyancy Forces ◽  
Unsteady Pressure Measurements

Abstract The flow in the heated rotating cavity of an aero-engine compressor is driven by buoyancy forces, which result in pairs of cyclonic and anticyclonic vortices. The resultant cavity flow field is three-dimensional, unsteady and unstable, which makes it challenging to model the flow and heat transfer. In this paper, properties of the vortex structures are determined from novel unsteady pressure measurements collected on the rotating disc surface over a range of engine-representative parameters. These measurements are the first of their kind with practical significance to the engine designer and for validation of computational fluid dynamics. One cyclonic/anticyclonic vortex pair was detected over the experimental range, despite the measurement of harmonic modes in the frequency spectra at low Rossby numbers. It is shown that these modes were caused by unequal size vortices, with the cyclonic vortex the larger of the pair. The structures slipped relative to the discs at a speed typically around 10% to 15% of that of the rotor, but the speed of precession was often unsteady. The coherency, strength and slip of the vortex pair increased with the buoyancy parameter, due to the stronger buoyancy forces, but they were largely independent of the rotational Reynolds number.

Wind tunnel unsteady pressure measurements using a differential optical fiber Fabry-Perot pressure sensor

2014 ◽  
Author(s):  
Ricardo Correia ◽  
Stephen E. Staines ◽  
Stephen W. James ◽  
Nicholas Lawson ◽  
Kevin Garry ◽  
...  
Keyword(s):  
Wind Tunnel ◽  
Optical Fiber ◽  
Pressure Sensor ◽  
Pressure Measurements ◽  
Unsteady Pressure ◽  
Fabry Perot ◽  
Unsteady Pressure Measurements
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