Near-wake measurement in a rotor/stator axial compressor using slanted hot-wire technique

 Three-dimensional near-wake structure behind a rotor was measured using slanted hot-wire technique in a large-scale, low-speed, rotor/stator axial compressor. Unsteady flow interaction between blade rows was varied by setting the axial gap between rows at 10% and 30% of rotor chord. Results show that stronger flow interactions between blade rows, or closer axial gap, produce more pronounced time variation within the rotor wake. All parameters measured – three component velocities, yaw and pitch angles – varied strongly within the wake, and are quantified. Received: 8 July 1996/Accepted: 29 May 1997     

Flow control of hub corner stall in a highly loaded axial compressor cascade

Three-dimensional corner stall is one of the key factors limiting the compressor performance. This paper presents a detailed experimental and computational study of a flow control strategy involving the endwall suction, aiming to eliminate the hub corner stall in a highly loaded axial compressor cascade. Various mass flow suction cases were parametrically tested with the aim of eliminating the corner stall by applying a minimum suction flow ratio. In the experiments, seven-hole pressure probe traverses, different loading distributions and surface oil flow visualizations were applied to address the flow and loss mechanisms in the cascade. Measurements were supplemented with numerical predictions from a commercially available CFD code. It was found that the corner stall, characterized by a large amount of reversed fluid, occupied a large region over the blade suction surface in the highly loaded compressor airfoil, rather than occurring at the junction of a blade suction surface and the endwall as in the conventionally loaded compressor airfoil. By applying flow control, the dominant flow structures, e.g. the flow separations and particularly the corner stall, within the compressor cascade were significantly affected. The maximum spanwise penetration depth of the endwall flow on the suction surface was significantly decreased once the endwall suction flow was applied. Furthermore, the corner stall was completely eliminated by suctioning the mass flow at a specific ratio of the inlet boundary layer flow rate. The midspan flow field was not notably affected, and a further increase in suction mass flow did not benefit the flow field approaching the endwall.     

Flow visualization in a laboratory vaned diffuser

An experimental model of a vaned diffuser with rectangular flow cross-sections was constructed of clear plastic for flow visualization studies. A swirl generator was used to induce fluid rotation without subjecting the diffuser to any unsteady and irregular impeller flow phenomena. The blades were of a thin circular arc shape. The clear plastic construction allowed large-scale flow visualization with tufts attached to the diffuser wall and dye injected into the separation regions. Four conditions were tested: a vaneless, a four-vaned, a six-vaned, and eight-vaned diffuser. Each test was conducted at an average Reynolds number of 20 000, based on passage thickness. In the absence of diffuser blades the flow angle was not radially constant, as a result of the viscous effects, varying as much as 11° from the ideal 16°. With four blades installed, separation began at 23% of the blade length from the leading tip. At the peak development of the separation regions 34% of the flow area was blocked. Separation began at 27% from the leading edge when six blades were used. Finally, with eight blades in place, separation began at 50% of the blade length from the leading tip; at the peak development of the separation regions 64% of the flow area was blocked.     

Flow visualization in a scanning electron microscope scale

This paper describes a method of visualizing the two-dimensional flow around small bodies by means of a scanning electron microscope (SEM). The advantages, compared to light microscopic methods, are, for example, a greater distance between objective lens and object, a higher depth of focus and the possibility of observing smaller details. The disadvantages are the restriction to certain fluids and the expensive equipment. The described method may be important for the investigation of stationary objects as well as that of unsteady flow. In the near future we shall try to visualize three dimensional flow in a scanning electron microscope scale.     

A three-dimensional blockage theory for the stability of an axial compressor row

The concept of ‘blockage’ between the blades of an axial compressor row as a theoretical device to replace loss was used by Stenning and Kriebel (1) to derive their well-known stability criterion for the stability of two-dimensional cascades. In their work they ignored the influence of that blockage upon the term describing the inertia of the fluid between the blades. The first part of the present work includes this influence and a modified stability criterion is derived that links the stability parameter with ‘stall cell’ speed and number. In the second part the blockage principle is extended to the problem of an isolated rotor row of arbitrary hub/tip ratio, treated as a ‘semi-actuator disc’, and perturbation of the upstream potential arising from changes of blockage are studied. The method proceeds by analysing the response of the flow to an imposed pressure discontinuity within the blade row. Rotating stall modes as such are not predicted but the theory demonstrates regions of the span which are undamped and on this basis a comparison can be made with two-dimensional ‘strip’ theory and it is shown that blades of low aspect ratio have broader undamped regions when zero damping is first encountered.     

An actuator-disc model for the prediction of abrupt stall in an axial compressor rotor

The representation of loss in a cascade by the appearance of blockage has been extended to deal with blade rows by the use of a source distribution to represent this blockage, and in the case of the actuator disc approximation, the presence of sources is confined to an axi-symmetric dìstribution over the actuator disc. It is found that if a typical dependence of loss (and consequently diffusion ratio) upon incidence for each section of an axial compressor rotor is represented in this manner, the influence of blockage on the axial velocity distribution may be found using the potential equation combined with the usual actuator disc approximation. Study of the behaviour of the controlling ordinary differential equation for the axial velocity ahead of the disc reveals that as the flow is reduced, the equation contains a singularity within the range of radius and a meaningful solution does not exist. This result is interpreted as the limit to continuous operation and reasonable agreement between this predicted limit and the appearance of abrupt stall (experimentally) is found.     

Separation control using synthetic vortex generator jets in axial compressor cascade

An experimental investigation conducted in a high-speed plane cascade wind tunnel demonstrates that unsteady flow control by using synthetic (zero mass flux) vortex generator jets can effectively improve the aerodynamic performances and reduce (or eliminate) flow separation in axial compressor cascade. The Mach number of the incoming flow is up to 0.7 and most tested cases are at Ma = 0.3. The incidence is 10° at which the boundary layer is separated from 70% of the chord length. The roles of excitation frequency, amplitude, location and pitch angle are investigated. Preliminary results show that the excitation amplitude plays a very important role, the optimal excitation location is just upstream of the separation point, and the optimal pitch angle is 35°. The maximum relative reduction of loss coefficient is 22.8%.The project supported by the National Natural Science Foundation of China (10477002 and 50476003) and the Ph.D. Innovative Foundation of Beihang University. The English text was polished by Yunming Chen.     

Stereoscopic PIV measurement of unsteady flows in an axial compressor stage

Stereoscopic particle-image velocimetry (SPIV) measurements are conducted in a Low-speed Large-scale Axial Compressor. During the experiment the two CCD cameras are placed at the different sides of the laser light sheet and it is proved that this configuration is more suitable for the investigation in multi-stage turbomachines. The measured results, including the overall performances of many typical flow structures near the rotor tip region and the phase locked unsteady flows inside the stator passage at both the design and near-stall conditions, are introduced. Some new features of the complicated flow structures, such as the breakdown of the tip leakage vortex, the formation of the compound corner vortex at the rotor suction tip corner, the interactions between the hub stall and the tip separation and the rotor wakes, and the evolutions of the tip corner anti-rotating streamwise vortices inside the stator passage, are revealed.     

Flow and heat transfer in a rotating enclosure with axial throughflow

An analysis is made of the fluid flow and heat transfer processes in a circular cylindrical enclosure rotating about its own axis. A coolant is passed through the enclosure, entering and leaving through centrally located apertures in the end walls. This configuration is intended as a model of rotating enclosures in devices such as gas turbines and air compressors. The Navier-Stokes and energy equations were solved by a finite-difference formulation which can accommodate either steady or transient conditions. Buoyancy forces associated with the rotational body forces were included in some cases. All solutions were performed for laminar flow. For the parameter ranges investigated it was found that rotation inhibited the recirculating motion within the enclosure and thereby decreased the heat transfer relative to that for the stationary enclosure. Buoyancy further reduced the heat transfer owing to the break up of residual circulatory motions in the outer portion of the enclosure. Still stronger buoyancy brought about a slight increase in the heat transfer. The coolant flow was confined to a corridor adjacent to the axis of the enclosure, and there was no mixing between the coolant and the fluid in the enclosure proper.     

Characterization of incipient cavitation in axial valve by hydrophone and visualization

As the effects of cavitation in valves are devastating, the choice of the correct valve for a given operating range is crucial. For this, the valve characteristic is needed, whereby one side of the operating range depends on the determination of the incipient cavitation.In this paper, the visualization method for incipient cavitation detection is presented. For the purpose of comparison, pressure oscillations inside the pipeline were simultaneously measured with a hydrophone. The effect of operating pressure was studied for two different openings of the valve.For each operating point of incipient cavitation, corresponding points were measured for developed cavitation and no-cavitation state, based on a constant-portion change of volumetric flow rate with regard to the incipient cavitation volumetric flow rate. The visualization and hydrophone signals were compared.The visualization method proved efficiency over hydrophone measurements because it is more sensitive to cavitation and the signal is independent of the operating pressure. The main drawback is the preparation of the observation window.     

Flow boiling visualization in a vertical circular minichannel at high vapor quality

This paper reports on an experimental study of saturated flow boiling of R134a inside a circular vertical quartz tube coated with a transparent heater. The inner diameter of the tube was 1.33 mm and the heated length 235.5 mm. The flow pattern at high vapor qualities and the dryout of the liquid film were studied using a high speed CCD camera at the mass fluxes 47.4 and 124.4 kg/m² s in up flow at 6.425 bar. The heat fluxes ranged from 5 to 13.6 kW/m² for the lower mass flux and from 20 to 32.4 kW/m² for the higher mass flux.

The behavior of the flow close to dryout was found to be different at low and high mass flux. At low mass flux the location of the liquid front fluctuated with waves passing high up in the tube. In between the waves, a thin film was formed, slowly evaporating without breaking up.

At high mass flux the location of the liquid front was more stable. In this case the liquid film was seen to break up into liquid streams and dry zones on the tube wall.     

Experimental investigation about the effect of non-axisymmetric wake impact on a low speed axial compressor

Non-axisymmetric wake impact experiments were carried out after the best exciting frequency for a low speed axial compressor had been found by axisymmetric wake impact experiments. When the number and circumferential distribution of inlet guide vanes （IGV） are logical, the wakes of non-axisymmetric IGVs can exert beneficial unsteady exciting effect on their downstream rotor flow fields and improve the compressor＇s performance. In the present paper, four non-axisymmetric wake impact plans were found working better than the axisymmetric wake impact plan. Compared with the base plan, the best non-axisymmetric plan increased the compressor＇s peak efficiency, and the total pressure rise by 1.1 and 2%, and enhanced the stall margin by 4.4%. The main reason why non-axisymmetric plans worked better than the axisymmetric plan was explained as the change of the unsteady exciting signal arising from IGV wakes. Besides the high-frequency components, the nonaxisymmetric plan generated a beneficial low-frequency square-wave exciting signal and other secondary frequency components. Compared with the axisymmetric plan, multifrequency exciting wakes arising from the non-axisymmetric plans are easier to get coupling relation with complex vortices such as clearance vortices, passage vortices and shedding vortices.     

Flow measurements inside a heated multiple rotating cavity with axial throughflow

This paper discusses experimental results from a multiple cavity test rig representative of a high pressure compressor internal air system. Measurements of the axial, tangential and radial velocity components are presented. These were made using a two component, laser doppler anemometry (LDA) system for a range of non-dimensional parameters representative of engine conditions (Re up to 4 × 10⁶ and Re_z up to 1.8 × 10⁵). Tests were carried out for two different sizes of annular gap between the (non-rotating) drive shaft and the disc bores.

The axial and radial velocities inside the cavities are virtually zero. The size of the annular gap between disc bore and shaft has a significant effect on the radial distribution of tangential velocity. For the narrow annular gap (d_h/b = 0.092), there is an increase of non-dimensional tangential velocity V/Ωr with radial location from V/Ωr < 1 at the lower radii to solid body rotation V/Ωr = 1 further into the cavity. For the wider annular gap (d_h/b = 0.164), there is a decrease from V/Ωr > 1 at the lower radii to solid body rotation further into the cavity. An analysis of the frequency spectrum obtained from the tangential velocity measurements is consistent with a flow structure in the r– plane consisting of pairs of contra rotating vortices.     

Flow birefringence visualization of transitions in circular Couette flow

Flow birefringence technique is an experimental method in fluid dynamics from which we can obtain information about the velocity fields in various devices. This method has been used here to observe transitions which occur in the classical Couette device when the angular velocity of the inner cylinder increases beyond a certain critical value. Although earlier experiments have been reported, the observation of the flow birefringence phenomena in the entirely illuminated annular gap of a classical Couette cell consists in a new access to these problems and may well bring important information on the changes occuring in the hydrodynamical field at the different transitions.     

Flow visualization of supersonic laminar flow over a backward-facing step via NPLS

An experimental study on a supersonic laminar flow over a backward-facing step of 5 mm height was undertaken in a low-noise indraft wind tunnel. To investigate the fine structures of Ma = 3.0 and 3.8 laminar flow over a backward-facing step, nanotracer planar laser scattering was adopted for flow visualization. Flow structures, including supersonic laminar boundary layer, separation, reattachment, redeveloping turbulent boundary layer, expansion wave fan and reattachment shock, were revealed in the transient flow fields. In the Ma = 3.0 BFS (backward-facing step) flow, by measuring four typical regions, it could be found that the emergence of weak shock waves was related to the K–H (Kelvin–Helmholtz) vortex which appeared in the free shear layer and that the convergence of these waves into a reattachment shock was distinct. Based on large numbers of measurements, the structure of time-averaging flow field could be gained. Reattachment occurred at the location downstream from the step, about 7–7.5 h distance. After reattachment, the recovery boundary layer developed into turbulence quickly and its thickness increased at an angle of 4.6°. At the location of X = 14h, the redeveloping boundary layer was about ten times thicker than its original thickness, but it still had not changed into fully developed turbulence. However, in the Ma = 3.8 flow, the emergence of weak shock waves could be seen seldom, due to the decrease of expansion. The reattachment point was thought to be near X = 15h according to the averaging result. The reattachment shock was not legible, which meant the expansion and compression effects were not intensive.     

Flow visualization and wall shear stress of a flapping model hummingbird wing

The unsteady low Reynolds number aerodynamics of flapping flight was investigated experimentally through flow visualization by suspended particle imagery and wall shear stress measurement from micro-array hot-film anemometry. In conjunction, a mechanism was developed to create a flapping motion with three degrees of freedom and adjustable flapping frequency. The flapping kinematics and wing shape were selected for dynamic similarity to a hummingbird during hovering flight. Flow visualization was used to validate the anemometry observations of leading edge vortex (LEV) characteristics and to investigate the necessity of spanwise flow in LEV stability. The shear sensors determined LEV characteristics throughout the translation section of the stroke period for various wing speeds. It was observed that a minimum frequency between 2 and 3.5 Hz is required for the formation and stabilization of a LEV. The vortex strength peaked around 30% of the flapping cycle (corresponding to just past the translation midpoint), which agrees with results from previous studies conducted by others. The shear sensors also indicated a mild growth in LEV size during translation sections of the wing’s motion. This growth magnitude was nearly constant through a range of operating frequencies.     

Flow and coupled heat transfer in the cavity between the rotor and stator

The turbulent flow and coupled heat transfer in the cavity between the rotor and stator is numerically simulated. Reynolds-averaged Navier-Stokes equations closed with equations of the k-ɛ turbulence model are used to calculate the viscous compressible gas flow characteristics and heat transfer; the unsteady heat conduction equation is used to calculate the temperature field in the metal. The influence of the mass flow rate of the coolant on the flow structure and efficiency of cooling of the rotor and stator walls is studied. The calculated results are compared with experimental data.     

Flow visualization of the elastic Taylor-Couette instability in Boger fluids

Flow visualization is performed on an elastically-dominated instability in several similar Boger fluids in Taylor-Couette flow. The onset and evolution of secondary flow are observed over a range of shear rates using reflective mica platelet seeding. Sequences of ambiently and sheet-illuminated images were digitally processed. Rotation of the inner cylinder was ramped from rest to its final value over a time on the order of a polymer relaxation time. Dilute solutions of high molecular weight polyisobutylene in oligomeric polybutene manifest a flow transition at a Deborah number, De _s = _s 1.5 with a Taylor number of 0.00022 in a cell with dimensionless gap ratio = 0.0963. At this transition, simple azimuthal shearing is replaced by steady, roughly square, axisymmetric counter-rotating vortices grossly similar to the well-known Taylor vortex flow that is observed at De _s = 0, Ta = 3612. At De _s = 3.75, Ta = 0.0014, an axisymmetric oscillatory secondary flow develops initially but is replaced by the steady vortices. At De _s = 7.5, Ta = 0.0054, the oscillatory and vortex flow coexist and possess an irregular cellular cross-section. A wide span of growth rates is observed: the ratio of onset to polymer relaxation time ranges from 170000 at De _s = 1.5 to O(10) at De _s > 5. The role of inertia was explored through changing the solvent viscosity. A transition similar to the one that occurs at De _s = 3.75, Ta = 0.0014, from the base azimuthal shearing flow to axisymmetric vortices, was also observed with a much lower viscosity fluid at De _s = 3.3, Ta = 74.     

Characterization of the tip clearance flow in an axial compressor using 3-D digital PIV

The accurate characterization and simulation of rotor tip clearance flows has received much attention in recent years due to their impact on compressor performance and stability. At NASA Glenn the first known three dimensional digital particle image velocimetry (DPIV) measurements of the tip region of a low speed compressor rotor have been acquired to characterize the behavior of the rotor tip clearance flow. The measurements were acquired phase-locked to the rotor position so that changes in the tip clearance vortex position relative to the rotor blade can be seen. The DPIV technique allows the magnitude and relative contributions of both the asynchronous motions of a coherent structure and the temporal unsteadiness to be evaluated. Comparison of measurements taken at the peak efficiency and at near stall operating conditions characterizes the mean position of the clearance vortex and the changes in the unsteady behavior of the vortex with blade loading. Comparisons of the 3-D DPIV measurements at the compressor design point to a 3D steady N-S solution are also done to assess the fidelity of steady, single-passage simulations to model an unsteady flow field.     

Flow visualization of a recirculating flow by rheoscopic liquid and liquid crystal techniques

Experiments were conducted in a three-dimensional lid-driven cavity flow to study the behavior of longitudinal Taylor-Görtler-like vortices. Flow visualization was accomplished by use of a rheoscopic liquid and of liquid crystals, together with laser-light and white-light sheets, respectively. Photographs of the lighted planes in the flow confirmed the existence of the vortices for a wide range of Reynolds numbers and for stable, neutrally-buoyant and buoyant global flow conditions. As usual the flow visualization revealed flow patterns not deducible by in situ measurements; the liquid crystal photographs give both flow pathlines and temperature distribution on any lighted plane.