Low-speed Axial Flow Research Articles

4336 タービン・ディスクキャビティへの主流巻き込み現象の検討(S60-1 ガスタービン(1),S60 ガスタービン)

This paper presents the result of static pressure distribution measurements on the surfaces of endwall and rim-side wall of the nozzle ring in a low-speed axial flow turbine. equipped a rotating disc with pin blades. The measurements were performed under a constant disc cavity gap and a constant blade Reynolds number with a secondary fluid flow from the disc cavity for the location of stationary pin blades moving between one pitch of nozzle blades. The possibility of the main flow enrolment to the disc cavity was discussed through the static pressure distribution characteristics on the endwall and rim-side wall surfaces.

Interaction of unsteady separated flow over multi-bodies moving relatively in the same flow field

Unsteady separated flow is one of research frontiers in current aerodynamic. Great accomplishments have been acquired; however, most studies are on single body in a stream, such as studies on unsteady separated flows over airfoils. There are typical cases in the nature and engineering applications, in which several interacting bodies with relative motions are within the same flow field. These interacting unsteady separated flow fields not only are closely related to the phenomena of noise and flutter induced by flows, but also have strong influences on aerodynamic performances. With axial flow compressors as background, the present paper carried out studies on ‘interaction of unsteady separated flow over multi-bodies moving relatively in the same flow field’. Experiment investigations carried out in the stationary annular cascade wind tunnel and the single-stage low-speed axial flow compressor experimental facility as well as relevant CFD simulations demonstrate that under properly organized interactions between all unsteady components, the time-space structure of unsteady separated flow field can be remarkably improved and the time-averaged aerodynamic performances be significantly enhanced accordingly. The maximum reduction of the loss coefficient reached 27.4% and 76.5% in the stationary annular cascade wind tunnel and the CFD simulation for single-stage axial flow compressor, respectively.

유량에 따른 축류홴의 익단누설와류 및 후류 특성

The flow characteristics in the blade passage and in the wake region of a low speed axial flow fan have been investigated by experimental analysis using a rotating hot-wire sensor for design and off-design operating conditions. The results show that the tip leakage vortex is moved upstream when flow rate is decreased, thus disturbing the formation of wake flow near the rotor tip. The tip leakage vortex interfaces with blade pressure surface, and results in high velocity fluctuation near the pressure surface. From axial velocity distributions downstream of the fan rotor, large axial velocity decay near the rotor tip is observed at near stall condition, which results in large blockage compared to that at the design condition. Although the wake flow downstream of the rotor blade is clearly measured at all operating conditions, the trough of the high velocity fluctuation due to Karmann vortex street in the wake flow is mainly observed at a higher flow condition than the design flow rate.

Long-to-Short Length-Scale Transition: A Stall Inception Phenomenon in an Axial Compressor With Inlet Distortion

A theoretical and experimental study of stall inception processes in a three-stage low-speed axial flow compressor with inlet distortion is presented in this paper. Since inlet distortion provides asymmetric flows imposing onto the compressor, the main goal of this research is to unveil the mechanism of how such flows initiate long and/or short length-scale disturbances and how the compression system reacts to those disturbances. It is found that the initial disturbances are always triggered by the distorted flows, yet the growth of such disturbances depends on system dynamics. While in many cases the stall precursors were the short length-scale spikes, there were some cases where the compressor instability was triggered after the disturbances going through a long-to-short length-scale transition. A Moore-Greitzer-based (system scale) model was proposed to qualitatively explain this phenomenon. It was found that, when the compressor operated in a region where the nonlinearity of the characteristics dominated, long length-scale disturbances induced by the inlet distortion would evolve into short length-scale disturbances before they disappeared or triggered stall. However, the model was not able to predict the fact that many disturbances that were triggered by the distorted sector(s) were completely damped out in the undistorted sector(s). It is thus suggested that in future research of compressor instability, one should consider the flows in blade passage scale, the dynamics in system scale, and their interaction simultaneously.

Experimental Analysis on Tip Leakage and Wake Flow in an Axial Flow Fan According to Flow Rates

The flow characteristics in the blade passage and in the wake region of a low-speed axial flow fan have been investigated by experimental analysis using a rotating hot-wire sensor and a five-hole probe for design and off-design operating conditions. The results show that the tip leakage vortex is moved upstream when the flow rate is decreased, thus disturbing the formation of wake flow near the rotor tip. That is, the tip leakage vortex interfaces with the blade suction surface and results in high velocity fluctuation near the blade suction surface. From axial velocity distributions downstream of the fan rotor, large axial velocity decay near the rotor tip is observed at near-stall condition, which results in a large blockage compared to that at the design condition. Finally, the wake flow downstream of the rotor blade is clearly measured at the design and off-design conditions. However, the trough of the high velocity fluctuation due to Karmann vortex street in the wake flow is observed at a higher flow condition than the design flow rate.

An Experimental and Numerical Investigation into the Mechanisms of Rotating Instability

This paper reports on an experimental and numerical investigation aimed at understanding the mechanisms of rotating instabilities in a low speed axial flow compressor. The phenomena of rotating instabilities in the current compressor were first identified with an experimental study. Then, an unsteady numerical method was applied to confirm the phenomena and to interrogate the physical mechanisms behind them. The experimental study was conducted with high-resolution pressure measurements at different clearances, employing a double phase-averaging technique. The numerical investigation was performed with an unsteady 3-D Navier-Stokes method that solves for the entire blade row. The current study reveals that a vortex structure forms near the leading edge plane. This vortex is the result of interactions among the classical tip-clearance flow, axially reversed endwall flow, and the incoming flow. The vortex travels from the suction side to the pressure side of the passage at roughly half of the rotor speed. The formation and movement of this vortex seem to be the main causes of unsteadiness when rotating instability develops. Due to the nature of this vortex, the classical tip clearance flow does not spill over into the following blade passage. This behavior of the tip clearance flow is why the compressor operates in a stable mode even with the rotating instability, unlike traditional rotating stall phenomena.

Stall Margin Improvement of an Axial Flow Fan with End Wall Injection and Suction. 2nd Report. A Study of Stall Recovery Margin Improvement.

This paper presents the experimental results of stall recovery margin improvements. Studies have been conducted on a low speed axial flow fan to investigate the stall cessation flow rate with several kinds of injection or suction holes on the end wall. First of all, the fan characteristics for the solid wall have been measured. The hysteresis loop has been observed, but the size of hysteresis loop was not large, so that the stall cessation flow rate was about 17% smaller than the stall onset flow rate for several rotor speeds. Stall recovery margin improvements for both injection and suction were about 3 to 5% larger than stall margin improvements. For the case of injection, the position of maximum stall recovery margin improvements has moved to downstream direction a little from the position of maximum stall margin improvements. For the case of suction, the largest improvement was found at the position of just upstream of leading edge of blade. And negative improvement was not found except very large suction. For the case of injection, comparing with stall recovery margin improvements and stall margin improvements, both values are almost the same with regard to the injection momentum relative to the chordwise.

Influence of deterministic stresses on flow prediction of a low-speed axial flow compressor rear stage

Deterministic stresses are used to describe the unsteady effects on the flow field in multistage axial flow compressors. In this paper, a circumferentially non-uniform mixing plane method implemented with deterministic stresses is used for the flow prediction inside the rear stage of an axial flow compressor in a multistage environment. The deterministic stress field is calculated with an overlapped solution approach for the same flow passage and implemented with the Navier-Stokes equations in order to provide continuous interfaces. The aerodynamic boundary condition is simplified with the application of a repeating stage model where only the mass flowrate and stage exit average static pressure are required. Two computational cases for the third stage of the Cranfield low-speed research compressor, one with the mixing plane model and the second with the mixing plane model in combination with deterministic stresses, are examined and compared with each other and also with corresponding experimental data. It is shown that the implementation of the deterministic stresses is beneficial for the flow prediction but the improvement in accuracy for low-speed axial flow compressor rear stages is not significant.

A functional neuroanatomy of sleepiness induced by sedative anthihastamine

A kind of foam metal casing treatment (FMCT) is proposed and experimentally tested on a low-speed axial flow compressor in this work. Foam metal is a foam-like substance with a lightweight structure but a certain strength. Both time-mean and high-response instrumentation are applied to capture the steady and unsteady performances of the compressor. Initially, parametric experimental measurements on axial location of foam metal over rotors are performed with respect to stability enhancement and noise reduction. It is found that the stall margin improvement (SMI) and the aerodynamic efficiency loss are functions of the axial location of the foam metal. Particularly, the SMI maxima appears at CT7 with a certain amount of aerodynamic efficiency loss. In analyzing the results measured over and downstream of the rotor, it is suggested that the release of rotor tip loading may be a stability improvement mechanism of FMCTs. Furthermore, comparison in the stall inception process of the axial flow between the compressor with the solid-wall and the implementation configuration shows that, in solid-wall casing condition, the compressor experiences a spike-type stall inception, however in CT7, the compressor goes through a modal-type process. Finally, conclusions and suggestions are given based on the findings.

軸流ファンにおける低周波吸音について

The acoustic treatment with a rather large cavity was discussed from both aspects of analysis and experiment for low-frequency noise reduction in a low-speed axial flow fan. The rectangular and annular ducts with treatments were used. It is tentatively concluded that the axial modes of sound wave travelling in the cavity played an important role in low-frequency noise reduction mechanism. The acoustic treatment with a biased cavity showed a promise from a view point of weight reduction in aeroengines and fluid machinery.

Experimental Study of Casing Boundary Layers in a Multistage Axial Compressor

Measurements of the casing boundary layers were obtained in a four-stage, low speed axial flow compressor, to verify the ‘law of the wall’ applicability to these complex flows. Some of the available shear stress models of the two-dimensional flows have been examined towards the quantitative assessment of skin friction. The shear stress prediction obtained from the Ludwieg-Tillmann relation applied to the streamwise or untwisted profile agreed closely with the measured shear stress by the hot wire. The skin friction was fairly constant for rotor and stator flows and was close to the flat plate values. The boundary layer profiles exhibited a well pronounced semi-logarithmic region with the universal constants of the law of the wall far removed from the standard two dimensional values, especially for rotor flows. Stator flows showed signs of similarity to two dimensional flows.

Experimental Investigation of Rotating Stall in a Mismatched Three-Stage Axial Flow Compressor

This paper reports on an examination of rotating stall in a low-speed three-stage axial flow compressor operating with various degrees of stage mismatch. The objective of this study was to simulate the mismatching that occurs in high-speed multistage compressors when operating near surge. The study of the stall zones involved the use of fast response measurement techniques. The study clearly shows how stages can operate in an axisymmetric fashion even when heavily stalled, since rotating stall inception requires the stall of more than one stage. The study also compares conditions required for full-span and part-span stall and suggests that the part-span stall structure is more relevant to high-speed multistage compressors.

End-Wall and Profile Losses in a Low-Speed Axial Flow Compressor Rotor

The blade-to-blade variation of relative stagnation pressure losses in the tip region inside the rotor of a single-stage, axial-flow compressor is presented and interpreted in this paper. The losses are measured at two flow coefficients (one at the design point and the other at the near peak pressure rise point) to discern the effect of blade loading on the end-wall losses. The tip clearance losses are found to increase with an increase in the pressure rise coefficient. The losses away from the tip region and near the hub regions are measured downstream. The losses are integrated and interpreted in this paper.

Rotating stall cells in a low-speed axial flow compressor

An experimental investigation of the flowfield inside a high hub-to-tip ratio, single-stage compressor is described. Fast-response instrumentation was used for the measurements in the absolute and relative frames of reference. The information was digitized for further analysis of the instantaneous and averaged flow parameters. The calculation of power spectra and the autocorrelation function, and the phase-locked averaging technique are used in this study. This technique consists of dividing the long sample in time intervals equal to the period as obtained by fast Fourier transforms and ensemble averaging these intervals. This procedure has been adopted because it can handle singleand multicell patterns. Two types of perturbation are distinguished: the small eight-cell stall and the singleor double-cell pattern with deep stall. The overall flow behavior, as well as the detailed flowfield structure, are presented for the two cases as measured with stationary probes. On-blade flow measurements suggest the existence of important radial velocities near the rotor blade surface when deep stall occurs.

Three-Dimensional Flowfield Inside a Low-Speed Axial Flow Compressor Rotor

T HE velocity and pressure fields were measured across the entire rotor passage at six axial locations and five radial locations inside the passage of an axial flow compressor rotor. A rotating five-hole probe was used to measure the three components of velocity and the static pressure. Contents The objective of this investigation is to increase our un- derstanding of the nature of the three-dimensional flowfield, and to give us a comprehensive set of data for computer code verification. The measurements reported in this paper were performed using the axial flow compressor facility located in the Tur- bomachinery Laboratory of The Pennsylvania State University. The hub/annulus wall diameter ratio of the facility is 0.5 with the diameter of the annulus wall equal to 0.932 m. The operating conditions are as follows: flow coefficient based on tip speed, 0.56; stage loading coefficient based on the tip speed, 0.486; speed of the rotor, 1088 rpm. A rotating five-hole probe similar (but much smaller in size) to that used in Ref. 1 was employed for all of the measurements reported in this paper. The probe head had a diamond shape with a diagonal distance of 1.67 mm. The pressures measured by the five holes were used to determine the velocity and pressure field by interpolation of the calibration curves.

Investigation of the Coupling of Unsteady Lift to Lower Order Acoustic Duct Modes in an Axial Flow Fan

Abstract : By using flow distortion screens, the plane wave (0,0) and first higher order (-1,0) and (1,0) spinning modes were generated in a low-speed axial flow fan. The perpendicular unsteady lift force on a segment of a nine-bladed rotor and the acoustic pressure along the duct were measured simultaneously by a strain gage sensor and a flush-mounted microphone, respectively. The total pressure field downstream of the screens was measured by circumferentially traversing a Kiel probe. The typical signal-to-noise ratio was sufficiently high for the desired signal isolation with a spectrum analyzer. A phase-locked ensemble averaging technique was used to obtain amplitude and phase measurements of the periodic unsteady signals. Standing waves set up in the duct between the rotor and duct inlet seemed to cause a back-reaction effect which resulted in an increase and reduction of the unsteady lift force above and below expected levels. If these effects are taken into account, theoretical predictions coupling unsteady lift to acoustic far field pressure levels conform to measured data. Therefore, it is concluded that there is a definite coupling between unsteady lift and acoustic pressure in the duct, yet determination of an unambiguous coupling factor would require more extensive data acquisition.

Measurement, in a duct, of the space-structure of the discrete-frequency noise generated by an axial compressor

A method for measuring the spatially coherent and incoherent structures of acoustic modes propagating in a duct, at a given frequency, is described. The first part of this paper deals with data acquisition in the laboratory; the second part concerns the analogical processing of the signals; the third part describes the numerical processing of the analogical information acquired, based on the fast Fourier transform. The method has first been developed and applied experimentally at ONERA for the characteristic frequencies generated by a six-bladed fan in a cylindrical duct 4 m long, 60 cm in diameter and carrying a low speed axial flow ( M o < 0·1). The results are presented and they show that in the near field of the fan, the azimuthal structure of the sound field is, in part, accounted for by Tyler and Sofrin's model. But other modes are also shown to exist and they propagate in the duct, their azimuthal structures being compatible with the propagation properties of the duct. The degree of coherence decreases with the axial distance from the source. The dispersion relation of the modes is discussed. For higher speed flows in a half scale model compressor, the method is being tested at SNECMA by SNECMA and ONERA.

Advanced through-flow analysis applied to a low-speed axial flow compressor

This paper describes some calculations using the digital computer programme written by Marsh 1 for the through-flow fluid mechanics of a turbomachine. The machine chosen as the subject for the calculation was a slow-speed single stage axial flow compressor in the Mechanical Engineering Department at Liverpool University. The deviations and losses for the blades were obtained from cascade correlations and the area blockages and secondary flow angle changes were derived from a simple annulus wall boundary layer calculation performed on an analogue computer.