Low-speed Axial-flow Fan Research Articles

軸流ファンにおける小スケール失速セルの発生点近傍での動翼先端流れ場(流体工学,流体機械)

Rotor tip flow-field in a low-speed axial-flow fan was investigated by steady numerical simulations to elucidate inception mechanisms of rotating stall initiated from a short length-scale stall cell. Tip leakage flow interacts with the incoming flow at the maximum pressure-rise point. Backflow from the trailing edge is initiated at the tip near the stall inception point and develops at the stall inception point. The interface between the tip leakage flow and the incoming flow becomes parallel to the leading edge plane near the stall inception point and moves upstream of the rotor leading edge at the stall inception point. Moreover, the tip leakage flow spills from the leading edge at the stall inception point. Tip-leakage vortex breakdown occurs and induces the three-dimensional separation on the suction surface of the rotor blade near the stall inception point. At the stall inception point, this separation grows into a three-dimensional separation vortex, which crosses the rotor passage near the trailing edge. It is concluded from the numerical results that the three-dimensional separation vortex and the spillage of tip leakage flow affect the inception of the short length-scale stall cell.

J0501-1-3 軸流ファンにおけるモーダル波の発生点近傍での動翼先端流れ場(流体機械の研究開発におけるEFD/CFD(1))

Numerical and experimental studies were carried out to clarify the mechanism behind a modal-type stall inception in a low-speed axial-flow fan. The modal disturbance was experimentally observed in the positive slope region of the pressure-rise characteristics of two rotors. Tip-leakage vortex breakdown was not numerically confirmed in the positive slope region. A three-dimensional separation vortex, which was developed from the leading edge separation or the separation at the mid-chord on the suction surface, was confirmed. The numerical and experimental results show that the three dimensional separation vortex induced by the leading edge separation or the separation at the mid-chord influences the development of the modal disturbance.

Design optimization of low-speed axial flow fan blade with three-dimensional RANS analysis

This work presents a numerical optimization procedure for a low-speed axial flow fan blade with polynomial response surface approximation model. Reynolds-averaged Navier-Stokes equations with SST turbulence model are discretized by finite volume approximations and solved on hexahedral grids for flow analyses. The blade profile as well as stacking line is modified to enhance blade total efficiency, i.e., the objective function. The design variables of blade lean, maximum thickness and location of maximum thickness are selected, and a design of experiments technique produces design points where flow analyses are performed to obtain values of the objective function. A gradient-based search algorithm is used to find the optimal design in the design space from the constructed response surface model for the objective function. As a main result, the efficiency is increased effectively by the present optimization procedure. And, it is also shown that the modification of blade lean is more effective to improve the efficiency rather than modifying blade profile.

Direct calibration framework of triple-hole pressure probes for incompressible flow

This paper carries out a mathematical analysis of the limits and data reduction techniques of three-hole pressure (THP) probes operating in a ‘non-nulling’ mode for incompressible flow. As a result of this analysis, a direct procedure is advanced, based on the distinction of several zones within the angular range, where different relations can be applied to obtain the flow variables. This proposal provides a considerable increment of the operative angular range of THP probes: about ±70° instead of the typical ±35° for a cylindrical probe. This may extend the application of these probes in highly unsteady flows, or reduce the acquisition and data reduction effort minimizing the necessity of probe reorientation. The influence of the data reduction technique on the uncertainty transmission is also presented in the paper. From detailed considerations, it is demonstrated that the results uncertainty depends on the specific probe, but it is unaffected by the mathematical procedure employed to calculate the flow variables. Validation measurements with pneumatic probes have been made for Reynolds numbers from 4 × 103 to 3.5 × 104. In addition, a highly unsteady measurement in a low-speed axial flow fan is succinctly analysed. Taking into account both attainable angular range and uncertainty, it is determined that the optimal construction angle for the holes of a low frequency response THP probe lies between 30° and 60°, while for fast response probes, in order to avoid the separated flow region, the optimal construction angle is around 30°.

Effect of Rotor Tip Leakage Flow on Spike-Type Stall Inception

We measured pressure fluctuations and distributions on the casing wall in a low speed axial-flow fan with relatively large tip clearance to clarify the criteria for a spike-type stall inception. The blade loading at the rotor tip and the tip leakage flow were compared between the two stagger angle settings for the rotor blade. Spikes appeared at the design stagger angle setting, and did not appear at the high stagger-angle setting, which was ten degrees larger than the design value. A rotating instability, which was induced by an interaction among the incoming flow, tip leakage flow, and end-wall backflow, was confirmed near the maximum pressure-rise point at both stagger angle settings. At the design stagger angle, the interface of this interaction became parallel to the leading edge plane. This interaction moreover generated a large end-wall blockage near the leading edge. This blockage developed a leading edge separation on the stalled rotor at the tip, and the leading edge separation then grew into a spike. In contrast, at the high stagger angle, the interface of this interaction did not become parallel to the leading edge plane, and the end-wall blockage was smaller than that at the design stagger angle. The leading edge separation therefore neither developed nor grew into a spike although the rotor blade at the tip stalled. We concluded from these results that the large end-wall blockage induced by the interaction among the incoming flow, tip leakage flow, and end-wall backflow was the main criterion for the spike type stall inception.

G601 動翼可変軸流ファンでのモーダル型旋回失速発生過程に及ぼす動翼先端流れの影響(2)(GS6 翼,渦,一般セッション)

Flow fields at the rotor tip region in a low-speed axial-flow fan were experimentally and computationally investigated to clarify the flow mechanism behind a modal stall inception pattern. The behavior of tip leakage flow at the modal stall inception was different from that at the spike stall inception. The interface between the incoming flow, tip leakage flow, and end-wall backflow did not parallel to the rotor leading edge plane. Leading edge separation occurred when the tip leakage vortex broke down. This separation grew and formed three-dimensional separation vortex, which stood on the suction side near the leading edge. We concluded from these results that this three-dimensional separation vortex influenced on the initiation of the modal disturbance.

Large Eddy Simulation of Acoustical Sources in a Low Pressure Axial-Flow Fan Encountering Highly Turbulent Inflow

A large eddy simulation (LES) was applied to predict the unsteady flow in a low-speed axial-flow fan assembly subjected to a highly “turbulent” inflow that is generated by a turbulence grid placed upstream of the impeller. The dynamic Smagorinsky model (DSM) was used as the subgrid scale (SGS) model. A streamwise-upwind finite element method (FEM) with second-order accuracy in both time and space was applied as the discretization method together with a multi-frame of reference dynamic overset grid in order to take into account the effects of the blade-wake interactions. Based on a simple algebraic acoustical model for axial flow fans, the radiated sound power was also predicted by using the computed fluctuations in the blade force. The predicted turbulence intensity and its length scale downstream of the turbulence grid quantitatively agree with the experimental data measured by a hot-wire anemometry. The response of the blade to the inflow turbulence is also well predicted by the present LES in terms of the surface pressure fluctuations near the leading edge of the blade and the resulting sound power level. However, as soon as the effects of the turbulent boundary layer on the blades become important, the prediction tends to become inaccurate.

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.

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.

Spike and Modal Stall-inception Patterns in a Variable-pitch Axial-flow Fan

Inception patterns of rotating stall in a low-speed axial-flow fan have been investigated by measuring pressure and velocity fluctuations. These experimental measurements were carried out at two different stagger-angle settings for the rotor blades. They show that at the design stagger angle, a short length-scale stall cell known as a “spike” appears first. This stall cell then grows into a long length-scale stall cell as flow rate decreases. At the small stagger angle, a long length-scale disturbance known as a “modal” is first observed, and the modal and the spikes then induce the long length-scale stall cell. The short length-scale stall cell and the modal are first observed at the rotor tip, because the end-wall blockage at the tip becomes large. At the small stagger angle, this blockage is smaller than that at the design angle. The modal is therefore generated before the short length-scale cells appear. It is concluded from these results that the rotating stall-inception patterns depend on the rotor stagger angle.

1314 動翼可変軸流ファンにおけるスパイク型とモーダル型旋回失速発生過程

Inception patterns of rotating stall in a low-speed axial-flow fan have been investigated by measurements of pressure and velocity fluctuations. Experiments have been carried out at two different stagger angle settings for rotor blades. At the design stagger angle, a short length-scale stall cell known as a "spike" and multiple short length-scale stall cells appear first. These stall cells then grow into a long length-scale stall cell as flow rate decrease. The spike and the multiple short length-scale stall cells induce a part-span stall, and the long length-scale stall cell induces a full-span stall. At the small stagger angle, a long length-scale disturbance known as a "modal" is observed first, and the modal and spikes then induces the long length-scale stall cell. It is concluded from these results that the rotating stall inception patterns depend on the rotor stagger angle.

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.

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 investigation of unsteady fan flow interaction with downstream struts

In the present study of the unsteady pressure field produced on fan rotor blades by interaction with downstream struts, a single stage, low speed axial-flow fan was instrumented with blade-mounted high frequency pressure transducers. In addition, stationary pressure problems were used to map out the flowfield. Fluctuating pressure measurements are presented for blade midspan and 85-percent span on both the suction and pressure surfaces of the rotor blades at several positions of the downstream struts, and for two different flow coefficients. The strut is found to produce an effect on the unsteady pressure field on the rotor blades; this effect exceeds that due to the stator at design rotor-stator-strut spacing, but it rapidly declines as the struts are moved downstream.

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.