Forward-skewed Blade Research Articles

Experimental and numerical comparisons among various skewed blades of low speed axial fan rotors

Literature indicates that there is lack of study related to the effects of axially skewed blades on the aerodynamic performance of axial flow fans. In this study rotors of various blade skews, namely, axial, circumferential and axial-circumferential types, are studied and their aerodynamic performances are compared to those of the baseline radial rotors of two types, namely, constant chord and varying chord. The skew in a blade can be considered as combination of sweep and dihedral. The skew change from the circumferential to axial type causes reversal of dihedral induced radial blade force and hence a reversal of meridional streamline curvature which in turn reverses the modulation of the end-wall loads. The inherent forward sweep in the forward skewed blades also modulates hub and tip loads. Thus, in a forward axial skew blade the forward sweep and positive dihedral combine favorably to unload the tip and to upload the hub. In the forward circumferentially skewed blades, forward sweep and the negative dihedral have mutually countering effects on end-wall load modulation. The highlight of this study is that it has demonstrated experimentally the unloading of the tip in the forward axially skewed blades which resulted in reduced losses and better performance including increased stall margin. Reduced tip loading was also verified from the casing pressure sensor measurements which showed weakening of tip vortex. The relative influence of sweep and dihedral on performance was additionally demonstrated in the axially-circumferentially skewed rotor of the constant chord type, with improvements attributable to higher sweep.

Aerodynamics and Aeroacoustics of Leading Edge Serration in Low-Speed Axial Fans With Forward Skewed Blades

Abstract Low-speed axial fans must comply with a wide number of standards and normative restrictions, often related to the maximum noise emission levels. Among the noise control techniques in axial fans, skewed fan blades and leading edge serrations have been found to be effective in leading edge noise control, which represents one of the dominant phenomena in axial fan broadband emissions. However, these solutions are usually applied separately, and literature is scarce on systematic studies on the coupling of the two modifications. In this work, a campaign of experimental measurements was conducted on unskewed and forward-skewed axial fan blades with and without leading-edge serrations. The tests were performed in undisturbed inflow conditions. The flow field and the turbulence characteristics were measured using three-dimensional hot-wire anemometry. The suction-side sound radiation of the fans was measured with microphones in an anechoic chamber. In addition, the rotating beamforming method was used to localize the sound sources at the axial fans. It was found that, regardless of the blade skew, the leading edge serrations lead to a reduction of the sound pressure level, whereby the aerodynamic properties of the fan decrease. At the same operating points, which were achieved by adjusting the rotational speed, the sound radiation through the leading edge serrations could be reduced at high-volume flows. This effect was more pronounced with the unskewed rotor, which indicates that the positive effect of the serrations is reduced by the already optimized shape of the forward skewed fan blade. Based on the experimental results, the four geometries were further considered for numerical investigations to understand how the serrations affect the fan operations and the overall aerodynamics of the rotor. All four geometries were simulated with RANS approach at the duty point to derive a flow survey and better understand the dynamics driven by serrations and blade skewing.

Detection of rotational speeds of sound sources based on array measurements

Rotating machinery such as fans feature a variety of sound generating mechanisms. In addition to stationary sound sources and sources moving synchronously to the measured object, sources rotating at different speeds can occur. This paper presents a method for detecting different rotational speeds of multiple dominant sources present in one data set. It is based on modal decomposition and processing of data measured with a circular microphone array. The method is applied to data sets with simulated point sources as well as measurement data of an axial fan with forward-skewed blades at two different operating points.

Prediction of Performance of a Variable-Pitch Axial Fan with Forward-Skewed Blades

For a single-stage variable-pitch axial fan, the aerodynamic performance and through flow with and without blade skewing are examined numerically. Simulated results show that the total pressure rise and efficiency increase by 2.99% and 0.16%, respectively, with the best forward-skewed angle of θ = 3° at the design conditions. At the blade pitch angles of β = 29° and 35°, the total pressure rises and efficiency of the fan with θ = 3.0° under the highest efficiency point change by −0.55%, −0.53% and 1.39%, 2.11%, respectively. At design and off-design conditions, the forward-skewed blades mitigate tip leakage and delay the emergence of separation flow at the blade root, these benefits are higher at the higher blade pitch angle. The θ = 3.0° forward skew effectively raises the stage performance of the impeller and guide vanes.

Influence of blade skew on axial fan component noise

Microphone arrays can be used to detect sound sources on rotating machinery. For this study, experiments with three different axial fans, featuring backward-skewed, unskewed, and forward-skewed blades, were conducted in a standardized fan test chamber. The measured data are processed using the virtual rotating array method. Subsequent application of beamforming and deconvolution in the frequency domain allows the localization and quantification of separate sources, as appear at different regions on the blades. Evaluating broadband spectra of the leading and trailing edges of the blades, phenomena governing the acoustic characteristics of the fans at different operating points are identified. This enables a detailed discussion of the influence of the blade design on the radiated noise.

The Effects of Radially Distorted Incident Flow on Performance of Axial-Flow Fans With Forward-Skewed Blades

In this study, experiments and numerical simulations were carried out to evaluate the effects of radially distorted incident flow on the performance of axial-flow fans, which were equipped with three kinds of forward-skewed blades. Loss coefficient and velocity components at exit section as well as overall performance are discussed, both at the design point and the lower mass flow rate point. Furthermore, rotating stall was also observed by use of three dynamic pressure sensors. All results reveal that there are beneficial effects of forward-skewed blades on the fan performance, but the fan performance and its unsteady aerodynamic characteristics are quite affected by the radially distorted incident flow.

An experimental study of the unsteady characteristics of tip-leakage flow of axial fans with circumferential skewed blades at off-design conditions

The unsteady characteristics of tip-leakage flow in axial fans with circumferential skewed blades were investigated at off-design conditions using particle image velocimetry (PIV) and wall pressure test. Two circumferential skewed fans, with the blade skew angles at 8.3° forward and backward, respectively, were investigated in this study. The instantaneous flow field in tip clearance region was measured using PIV at peak pressure condition. The static pressure fluctuations near blade tip were captured at off-design conditions using pressure sensors mounted on casing wall. From the measured results, the structure and instantaneous characteristics of tip-leakage flow in circumferential skewed fans were analysed. The tip-leakage vortex instability and its relation with circumferential skewed blades, to expand stall-free operation range of axial fans, were discussed. The forward-skewed blade is found to be effective in controlling tip-leakage flow to expand stall-free operation range at off-design conditions.

Effect of skewed blades on tip-clearance for different flow rates in axial fans

Tip-clearance noise induced by tip-leakage flow in axial flow fans, with the blades skewed circumferentially at angles of ±8.3°, has been investigated under off-design conditions by numerical simulation and experiments. Aerodynamic and aeroacoustic performances have been measured. Tip-leakage flow has been investigated by Navier-Stokes flow simulations at off-design conditions. The characteristics of noise sources in the tip-clearance region, obtained numerically, have been analyzed using vortex sound theory. Synergy between the tip-leakage vortex and the noise source in the tip region is discussed. Moreover, the noise source in the tip region has been measured by dynamic pressure sensors mounted in the casing wall near the blade tip and the acoustic far field has been obtained simultaneously as a function of flow rate. The frequency characteristic of the tip noise component is analyzed by measured results. The characteristics of noise source reveal that tip-clearance noise is associated with skewed blades, which are found to be effective in controlling tip-leakage flow and in eliminating the noise source in the tip-clearance region at off-design conditions. Forward-skewed blade are more effective than backward-skewed blades in reducing noise

Research on the Multi-variable Optimization of Forward-skewed Blade

Research on the Multi-variable Optimization of Forward-skewed Blade

Experimental and numerical investigations of the tip leakage flow of axial fans with circumferential skewed blades under off-design conditions

Experimental and numerical investigations of tip leakage flow of circumferential skewed axial fans were conducted under off-design conditions. Two circumferential skewed fans, with the blade skew angles of 8.3° forward and backward, respectively, and a base fan were investigated in this study. Aerodynamic and aeroacoustic performances were measured. The Navier—Stokes flow simulations were validated experimentally and the key analysis of tip leakage flow was based on computational fluid dynamics results. The simulations show that with a decrease in flowrate, the start of the tip leakage vortex moves towards the leading edge in the chordwise direction and towards the hub in the spanwise direction. These movements are less significant for the forward-skewed blade than for the backward-skewed blade. The strength of the tip leakage vortex decreases along the vortex line. The vortex strength for the forward-skew blade is significantly less than that for the backward-skewed blade. The aeroacoustic source intensity in the tip clearance region is reduced by employing circumferential skewed blades and changes with a change in flowrate in the same manner as the measured sound pressure level. The forward-skewed blade is found to be effective in eliminating noise sources in the tip clearance region and in controlling tip leakage flow to expand the stall-free operation range under off-design conditions.

Internal flow mechanism and experimental research of low pressure axial fan with forward-skewed blades

This article presents the flow mechanism analysis and experimental study of a forward-skewed impeller and a radial impeller in low pressure axial fan. The forward-skewed blade was obtained by the optimization design of the radial blade and CFD technique. Measurement of the two blades was carried out in aerodynamic and aeroacoustic performance. Compared to the radial blade, the forward-skewed blade has demonstrated the improvements in efficiency, total pressure ratio, Stable Operating Range (SOR) and less aerodynamic noise. Detailed flow measurement and computation were performed for outlet flow field for investigating the responsible flow mechanisms. The results show the forward-skewed blade can cause a spanwise redistribution of flow toward the blade mid-span and reduce tip loading. This results in reduced significantly total pressure loss near hub and shroud endwall region, despite the slight increase of total pressure loss at mid-span.

Experimental research on aerodynamic performance and exit flow field of low pressure axial flow fan with circumferential skewed blades

In this article, the low pressure axial flow fan with circumferential skewed rotor blade, including the radial blade, the forward-skewed blade and the backward-skewed blade, was studied with experimental methods. The aerodynamic performance of the rotors was measured. At the design condition at outlet of the rotors, detailed flow measurements were performed with a five-hole probe and a Hot-Wire Anemometer (HWA). The results show that compared to the radial rotor, the forward-skewed rotor demonstrates a wider Stable Operating Range (SOR), is able to reduce the total pressure loss in the hub region and make main loading of blade accumulating in the mid-span region. There is a wider wake in the upper mid-span region of the forward-skewed rotor. Compared to the radial rotor, in the backward-skewed rotor there is higher total pressure loss near the hub and shroud regions and lower loss in the mid-span region. In addition, the velocity deficit in the wake is lower at mid-span of the backward-skewed rotor than the forward-skewed rotor.

Optimization Design and Experimental Study of Low-Pressure Axial Fan with Forward-Skewed Blades

This paper presents an experimental study of the optimization of blade skew in low pressure axial fan. Using back propagation (BP) neural network and genetic algorithm (GA), the optimization was performed for a radial blade. An optimized blade is obtained through blade forward skew. Measurement of the two blades was carried out in aerodynamic and aeroacoustic performance. Compared to the radial blade, the optimized blade demonstrated improvements in efficiency, total pressure ratio, stable operating range, and aerodynamic noise. Detailed flow measurement was performed in outlet flow field for investigating the responsible flow mechanisms. The optimized blade can cause a spanwise redistribution of flow toward the blade midspan and reduce tip loading. This results in reduced significantly total pressure loss near hub and shroud endwall region, despite the slight increase of total pressure loss at midspan. In addition, the measured spectrums show that the broadband noise of the impeller is dominant.

Aerodynamic effects of forward blade skew in axial flow rotors of controlled vortex design

Comparative studies have been carried out on two axial flow fan rotors of controlled vortex design (CVD), at their design flowrate, in order to investigate the effects of circumferential forward skew on blade aerodynamics. The studies were based on computational fluid dynamics (CFD), validated on the basis of global performance and hot wire flow field measurements. The computations indicated that the forward-skewed blade tip modifies the rotor inlet condition along the entire span, due to its protrusion to the relative inlet flow field. This leads to the rearrangement of spanwise blade load distribution, increase of losses along the dominant part of span, and converts the prescribed spanwise blade circulation distribution towards a free vortex flow pattern. Due to the above, reduction in both total pressure rise and efficiency was established. By moderation of the radial outward flow on the suction side, being especially significant for non-free vortex blading, forward sweep was found to be particularly useful for potential reduction of near-tip loss in CVD rotors. Application of reliable CFD-based design systems was recommended for systematic consideration and control of both load- and loss-modifying effects due to non-radial blade stacking.

フォワードスキューをつけた直進翼周辺の可視化

In recent years researchers have reported the ship hull vibration and the noise induced by the propeller are reduced by using the highly skewed propeller.In this investigation, a forward skewed propeller blade is tested expecting better characteristics to reduce the vibration and the noise. A protoype, a backward skewed blade and combined skew ones are tested as well.Measurements of hydrodynamical forces and flow visualizations by oil film method and tuft grid method are carried out on the blades. As a result, we found the forward skewed blade does not stall at the high attack angle and the location of the voltex from the tip of the forward skewed blade is closer to the root of the blade than that of the backward skewed blade.