Patent Application
20230138644
The present disclosure relates to a fan and a fan blade, in particular, for a centrifugal fan.
Fans are used in heat exchangers, for example, with suction occurring via the exchanger package. However, this makes the inflow to the fan very turbulent. The turbulent inflow to the fan leads to a significant increase in noise emissions, which causes disturbing noise. To characterize the inflow, the degree of turbulence Tu and the so-called turbulent length A can be determined by measurement. The degree of turbulence is the ratio of the magnitude of the fluctuation in velocity to the mean value. The turbulent length is the mean dimension of the turbulent structures. It corresponds to the path length over which velocity fluctuations are correlated. The heat exchanger is essentially composed of tubes with fins.
With current fans or fan wheels, fan blades that are favorable in terms of flow mechanics enable high performance, especially with regard to the flow volume achieved or the pressure build-up. Basically, there is a need for low-noise fans with good aerodynamics, despite turbulent flow. However, a strong noise generation during the operation of a fan often remains problematic. The noise occurs when the turbulent inflow hits the fan blade.
In the prior art, there are various design solutions to reduce these problems. To reduce the operation noise, DE 19948075 A uses an axial fan with blades that have a double-sickled, leading blade edge with a protruding outer corner. U.S. Pat. No. 3,416,725 A shows a blade shape with a double-sickled leading edge and a slightly single-sickled trailing edge.
DE 10326637 B3 describes a further solution, namely a fan with alternating direction of rotation, which has S-shaped sickled blades with the leading edge receding sharply outwards. WO 1998005868 A1 discloses a numerical method for the aero-acoustic optimization of an axial fan or its blade geometry, and U.S. Pat. No. 2,649,921 provides a fan with very short and wide blades and triple curved leading and trailing edges. Furthermore, U.S. Pat. No. 5,533,865 A discloses a rotor for a windmill, the blades of which have sawtooth-shaped trailing edges. Jagged or undulated trailing edges are used to reduce trailing edge noise (e.g. GB 2497739 or EP 1801422 A2). DE 102009044824 A1 uses porosities in the form of holes in the region of the trailing edge to reduce the generation of noise at the trailing edge.
With turbulent inflow, however, the sound that occurs at the trailing edge. However, in a turbulent inflow the noise generated at the trailing edge is of secondary importance compared to the generation of noise at the leading edge.
Undulated or jagged leading edges are also known as a means of reducing noise in turbulent flows. U.S. Pat. No. 6,431,498 B1 describes an undulated leading edge that is created by various cuts in the spanwise direction. The front region is lengthened in the direction of the chord up to the maximum thickness. U.S. Pat. No. 9,249,666 B2 describes an alternative design of the wave on the leading edge, where the profile is not lengthened in the direction of the chord, but instead leaves the reference profile towards the pressure or suction side. A special leading edge wave in the form of a double sine is described in EP 3121 376 B1. WO2013/180296 uses jagged leading edges with a triangular shape. DE 102017212231 A1 describes a combination of an undulated leading edge with an undulated trailing edge. The waves on the leading edge have larger wavelengths compared to the trailing edge.
The wave trough is an important place where noise is generated in the case of undulated or jagged leading edges. Other publications deal with modifications of the vane in the region of the trough. JP6409666B2 uses additional leading elements on the blade in the trough region. In JP5978886B2 a recess of the jagged leading edge in the trough is described.
Against this technical background, the disclosure deals with the problem of providing a fan or fan blade that operates with little noise, particularly in the case of turbulent inflow, and which at the same time has good aerodynamic properties.
The disclosure solves this problem with a fan according to the independent claims. The dependent claims contain advantageous developments.
Before the disclosure is described in more detail, some terms and the terminology used are explained for a better understanding of the disclosure. A typical centrifugal fan usually has several fan blades arranged around the circumference for aerodynamic suction and/or discharge of the air surrounding the fan or a gas to be conveyed by the fan. The fan blades can be connected to one another by a bottom disk or a cover disk or both.
Each fan blade has a radially inner leading edge and a radially outer trailing edge. Depending on the direction of rotation and the fan blade profile, there is a suction side and a pressure side. The pressure side leads the suction side in the intended direction of rotation when the fan is operating. In this respect, the fan blade has a suction side, that sucks in the inflowing air during operation, and a pressure side opposite the suction side, where the pressure for ejecting the air builds up.
The fan according to the disclosure is distinguished from a comparable conventional fan by a noise-reduced operation with turbulent inflow. As already mentioned above, a fan according to the disclosure uses at least one fan blade according to the disclosure, that has a reduced noise generation during operation compared to comparable conventional fans due to its shape.
The mechanism of increased noise generation is based on the fact that the turbulent flow is linked to a change in the flow of the fan over time. The turbulence leads to fluctuations in the forces occurring on the blade over time, as a result, a corresponding vibration-like sound emission is triggered. Of particular importance is the intensity of such fluctuations. The higher the degree of turbulence in the inflow and thus the fluctuation of the relevant flow variables in the inflow of the fan, the more sound is emitted or, to put it another way, the louder one perceives the operation of such a fan.
Investigations of various modifications of the front leading edges of blade profiles in turbulent inflow show positive acoustic effects if the leading edge is designed with an undulated or undulated jagged shape. According to the disclosure, it could be assessed that the essential mechanism leading to the reduction of the emitted sound is that the sources on the blade are decorrelated. A turbulent inflow appears chaotic, but is not completely chaotic, but correlates to geometric factors. The previously mentioned length A is the path length over which a concrete correlation of the turbulent fluctuations can be determined.
Studies show, however, that acoustically effective undulated leading edges have poorer aerodynamic properties. Thus, one skilled in the art is initially discouraged from forming such shapes. The present disclosure aims at a specific design of the leading edge wave that is acoustically and aerodynamically advantageous. It has been shown, according to the disclosure that the formation of a specific waveform is particularly advantageous.
A basic idea of the disclosure is that the leading edge has, at least in portions, a three-dimensional wave-shaped form or is three-dimensionally undulated. The claimed design of the wave differs significantly from the prior art. It is also advantageous if the undulated leading edge is also designed with a porosity. According to the disclosure, a fan blade has a leading edge and a trailing edge. The fan blade has an undulated leading edge at least in a region, of the edge. The edge has a periodically repeating waveform of period length A. The period length A deviates from a sinusoidal or almost sinusoidal waveform, in particular, deviating from a sinusoidal or almost sinusoidal waveform with the same period length A.
In the case of a regularly recurring physical occurrence, the period (period length) is the smallest local distance after which the phenomenon is repeated.
It is also advantageous if the undulated leading edge has two or more periodically repeating waveforms. The effect according to the disclosure occurs when the desired waveform is formed over a number of periods.
Thus, alternating troughs and crests can be formed at the leading edge or inflow edge, that are provided with a certain periodicity.
The optimal range of wavelength and amplitude was determined from experimental tests, that bring both aerodynamic and acoustic improvements. The so-called peak-to-trough value H of the wave is the distance from the highest point to the lowest point. In order to focus on the reduction of the sound power, waves with large height(large peak-to-trough value H) and smaller wavelengths (small λ/H) are used. Small peak-to-trough values H and larger wavelengths (larger λ/H) are advantageous for reducing the received power. In relation to the impeller diameter D, preferred peak/trough values H in the range of 0.01≤H/D≤0.1 are advantageous.
In a correspondingly advantageous embodiment of the disclosure, the peak-trough value H of the wave troughs is defined from the leading edge, in this region of the undulated leading edge, up to the respective wave trough (viewed in flow direction). Values for the ratio between the period length λ and the peak-to-trough value H lie in the range of 0.2≤λ/H≤2. The values can vary along the leading edge.
A solution has proven to be particularly effective where a waveform deviating from a sinusoidal shape, includes deep-cut wave troughs, for each period, namely sufficiently large pronounced troughs. For this purpose, the amplitude or the peak-trough value should have a specific value compared to the chord length of the fan blade. Slightly pronounced or only sinusoidal wave troughs have not proven to be sufficiently effective. Rather, the peak-trough value of the wave troughs in the region of the undulated leading edge should preferably be approximately 10%-30% of the chord length SL, more preferably 10% to 20% of the chord length SL. Compared to an imaginary sine wave with the same number of periods, the peak-trough value should therefore be larger, which leads to steeper flanks compared to the direction of flow in the wave trough.
It is therefore preferred if the repetitive waveforms forms at least one wave trough per period with two “steep” wave flanks running towards one another and each at an angle to the direction of flow. It is particularly advantageous here if the (lateral) wave flanks running obliquely to the wave trough in this region of the leading edge (particularly in a portion near the middle of the flank) form a tangential angle β of between 15° and 35° relative to the direction of flow, preferably a tangential angle β of 25° to 30°.
This also requires a significantly stronger curvature in the trough of a wave form according to the disclosure compared to the course of the curvature in a sinusoidal wave trough.
In another alternative embodiment, the repeating waveform (the waveform that is periodically arranged) forms two adjacent wave troughs with an intermediate crest extending upstream toward the inflow leading edge. The two lateral flanks that delimit this waveform are correspondingly slanted, as previously explained.
According to the disclosure, the design of the wave can be as follows. The waveform in the region of the undulated leading edge runs at least in portions or completely through several, in particular six, common points of intersection (support points) with an imaginary sine wave, while its form deviates from a sine wave.
In the alternative exemplary embodiment, where there is a wave crest between two wave troughs. The peak-trough value h2 of such a wave crest is approximately 10% to 80% of the peak-trough value H of the immediately adjacent wave crest or crests.
In addition, it is favorable to adapt the undulated leading edge locally to the flow approximately in the middle of a period, namely at half wavelength. This introduces an offset of the leading edge perpendicular to the center line between the pressure side and the suction side of the blade. This offset improves the flow towards the leading edge and helps to avoid flow separation in this region. This offset is preferably implemented in the direction of the suction side. In this respect, it is advantageous if the blade profile also has a specific, in particular undulated, structure in certain portions in addition to the undulated leading edge. The blade profile (viewed in a profile section in the region of a wave crest) can form a bulge protruding on the pressure side (DS) and a dent on the corresponding opposite position of the suction side (SS). The surface profile is defined in such a way that, as viewed in the flow direction (V), the surface curvature changes twice. If the surface profile on the upper side (pressure side) corresponds approximately to the opposite surface profile on the underside (suction side), the blade profile has an approximately constant thickness, but bulges slightly on the pressure side.
A further improvement can lie in the fact that the blade profile curves further towards the suction side at the front in the region of the leading edge (viewed in a profile section) compared to the one or a neighboring region that is less curved towards the suction side. As a result, a specific additional undulated structure of the blade is achieved, preferably with a spacing of one period, more preferably from one period center to the next.
An additional improvement in the noise behavior can be achieved by equipping the fan blade, in the region of the leading edge, with a large number of channels running through the fan blade from the pressure side to the suction side (region with porosity). A further reduction in noise can be achieved by using porosities in the region of the blade edges interacting with the turbulence. These can be formed by holes or slots. These are through-openings that enable pressure equalization between the suction and the pressure side of the fan blade. In a preferred embodiment, the hole diameter or the width of the slots assume values in a range of up to approximately 2 mm. The porous region preferably comprises only a partial region of the inflow edge, the partial region being less than about 20% of the blade length.
A combination of a plurality of geometric design elements according to the disclosure is particularly advantageous, with the particular characteristics of the disclosure taken into account in each case. For example, a combination of porosity and a three-dimensional undulation in the region of the leading edge is possible.
The present disclosure relates in particular to a centrifugal fan with one or more fan blades as described above.
Other advantageous developments of the disclosure are described in the dependent claims or are presented in more detail below together with the description of the preferred embodiment of the disclosure with reference to the figures.
The disclosure is explained in more detail in the following using an exemplary embodiment with reference to
The position of the profile section A is chosen such that the chord length of the fan blade 1 with the undulated leading edge 4 corresponds approximately to the chord length of a reference blade with a non-undulated leading edge 2*. The position of the profile section B is chosen so that it runs through a wave crest of the fan blade 1 with the undulated leading edge 4. The position of the profile section C is chosen so that it runs through a trough of the undulated region (4) of the leading edge of the fan blade 1.
A measure is described below as to how the waveform at the leading edge can be derived or optimized based on a sine wave. The waveform is defined by several points S of the sine curve and the course of the curve results from a spline interpolation.
In comparison to the sine wave shape, the two flanks K1, K2 falling towards the trough 7 are closer together and their angle of attack is steeper in comparison to the sine wave in relation to the direction of the inflow velocity v. The effective inflow velocity, i.e. the component of the inflow velocity v perpendicular to the leading edge, with which the disturbance hits the leading edge of the fan blade, is reduced if the flanks K1, K2 are steeper. This leads to a more effective reduction of the emitted sound. Preferred values for h1 are in the range 0<h1<h with h=amplitude of the sine wave.
The implementation of the disclosure is not limited to the preferred exemplary embodiments specified above. Rather, a number of variants are conceivable that make use of the solution shown even in the case of fundamentally different designs.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2020 106 534.5 | Mar 2020 | DE | national |
This application is a 371 U.S. National Phase of International Application No. PCT/EP2021/055474, filed Mar. 4, 2021, which claims priority to DE Application No. 102020106534.5, filed Mar. 10, 2020. The entire disclosures of the above applications are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/055474 | 3/4/2021 | WO |
