Patent Application
20050147496
Fan noise has been identified as a primary component of overall noise generated by various types of machinery, including heat exchanger equipment. For example, low speed, low pressure axial flow fans are typically used in heat exchanger applications, such as for moving ambient air over commercial and residential air conditioning condenser heat exchangers. In residential air conditioning systems, low speed, low pressure axial flow fans typically meet the requirements for effective operation in terms of performance capability, durability, and cost.
Although relatively low speed, low pressure axial flow fans have achieved noticeable reduction in noise generation through the design of the fan blading and reductions in turbulence from motor supports and fan shrouding, many of such fans continue to generate noise at frequencies which are perceived by the human ear as somewhat annoying. Moreover, the application of axial flow, low speed, low pressure fans in residential air conditioning systems, where relatively high density dwellings result in a condenser unit for one residence being within a few feet of an adjacent residence, has mandated further reductions in noise generated by air conditioning condenser cooling fans, in particular.
Fan self induced tonal noise in a frequency range of about 2300-3500 Hz has been identified during operation of low speed, low pressure, axial flow fans. Reduction of noise in this frequency range as well as over a relatively broad range of frequencies normally audible to humans is always sought. One source of noise in axial flow fans, in particular, is due to a phenomenon known as laminar boundary layer shedding. This phenomenon is similar in some respects to the generation of the well-known von Karman vortex streets which occur when fluid flows around a body disposed in the fluid flow path. In accordance with the present invention, tonal noise generated by laminar boundary layer shedding has been measurably decreased thereby providing advantages in fans used in various air-moving applications and, particularly, in applications associated with heat exchange equipment in air conditioning systems and the like.
The present invention provides an air-moving fan having reduced acoustic emissions or “noise” perceptible to the human ear.
The present invention also provides an improved heat exchanger unit including an axial flow low speed, low pressure fan having reduced noise generation and being generally of the type used in applications, such as commercial or residential air conditioning unit condenser units.
In accordance with one aspect of the present invention, generally axial flow type fan propellers are provided with roughness on the fan blade surfaces on the so-called pressure side of the blades adjacent the trailing edges of the blades, which roughness disrupts the boundary layer shedding phenomena and also reduces tonal noise generated by the fan blade in a frequency range perceptible to human hearing. The roughness is placed on the pressure side or surface of the blade, which is the surface substantially facing the general direction of air movement discharged from the fan, adjacent the blade trailing edge and preferably extends over a major portion of the trailing edge between the radially outermost part of the blade and the fan hub. The roughness may take various forms, such as that created by relatively sharp edged curbs or trip surfaces or other portions of the blade forming a surface interruption or discontinuity, or a strip of abrasive paper or cloth, such as so-called sandpaper, suitably secured to the blade surfaces. The height of the roughness is preferably at least that of the thickness of the boundary layer of the air moving over the blade surface.
Still further, the blade surface roughness may be generated by plural ridges extending generally parallel to the contour of the blade trailing edge or by a so-called cross-hatched or gridlike arrangement of ridges similar to the geometry of knurled surfaces. It is contemplated that the blade surface roughness may also be provided by upturning or offsetting the trailing edge of the blade to also provide a curb or trip surface extending somewhat normal to a major portion of the blade surface.
Although the reduction in noise generation is deemed to be particularly noticeable for fan propellers with forward-swept blades, it is contemplated that the invention may be applied to propellers with substantially straight, radially projecting blades as well as backward-swept blades. The present invention also contemplates that fans having blades of other configurations may benefit from the provision of “roughened” trailing edge portions which are operable to disrupt laminar boundary layer shedding.
Those skilled in the art will further appreciate the above-mentioned advantages and superior features of the invention together with other important aspects thereof upon reading the detailed description which follows in conjunction with the drawings.
In the description which follows, like parts are marked throughout the specification and drawing with the same reference numerals, respectively. The drawing figures are not necessarily to scale and certain features may be shown in somewhat generalized or schematic form in the interest of clarity and conciseness.
Referring to
Mounted partially within the opening 20 is an axial flow fan of the multiblade propeller type, generally designated by the numeral 24 and which is mounted for rotation on and with a shaft 26,
The fan propeller 24 is shown by way of example as a three-bladed member having respective forward-swept circumferentially spaced blades 25 which are suitably mounted on a hub 27. Hub 27 has a suitable core part 29 which is mounted directly on shaft 26. The configuration of the fan propeller 24 as shown in
Referring now to
The characteristics of the roughness or roughened surfaces 25f on the so-called pressure sides 25a of blades 25 may be varied. As shown in
The so-called roughened surfaces of each blade 25 may also be formed as an area of cross-hatched serrations similar in some respects to what is known as a knurled surface, and as indicated by the roughened surface 25h shown in
Still further, the roughened surface or boundary layer trip may be formed by merely curling or bending the trailing edge 25e upward away from and generally normal to the surface 25a, as indicated at 25j in
Referring now to
Referring now to
Referring still further to
Each of the roughened surface portions formed at or by elements 25f, 25g, 25h, 25j, 25m, 25n and 25p is formed such as to interrupt a generally laminar boundary layer of air flowing over the surface 25a of each of the blades 25 so as to prevent so-called laminar vortex shedding from the trailing edges of the blades.
A twenty-four inch diameter air conditioning system condenser cooling fan operating at 847 rpm to 859 rpm and having a geometry of the fan propeller 24 was tested with and without the roughened surface 25f. The blades 25 were of aluminum and of about 0.040 inch to 0.050 inch thickness. By applying a 0.375 inch width strip of 120 grit sandpaper of about 4.0 inches length to the blade surface 25a of each blade 25 directly adjacent the blade trailing edge 25e, a reduction in sound pressure level was observed within the human audible acoustic frequency range from about 200 Hz to 10,000 Hz. In particular, a bulge in the acoustic vibration one-third octave spectrum of the fan between 2400 Hz and 3150 Hz and a characteristic hissing sound generated thereby, was eliminated by a roughened blade surface treatment as described above. Accordingly, it is indicated that using surface roughness to force transition of fan blade surface air flows from laminar-to-turbulent flow may be achieved without significant modification to blade geometry and without any significant effect on fan propeller performance. It is noted that the highest frequency and sound power contribution of laminar flow shedding occurs at the highest speed portion of the fan blade.
A condenser cooling fan having generally the same geometry as the fan described above for Example 1 was tested over the same operating speed range. Each blade was provided with two rows of trips 25p and extending along the trailing edges 25e of the blades 25, respectively, and as shown in
Referring again briefly to
One preferred way to characterize the height of roughness or boundary layer trip elements on the surface of a fan blade which are intended to generate a level of turbulence in the fluid boundary layer sufficient to destroy the coherence and flow pattern of naturally laminar flow is as follows.
Define the “roughness” or height of the disruption or discontinuity of the blade surface as E and normalize the value by some physical reference dimension on the blade surface. The blade chord distance may be used to normalize E where C is the distance from the blade leading edge to its trailing edge in the peripheral or rotating direction along the blade. Normalized roughness is, then: ε/C
Also needed is a characteristic measure of the boundary layer flow to be disrupted with the presence of roughness elements on the blade surface. This dimension is properly the thickness of the boundary layer, readily associated with the classical displacement thickness or the momentum thickness of the laminar layer. The choice is not very critical since they are all related.
Displacement thickness may be defined as δ*, and normalized as before as: δ*/c
On the blade surface the thickness of the laminar boundary layer is a function of the Reynolds number for the blade and the chord-wise position on the blade, defined by X or normalized as X/C that is being considered. It is also a function of the chord-wise pressure gradient along the blade, which may be defined as dp/dX.
Considering blades for which the boundary layer on the suction surface is laminar, in order to restrict attention to blades for which laminar vortex shedding can occur at the blade trailing edge, the analysis is restricted to flow conditions when dp/dX is small enough to allow the continuation of natural laminar flow to the blade trailing edge. To that end, it may be assumed that dp/DX≈0. This assumption allows use, with acceptable accuracy, of the flat plate boundary layer formula, where
δ*/X=1.721/Rex1/2
where the Reynolds number is
Rex=ρVX/μ=VX/ν
where ν=μ/ρ
The traditional 99% boundary layer thickness is given by δ/X=5.0/Rex1/2 or δ/δ*≈3. Here, ρ and μ are the fluid properties of density and viscosity and V is the air velocity onto the blade, approximately equal to the rotating speed, U=(r/R)ND/2. r/R is the normalized radial station being examined, clearly lying between 0 and 1.0 R=D/2.
These formulas may be used for sizing the roughness height to be placed on the blade, by requiring that the height E be of the order of the thickness δ*, or ε/δ*≈1.
The frequency of vortex shedding from a blade that has not been sufficiently roughened is characterized by a Strouhal number of approximately St≈0.21. The value of St is only weakly dependent on the value of Rex, so that:
St=ωd/2πU≈0.21=fd/U
Here, f=ω/2π, U≈ND/2 and d is the diameter of a cylinder immersed in a laminar flow field; the classic Strouhal experiment, later theoretically explained by T. von Karman. It can be estimated that d is the order of the displacement thickness plus blade thickness, t. Thus one can calculate: f≈0.21(U/d)=0.21(U/(δ*+t))
Typical values for fan blades of the type described herein are: blade thickness, t=0.040 inches, X=19 inches=1.6 ft, U=88 ft/s, ν=μ/ρ −1.6×10−4 ft2/s which gives an Rex≈106. Then δ*/X≈0.017 and δ*≈0.0027 ft=0.035″. So with d=δ*+t, then f=2956 Hz. This is reasonable agreement with experimental results.
The criterion for turbulent flow at relatively low Reynolds number is that the pressure gradient on the suction surface of the blade be “sufficiently adverse.” Hence, it is required that the “diffusion” on the suction surface be small enough to allow laminar flow to exist on the blades.
The turbomachinery value of diffusion can be described as Dp=1−V2/Vp or one minus the inverse of the ratio of the peak surface velocity to the value of velocity as the flow exits the blade row. These velocities can be described as functions of rotating speed, flow rate and pressure rise for the fan.
The value of Vp is defined as
Vp=[(xVT)2+Va2]1/2+Vg
Where x=r/R, VT is the fan tip speed and Vg=Vθ/2 is the “circulation velocity” related to pressure rise. Rewriting,
Vp=VT[x2+Φ2)½+ψT/(4σxηT)]
Similarly V2≈VT−Vθ and can be written as
V2=VT[1−ψT/(2σxηT)]
In these forms, the flow coefficient, Φ is
Φ=V2/VT=Q/AVT
and the pressure coefficient, ψT is
ψT=ΔpT/(ρVT2/2)
Q is the volume flow rate in ft3/s and ΔpT is the total pressure rise in lbf/ft2 (including the axial flow velocity pressure).
The Diffusion Factor, or the velocity ratio is thus written as
Dp=1−V2/Vp=1−[1−ψT/2σxηT]/[x2+Φ2)1/2+ψT/(4σxηT)
The value of Dp is a traditional measure of blade loading and a design criterion for sizing the blade row solidity, σ=NBC/(2n r). NB is the number of blades, C is the blade chord and r is the blade radial station. ηT is the fan efficiency based on total pressure rise.
The diffusion factor provides an upper limit on pressure rise at a given speed size and flow rate, since a blade row is prone to stall at values of Dp≈0.55. In practice, blade design and stall margin concerns require Dp to be less than about 0.45. However, diffusion should be kept below the transition level for laminar flow. A suitable value is: 0.1≦Dp≦0.2.
The amount of surface area which should be “roughened” to trip the laminar boundary layers is not obvious. Tests suggest that the roughness treatment should start at the blade tip at or near the trailing edges of the blades, since the highest peripheral speeds are at the blade tip. The influence of speed on the sound power level can be written as: Lp=55 log10VT+Constant. The value at x=r/R<1.0 becomes ΔLp=55log10x. The blade needs to be treated up to the point where a noise signature is negligibly small, perhaps a reduction of 10 dB. This implies a minimum value of x given by x=10−(10/55)=0.66. Tests on a 12.0 inch radius fan confirmed the relationship of tonal sound power and tonal frequency to several x locations of boundary layer trips. If a 5 dB reduction in emissions is the criterion, then the roughness should extend to about x=0.8 or about 3.0 inches in toward the hub, for example, on a 12.0 inch radius fan.
The extent of roughness needed in the chord-wise direction is not as clearly defined. The hypothesis that laminar flow exists all the way to the trailing edge in the absence of added roughness suggests that the coherent vortex shedding can be prevented with the roughness added to the blade surface exactly at or directly adjacent to the trailing edge and extending over at least about three percent of the blade chordwise length.
Referring briefly to
Referring to
Fabrication of the fan propellers 24, 44 and 54 may be carried out using conventional manufacturing processes known to those skilled in the art of air-moving fans and as reinforced by the description hereinbefore. Conventional engineering materials may be used for fabricating the propeller fans 24, 44 and 54.
Although preferred embodiments of the invention have been described in detail herein, those skilled in the art will recognize that various substitutions and modifications may be made without departing from the scope and spirit of the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09994294 | Nov 2001 | US |
| Child | 11055469 | Feb 2005 | US |
