Noise attenuating sound resonator for automotive cooling module shroud

[0002] This invention relates to cooling systems of an internal combustion engine and more particularly to sound absorption resonators incorporated into a fan shroud of the cooing system.

[0003] Noise, particularly blade passing tone, produced by an axial flow fan of an engine cooling system has been a concern in the automotive industry. The term “axial flow fan” used herein refers to any fan of the general type in which the flow of air or other gas is in a direction parallel to the axis about which the fan blades rotate.

[0004] A technique for reducing noise in axial flow fans includes employing noise absorption material in regions near the fan or otherwise adjacent to the fluid flow. This technique is helpful but the effectiveness is limited in certain frequency ranges such as absorbing the passing blade tonal frequency.

[0005] Accordingly, there is a need to provide a cooling structure including at least one resonator thereon so as to reduce or eliminate blade passing tonal noise.

[0006] An object of the present invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is obtained by providing a cooling structure for cooling an engine. The cooling structure includes an axial flow fan having a plurality of blades. A shroud is spaced from and is generally adjacent to the blades. A plurality of Helmholtz resonators is carried by the shroud. Each of the resonators has an opening disposed substantially perpendicular with respect to a direction of air flow resulting from rotation of the fan. The resonators are disposed at locations on the shroud which are generally adjacent to tips of the blades. The resonators are tuned to reduce the blade passing tone of the fan.

[0007] In accordance with another aspect of the invention, a method of absorbing the blade passing tone noise produced from an axial flow fan for cooling an engine includes providing a shroud spaced from and generally adjacent to blades of the fan. A plurality of Helmholtz resonators are provided and carried by the shroud. Each of the resonators has an opening disposed substantially perpendicular with respect to a direction of air flow resulting from rotation of the fan. The resonators are disposed at locations on the shroud which are generally adjacent to tips of the blades. The resonators are tuned to reduce the blade passing tone of the fan.

[0008] Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.

[0009] FIG. 1 is front view of a cooling structure provided in accordance with the principles of the present invention;

[0010] FIG. 2 is a view of the cooling structure taken along the line A—A of FIG. 1;

[0011] FIG. 3 is an enlarged view of the portion B encircled in FIG. 1;

[0012] FIG. 4 is a perspective view of a shroud of the cooling structure of FIG. 1;

[0013] FIG. 5 is a front view of a second embodiment of the cooling structure of the invention;

[0014] FIG. 6 is a schematic view of a resonator in a flow path showing various pressures and areas; and

[0015] FIG. 7 is a schematic view of a Helmholtz resonator showing certain lengths for calculating effective length.

[0016] Referring to FIG. 1, a cooling structure for cooling a vehicle engine is shown, generally indicated 10, provided in accordance with the principles of the present invention. The cooling structure 10 includes a shroud 12, formed preferably of lightweight material such as plastic. In the conventional manner, the shroud structure 12 is spaced from and generally adjacent to blades 14 of a fan 16 of the cooling structure 10 to guide the air flow and to prevent foreign objects from contacting the rotating blades. The shroud 12 is constructed and arranged to be coupled to a radiator (not shown) of an engine cooling system.

[0017] With reference to FIG. 2, the cooling structure 10 includes a dc electric motor 20 coupled to the fan 16 via shaft 22 so as to cause rotation of the fan 16.

[0018] In accordance with the principles of the present invention, a plurality of Helmholtz resonators 24 are carried by the shroud 12. As shown in FIGS. 1 and 4 the shroud 12 is generally square in shape having four sides 25, 27, 28 and 29. In the embodiment, two resonators 24 are disposed on each side 25, 27, 28 and 29 of the shroud 12 such that the opening 30 (FIG. 3) is directed towards tips 32 of the blades 14. With reference to FIG. 3, each resonator 24 is generally of cylindrical, having an orifice opening 30 which communicates with an internal cavity 34. Each resonator 24 extends outwardly from a respective side of the shroud 12 with opening 30 defined in internal wall 36 of the shroud 12.

[0019] In addition, certain of the resonators 24′ are disposed in an interior chamber 35 defined by the internal wall 36 of the shroud 12. Openings 30 of the resonators 24 and 24′ are in open communication with the internal chamber 35. As best shown in FIG. 1, resonators 24′ are disposed on a first circle 38 which is concentric with a circle 40 defined by the blade tips 32. As shown, the first circle 38 has a radius greater than a radius of the second circle 40.

[0020] Each of the resonators 24 and 24′ has its opening 30 disposed substantially perpendicular with respect to a direction of air flow which results from rotation of the fan 16. With reference to FIG. 2, air flow is directed along axis C. The resonators 24 and 24′ are disposed at locations on the shroud 12 beyond the extent of the blade tips 32 and generally adjacent thereto.

[0021] As noted above, the resonators 24 and 24′ of the invention are Helmholtz resonators which are acoustic oscillators having a resonant frequency tuned to the fan blade passing tone. The geometry of each resonator is determined by the size of the cavity 34 of the resonator and the size of the orifice or opening 30 through which air may enter and escape from the cavity 34. In the illustrated embodiment and with reference to FIG. 3, each resonator 24 and 24′ is of generally cylindrical shape. The specific resonator dimensions are chosen for the particular cooling structure so as to be tuned to the fan blade passing tone.

[0022] The Helmholtz resonator may be compared to a typical mechanical spring mass system. The equivalent of the spring is the compressibility of the gas in the cavity 34 and the equivalent of the mass of the spring-mass system is the effective mass of the air in the orifice 30. If the resonator is tuned to the fan blade passing rate, then any pressure disturbances at the blade passing rate will cause the resonator to oscillate, thereby acting as a large air source and sink at that frequency to effectively absorb the pressure disturbances rather than permitting the pressure disturbance from passing outwardly of the shroud 12.

[0023] For a spring mass system where M is mass and K is the spring constant, the natural frequency f of the system is: $f_{\mathrm{nat}}=\frac{1}{2\pi }\sqrt{\frac{k}{m}}$

[0024] With reference to FIG. 7 for a Helmholtz resonator wherein A₀is the orifice cross-sectional area, A_cis the cavity cross-sectional area, L is the length from the opening to the cavity and V_cis the volume of the cavity, then

[0025] M=ρAL_eff

[0026] L_eff=L+0.8{square root over (A

[0027] where L_effis effective length,

[0028] As shown in FIG. 7, L_eff=ΔL₀+ΔL_c+L₀ $k=\frac{{\mathrm{BA}}_o^2}{V_c}$

[0029] where B is the Bulk modulus, $f_{\mathrm{nat}}=\frac{C_o}{2\pi }\sqrt{\frac{A_o}{L_{\mathrm{eff}}{V}_c}}$ $Z_{\mathrm{helm}}=\frac{P_{\mathrm{helm}}}{\mathrm{Vel}}=|\frac{\gamma M\omega -\gamma k/\omega }{A_o}$

[0030] where Z_helmif acoustic impedance of a Helmholtz resonator.

[0031] Modeling for sound transmission loss from a Helmholtz resonator R will be explained with reference to FIG. 6. The following equations are applicable:

[0032] P_I=ρcV_I

[0033] P_R=−ρcV_R

[0034] P_T=ρcV_T

[0035] At the resonator:

[0036] P_I+P_R=P_T=P_B

[0037] A₁(V₁+V_R)=A₁V_R+A₂V_B

[0038] Accordingly, $P_T=\frac{P_I}{1+\frac{\rho {\mathrm{cA}}_2}{2Z_{\mathrm{helm}}{A}_1}}$

[0039] Note that when Z_helm=0 $\tau \left(\mathrm{transmission}\mathrm{loss}\right)=10{{\log }_{10}\left[\frac{P_I}{P_T}\right]}^2$ $\tau =20\log \left(1+\frac{{\mathrm{pcA}}_2}{2A_{2}R}\right)$

[0040] Where R_a=acoustic resistance: $R_a=\frac{{\mathrm{pcA}}_o^{5/2}}{2\pi V_c}=Z_{\mathrm{helm}}$

[0041] for absorption at f=f_n.

[0042] The dimension of the resonator is selected to meet f/f_n=1.

[0043] Returning to the description of the invention, the shroud 12 is preferably molded from plastic. In the embodiment of FIGS. 1-5, the resonators 24 and 24′ are unitary members formed preferably from plastic and mounted by clips, adhesive or any conventional manner to the shroud 12. Alternatively, the resonators may be molded integrally with the shroud. FIG. 6 shows a second embodiment of the invention wherein the shroud includes a plurality of segments with each segment disposed beyond an extent of tips 32 of the blades 14. Each segment carries at least one resonator 24 such that the opening 30 of the resonator 24 generally faces the tips 32 of the blades 14. The segments include linear segments 40 disposed between generally radial segments 42 and all segments are joined to define a continuous resonator carrying structure, generally indicated at 44. Each segment 40 and 42 is molded from plastic with the resonator(s) being coupled thereto or molded integrally therewith. The segments 40 and 42 are secured to the shroud 12 and add rigidity to the shroud. In the embodiment, each linear segment 40 carries two resonators 24 and each radial segment 42 carries one resonator 24.

[0044] In the embodiment, all resonators are configured generally identically so as to absorb the blade passing tone within a certain range of frequencies. If noise is generated at other frequency ranges, one or more additional set(s) of resonators, configured to absorb these frequencies can be provided on the shroud.

[0045] Although a certain number of resonators were shown at certain locations on the shroud, the locations and number of resonators on the shroud may vary due to the particular shroud and fan configuration.

[0046] The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.