Air moving device

Abstract: An air moving device which achieves an improvement in the P-Q characteristic and a reduction in noise by suppressing the production of vortices. An annular wall (2) is constructed such that it is spaced from blade tips. This annular wall is formed of a plurality of annular plates (7 1 ,...7 5 ) and spacers with a slit (6) defined between adjacent annular plates (7 1 ,...7 5 ). The width of the slits (6) is set such that as a fan (1) is rotated, air is drawn in a laminar flow through said slits (6) to the inner periphery of the annular wall (2). Thereby, it is possible to suppress the separation of air flow on the suction side of blade surfaces and the production of vortices, thereby to improve the air moving state.

Description: The present invention relates toa method for forming an air moving device for cooling purposes used in electronic apparatuses and instruments and so on. Such a method is already known from document US-A-5407324. This document already shows an annular wall that is spaced from the blade tips of a fan, wherein at the portions of said annular wall opposed to the blade tips slits are formed. With miniturization or downsizing and electronic design changes of apparatuses and instruments, high density packaging of electronic circuits has become popular. With this trend, the heat generating density of electronic apparatuses and instruments increases, so that axial flow type or diagonal flow type air moving devices are used for cooling them. A conventional air moving device, as shown in Fig. 9, has an annular wall 2 spaced from the blade tips of an axial flow fan 1. And in the air moving state with the motor 3 energized, the axial flow fan 1 is rotated around the axis of the shaft 4, producing an air flow 5 moving from the inlet side to the outlet side. Further, US Patent Nos. 2,628,020 and 5,292,088 disclose an arrangement wherein said annular wall is replaced by a plurality of rings stacked to define spacings each between adjacent rings, said spacings serving as an air flow inlet to draw air also from the outer periphery of the fan. However, in the air moving state shown in Fig. 9, the speed of the air flow increases on the suction side of the blade tips, and at the trailing edges of the blades where the velocity is converted into pressure energy, a low energy region is created due to the influence of interblade secondary flow. In this region, the loss is high and separation of flow tends to occur, with the air flow separating from the blade surfaces, and vortices are created in the separated region, increasing turbulent flow noise to aggravate the noise level and static pressure-quantity of flow characteristic (hereinafter referred to as P-Q characteristic). This phenomenon is frequently observed particularly when the outlet flow side is subjected to a flow resistance (i.e. system impedance), in which case the leakage vortices at the blade tips increase, until the fan is forced to stay in a stalling condition. However, the air moving device disclosed in US Patent No. 2,628,020 is so designed that air introduced from the outer periphery flows obliquely rearward so as to allow air flowing in through the air flow inlet to meet the fan delivery air. However, this is not intended to suppress the production of vortices, little contributing to improvement of the P-Q characteristic and reduction in noise. Further, the air moving device disclosed in US Patent No. 5,292,088 is so designed that air introduced from the air flow inlet between rings forms vortices around the outer periphery of the fan for increasing the flow rate, or the vortices present around the outer periphery of the fan are utilized for increasing the flow rate by enhancing the flow of vortices. It is the object of the present invention to provide a method of forming an improved air moving device with an improved P-Q characteristic and quietness. This object is met by the features of claim 1. Contrary to this, according to the present invention that is different in technical concept from said US Patent No. 5,292,088 which utilizes vortices for increasing the flow rate, the production of vortices is suppressed thereby to improve the P-Q characteristic and quietness. According to this arrangement, the annular wall is constructed such that it is separated from the blade tips, said annular wall being formed at its portions opposed to the blade tips with slits which establish communication between the inner and outer peripheral portions of the annular wall, and the width of said slits is set such that as the fan is rotated, air is drawn in a laminar flow through said slits to the inner periphery of the annular wall, suppressing the aforementioned separation of air flow and the aforementioned production of vortices on the suction side of the blade surface, thereby making it possible to improve the air moving state and to improve the P-Q characteristic and reduce noise as compared with the conventional air moving device. Preferred embodiments are shown in the subclaims. Fig. 1 is a front view, a side view and a sectional view of an axial flow type air moving device according to an embodying form 1 of the invention; Fig. 2 is an external perspective view of said embodying form; Fig. 3 is a view for explaining the operating principle of said embodying form; Fig. 4 is a view for explaining the operating principle of said embodying form; Fig. 5 is an external perspective view of an axial flow air moving device according to an embodying form 2 of the invention; Fig. 6 is a front view and a side view of an axial flow air moving device according to an embodying form 3 of the invention; Fig. 7 is a front view and a side view of an axial flow air moving device according to an embodying form 4 of the invention; Fig. 8 is a front view and a side view of an axial flow air moving device according to an embodying form 5 of the invention; Fig 9 is a sectional view of conventional axial flow type air moving device; and Fig. 10 is a graph showing measured characteristics of a conventional axial flow type air moving device and the embodied model according to the embodying form 1. DESCRIPTION OF EMBODIMENTS Embodiments of the invention will now be described with reference to Figs. 1 through 8 and Fig. 10. (Embodying form 1) Figs. 1 through 4 shows an embodying form 1. This air moving device has slits 6 formed in an annular wall surrounding an axial flow fan 1. Stated concretely, annular disks or plates 7 1 , 7 2 , 7 3 , 7 4 ,7 5 are laminated with spacers 8 held between adjacent annular plates, with slits 6 each formed between adjacent annular plates. As shown in (c) of Fig. 1, the width of the laminated annular plates 7 1 - 7 5 is set equal to or substantially equal to the axial width of the axial flow fan 1. Further, the width w of each slit 6 is set as follows. Fig. 3 schematically shows a case where the width W of the slit 6 is greater than necessary. In this case, leakage vortices 10 are produced to move from the pressure side to the suction side at the blade tips as the axial flow fan 1 is driven for rotation in the direction of an arrow 9. Further, as the axial flow fan 1 is driven for rotation, there is produced an inflow of air moving from each slit 6 toward the inner side. In the case where the width W of the slit 6 is greater than necessary, the air flow from each slit 6 is a turbulent flow A, which passes through the clearance between the blade tips and the inner peripheral surface of the annular wall 2 to become a leakage flow 12 which enters the suction side, where the air flow separates from the blade surfaces. The numeral 19 denotes the suction surface separation boundary line, and vortices 13 are produced in the separating region, aggravating the P-Q characteristic and increasing noise. In this case, a disk circulation 18 is also created in which air flow which once flows in through one slit flows out through the next slit, leading to further aggravation of the P-Q characteristic and further increase in noise. In contrast, Fig. 4 shows a case where the width W of the slit 6 is properly set. In the case where the width W of the slit 6 has been properly set such that the air flow which moves in from each slit 6 toward the inside becomes a laminar flow B, the leakage vortices 10 flowing at the blade tips from the pressure side to the suction side are suppressed more than in the case shown in Fig. 3 to the extent that there is no separation of air flow at the suction surface. The numeral 14 denotes a suction surface non-separation streamline, which improves the P-Q characteristic and reduces noise. The value of the width W of the slit 6 which ensures that the air flow moving in through the slit 6 is a laminar flow will now be described making concrete examples. Dimensionless Reynolds' number having to do with the determination of whether an air flow is laminar or turbulent is: R e = (v · w) / ν In the formula, ν is the kinematic viscosity of air (15.6 mm 2 / s at 20°C); v is the peripheral speed of the blade tips; and w is the width of the slit. Therefore, W = (R e · ν) / v Let R e c be the critical Reynolds' number at which a change from laminar to turbulent flow takes place, and with R e c taken to be about 2000 (precisely, 2320: approximate value for a flow in a pipe), the width W of the slit is computed below. Suppose that the diameter d of the axial flow fan of a common axial flow type fan motor having a housing size of 92 x 92 mm is about 86.5 mm and the speed of rotation N is 3000 rpm. The peripheral speed v of the blade tips of this axial flow fan is: v = (π · d · N) / (1000 x 60) = (π · 86.5 · 3000) / (1000 x 60) = 13.58 m/s. Substitution of these values into the above formula gives w = (2000 x 15.6) / (13.58 x 1000) = 2.297 x 10 -3 m = 2.297 mm. Therefore, it is seen that in the case of a common axial flow type fan motor having a housing size of 92 X 92 mm, if the spacers 8 are produced to set the width of the slits to "W ≤ 2.297", then the air flow moving in through the slits 6 toward the inside is a laminar flow. It goes without saying that if the width W of the slits is too small, the slits present a resistance to inflows of air, making it impossible to expect the aforesaid improved P-Q characteristic or reduced noise. It is seen that forming the slits 6 in the annular wall 2 in this manner and properly setting the width W of the slits improves the P-Q characteristic and reduces noise. Fig. 10 is a graph making a comparison between a conventional model which is a common axial flow type air moving device having a housing size of 92 x 92 and the embodied model according to the embodying form 1, as to measured values obtained when the models are subjected to a back pressure during operation in practical use. The broken lines refer to the conventional model and the solid lines to the model of the embodying form 1 for the N (rpm) - Q characteristic, S (noise) - Q characteristic, and P - Q characteristic, where Q stands for quantity of air flow and S for sound pressure level. It is obvious from this comparison that the embodied model has a great advantage. (Embodying form 2) Fig. 5 shows an embodying form 2. In the embodying form 1, the spacers 8 for holding the annular plates 7 1 - 7 5 forming the annular wall 2 spaced from each other have been disposed in the same circumferential position in the upper layer (upstream side of the flow) and the lower layer (downstream side of the flow). This second embodying form 2 differs from the embodying form 1 in that, as shown in Fig. 5, the spacers 8 in the upper and lower layers are shifted in the direction reverse to the inclination of the blade tips. Properly setting the width W of the slits is the same. With this arrangement, the spacers can be made to be out of synchronism with the air passing position of the blade tips, whereby noise can be further reduced. (Embodying form 3) Shown in (a) and (b) of Fig. 6 is an embodying form 3. This embodying form 3 is a modification of the embodying form 1. The annular wall 2 of the embodying form 1 has been such that its outer shape projects further outward from the rectangular casing body 15 in the vicinity of the middle of each of the upper and lower and right and left edges 16. However, in this embodying form 3, the annular plates 7 1 - 7 5 constituting the annular wall 2 have their portions corresponding to the middle regions of the upper and lower and right and left edges 16 shaped flush with the casing body 15. The rest of the arrangement is the same as in the embodying form 1. In addition, in (b) of Fig. 6, the axial flow fan 1 is omitted from the illustration. With the arrangement thus made, although the function of drawing laminar air flow through the slits 6 is a little lower than that of the embodying form 1, the P-Q characteristic is improved any noise is reduced as compared with the conventional axial flow fan. Further, another merit is that the installation space required in practice is the same as in the conventional model. (Embodying form 4) Shown in (a) and (b) of Fig. 7 is an embodying form 4. This embodying form 4 is a modification of the embodying form 2, and as in the embodying form 3, the annular plates 7 1 - 7 5 constituting the annular wall 2 have their portions corresponding to the middle regions of the upper and lower and right and left edges 16 shaped flush with the casing body 15. The rest of the arrangement is the same as in the embodying form 2. In addition, in (b) of Fig. 7, the axial flow fan is omitted from the illustration, and it is well seen that the spacers 8 in the upper and lower layers are inclined from the upper to the lower layer as they are shifted in the direction reverse to the inclination of the blade tips. With the arrangement thus made, although the function of drawing laminar air flow through the slits 6 is a little lower than that of the embodying form 2, the P-Q characteristic is improved and noise is reduced as compared with the conventional axial flow fan. Further, another merit is that the installation space required in practice is the same as in the conventional model. Further, since the air flowing in through the outer peripheries of the slits is allowed to flow in at the tip surfaces of the fan blades in a couterattack manner, an additional improvement in the P-Q characteristic can be expected, though just a little. (Embodying form 5) Shown in (a) and (b) of Fig. 8 is an embodying form 5. This embodying form 5 is a modification of the embodying form 3 shown in Fig. 6, and the only difference from the embodying form 3 is that the spacers 8 are curved in the diametrical direction of the axial flow fan 1. In addition, in (b) of Fig. 7, the axial flow fan is omitted from the illustration. With this arrangement, the air flowing in through the slits is subjected to contraction effect in advance, making it possible to expect a further improvement in the P-Q characteristic. As for the curving of the spacers, they are curved by using a line segment which is straight or curved or of a combined shape with respect to the diametrical direction of the axial flow fan. Further, curving the spacers 8 diametrically of the axial flow fan 1 as in this embodying form 5 may also be employed in the embodying forms 1 through 4. In each of the above embodying form, if an arrangement is employed in which the number of radial spacers is a prime number which is 3 or above and the number of fan blades and the number of spokes 17 are not synchronized with said prime number, then a resonant phenomenon (in this case, air resonance) can be avoided, contributing much to noise reduction. The form of each embodiment above has been described as an axial flow fan, but the invention is likewise applicable to a diagonal flow fan.

Claims: A method for forming an air moving device comprising the following steps: forming an annular wall such that it is spaced from blade tips of a fan, forming said annular wall at its portions opposed to the blade tips with slits which establish communication between inner and outer peripheral portions of the annular wall, characterized by supposing the peripheral speed v of the blade tips, setting the width of said slits to satisfy the formula W ≤ (ν · R e c /v) where ν is the kinematic viscosity of air; v is the peripheral speed of the blade tips, w is the width of the slits, and R e c is the critical Reynolds' number, such that when the fan is rotated, air is drawn in a laminar flow through the slits to the inner periphery of the annular wall. A method according to claim 1, wherein a plurality of annular plates are laminated axially of the fan to define spacings W each between adjacent annular plates, thereby forming a slitted annular wall. A method according to claim 1, wherein spacers which define the slits are inclined with respect to the axis of the fan. A method according to claim 1, wherein the spacers are curved by using a line segment which is straight or curved or of a combined shape with respect to the diametrical direction of the fan. A method according to one of claims 1 through 4, wherein the number of radial spacers is chosen to be a prime number which is 3 or above. A method according to one of claims 1 through 5, wherein an axial flow fan or an diagonal flow fan is formed.

Inventors: OTSUKA SHIGERU, OTSUKA, SHIGERU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1997
Main classification: F04D29
IPC codes: F04D29, F04D19