Shrouded fan impeller with reduced cover overlap

The described embodiments relate to improving efficiency of a low-profile cooling fan. In one embodiment, an impeller of the cooling fan includes a shroud which covers a central portion of the impeller, thereby allowing a central inlet portion of the blades to have an increased fan blade height when compared to a cooling fan constrained by minimum part tolerances between the fan blades and a portion of the fan housing. In some embodiments, the impeller includes splitter blades that can improve performance of the low-profile cooling fan.

FIELD

The described embodiments relate generally to fan designs that allow for an overall reduction in height of a fan assembly. More particularly, the present embodiments relate to maintaining an effective blade height of the fan assembly by utilizing a shroud to cover part of a bottom portion of the fan assembly.

BACKGROUND

As computer systems are reduced in thickness, the thickness of the modules and components inside must also be correspondingly reduced. Although these modules and components must get thinner, reduced performance is generally not an acceptable consequence and, hence, new methods are sought to improve performance of these modules. One particular component module that continues to need a relatively substantial amount of vertical height is a fan assembly. Unfortunately, a reduction in height of the fan assembly generally corresponds to a reduced effective blade height of the fan assembly, thereby reducing an effective flow rate of the fan assembly.

Therefore, what is desired is a configuration that allows for a reduction in fan assembly height without reducing the effective flow rate of the reduced height fan assembly.

SUMMARY

This paper describes various embodiments that relate to designs for efficient low profile fan assemblies.

According to one embodiment, an impeller enclosed within a cover is described. The impeller includes a central hub and a number of blades extending radially from the central hub. The impeller also includes a ring shaped shroud attached to the blades separated from the cover by a radial gap that allows the ring shaped shroud to rotate with the plurality of blades without contacting the cover. The shroud extends towards the tip of each of the blades, allowing an increase in the effective height of the blades.

According to another embodiment, a fan assembly is disclosed. The fan assembly includes at least the following: a housing; a cover that cooperates with the housing to define a fan assembly interior portion, the cover defining a fan inlet zone external to the fan assembly suitable for receiving an air flow in accordance with a pressure difference; and an impeller arranged to rotate in a manner that creates the pressure difference to drive the air flow and disposed within the interior portion of the fan assembly, the impeller including a number of fan blades that are integrally formed with a shroud that extends toward leading edges of the fan blades to allow an increase in an effective height of the fan blades. The shroud and cover are separated by a radial gap. This gap is designed to be as small as possible to maximize the impedance to air flow through the radial gap from the relatively high pressure zone proximate to the blades to the relatively low pressure zone proximate to the fan inlet.

According to a further embodiment, a fan for an electronic device is described. The fan includes a cover. The fan also includes an impeller arranged to rotate around a center of rotation independent of the cover. The impeller includes a ring shaped shroud that cooperates with the cover to define an interior portion of the fan. The ring shaped shroud includes blades and splitter blades radially positioned around the center of rotation, each of the splitter blades having a length that is less than a length of each of the blades. At least one of splitter blades is radially positioned between every two blades.

DETAILED DESCRIPTION

As computer systems are reduced in thickness, the thickness of the modules and components inside the computer systems must also be correspondingly reduced. Although these modules and components must get thinner, reduced performance is generally not an acceptable consequence and, hence, new methods are sought to improve performance of these modules. Fan modules and assemblies, in particular, can be difficult to make thinner without dramatic loss in air throughput and cooling performance.

The fans and fan systems described herein include features that can provide a thin fan profile while providing high cooling efficiency. In some embodiments, the fans include impellers with shrouds that rotate independently from stationary covers of the fans. The shrouds cooperate with the stationary covers to define interior portions of the fans. The shrouds can include blades that are fixedly coupled to the shrouds or integrally formed with the shrouds. In some embodiments, the shrouds include splitter blades, which are generally shorter than the regular blades of the fans and which can increase efficiency of the fans.

These and other embodiments are discussed below with reference toFIGS. 1-14. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting.

FIG. 1shows a fan100for which such a method would be useful. Fan100can have many uses. For example, fan100can be used in portable computing device such as a laptop computer or other portable computing devices having limited internal volumes due to external size constraints. It should be noted that while a centrifugal fan is utilized for exemplary purposes, it should be understood that the described embodiments could be applied to both axial and mixed flow fans. Fan100can include exhaust opening102for expelling exhaust air flow103to an external environment and inlet opening104for receiving inlet air flow105. It should be noted that, in general, inlet air flow105and outlet air flow103are generally about the same. Also depicted are cover106and impeller108. Impeller108can be rotationally coupled to a bearing (not shown) within cover106that can impart a rotational force to impeller108causing blades110to rotate in such a way as to convert inlet air flow105into exhaust air flow103.

FIG. 2shows a partial cross-sectional view of fan100(as indicated by section line A-A ofFIG. 1) that is installed within enclosure201. More specifically, impeller108is depicted bringing a stream of cooling air202through opening104. Fan blade204is depicted with dashed lines as only a portion206of fan blade204extending from impeller108is contained within the depicted cross-section. Each of fan blades204can have a curved geometry, as is depicted inFIG. 1. Inlet air flow105is constrained by enclosure201, which leads to a loss of flow rate of air through fan100. One way to attempt to increase the flow rate of air through fan100is to increase the height H of fan blades204within fan100without increasing the thickness l of fan100. A consequence of increasing the blade height H in this manner is a reduction in blade/cover clearance208as shown inFIG. 2. Unfortunately, this clearance reduction increases the risk of fan blades204interfering and/or causing rubbing noise between fan blades204and cover106.

It may also be desirable to improve a number of other performance parameters of fan100, especially when factors such as fan noise and thermal performance are important. Two such performance parameters include a volumetric flow rate of air through fan100, and an acoustic output (otherwise referred to as fan noise) of the fan100under operating conditions. In applications noted above where fan100is anticipated for use in a laptop computer environment, it can be of particular importance that fan100remove as much heat as possible with as little fan noise as possible in keeping with a desired computer user's experience. For example, if a thickness T of the computer system surrounding fan100and a thickness l of fan100are reduced in such a way that the ratio of fan thickness to computer system thickness (l/T) remains constant, the change in air flow performance of fan100can be calculated using known scaling equations, such as scaling equations found in Chadha, Raman (2005), Design of High Efficiency Blowers for Future Aerosol Applications, M.S. Thesis, Texas A&M University, College Station, Tex., USA, which is incorporated herein by reference in its entirety. In particular, using scaling equation 36 of Chadha, Raman (2005), a fan having a thickness l of 6.0 mm would be expected to deliver 71.1% of the volumetric flow rate that of a fan having a thickness l of 8.0 mm. That is, the volumetric flow rate is significantly reduced by such thickness change. The static pressure is less sensitive to thickness changes. Specifically, a fan having a thickness l of 6.0 mm is calculated to produce 99.0% of the static pressure compared to a fan having a thickness l of 8.0 mm.

The fan and fan assemblies described herein are thin such that they can be positioned within small spaces such as enclosures of laptops and other portable computing devices, yet can deliver exceptional cooling needed for modern high performance computer systems. The fans include fan blades that are incorporated with or attached to a shroud. The shroud can function as a portion of the cover of the fan, thereby providing a configuration that allows for an increased fan blade area compared to conventional fans. To illustrate,FIG. 3shows a cross-sectional view of a fan300in accordance with some embodiments. Fan is positioned within enclosure301, which can correspond to an enclosure for a computer system or an enclosure of a subsystem that is further encased within one or more enclosures of a computer system. In this way, fan300and enclosure301form a fan assembly. Fan blade304is represented with dashed lines since the cross-section view ofFIG. 3shows a portion of impeller308that does not include fan blade304. Fan blade304is one of multiple fan blades that are not depicted inFIG. 3. Fan blade304is coupled with shroud302such that shroud302can rotate with fan blade304and independent of cover306. Shroud302can be located proximate to and separated from cover306by shroud/cover radial gap303. Pathlines310indicate air flow between enclosure301and fan300, and toward interior portion316of fan300. Shroud302can function as a portion of cover306in that shroud can physically prevent ingress of air flow into an interior of fan300other than as depicted by pathlines310.

It should be noted that fan300shows a particular technique for increasing blade height H compared to fan100ofFIG. 2without decreasing a blade/cover clearance. That is, incorporating shroud302with blade304allows blade304to be taller compared to a blade height that would be possible if a stationary cover is used, such as fan100ofFIG. 2. This increases the effective height of blade304, which corresponds to the height of the blade304that is effective in moving air. In addition, this configuration eliminates the need for a clearance between fan blade304and the portion of the cover that makes up shroud302. The extra blade height H (corresponding to increased blade area) afforded by shroud302allows more momentum to be imparted to the incoming air, which can result in the development of higher static pressures and increased flow rates. The blade height inboard of shroud302can also be increased, resulting in additional useful blade surface.

In some embodiments it may be beneficial to avoid having shroud302extend all the way to the blade tips, as shown inFIG. 3. This is because this configuration could result in shroud/cover radial gap303being located at a region where the pressure difference between the inside and outside of the fan would be at its highest. In some configurations, shroud/cover radial gap303can be on the order of between about 0.3 mm and 0.5 mm wide. Alternatively, to ensure a properly functioning shrouded impeller, the ratio of shroud/inlet radial gap (g) to impeller blade tip diameter (D) should be less than 0.01. That is, g/D<0.01. This is because the pressure can increase significantly with distance from a rotational axis of the impeller due to the action of the fan blade304being rotated through the air. This is illustrated with atFIG. 4, which shows an isometric view of impeller400. Impeller includes a central portion or central hub412, and fan blades that extend radially from central hub412. V represents the air velocity as experienced by fan blades402, r represents the distance from rotational axis404of the impeller400to tips410fan blades402, and ω represents the rotational speed of impeller400. The pressure increases significantly with distance r from the rotational axis due to the action of the fan blades402being rotated through air. Rotation of impeller causes higher static pressure to develop in “pressure side”406compared to “suction side”408of fan blades402. This results in creating different pressure gradients within a fan.

FIG. 5shows a cross-section partial view of fan500positioned within enclosure501illustrating how different pressure differentials can be formed. Fan500includes impeller502and cover504. Impeller502includes blades506and shroud508, with shroud508extending to tips510of blades506. Air flow into fan500is represented by pathlines512. Fan inlet zone518corresponds to a region external to fan500where air enters the fan500. Air pressure gradually decreases as air flows from outer edge514to inner edge516of cover504. Then, air pressure gradually increases as air flows from fan inlet zone518to tips510of blades506. The region of blades506immediately proximal to shroud/cover radial gap505experiences the highest static pressure. In particular, region of blades506immediately proximal to shroud/cover radial gap505experiences much higher static pressure compared to fan inlet zone518. This significant difference in static pressure is separated by only shroud/cover radial gap505.

Providing some amount of radial overlap between fan blades506and cover504can reduce this pressure difference. The reduced pressure difference results in a lower likelihood of recirculating air from fan blades506back out into the fan inlet zone518. The compromise required by this solution is the need to maintain a blade-cover axial clearance outboard of shroud508, which results in less available blade area for moving air when compared to an impeller that has shroud508that extends to tips510of blades506. In some embodiments, shroud508can extend across a bottom surface of cover504in more traditional configurations.

An example of an impeller that is shrouded and yet maintains some blade-cover overlap is shown inFIG. 6, which shows a partial cross-section view of fan600within enclosure603. Fan600includes impeller608and cover601. Shroud/cover radial gap612separates cover601and shroud610. Pathlines614indicate air flow between enclosure603and fan600, and toward interior portion616of fan600. An isometric view of the impeller608is shown inFIG. 7. As shown in embodiments ofFIGS. 6 and 7, shroud610can be positioned relative to fan blades606such that portions of fan blades606overlap with cover601(indicated by overlap602), which reduces a likelihood of recirculating air from fan blades606into fan inlet zone605.FIG. 7shows how shroud610can have a ring or disc shape that can be characterized as having a first side702and opposing second side704. Fan blades606each have a leading edge706and trailing edge708. Fan blades606can be circularly arranged with respect to shroud610such that leading edges706define a leading edge diameter and the trailing edges708define a trailing edge diameter. Fan blades can be positioned on first side702positioned, while second side704can correspond to a surface of shroud610that cooperates with cover601to prevent ingress of air into an interior of the fan until it reaches the fan inlet opening.

In some embodiments, shroud610is positioned at a central portion of fan blades606corresponding to a portion of fan blades606between leading edges702and trailing edges704. For example, shroud610can be characterized as having outer edge710and inner edge712. Outer edge710can define an outer diameter of shroud610, and inner edge712can define an inner diameter of shroud610that acts as the fan inlet. Fan blades606can be arranged with respect to the shroud such that the trailing edge diameter (corresponding to trailing edges708) is larger than the outer diameter of shroud610(corresponding to outer edge710). In some embodiments, the leading edge diameter (corresponding to leading edges706) is smaller than the inner diameter of shroud610(corresponding to inner edge712).

FIGS. 8A-8Eshow alternative embodiments in which a shroud and/or a cover are designed to prevent air flow within a shroud/cover radial gap, thereby improving the efficiency of the fan.FIG. 8Ashows a cross section view of fan800positioned within enclosure801. Fan800includes cover802and impeller804. Impeller804includes blades806and shroud808. Pathlines805indicate air flow between enclosure801and fan800, and toward interior portion807of fan800. Shroud808is separated from cover802by shroud/cover radial gap812. Shroud808includes outlet surface810that is tapered to guide air flow (indicated by pathlines805) away from shroud/cover radial gap812preventing recirculating of air through shroud/cover radial gap812. That is, shroud outlet surface810is angled to impart a vertical velocity component to the air flow near shroud/cover radial gap812, thereby biasing air flow away from shroud/cover radial gap812. For example, shroud outlet surface810can be arranged to direct air flow above and away from shroud/cover radial gap812. In some embodiments, this can be accomplished by increasing a thickness of shroud808when traveling from inner edge814to outer edge816of shroud808. Specifically, the thickness of shroud808increases from a first thickness818at inner edge814to a second thickness819at outer edge816. In some embodiments, shroud outlet surface810has a straight or linear shape while in other embodiments shroud outlet surface810is curved. In some embodiments, shroud outlet surface810includes one or more steps that provide a desired amount of taper. In some embodiments, shroud outlet surface810has a combination of linear segments, curved segments and/or stepped segments.

FIG. 8Bshows fan820having another alternative configuration in accordance with described embodiments. Fan820includes cover822and impeller824. Impeller824includes blades826and shroud828. Pathlines825indicate air flow between enclosure821and fan820, and toward interior portion827of fan820. Shroud828is separated from cover822by shroud/cover radial gap832. Shroud828, in addition to having a tapered shroud outlet surface830, also includes an overlapping feature838that overlaps with cover822proximate shroud/cover radial gap832. Overlapping feature838can force air out of shroud/cover radial gap832and back toward interior portion827of fan820. This can prevent undesirable leakage of air through radial gap832. Overlapping feature838can correspond to a ledge or lip positioned at inner edge836of shroud828.

FIG. 8Cshows fan840having another configuration in accordance with described embodiments. Fan840includes cover842and impeller844. Impeller844includes blades846and shroud848. Pathlines845indicate air flow between enclosure841and fan840, and toward interior portion847of fan840. Fan840is configured such that surfaces defining shroud/cover radial gap852are slanted in a way to prevent air flow into shroud/cover radial gap852. Specifically, outer edge850of shroud848and surface851of cover842define a shroud/cover radial gap852having a diagonal geometry that is slanted in a direction different than the air flow into the fan (represented by pathlines845). This diagonal configuration forces air out of shroud/cover radial gap852and back toward interior portion847of fan840, which as in fan820ofFIG. 8Breduces a likelihood of a parasitic flow path from being established through shroud/cover radial gap852.

FIG. 8Dshows fan860having another configuration in accordance with described embodiments. Fan860includes cover862and impeller864. Impeller864includes blades866and shroud868. Pathlines865indicate air flow between enclosure861and fan860, and toward interior portion867of fan860. Fan860shows a configuration in which outer edge876of shroud868extends past trailing edges869of fan blades866. This configuration prevents high pressure air exiting fan blades866and entering interior portion867from recirculating through shroud-/cover radial gap872. In some cases this configuration adds more length to shroud868compared to the shrouds shown inFIGS. 8A-8C.

FIG. 8Eshows fan880having another alternative configuration in accordance with described embodiments. Fan880includes cover882and impeller884. Impeller884includes blades886and shroud888. Pathlines885indicate air flow between enclosure881and fan880, and toward interior portion887of fan880. Fan880shows a configuration in which shroud888has a tapered shroud interior surface890and a tapered shroud exterior surface891. One or both of tapered shroud interior surface890and a tapered shroud exterior surface891can have a linear shape, curved shape, stepped shape, or a combination of linear, curved and/or stepped segments. The tapered shroud exterior surface891directs air away from the shroud/cover radial gap892on one side of shroud888, and curved shroud interior surface890directs air that has a tendency to recirculate within interior portion887away from shroud/cover radial gap892on another side of shroud888.

Note that any suitable combination of the shroud and cover configurations described above with reference toFIGS. 8A-8Ecan be utilized. For example, the shrouds can have any suitable combination of the above-described varying thicknesses, tapered shroud outlet surfaces, tapered shroud inlet surfaces, slanted outer edges, overlapping features and outer edges that extend past trailing edge of the blades.

FIG. 9shows a graph depicting both air flow performance of a fan using a shrouded impeller, such as the one shown inFIG. 7and performance of an unshrouded, or conventional, impeller such as the one used in the fan of prior artFIG. 1. The solid line shows the fan curve of a shrouded impeller with similar overall geometry and fan speed, but with a shroud. A large increase in the air flow delivered is observed for a significant portion of the fan operating range. The dotted line shows an example of a conventional impeller. As depicted, the shrouded impeller can have various effects on fan performance and can be beneficial for certain air flow rates and static pressures.

In some embodiments, the fan includes splitter blades that can be coupled to the shroud or other portions of the impeller in order to increase the efficiency of the fan.FIG. 10shows a front view of impeller1000, which includes a number of blades1002radially positioned around an axis of rotation of impeller1000. Central portion1004covers an impeller motor and bearing when impeller1000is assembled within a fan. Blades1002can have any suitable shape, including curved geometries that can be curved into the direction of rotation. Each of blades1002includes leading edges1002athat are positioned more proximate to the center of rotation than trailing edges or tips1002b. In some embodiments, impeller1000includes blade support disc1012that is coupled with and supports leading edges1002aof blades1002. The center of blade support disc1012can correspond to a center of rotation of impeller1000.

Impeller1000includes shroud ring1006that can constitute part of a cover and reduce the overall height of a fan, as described above. Shroud ring1006can be rigidly coupled with and support blades1002, or formed integrally with blades1002. In this way, shroud ring1006can rotate with blades1002during fan operation. In addition to blades1002, impeller1000includes splitter blades1008/1010, which are also radially positioned around an axis of rotation. In some embodiments, splitter blades1008/1010are coupled with shroud ring1006. Like blades1002, splitter blades1008/1010can guide air flow when impeller1000is rotated. However, splitter blades are generally shorter in length than blades1002and can thus be referred to as partial blades. The shorter length of splitter blades1008/1010allows for optimized flow guidance in the channels formed between adjacent blades1002.

To illustrate,FIG. 11shows a view of impeller1000with dashed lines representing portions of blades1002and splitter blades1008/1010that are not visible from a front view. Blades1002and splitter blades1008/1010each have trailing edges that are defined by fan blade diameter1108. However, splitter blades1008/1010have different lengths than blades1002. In particular, the leading edges of splitter blades1010are defined by a first diameter1102, the leading edges of splitter blades1008are defined by a second diameter1104, and the leading edges of blades1002are defined by a third diameter1106. The shorter lengths of splitter blades1008/1010keep them from impeding air flow entering from interior region1110. At the same time, the additional trailing edges or tips of splitter blades1008/1010being positioned along the fan blade circumference corresponding to diameter1108allows for improved guidance of air into the fan compared to blades1002alone. This can be important since the guidance provided by the tips of blades1002and splitter blades1008/1010are critical in determining the amount of air pressure produced by impeller1000. In some embodiments, the leading edges of one or both of splitter blades1008and splitter blades1010do not overlap with blade support disc1012. That is, one or both of diameters1102and1104can be larger than a diameter defined by an outer edge1107of blade support disc1012.

FIGS. 12 and 13show isometric section views of a portion of impeller1000showing additional details of blades1002and splitter blades1008/1010. As shown, blades1002and splitter blades1008/1010are coupled with shroud ring1006. A top surface of shroud ring1006can correspond to a portion of a cover that impeller1000is assembled in. Blade support disc1012is positioned below shroud ring1006and is coupled with the leading edges of blades1002, which provides additional structural support for the longer length of blades1002. In some embodiments support disc1012has a tapered shape such that surface1302of support disc1012is substantially parallel or divergent with respect to surface1304of shroud ring1006. Splitter blades1008/1010are shorter than blades1002and circumferentially positioned between blades1002. The shorter length of splitter blades1008/1010provides improved flow guidance within interior region1110of impeller1000, thereby providing more efficient air flow through impeller1000.

Note that since shroud ring1006supports splitter blades1008/1010, splitter blades1008/1010do not need to extend from a location closer to the center of rotation, thereby allowing splitter blades1008/1010to be shorter and thus reduce impedance of air into the channel between consecutive blades1002. In embodiments that do not include shroud ring1006, splitter blades1008/1010can be coupled with support disc1012. In these embodiments, support disc1012can include gaps between splitter blades1008/1010to allow for low-impedance air flow within interior region1110. However, removal of shroud ring1006may mean losing some extra blade height afforded by the addition of shroud ring1006, as describe above with reference toFIG. 3. In addition, there can be some loss of blade area near support disc1012.

Impeller1000shown inFIGS. 10-13is configured such that two shorter splitter blades1010and one longer splitter blade1008are positioned between blades1002(i.e., short-long-short). It should be noted that this configuration is exemplary and other configurations can be used. For example, in some embodiments, an impeller can include splitter blades that each has one length, or the impeller can include splitter blades having more than two different lengths. In some embodiments, the splitter blades are arranged in other orders, such as long-short-long, short-short-long, long-long-short, long-medium-short, etc. In some embodiments, there is one splitter blade between each blade1002, while in other embodiments there are two, three, four, or more splitter blades between each blade1002. That is, the number and order of splitter blades can vary depending on design choice. Generally, the larger the fan blade diameter1108is, the more blades1002and splitter blades1008/1010can be positioned within the impeller to optimize air flow. The optimal number, order and shape of blades and splitter blades can be calculated for a given impeller by considering parameters such as the fan blade diameter and divergence angle between consecutive blades.

FIGS. 14A-14Dillustrate how a divergence angle between blades1402and1404can affect air flow.FIG. 14Ashows reference circle1408, which is at a first radial distance from the center of rotation of the impeller.FIG. 14Bshows reference lines1412and1414, which are tangential to reference circle1408. Angle1416corresponds to the angle between reference lines1412and1414, also referred to as a divergence angle. If divergence angle1416is too large, the air flow between blades1402and1404becomes inefficient. This is illustrated inFIG. 14C, showing air flow pathlines1418and1420passing between blades1402and1404. Pathline1418shows that some air passes over and follows a surface of blade1404. However, pathline1420shows that some air does not follow the surface of blade1404but instead reverses direction, also known as flow separation. This flow separation can occur if the divergence angle1416between blades1402and1404is too large, which decreases the air flow efficiency of the fan.

FIG. 14Dshows insertion of splitter blade1422. Reference circle1423is at a second radial distance from the center of rotation, which is greater than the first radial distance of reference circle1408. Reference lines1412and1414, which are tangential to circle1408define divergence angle1424. As shown, divergence angle1424between blade1404and splitter blade1422is less than divergence angle1416without splitter blade1404. The reduced divergence angle1424reduces or eliminates any flow separation and improves the air flow efficiency of the fan. In general, the larger the divergence angle1416between blades1402and1404, the more splitter blades1422should be used. Another words, at each radial location there can be calculated an optimal number of blades. When that optimal number reaches an integer, another splitter blade can be added.