US2405768A - Axial blower - Google Patents

Description

13, 1946- E. A. STALKER 2,405,768

AXIAL BLOWER Filed June 11, 1945 A TTORNE Y6 Patented Aug. 13, 1946- AXIAL BLowEa Edward A. Stalker, Bay City, Mich. Application June 11,

1 12 Claims. 1. Y

My invention relates to blowers and particularly tothe form of the blades for use therein. It is the principal object of the invention to provide a. blower having blades of such construction and arrangement that in operation the pressure is well distributed over the surface of the blade substantially eliminating a sharp peak pressure condition at any point and making it possible to operate the blade at a high speed with highly effective and efficient results.

It is also an object to provide such a blower capable of satisfactory operation at a relative flow speed approaching the speed of sound. I

It is a further object to provide a blade of this character having an airfoil section which favors the control of the boundary layer. Other objects will appear from the description and drawing.

I accomplish the above objects by the means illustrated in the accompanying drawing in which Fig. 1 is a diagrammatic view of the pressure distribution over a blade of conventional construction;

Fig. 2 is a section through a blade constructed in accordance with the present invention and ineorporating such blades along the line 2-2 in Fig. 3;

Fig. 2Ais a sectional view through the completed airfoil section developed from Fig. 2 in accordance with the present invention;

Fig. 3 is an axial section through a blower;

Fig. 4 is a diagrammatic view showing the manner of developing the preferred blade form; and

Fig. 5 is a; schematic view showing the inter-' connection of the blades of two alternate stages of the blower.

In an axial flow blower the blades operate with a rather wide range of angles of attack. The largest useful angle of attack occurs when the blower is pumping fluid against a large pressure.

For this condition the conventioal blade I has a pressure distribution curve 2 substantially as shown in Fig. 1, negative pressures being used as ordinates at the different stations along the blade as shown. Itis to be observed that this curve has a peak more than 50% higher than- 1943, Serial No. 490,419

at a higher speed provided this peak pressure condition is overcome and a maximum pressure obtained which is distributed over a substantial area of the blade and made as small as possible above the average pressure. The pressure is of course directly related to the square of the velocity according to Bernoullis equation.

The basic form of the blade which will provide the uniform velocity and properly distributed 1o pressure across the major portion of the chord is shown in Fig. 4 which gives the upped half of the section. In the preferred form the forward half of the contour I3 is substantially elliptical. That is, the contour ahead of the ordinate at the maximum thickness of the section, which in this case is located at the point 0.50, corresponds to a semi-ellipse.

The airfoil section of elliptic form, Fig. 4, and no arching of the mean camber line I call the basic airfoil section. The ordinate above the line 0A represents one-half of the thickness of the airfoil section. 1

An airfoil section of any form'may be readily converted to a section with an arched mean camher line, and vice versa. This may be done by laying off the abscissa from the origin 0 along the camber line and describing a circle so that each radial length equals the ordinate of the corresponding basic airfoil section. It is important in so doing that the abscissa be laid off along the mean camber line rather than along the subtending chord line.

The mean camber line preferably has the substantially arched form l2 shown in Fig. 2 (dot and dash lines). Its ordinates above the subtending chord are greater at the extremities of the wing than those of a circular are. it (shown by dotted lines) of the same maximum height passing through the ends of the chord line. The abscissae are laid off along the mean camber line and the ordinates II for upper and lower surfaces are then struck as arcs with their centers on such mean camber line. This defines the upper and lower airfoil surfaces. Following this procedure with the elliptic curve l3 and transferring the same to the arched mean camber line it of Fig. 2, the final upper and lower airfoil surfaces for blade '6 are obtained.

This blade 8 shaped as shown in Fig. 2 develops the same lift as that of Fig. 1, that is, the average ordinate of the sub-pressure curve 3 is equal to the average ordinate of the sub-pressure curve 2 in Fig.1. It is to be noted that the curve? 3 has a substantially constant value over a major part, approximately per cent of the chord higher permissible rate of rotation than for the blade of Fig, 1.

The elliptic curve defining the asic airfoil section may vary within a prede rmined range. Fig. 4 shows by the contours l6 and H, the limits of variation in the nose contour which permit of keeping the velocity substantially constant along the section. If this velocity is not to vary substantially'the flow should divide at the forward end of the mean camber line. When the division occurs at a point nearer the lower contour the velocity over the nose is increased and while a man increase can be tolerated, it should be kept to a minimum. The contour I6 is an arc of an ellipse whose minor semi-axis is located at 0.3C instead of at 0.5C as is the case with curve l3. Contour I6 is faired into the aft portion of the contour i3-l3a. This curve defines the upper limit to the ordinates of the nose contour. For the purposes of further discussion the curve portion I6 is considered to extend to the end of the maximum thickness ordinate. The upper aft portion 83a of the curve has the convex curvature as shown up to the end of the section where it terminates with a relatively sharp trailing edge as indicated at A in Fig. 4.

The contour I! is a curve laid off below curve l3. At any station along the chord line the differences between the ordinates of curves l3 and I6 and between l3 and I! are equal. The curve 11 then defines the lower limit to the ordinates of the nose contour. 3

It may thus be stated that the preferred contour for the nose portion of the blade section lies within a family of substantially elliptical curves whose mean curve is if! the minor semiaxis of which is located at 0.50, and whose outer or boundary curve (of the family) is the elliptic arc l6 extended to the maximum thicknes ordinate at the mid-point of the chord. That is. by defining one boundary curve and the mean curve. the other boundary curve is immediately defined. The actual blade section is then determined by transferring such curves from a straight line to a highly arched mean camber line as above described. 4

The location of the maximum thickness ordinate at the mid-point of the chord give optimum results but it is possible to place the maximum thickness ordinate further forwardwith some impairment of the beneficial results. If it is placed as far forward as the 40 per cent point of the chord the benefits obtained by the special airfoil shape are only slight and therefore this location at 40 per cent of the chord is taken as the forward desired limit.

It istheoretically possible to place the maximum thickness ordinate far to the rear of the mid-point of the chord when slots are used to compel the fluid to follow blade contour. However when the maximum thickness ordinate is aft of the 60 per cent point of the chord the upper and lower contours of the blade section converge so rapidly that very large suction is required at the slots to compel the fluid to follow the upper contour. For discharg slots the pressure must be very high to effect a like resu t- The practical rearward location is near the 60 per cent point and accordingly that is taken as the rearmost desired limit.

For further discussion I define the portion of the circumference of an ellipse between the ends of the major and minor axes as an elliptic quadrant.

The effectiveness of the blade is further enhanced by the provision of slots l8 and IQ for controllingthe boundary layer. The slots may be located well rearward because the sub-pressure curve 3 is favorable to the flow followin the blade contour back to at least the halfway station along the chord line 03. That is, the pressure gradient is favorable over the front half of the blade section as indicated by the increasing suction (sub-pressure ordinate) in going from 0 toward B. In Fig. 1 the conventional blade has an unfavorable pressure gradient from C to D and favorable only from 0 to C, the maximum suction acting on a particle flowing from C to D acting forward and tending to stop the particle and cause separation of the flow from the surface. On blade form 6, Fig. 2, the maximum suction ordinate is always ahead of the par. ticle at least until the mid-point of the section is reached. This is an important feature of the invention. .Also the blade form makes it possible to locate the slots well back on the chord where the external sub-pressure is low and will therefore offer smaller opposition to the induction of the fluid into the blade, providing additional improved characteristics.

The greater the maximum ordinate of the mean camber line of the airfoil section the more significant it becomes to maintain a substantially constant velocity across a large part of the airfoil section chord-0r to bring the maximum suction'ordinate well aft along the wing. Conven- 40 tional airfoil sections do not employ, in practice,

mean camber maximum ordinates greater than 5 per centof the length of the chord subtending the mean camber line. However by employing the special airfoil shape as described herein, I

; have found that the airfoil sections for the blower blades operate best with a mean camber maximum ordinate greater than 5 per cent of the chord, and that with such special shape the peak efliciency isshifted into the regime of blower operation where the pressure is highest. This corresponds to the flow condition on the airfoil section when the velocity across the blade is substantially constant for the forward half of the section. The provision for the coincidence of the maximum efficiency with the high pressure regime of the blower is a feature of this invention.

As the maximum camber is increased beyond 10 per cent of the chord length it becomes important to add the boundary layer control slots but they give some benefit at all camber values. The mean camber line should have its maximum ordinate in the neighborhood of the mid-point of the chord in order to promote a substantially constant velocity over a large forward portion of the blade section; the preferred location is at the mid-point. A shift forward from this location-is more disadvantageous than a shift rearward. When the ordinate has been shifted as far forward as the 40 per cent point the same beneficial results are not secured.

For blades in a rotor which are placed closer than a gap to chord ratio of unity it is somewhat desirable to flatten the forward portion of the ean camber line and shift the maximum ordiasomee nets of the mean camber line rearward. should however be between 50 per cent and 60 per cent of the chord length. Preferably this flattening does not extend substantially beyond distorting the mean camber line I! into the circular are It.

The leading edge should be Well rounded and may be approximated by a circular. segment. The radius should be larger than 8 per cent of the maximum thickness of the airfoil section and substantially less than the maximum thickness. A preferred value would be of the order of 24 per cent of the maximum thickness with a preferred range of 20 to 40 per cent. If the blades are placed close together so that the ratio of gap to chord is substantially less than unit the greater nose radii are beneficial.

6 constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise forms of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

scribed. Fluid is inducted at 8 and compressed exit l0.

In order to provide a fiow of air through slots l8 and IS, a blower or other suitable means may be provided. However the arrangement as shown in copending application Serial No. 447.822 filed June 20, 1942 (with respect to which Patent No. 2,344,835 has issued as a continuation thereof) is preferably used. In accordance therewith and as shown in Figs. 3 and 5 the rotor blade 6 of an upstream rotor is connected by duct 4 to the interior of blade 6a of a downstream rotor. Each pair of alternate blades in the same axial plane is thus connected by separate ducts 4 which communi cate between the interiors thereof, the remainder of the ducts not showing in Fig. 3 because they lie out of the plane of the section of the drawing.

The blades of the first and third rotor stage are in line with each other but are staggered peripherally relative to the blade of the second rotor stage to permitthe passage of ducts 4 across the plane of the second rotor. The staggering of the blades of one rotor stage relative to those of the next permits alternate rotor stages to have their respective blades interconnected by the ducts.

The stator blades are arranged throughout in the same manner as the rotor blades to accommodate the ducts 4a.

The boundary layer of the blades enters the slots l8 and I9 of the downstream blades where the pressure is high and flows to the discharge slots 24 in the upstream blades where the pressure is low. Slot 24 is similar to slots l8 and I9 except that preferably the walls of the slot overlap to direct the outflowing jet rearwardly along the trailing blade surface. Both sets of slots serve to make the external flow follow the blade surfaces to very large angles of attack. The provision of such boundary layer control serves to energize the boundary layer on the trailing portion of the blade, in that range where the layer would tend to lose energy by reason of the adverse pressure gradient encountered, thereby improving the aerodynamic characteristics, and giving improved lift and drag characteristics. The provision of boundary layer control on a blade having the airfoil contour in accordance with the present invention is particularly advantageous as overcoming what would otherwise be' a seriously limiting condition in operating a blade under the desired high speed conditions. i

While the forms of apparatus herein described What is claimed is:

1. In an axial flow blower, an enclosing casing, a blade having a basic airfoil section whose maximum thickness lies between stations at 40 per cent and 60 per cent of the chord length from the leading edge and whose upper contour ahead of the maximum thickness ordinate lies An major part within bolindary curves whose mean curve is an elliptic quadrant passing through the nose point of the section and the end point of the said maximum thickness ordinate, "said maximum thickness ordinate serving as the minor semiaxis of said quadrant, the outer curve of said boundary curves being an auxiliary elliptic quadrant passing through said nose point and the end of an auxiliary minor semi-axis at the 0.3 point of the chord and extending on to the outer end of said maximum thickness ordinate, said auxiliary minor semi-axis being equal to one-half the said maximum thickness of the said airfoil section, and means to mount said blade within said casing to induce a flow therethrough.

2. The structure of claiml wherein saidblade has a slot in its surfaces, and means to induce a flow of fluid therethrough to control the boundary layer on said airfoil surface.

3. In an axial flow blower, an enclosing casing, an axial flow blade in said casing, said blade having a basic airfoil section over the portion thereof ahead of the maximum thickness ordinate, said basic section being defined by a curve lying within a family of substantially elliptical curves the mean curve of which has its minor semi-axis located between stations at the 0.4 and the 0.6 points of the chord and whose outer boundary curve has its minor semi-axis located approximately at the 0.3 point of the chord, the upper aft portion of said airfoil section having a convex curvature and ending in a relatively sharp trailing edge.

4. In an axial flow blower, an enclosing casing, an axial flow blade in said casing, said blade having a basic airfoil section over the portion thereof ahead of the maximum thickness ordinate, said basic section being defined by a curve lying within a family of substantially elliptical curves the mean curve of which has its minor semi-axis located between stations at the 0.4 and the 0.6 points of the chord and whose outer boundary curve has its minor semi-axis located ap-' proximately at the 0.3 point of the chord, the inner boundary curve of said family being a curve laid off below said mean curve in such relation thereto that said mean curve lies vertically midway between said boundary curves, the upper aft portion of said airfoil section having a convex curvature and ending in a relatively sharp trailing edge.

5. In an axial fiow blower, an enclosing casing, an axial flow blade in said casing, said blade having a basic airfoil section over the nose portion thereof the maximum thickness ordinate of which is located between approximately th 40% and the 60% points of the chord, said basic section being defined by a curve lying within a family of substantially elliptical curves the mean curve of which has its minor semi-axis located at approximately the point of maximum thickness of the section and whose outer boundary curve has its minor semi-axis located approximately at the 0.3 point of the chord, the upper aft portion of said airfoil section having a convex curvature and ending in a relatively sharp trailing edge.

6. In an axial flow blower, an enclosing casing, an axial flow blade in said casing, said blade having a basic airfoil section over the portion thereof ahead of the maximum thickness ordinate based on a mean camber line which is a straight line, said basic section being defined by a curve lying within a family of substantially elliptical curves the mean curve of which has its minor semi-axis located between said sections at the 0.4 and the 0.6 points of the chord and whose outer boundary curve has its minor semi-axis located at the 0.3 point of the chord, the surface of said blade determined by said basicairfoil section being developed with respect to an arched mean camber line the maximum ordinate of which above the subtending chord is greater than approximately of the length of the chord.

7. In an axial flow blower, an enclosing casing, an axial flow blade in said casing, said blade having a basic airfoil section over the portion thereof ahead of the maximum thickness ordinate based on a meancamber line which is a straight line, said basic section being defined by a curve lying within a family of substantially e1- llptlcal curves the mean curve of which has its minor semi-axis located between said stations at the 0.4 and the 0.6 points of the chord and whose outer boundary curve has its minor semi-axis located at the 0.3 point of the chord, the surface of said blade determined by said basic airfoil section being developed with respect to amean camber line which is arched above a circular arc of the same maximum height passing through the ends of the chord line.

8. In an axial flow blower, an enclosing casing, an axial flow blade in said casing-said blade having a basic airfoil section over the' portion thereof ahead of the maximum thickness ordinate based on a mean camberline which is a straight line, said basicsection being defined by a curve lying within a family of substantiallyelliptical curves the mean curve of which has its minor semi-axis located between said stations at the 0.4 and the 0.6 points of the chord and whose outer boundary curve has its minor semi-axis located atthe 0.3 point of the chord, the surface of said blade determined by said basic airfoil section being developed with respect to an arched mean camber line, said mean camber line having a height above the subtending chord greater than about 5% of said chord, the upper aft portion of said airfoil section having a convex curvature and ending in a relatively sharp trailing edge.

9. In an axial flow blower, an enclosing casing, an axial flow blade in said casing, said blade having a basic airfoil section over'the portion thereof ahead of the maximum thickness ordinate, said basic section being defined by a curve lying within a family of substantially elliptical curves 8, the mean curve of which has its minor semi-axis located between stations at the 0.4 and the 0.6 points of the chord and whose outer boundary curve has its minor semi-axis located approximately at the 0.3 point of the chord, said blade having a slot in its surface. and means to induce a flow of fluid therethrough to control the boundary layer on' the surface thereof.

10. In an axial flow blower, an enclosing casing, an axial fiow blade in said casing, said blade having a basic airfoil section over the portion thereof ahead of the maximum thickness ordinate based on a mean camber line which is a straight line, said basic section being defined by a curve lying within a family of substantially elliptical curves the mean curve of which has its minor semi-axis located between said stations at the 0.4 and the 0.6 points of the chord and whose outer boundary curve has its minor semiaxis located at the 0.3 point .of the chord, the surface of said blade determined by said basic airfoil section being developed with respect to an arched mean chamber line, said mean camber line-having a height above the subtending chord greater than about 5% of said chord length, the upp r aft portion of said airfoil section having a convex curvature and ending in a relatively sharp trailing edge and the lower aft portion of said section having a substantial concavity therein.

11. In an axial flow blower, an enclosing casing, anaxial fiow blade in said casing, said blade having a basic airfoil section over the portion thereof ahead of the maximum thickness ordinate based on a mean,camber line which is a straight line, said basic section being defined by a curve lying within a family of substantially elliptical curves,.the mean curve of which has its minor semi-axis located within the range of about the 0.4 and the 0.6 points of the chord and the outer boundary curve of which has its minor semi-axis located at the 0.3 point of the chord, the surface of said blade determined by said basic airfoil section being developed with respect to a mean camber line which is arched at its aft end above a circular arc of the same maximum height passing through the ends of 1 the chord line.

12. In an axial flow blower, an enclosing casing, an axial flow blade in said casing, said blade having a basic airfoil section over the portion thereof ahead of the maximum thickness ordinate, said basic section being defined by a curve lying within a family of substantially elliptical curves and below the mean curve thereof, said mean curve of said family having its minor semiaxis located withina range of about the 0.4 and the 0.6 points of the chord, the outer boundary curveof said family having its minor semi-axis located approximately at the 0.3 point of the control the boundary layer on the surface thereof.

EDWARD A. STALKER.

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