US2468723A - Axial flow fan - Google Patents

Description

April 26, 1949- J. BAR1I'LETIT, JR 2,468,723

AXIAL FLOW FAN Filed Jan. 24, 1945 2 Sheets-Sheet 1 Hub Cyhnder INVENTOR. Lame-5 L. Barf/671% April 26, 1949. J. L. BARTLETT, \JR

AXIAL FLOW FAN 2 Sheets-Sheet 2 Filed Jan. 24, 1945 W N mm? 150W Patented Apr. 26, 1949 AXIAL FLOW FAN James L. Bartlett,

Jr., Westwood, Mass,

assignor,

by mesne assignments, to Westinghouse Electric Corporation, a corporation of Pennsylvania Application January 24, 1945, Serial No. 574,269

1 Claim.

This invention relates to axial flow fans and has as it object, the provision of a relatively inexpensive, axial flow fan wheel having high efficiency and great strength.

Axial flow fans are now commonly rotated at high speeds for providing high pressures for such duties as supplying air under forced draft for steam plants. The axial flow fan blades now used for such duties have air foil sections, every dimension of which changes at every radius, resulting in high fabrication costs.

This invention provides a fan wheel having axial flow blades of sheet metal and which are formed as conoids, the straight line directrix of each blade being the axis of the Wheel, and the curved directrix of each blade being a curve laid out on the surface of a cylinder at the hub and coaxial therewith, the curve being derived as a smooth curve formed tangent to the entering and leaving velocity vectors at the radius of the hub cylinder. The chord of the blades is tapered for providing constant circulation.

The blades have great structural strength since all outer elements lie on radial lines extending through corresponding inner elements, and are easily and inexpensively fabricated. The pattern maker has to follow but a single curve in making up the pattern for the blade die. The aerodynamic efliciency is high since although the blade curvature is derived at one radius only, the fact the blades are conoids, results in the blades having substantially the correct shape at every radius when the chords are adjusted for constant circulation.

The invention will now be described with reference to the drawing, of Which:

Fig. 1 is a vector diagram illustrating how the air entering and leaving velocities at the hub are determined;

Fig. 2 is a vector diagram illustrating how the curvature of the blade at the hub radius is plotted;

Fig. 3 is a diagram illustrating how the blade element is rotated through a selected angle of attack;

Figs. 4, 5 and 6 are views in geometric projection illustrating how the curved directrix is laid out on a cylindrical surface at the hub, using the curve of Figs. 2 and 3;

Fig. 7 is a front elevation half of a hub of a fan invention;

Fig. 8 is a view of the path swept by the blade and hub of Fig. 7, and

Fig. 9 is a side elevation of the wheel of Fig. 7.

In the following explanation:

of one blade and onewheel embodying this It is desired to design a fan wheel embodying this invention to have an overall diameter of 24", a hub diameter of 6", and to be rotated at 1080 revolutions per minute for providing 4300 cubic feet of air per minute.

'%% =2.94 sq. ft. free area =g gi=1iso ft./min.=24.4 mm.

W, 24.4 2 VP= %=9.25 ft. output head Assuming a design total efficiency of then T 'P-= =13.3 ft. input head 21r 1080 60 113 radians per second Referring now to Fig. 1, the angles ,8 and 0 are determined as follows:

24.4 tan. 5-m-865 24.4 tan. m1.877 fi+0=620' From the foregoing data, the velocity vectors V1 and V2 are drawn in on the vector diagram of Fig. 1. These vectors are then transferred to Fig. 2 and the smooth curve C-L is drawn in tangent thereto. This curve is the center line of the blade at the hub.

The camber of the center line C-L is determined from Fig. 2 by scaling the height h of the curve above the chord line at a point one-half the length thereof.

Percent camber 4 From the N.. A. C. A. Reports a CL of 0.8 and an angle of attack of 3.5" is taken from the data for the air foil #4506.

The blade center line C-L is shifted through the angle of attack our-3.5 as illustrated by Fig. 3.

g The blade center line shown by Figs. 2 and 3 is what is known as a developed curve. to say it is a curve plotted on the surface of a developed or unrolled cylinder. The procedure for applying this curve to the hub is illustrated by Figs. 4, and 6. Fig. 4. shows. the curve on the developed cylinder. Fig. 5 is a circle having the the radius of the cylinder, and Fig. 6 is a plan view of the cylinder and also represents a plan view of the hub cylinder. Three points are used for establishing the curve on the hub cylinder, the ends of the curve on Fig. 4 and the point in alignment with the center of the cylinder. The points on the curve of Fig. 4 are scaled as illustrated and applied to Fig. 5 from which they are projected over to Fig. 6, the axial positions of the points being scaled as illustrated, from Fig. 4.

Fig. 6 is a plan view of the intersection of the blade center line with the surface of the hub cylinder. Using the curved center line of Fig. 6 as the curved directrix, and the axis of the wheel as the straight line directrix, the blade ex- That "is 4 tending outwardly from the hub is formed as a. conoid as illustrated by Figs. 7 and 9.

For determining the chord lengths of the\ blades at different radii, the formula for cir-\ culation given in the foregoing is used, the circulation being constant at every radius. For each radius the values of the velocity vectors will vary but their values can be determined as outlined in the foregoing and the chord length for each radius calculated. This will result, through the blade curvature having been established at the hub, in the elements of the blades having chord lengths which increase as the radius increases as illustrated by Fig. 7. It also results in the path swept by the blades having a width which decreases from a maximum at the hub to a minimum at the tips as illustrated by Fig. 8.

The hub can be cylindrical or as illustrated by Figs. '7, 8 and 9, it can be curved for smooth air flow so as to have air entering portions of smaller diameter than that of the cylinder upon which the curved center line is laid out. The intersections of the blades at their roots with the rounded portion of the hub are formed by radial lines extending parallel to each other and through the curve laid out upon the hub cylind'er as described.

While the air entering edgesof the blades have been illustated as extending radially with the air leaving edges curved to provide the proper chord lengths for constant circulation, the entering edge or both edges could be cut to provide the proper chord lengths.

While one embodiment of the invention has been described for the purpose of illustration, it should be understood that the invention is not limited to the exact arrangement illustrated as modifications thereof may be suggested by those skilled in the art without departure from the essence of the invention.

What is claimed is:

An axial flow fan wheel comprising a hub, and a sheet metal blade formed substantially as a conoid extending radially from said hub, the width of the path of the air swept by the elements of said blade decreasing as their; distances from said hub increase, the curved directrix of said conoid being a curve on the surface of a cylinder coaxial with said hub and having a diameter equal substantially to the largest diameter of the hub, said curve being derived by being formed substantially tangent to the velocity vectors V1 and V2 in a vector diagram whereVi=the velocity of the air entering the blade element at the radius of the said cylinder, and Vz=the velocity of the air leaving said blade element.

JAMES L. BARTLETT, JR.

REFERENCES CITED The following references are of record the. filev of this patent:

UNITED STATES PATENTS Number Name Date 1,504,710 Roberts Aug. 12, 1924 2,004,516 Hagen June 11, 1935 FOREIGN PATENTS Number Country Date 279,426 Great Britain Aug. 9,, 1.928.

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