US2090888A - Screw rotor for high speed axial flow machines - Google Patents

Description

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SCREW RQTOR FOR HIGH SPEED AXIAL FLOW IACHINES Filed April 22, 1933 3 ShOOtS-ShQOt 1 Fig. 2.

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SCREW ROTOR FOR HIGH SPEED AXIAL FLOW MACHINES Filed April 22, 1933 s sneetfls gege j.

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Patented Aug. 24, 1937 UNITED STATES Search Room PATENT OFFICE SCREW ROTOR FOR. HIGH SPEED AXIAL FLOW MACHINES James Herbert Wainwright Gill, London, England Application April 22, 1933, Serial No. 667,475 In Great Britain May 30, 1932 7 Claims.

This invention relates to screw rotors for high speed machines wherein the general direction of flow of the working fluid (whether liquid or gaseous) through the rotor is axial or more nearly axial than radial. Such rotors may be employed alone as fans or propellers or fluid motors or may operate in combination with suitably designed and arranged guide vanes or with complementary rotors of opposite rotational sense in blowers, pumps, reaction turbines or other machines, two or more rotors being employed in the case of multi-stage machines.

Various proposals have been made, involving radial variation of pitch in the rotor blades, with the object of improving the efliciency of axial flow screw rotors by providing for a regulated distribution of dynamic head at the various radii of the rotor disc annulus,'more especially in pumps, turbines and blowers, but such proposals have not hitherto taken into account the special conditions obtaining for operation at high specfic speeds. Thus it has been found that the width of the rotor blades and the distribution of this width, plays a much more important part in determining the efliciency of the rotor when high speeds are employed, and to regulate the dynamic loading over the blades whilst reducing to a minimum the effect of frictional drag, it is desirable to modify the incidence of pitch in accordance with a suitably disposed radial distribution of blade width. It should be mentioned that the term specific speed is employed more especially in connection with hydraulic turbines to provide a basis of comparison between machines having different dimensions and operating characteristics, and may be defined as the speed at which the rotor of a turbine would operate if reduced geometrically (without altering its design) to such a size that it would develop unit horse power under unit working head. The term specific speed has also been applied to pumps, in which case it is defined as the speed at which a geometrically similar rotor reduced in size so as to deliver unit quantity under unit head at conditions of maximum efficiency would require to operate. Analogous definitions of specific speed can also be given for other types of machine employing screw rotors.

The present invention has for its object to provide an axial flow screw rotor in which the pitch and the width of the blades are so chosen in relation to one another and to the radius as to promote high efiiciency, accompanied by a minimum liability to mutual interference and to cavitational effects, when the rotor is operated at a. high specific speed.

In the screw rotor according to the present invention the pitch at the working edge of each blade varies radially in such a manner that the product of the pitch and the radius either is constant at all radii or has a minimum value at a position whose radial distance from the axis is not greater than the normal designed mid-blade radius and increases from such value both inwards towards the boss and outwards towards the blade tip, whilst the width of the blade either is constant at all radii or decreases radially both inwards and outwards from a maximum value at a position whose radial distance from the axis is not greater than the normal designed mid-blade radius. In a preferred arrangement according to the invention the pitch at the working edge of each blade varies radially in such a manner that the product of the pitch and the width and the radius is substantially constant at all radii.

It should be made clear that the mid-blade radius is to be understood as the arithmetic mean between the boss radius and the radius of the blade tip at the working edge of the blade. The term working edge of the blade is to be taken to mean the trailing edge (or marginal portion) in the case of a fan or propeller or the discharge edge in the case of a pump or blower or the inlet edge in the case of a turbine or fluid motor. The term pitch relates to the pitch of the blade at the working face of the blade and may be defined by the mathematical expression 21rr tan ,6, where r is the radial distance from the axis of the point at which the pitch is to be measured and ,3 is the blade angle, i. e. the angle made by the line of intersection of the blade face and the stream tube through the point to a surface through the point normal to the stream tube. A stream tube may be defined as any one of a series of infinitely thin tubular sheets of fluid, disposed one within the other and together making up the whole body of flowing fluid, the thickness and direction of each stream tube varying in conformity with changes in the crosssection and direction of the whole body of fluid in the flow stream. Thus when the arrangement is such that the stream tubes are cylindrical surfaces, the pitch is the axial distance between successive convolutions of the helix of which the portion ,of the blade face where the pitch is to be measured forms part.

In the case of rotor blades of simple construction, the width of a. blade at a given radius may be defined as the actual distance between the point on the working edge of the blade at that radius and the point on the other edge which lies on the same stream tube, as measured along the line of intersection of the stream tube and the working face of the blade. This definition is satisfactory for rotor blades such that the line of intersection of the stream tube and the working face is a straight line on a plane development of the stream tube, but for blades of more complex structure where the intersecting line is not straight on the tube development, this definition does not give a strictly accurate measure of the effective blade width, which should more correctly be taken to be the straight line distance between the edge points on the tube development. For all blades therefore the width may be properly defined as the shortest distance as measured along. the stream tube between the point on the working edge of the blade at the given radius and the point on the other edge of the blade on the same stream tube.

The maximum width of each blade is preferably not less than 10% greater than the Width of the blade at the boss and similarly the minimum value of the product of the radius and the pitch is not greater than 90% of the value of the product at the boss. At the blade tip the designed width may be equal to that at the boss or may differ therefrom by an amount preferably not greater than of the width at the boss. It should be mentioned that it is usually desirable in practice to round off the corners of the blade at the tip, so that the actual Width thereat is somewhat less than the designed width.

The pitch of the blade on any stream tube may be constant throughout the axial length of the rotor, but except with very low ratios of mean pitch to diameter it will usually be desirable to include axial variation in pitch, so that the pitch gradually decreases from the working edge of the blade to the other edge, more especially at the shorter radii, in order to ensure the most v favourable angles of incidence between the blade face and the fluid flow relative thereto, namely between 3 and 8 in most cases. The amount of axial variation in pitch will be determined in accordance with the particular purpose for which the rotor is to be used, but it will usually be preferable to decrease the pitch from the working edge to the other edge of the rotor more rapidly in the neighbourhood of the boss than in the outer parts of the rotor, where such decrease may not be necessary or where, in an extreme case, a slight increase of pitch at the tip might be indicated. Thus for example in the case of a pump, in order to avoid shock due to unfavourable angles of entry at inlet, it is often desirable to have the pitch approximately constant along the inlet edge of the blade or even decreasing slightly towards the boss.

The number of blades employed will vary according to the conditions of the particular in stallation, and the blades may overlap one another in face projection more especially in the inner parts of the rotor when the rotor is to operate against high pressures. Cylindrical sections through the blades may be of uniform thickness from edge to edge or may have lenticular or aerofoil or other suitable shape, a special streamlined form being preferred.

The boss will usually be cylindrical or conical, but a streamlined or other shape may alternatively be employed.

The invention may be carried into practice in various ways, but three convenient constructions according thereto are illustrated in the accompanying drawings, in which Figures 1 and 2 respectively show a face projection and a profile projection of a blade of one construction of rotor more especially intended for use as an air fan,

Figures 2a, 2b, and 20 show cylindrical sections of the blade respectively on the lines 2a, 2b, and 2c of Figure 2,

Figure 3 is a graph showing both the blade width and the pitch at the working edge of the blade shown in Figures 1 and 2 plotted against radial distance from the axis,

Figures 4, 5, and 6 are drawings similar to Figures 1-3 but for a construction of rotor more especially intended for use in a hydraulic pump,

Figures 5a, 5b, and 50 show cylindrical sections (corresponding to those of Figures 2m, 25, and 20) respectively on the lines 5a, 5b, and 5c of Figure 5,

Figures 7, 8, and 9 are similar drawings of a third rotor construction which is suitable for use in a pump or in a turbine, and

Figures 8a, 8b, and show cylindrical sections (corresponding to those of Figures 2a, 2b, and 20) respectively on the lines 8a, 8b, and 8c of Figure 8.

In the construction of Figures 1-3, the rotor has six blades, similar to the blade A shown, carried by a cylindrical boss B whose radius is approximately 0.43 taking the peripheral radius of the rotor as unit.

The curve D in Figure 3 shows the pitch at the working edge 0 of the blade A plotted against the radius and the curve E shows the width of the blade A plotted against the radius, the dotted ordinates in Figure 3 corresponding to the dotted circles and lines of Figures 1 and 2 at equally spaced radial distances from the axis of the rotor. The left-hand ordinate F in Figure 3 corresponds to the blade root at the surface of the boss B whilst the right-hand ordinate G corresponds to the blade tip. Figure 3 is drawn to twice the scale of Figures 1 and 2.

From the curve E of Figure 3 it will be seen that the width of the blade increases from the boss at F to a maximum at a radius of about 0.6 and then decreases again to the blade tip G where it has a value nearly 30% less than the width at the boss, the maximum width being nearly 15% greater than the boss width.

The pitch at any radius is constant over the whole width of the blade and decreases, as shown by the curve D, from the boss at F to a minimum at a radius of about 0.75 from which it increases slightly towards the blade tip G. The rate of radial variation in pitch is such that the product of the pitch and the width is substantially inversely proportional to the radius. The pitch of the blade near the mid-blade radius (0.715) has a value approximately equal to four-fifths of the rotor diameter at the blade tips.

Cylindrical sections through the blade at radii 0.5, 0.7, and 0.9 are shown respectively in Figures 2a, 2b, and 20.

In profile view (see Figure 2) the apparent width of the blade decreases from the boss to the blade tip, the rate of change of profile width decreasing towards the tip. The surfaces containing the leading and following edges H, C, of the blades are preferably symmetrically disposed with respect to a transaxial plane through the centre of the length of the rotor.

In face projection (see Figure 1) the blades do not overlap, and the leading and following edges H, C of each blade are symmetrically disposed about the median radial line of the face projection. The angle subtended at the axis of the rotor by the face projection of the blade varies from about 40 at the boss to a maximum of about 44 in the inner part of the blade and thence to about 18 at the blade tip.

The chain lines J in Figure 2 are a conventional representation of the incidence and position of maximum thickness for the blade, the upper line showing the position of maximum thickness whilst the distance between the two lines at any rad us represents the actual maximum thickness of the blade at that radius. Thus the thickness decreases gradually from the boss to the tip and the line of maximum thickness lies somewhat nearer the leading edge H than the following edge C. The positions of maximum thickness are indicated by chain lines in the sections of Figures 2a, 2b, and 2c.

The second rotor construction shown in Figures 4-6 is intended more particularly for use in a hydraulic pump and differs from the construction of Figures 1-3 mainly in respect of the number and width of the blades and also in the radial distribution of pitch which changes somewhat less rapidly in the neighbourhood of the boss B and the tip, the pitch near the mid-radius again being approximately four-fifths of the rotor diameter.

In this construction as shown in curve E of Figure 6, the width of the blade A increases from the boss at F (at radius about 0.43) to a maximum value at a radius of about 0.6 and decreases again to the tip G the maximum width being rather more than 15% greater than the boss width, whilst the tip width is about 20% less than the boss width.

The pitch at any radius is again constant over the width of the blade and decreases, as shown by curve D, from the boss to a minimum value at a radius of about 0.75, the pitch remaining substantially constant from this radius outwards to the tip. The product of the pitch and the width is substantially inversely proportional to the radius.

As will be clear from Figure 5, the leading edges H of the blades lie on the surface of a cone and the following edges C lie on a curved surface of hollowed generally conical shape, the two cones being oppositely disposed.

Four blades are employed, each blade in face projection (see Figure 4) being symmetrical about its median line and subtending at the axis of the rotor a considerably larger angle than in the construction of Figures 1-3. Thus the subtended angle increases from about 78 at the boss to a maximum of nearly 88 in the inner part of the blade and then decreases again rapidly to about 39 at the tip.

As indicated by the chain lines J in Figure 5, the blade thickness decreases from root to tip and has its maximum towards the inlet edge H of the blade.

The rotor construction of Figures 7-9, which is intended for use in a hydraulic pump or turbine, differs from the preceding constructions mainly by the inclusion of axial variation in pitch and in having the radially outward decrease in pitch at the working edge of the blade extended to the blade tip.

The rotor has four blades similar to the blade A shown, carried by a cylindrical boss B whose radius is approximately 0.4, taking the normal designed peripheral radius as the unit.

As shown by the curve E of Figure 9, the width of the blade A increases from the boss at F to a maximum value at a radius of about 0.6 and thence decreases outwards to the blade tip G the maximum width being about 18% greater than the boss width, whilst the tip width is only slightly less than the boss width.

The pitch at the working edge C of the blade (see curve D of Figure 9) decreases at first rapidly and then more slowly right to the blade tip, and the product of the pitch at the working edge and the blade width and the radius is substantially constant at all radii.

In the outer part of the rotor, say from a radius 0.8 outwards, the pitch at any radius is approximately constant over the Whole width of the blade, but the pitch changes axially in the inner part of the blade. The curve K of Figure 9 shows the pitch along the edge H of the blade and, in this part of the rotor the pitch changes gradually from the value defined by the curve K at the edge H to the value defined by the curve D at the working edge C of the blade. The pitch at the edge H is chosen to preserve a nearly uniform value of slip angle at all radii, and it will be seen from the curve K that this pitch increases slightly from the boss to the intermediate radius 0.8 from which it decreases outwardly with the pitch at the working edge C The working edges C of the blades lie on the surface of a cone, whilst the edges H are curved. The thickness of each blade decreases from root to tip, and the maximum thickness occurs towards the inlet edge of the blade and at to therefrom in most cases. Thus in the case of a turbine the maximum thickness is nearer the working edge C whilst in the case of a pump the maximum thickness is nearer the lead ing edge H Figures 8a, 8b, and 80 show cylindrical sections (corresponding to those of Figures 2a, 2b, and 20) respectively on the lines 8a., 8b, and 8c of Figure 8. Thesesections are drawn for the case when the rotor is employed in a pump, the maximum thickness (indicated by chain lines) being nearer the edge H These sections clearly show the axial change in pitch at the inner radii.

In face projection the blades, which overlap slightly in the region of radius 0.5, are symmetrical about their median radial lines and the subtended angle varies from 84% at the boss to a maximum of approximately 91 and then to 42 at the normal designed tip radius.

Since the characteristic features of the types of rotor described above, and more especially of the rotor of Figures 7-9, are such that-the pitch, radius and Width of blade are correctly related to each other for the most effective working at each radius, a wide range of volumetric duties can be catered for without material change of efficiency merely by extending or reducing the peripheral radius of the rotor leaving other conditions unchanged, thus obviating the necessity for a multiplicity of designs to meet various volumetric duties at given revolution speeds and heads. Thus for diminished volumetric duty the peripheral radius may be reduced by 20% or even more, so that the rotor will be identical with the rotor above described having the appropriate portion cut off the blade tips. Similarly for an increased volumetric duty the peripheral radius may be increased by, say, 10% as indicated in dotted line in Figures 7-9, or even by a greater percentage in some instances. It should be mentioned that when such changes are made, the various radial dimensions referred to above still have the values stated but with reference to 5 the normal designed tip of the blade and not to the actual tip of the blade.

It will be appreciated that these constructions have been described by way of example only and may be modified in a variety of ways within the scope of the invention. Thus for instance the blades of the rotor may be so mounted on the l boss as to permit rotational adjustment through a small angle of say, 5 in either direction about their longitudinal axes in order to accommodate changes in working conditions. Arrangements of this character are conventional, reference being directed, for instance, to the patent to Boutwell No. 1,031,415, granted July 2, 1912. Again the projected face view of the blade may be arranged otherwise than symmetrically about a median radial line.

What I claim as my invention and desire to secure by Letters Patent is:-

1. A screw rotor having a boss, and blades carried thereby, wherein the pitch of the blade at the working edge of each blade varies radially in relation to the width of the blade and to the radius in such a manner that the product of the pitch and the radius increases radially both inwards towards the boss and outwards towards the blade tip from a minimum value at a position whose radial distance from the axis is not greater than the normal designed mid-blade radius, while the width of the blade decreases radially both inwards and outwards from a maximum value at a position whose radial distance from the axis is not I greater than thenormal designed mid-blade radius.

2. A screw rotor having a boss, and blades 4 carried thereby, wherein the pitch of the blade at the working edge of each blade varies radially in relation to the width of the blade and to the radius in such a manner that the product of the pitch and the width and the radius is substantially constant at all radii, the width at any radius between the blade tip and a chosen intermediate position whose radial distance from the axis is not greater than the normal designed mid-blade radius being not greater than the width at the intermediate position and not less than the width at the blade tip, while between the chosen intermediate position and the boss the width is not greater than the width at the intermediate position and not less than the width at the 10055.

3. A screw rotor having a boss, and blades carried thereby, wherein the pitch of the blade at the working edge of each blade varies radially in relation to the width of the blade and to the radius in such a manner that the product of the pitch and the radius increases radially both inwards towards the boss and outwards towards the blade tip from a minimum value at a position whose radial distance from the axis is not greater than the normal designed mid-blade radius, while the width of the blade decreases radially both inwards and outwards from a maximum value at a position whose radial distance from the axis is not greater than the normal designed mid-blade radius, such maximum value being not less than 10% greater than the width at the boss.

4. A screw rotor having a boss, and blades carried thereby, wherein the pitch of the blade at the working edge of each blade varies radially in relation to the width of the blade and to the radius in such a manner that the product of the pitch and the radius increases radially both inwards towards the boss and outwards towards the blade tip from a minimum value at a position whose radial distance from the axis is not greater than the normal designed mid-blade radius, such minimum value being not greater than of the value of the pitch-radius product at the boss, while the width of the blade decreases radially both inwards and outwards from a maximum value at a position whose radial distance from the axis is not greater than the normal designed mid-blade radius.

5. A screw rotor having a boss, and blades carried thereby, wherein the pitch of the blade at the working edge of each blade varies radially in relation to the width of the blade and to the 1 radius in such a manner that the product of the pitch and the width and the radius is substantially constant at all radii, the width of the blade decreasing radially both inwards and outwards from a maximum value at a position whose radial distance from the axis is not greater than the normal designed mid-blade radius, the designed width of the blade at the tip differing by less than 30% from the width at the boss.

6. A screw rotor having a boss, and blades carried thereby, wherein the pitch of the blade at the working edge of each blade varies radially in relation to the width of the blade and to the radius in such a manner that the product of the pitch and the width and the radius is substantially constant at all radii, the pitch of each blade also decreasing gradually in an axial direction from the working edge of the blade to the other edge at least in the inner parts of the rotor, the width of any radius between the blade tip and a chosen intermediate position whose radial distance from the axis is not greater than the normal designed mid-blade radius being not greater than the width at the intermediate position and not less than the width at the blade tip, while between the chosen intermediate position and the boss the width is not greater than the width at the intermediate position and not less than the width at the boss.

7. A screw rotor having a boss, and blades carried thereby, wherein the pitch of the blade at the working edge of each blade varies radially in relation to the width of the blade and to the radius in such a manner that the product of the pitch and the radius increases radially both inwards towards the boss and outwards towards the blade tip from a minimum value at a position whose radial distance from the axis is not greater than the normal designed midblade radius, while the width of the blade decreases radially both inwards and outwards from a maximum value at a position whose radial distance from the axis is not greater than the normal designed mid-blade radius, the pitch of each blade also decreasing gradually in an axial direction from the working edge of the blade to the other edge at least in the inner parts of the rotor.

JAMES HERBERT WAINWRIGHT GILL.

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