Patent Description:
Fluid pumps for pumping fluids, for example liquid fuel, are known in the art. One example of such a fluid pump is shown in <CIT>et al or in <CIT>. In such arrangements, an impeller is rotated, for example by an electric motor. The impeller is sandwiched between two plates which each have a respective flow channel formed in a face thereof such that each flow channel faces toward the impeller. The impeller includes a plurality of blades arranged in a polar array such that the blades are aligned with the flow channels of the two plates. Each blade may be a V-shape such that the concave side of the V-shape faces toward the direction of rotation of the impeller and the convex side of the V-shape faces away from the direction of rotation of the impeller. The impeller, including the plurality of blades, may be made as a unitary piece of plastic in an injection molding process where a pair of opposing molds form upper and lower halves of each blade. In order to allow for extraction of the impeller from the molds, a draft angle, typically about <NUM>°, is provided between each adjacent pair of blades. Furthermore, this draft angle is maintained along the radial length of each blade. This draft angle minimizes friction as the molds are extracted, thereby minimizing the likely hood of damage to the blades. However, this arrangement also causes the distance between adjacent blades to widen further from the center of the impeller, much like spokes on a bicycle wheel. In operation, fuel enters between adjacent blades on the inboard half of the blade radial length and centrifugal forces causes the fuel to exit the blade on the outboard half of the of the blade radial length. Since the distance between adjacent blades widens from inboard to outboard, the flow stream exiting the blade diverges which may be undesirable for momentum transfer of the fuel, thereby leading to decreased pumping efficiency.

What is needed is a fluid pump and impeller which minimizes or eliminates one or more of the shortcomings as set forth above.

Briefly described, the present disclosure provides an impeller for a fluid pump. The impeller includes a hub configured to be rotationally coupled to a shaft of the fluid pump such that the shaft provides rotational motion in a rotational direction about an axis, the hub having an outer surface; an outer ring which is concentric with the hub, the outer ring having an inner surface; and a plurality of blades extending from a root at the outer surface of the hub to a tip at the inner surface of the outer ring, each one of the plurality of blades having a first leg and a second leg which meet at a vertex, thereby forming a V-shape such that a concave side of the V-shape faces toward the rotational direction and such that a convex side of the V-shape faces away from the rotational direction. The first leg, at the concave side of each one of the plurality of blades, forms a draft angle with the first leg at the convex side of another one of the plurality of blades which is immediately adjacent thereto in the rotational direction. The draft angle at the inner surface of the outer ring is less than or equal to <NUM>% of the draft angle at the outer surface of the hub.

The present disclosure also provides a fluid pump which includes a housing; an electric motor within the housing, the electric motor having a shaft which rotates when electricity is applied to the electric motor; and an impeller located between an inlet plate having an inlet plate flow channel facing toward the impeller and an outlet plate having an outlet plate flow channel facing toward the impeller. The impeller includes a hub rotationally coupled to the shaft such that the shaft provides rotational motion in a rotational direction about an axis, the hub having an outer surface; an outer ring which is concentric with the hub, the outer ring having an inner surface; and a plurality of blades extending from a root at the outer surface of the hub to a tip at the inner surface of the outer ring, each one of the plurality of blades having a first leg and a second leg which meet at a vertex, thereby forming a V-shape such that a concave side of the V-shape faces toward the rotational direction and such that a convex side of the V-shape faces away from the rotational direction. The first leg, at the concave side of each one of the plurality of blades, forms a draft angle with the first leg at the convex side of another one of the plurality of blades which is immediately adjacent thereto in the rotational direction. The draft angle at the inner surface of the outer ring is less than or equal to <NUM>% of the draft angle at the outer surface of the hub.

The fluid pump and impeller as described herein provides for increased pumping efficiency while maintaining manufacturability of the impeller.

This invention will be further described with reference to the accompanying drawings in which:.

Referring initially to <FIG>, a fluid pump is illustrated, by way of non-limiting example only, as a fuel pump <NUM>. Fuel pump <NUM> may be, by way of non-limiting example only, submersed in a fuel tank (not shown) which pumps fuel to a fuel consuming device (also not shown) such as an internal combustion engine. The fuel pumped by fuel pump <NUM> may be any liquid fuel customarily used, for example only, gasoline, diesel fuel, alcohol, ethanol, and the like, and blends thereof.

Fuel pump <NUM> generally includes a pump section <NUM> at one end, a motor section <NUM> adjacent to pump section <NUM>, and an outlet section <NUM> adjacent to motor section <NUM> at the end of fuel pump <NUM> opposite pump section <NUM>. A housing <NUM> of fuel pump <NUM> is tubular and retains pump section <NUM>, motor section <NUM> and outlet section <NUM> together. Fuel enters fuel pump <NUM> at pump section <NUM>, a portion of which is rotated by motor section <NUM> as will be described in more detail later, and is pumped past motor section <NUM> to outlet section <NUM> where the fuel exits fuel pump <NUM>.

Motor section <NUM> includes an electric motor <NUM> which is disposed within housing <NUM>. Electric motor <NUM> includes a shaft <NUM> extending therefrom into pump section <NUM>. Shaft <NUM> rotates in a rotational direction <NUM> about an axis <NUM> when an electric current is applied to electric motor <NUM>. Electric motors and their operation are well known to those of ordinary skill in the art and will not be described in greater detail herein.

Pump section <NUM> includes an inlet plate <NUM>, a pumping element illustrated as impeller <NUM>, and an outlet plate <NUM>. Inlet plate <NUM> is disposed at the end of pump section <NUM> that is distal from motor section <NUM> while outlet plate <NUM> is disposed at the end of pump section <NUM> that is proximal to motor section <NUM>. Both inlet plate <NUM> and outlet plate <NUM> are fixed relative to housing <NUM> to prevent relative movement between inlet plate <NUM> and outlet plate <NUM> with respect to housing <NUM>. Outlet plate <NUM> defines a spacer ring <NUM> on the side of outlet plate <NUM> that faces toward inlet plate <NUM>. Impeller <NUM> is disposed axially between inlet plate <NUM> and outlet plate <NUM> such that impeller <NUM> is radially surrounded by spacer ring <NUM>. Impeller <NUM> is fixed to shaft <NUM> such that impeller <NUM> rotates with shaft <NUM> in a one-to-one relationship. Spacer ring <NUM> is dimensioned to be slightly thicker than the dimension of impeller <NUM> in the direction of axis <NUM>, i.e. the dimension of spacer ring <NUM> in the direction of axis <NUM> is greater than the dimension of impeller <NUM> in the direction of axis <NUM>. In this way, inlet plate <NUM>, outlet plate <NUM>, and spacer ring <NUM> are fixed within housing <NUM>, for example by crimping the axial ends of housing <NUM>. Axial forces created by the crimping process will be carried by spacer ring <NUM>, thereby preventing impeller <NUM> from being clamped tightly between inlet plate <NUM> and outlet plate <NUM> which would prevent impeller <NUM> from rotating freely. Spacer ring <NUM> is also dimensioned to have an inside diameter that is larger than the outside diameter of impeller <NUM> to allow impeller <NUM> to rotate freely within spacer ring <NUM> and axially between inlet plate <NUM> and outlet plate <NUM>. While spacer ring <NUM> is illustrated as being made as a single piece with outlet plate <NUM>, it should be understood that spacer ring <NUM> may alternatively be made as a separate piece that is captured axially between outlet plate <NUM> and inlet plate <NUM>.

Inlet plate <NUM> is generally cylindrical in shape, and includes an inlet passage <NUM> that extends through inlet plate <NUM> in the same direction as axis <NUM>. Inlet passage <NUM> is a passage which introduces fuel into fuel pump housing <NUM>. Inlet plate <NUM> also includes an inlet plate flow channel <NUM> formed in the face of inlet plate <NUM> that faces toward impeller <NUM>. Inlet plate flow channel <NUM> is a segment of an annulus and is in fluid communication with inlet passage <NUM>.

Outlet plate <NUM> is generally cylindrical in shape and includes an outlet plate outlet passage <NUM> that extends through outlet plate <NUM> where it should be noted that outlet plate outlet passage <NUM> is an outlet for pump section <NUM>. Outlet plate outlet passage <NUM> is in fluid communication with outlet section <NUM>. Outlet plate <NUM> also includes an outlet plate flow channel <NUM> formed in the face of outlet plate <NUM> that faces toward impeller <NUM>. Outlet plate flow channel <NUM> is a segment of an annulus and is in fluid communication with outlet plate outlet passage <NUM>. Outlet plate <NUM> also includes an outlet plate aperture, hereinafter referred to as lower bearing <NUM>, extending through outlet plate <NUM>. Shaft <NUM> extends through lower bearing <NUM> in a close-fitting relationship such that shaft <NUM> is able to rotate freely within lower bearing <NUM> and such that radial movement of shaft <NUM> within lower bearing <NUM> is limited to the manufacturing tolerances of shaft <NUM> and lower bearing <NUM>. In this way, lower bearing <NUM> radially supports a lower end of shaft <NUM> that is proximal to pump section <NUM>.

With continued reference to <FIG> and now with additional reference to <FIG>, impeller <NUM> includes a plurality of blades <NUM>, as can be most clearly seen in <FIG>, arranged in a polar array radially surrounding, and centered about axis <NUM>, such that blades <NUM> are aligned with inlet plate flow channel <NUM> and outlet plate flow channel <NUM>. Blades <NUM> are each separated from each other by a respective blade chamber <NUM> that passes through impeller <NUM> in the general direction of axis <NUM>. Impeller <NUM> may be made, for example only, by a plastic injection molding process in which the preceding features of impeller <NUM> are integrally molded as a single piece of plastic. Impeller <NUM> and blades <NUM> will be described in greater detail later.

Outlet section <NUM> includes an end cap <NUM> which closes the upper end of housing <NUM>. End cap <NUM> includes an outlet conduit <NUM> which provides fluid communication out of housing <NUM> such that outlet conduit <NUM> is in fluid communication with outlet plate outlet passage <NUM> of outlet plate <NUM> for receiving fuel that has been pumped by pump section <NUM>. Rotation of impeller <NUM> by shaft <NUM> causes fluid to be pumped from inlet passage <NUM> to outlet conduit <NUM> and to be pressurized within housing <NUM> such that pressurized fuel is communicated out of housing <NUM>. In order to prevent a backflow of fuel into housing <NUM> through outlet conduit <NUM>, fuel pump <NUM> may also include a check valve assembly <NUM> which allows fuel to flow out of fuel pump <NUM> through outlet conduit <NUM> but prevents fuel from flowing into fuel pump <NUM> through outlet conduit <NUM>.

Impeller <NUM> will now be described in greater detail with particular reference to <FIG>. Impeller <NUM> includes a hub <NUM> defining an aperture <NUM> extending axially therethrough at a center of hub <NUM>. As embodied herein, shaft <NUM> extends into aperture <NUM> and shaft <NUM> is rotationally coupled to hub <NUM> by way of complementary features which cause rotational motion of shaft <NUM> to be transferred to impeller <NUM> in rotational direction <NUM> about axis <NUM>. However, it should be understood that any known method of rotational coupling may be provided in the alternative. Hub <NUM> includes an outer surface <NUM> which surrounds, and extends along, axis <NUM> and which may be cylindrical. Impeller <NUM> also includes an outer ring <NUM> which is concentric to hub <NUM>. Outer ring <NUM> includes an inner surface <NUM> which surrounds, and extends along, axis <NUM> and which may be cylindrical.

Each blade <NUM> extends radially outward from a respective root 44a at outer surface <NUM> to a tip 44b at inner surface <NUM>. Each blade <NUM> includes a first leg 44c and a second leg 44d, which meet at a vertex 44e, thereby forming a V-shape such that a concave side 44f of the V-shape faces toward rotational direction <NUM> and such that a convex side <NUM> of the V-shape faces away from rotational direction <NUM>.

For each blade <NUM>, concave side 44f of first leg 44c forms a draft angle <NUM>n with convex side <NUM> of first leg 44c of the blade <NUM> which is immediately adjacent thereto in rotational direction <NUM> where n is used to represent different radial locations between outer surface <NUM> and inner surface <NUM> because draft angle <NUM>n varies between outer surface <NUM> and inner surface <NUM> and therefore is not uniform. As illustrated in <FIG>, draft angle <NUM>n at outer surface <NUM> of hub <NUM>, i.e. root 44a, is represented as draft angle <NUM><NUM> and is in a range of <NUM>° to <NUM>° with preference of being closer to <NUM>° in order to facilitate extraction of a mold (not shown) used in a plastic injection molding manufacturing process since a larger draft angle is desirable for manufacturing because it quickly separates the surfaces of blades <NUM> from the mold, thereby minimizing friction and the likelihood of causing damage to blades <NUM>. Now, as illustrated in <FIG>, draft angle <NUM>n at inner surface <NUM> of outer ring <NUM>, i.e. tip 44b, is represented as draft angle <NUM><NUM> and is less than or equal to <NUM>% of draft angle <NUM><NUM> such that draft angle <NUM><NUM> is preferably less than <NUM>° in order to promote high momentum transfer of the fuel during operation. Now, as illustrated in <FIG>, draft angle <NUM>n at a midpoint, i.e. equidistant, between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM> is represented as draft angle <NUM><NUM> and is greater than or equal to <NUM>% of draft angle <NUM><NUM>. As a result, draft angle <NUM>n changes very little from inner surface <NUM> of outer ring <NUM> and the midpoint between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM> which facilitates extraction of the mold. Also as a result, draft angle <NUM>n decreases primarily from the midpoint and inner surface <NUM> of outer ring <NUM>. While draft angle <NUM>n between the midpoint and inner surface <NUM> of outer ring <NUM> decreases to values which are typically not desirable for manufacturability, these lower draft angles occur for less than half of the radial distance from outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM> and therefore increases friction over a small area which still permits satisfactory extraction of the mold during manufacturing.

Each blade <NUM> has a thickness <NUM> which is measured in a direction perpendicular to the radial direction relative to axis <NUM>, i.e. perpendicular to a radius extending perpendicular from axis <NUM> through the center of blade <NUM> at the point at which thickness <NUM> is being measured. Furthermore, thickness <NUM> is measured at a blade axial face <NUM> of each blade <NUM> which is proximal to outlet plate <NUM>. Thickness <NUM> is substantially uniform from outer surface <NUM> of hub <NUM> to the midpoint between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM>, however, thickness <NUM> increases between the midpoint and inner surface <NUM> of outer ring <NUM> where substantially uniform is not varying by more than ±<NUM>%. This relationship of thickness <NUM> provides for a blade chamber distance <NUM>n which varies between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM> where n is used to represent different radial locations between outer surface <NUM> and inner surface <NUM>. Blade chamber distance <NUM>n is the measure from concave side 44f of one blade <NUM> to convex side <NUM> of another blade <NUM> which is immediately adjacent thereto in rotational direction <NUM> and is measured in a direction perpendicular to the radial direction relative to axis <NUM> (i.e. perpendicular to a radius extending perpendicular from axis <NUM> through the center of blade chamber <NUM> at the point at which blade chamber distance <NUM>n is being measured). Furthermore, blade chamber distance <NUM>n is measured at blade axial face <NUM>. As illustrated in <FIG>, blade chamber distance <NUM>n at outer surface <NUM> of hub <NUM>, i.e. root 44a, is represented as blade chamber distance <NUM><NUM>. Since thickness <NUM> is substantially uniform from outer surface <NUM> of hub <NUM> to the midpoint between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM>, blade chamber distance <NUM>n increases from outer surface <NUM> of hub <NUM> to the midpoint between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM>. However, blade chamber distance <NUM>n decreases from the midpoint to outer surface <NUM> of hub <NUM> such that a blade chamber distance <NUM><NUM>, illustrated in <FIG>, at inner surface <NUM> of outer ring <NUM>, i.e. tip 44b, is substantially equal to blade chamber distance <NUM><NUM> at outer surface <NUM> of hub <NUM> where substantially equal to is ±<NUM>% of blade chamber distance <NUM><NUM>.

Fuel is drawn into each blade chamber <NUM> at a location between outer surface <NUM> of hub <NUM> and the midpoint of outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM> and centrifugal force causes the fuel to be expelled from each blade chamber <NUM> at a location between the midpoint and inner surface <NUM> of outer ring <NUM> where the fuel continually recirculates in this way as the fuel travels through, and is pressurized within, outlet plate flow channel <NUM> before exiting through outlet plate outlet passage <NUM>. Due to the previously mentioned characteristics of draft angle <NUM>n from the midpoint to the inner surface <NUM> of outer ring <NUM> and of blade chamber distance <NUM>n from the midpoint to the inner surface <NUM> of outer ring <NUM>, momentum transfer of fuel exiting blade chamber <NUM> and entering outlet plate flow channel <NUM> is promoted which increases pumping efficiency. It should be recognized that the draft angles at the entrance region of the blade length and the outlet region of the blade length can be independently adjusted to tune the flow path for efficient flow of the fluid entering the blade and efficient momentum transfer of fluid exiting the blade, i.e. adjust to the optimum spot for the operating point of the fuel pump. Computational Fluid Dynamics (CFD) analysis has indicated that this arrangement yields <NUM>% efficiency in comparison to <NUM>% efficiency for a fuel pump which did not include impeller <NUM> as describe herein but was otherwise equivalent in design. This results in increasing efficiency by <NUM>%.

The characteristics of first legs 44c as described above are also provided to second legs 44d, however, for the sake of completeness, these characteristics will now be described with respect to second legs 44d. For each blade <NUM>, concave side 44f of second leg 44d forms a draft angle <NUM>n with convex side <NUM> of second leg 44d of the blade <NUM> which is immediately adjacent thereto in rotational direction <NUM> where n is used to represent different radial locations between outer surface <NUM> and inner surface <NUM> because draft angle <NUM>n varies between outer surface <NUM> and inner surface <NUM> and therefore is not uniform. As illustrated in <FIG>, draft angle <NUM>n at outer surface <NUM> of hub <NUM>, i.e. root 44a, is represented as draft angle <NUM><NUM> and is in the range of <NUM>° to <NUM>° with preference of being closer to <NUM>° in order to facilitate extraction of a mold (not shown) used in a plastic injection molding manufacturing process since a larger draft angle quickly separates the surfaces of blades <NUM> from the mold, thereby minimizing friction and the likelihood of causing damage to blades <NUM>. Now, as illustrated in <FIG>, draft angle <NUM>n at inner surface <NUM> of outer ring <NUM>, i.e. tip 44b, is represented as draft angle <NUM><NUM> and is less than or equal to <NUM>% of draft angle <NUM><NUM> such that draft angle <NUM><NUM> is preferably less than <NUM>° in order to promote high momentum transfer of the fuel. Now, as illustrated in <FIG>, draft angle <NUM>n at a midpoint, i.e. equidistant, between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM> is represented as draft angle <NUM><NUM> and is greater than or equal to <NUM>% of draft angle <NUM><NUM>. As a result, draft angle <NUM>n changes very little from inner surface <NUM> of outer ring <NUM> and the midpoint between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM> which facilitates extraction of the mold. Also as a result, draft angle <NUM>n decreases primarily from the midpoint and inner surface <NUM> of outer ring <NUM>. While draft angle <NUM>n between the midpoint and inner surface <NUM> of outer ring <NUM> decreases to values which are typically not desirable for manufacturability, these lower draft angles occur for less than half of the radial distance from outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM> and therefore increases friction over a small area which still permits satisfactory extraction of the mold during manufacturing.

Each blade <NUM> has a thickness <NUM> which is measured in a direction perpendicular to the radial direction relative to axis <NUM>, i.e. perpendicular to a radius extending perpendicular from axis <NUM> through the center of blade <NUM> at the point at which thickness <NUM> is being measured. Furthermore, thickness <NUM> is measured at a blade axial face <NUM> of each blade <NUM> which is proximal to inlet plate <NUM>. Thickness <NUM> is substantially uniform from outer surface <NUM> of hub <NUM> to the midpoint between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM>, however, thickness <NUM> increases between the midpoint and inner surface <NUM> of outer ring <NUM> where substantially uniform is not varying by more than ±<NUM>%. This relationship of thickness <NUM> provides for a blade chamber distance <NUM>n which varies between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM> where n is used to represent different radial locations between outer surface <NUM> and inner surface <NUM>. Blade chamber distance <NUM>n is the measure from concave side 44f of one blade <NUM> to convex side <NUM> of another blade <NUM> which is immediately adjacent thereto in rotational direction <NUM> and is measured in a direction perpendicular to the radial direction relative to axis <NUM> (i.e. perpendicular to a radius extending perpendicular from axis <NUM> through the center of blade chamber <NUM> at the point at which blade chamber distance <NUM>n is being measured). Furthermore, blade chamber distance <NUM>n is measured at blade axial face <NUM>. As illustrated in <FIG>, blade chamber distance <NUM>n at outer surface <NUM> of hub <NUM> is represented as blade chamber distance <NUM><NUM>. Since thickness <NUM> is substantially uniform from outer surface <NUM> of hub <NUM> to the midpoint between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM>, blade chamber distance <NUM>n increases from outer surface <NUM> of hub <NUM> to the midpoint between outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM>. However, blade chamber distance <NUM>n decreases from the midpoint to inner surface <NUM> of outer ring <NUM> such that a blade chamber distance <NUM><NUM>, illustrated in <FIG>, at inner surface <NUM> of outer ring <NUM> is substantially equal to blade chamber distance <NUM><NUM> at outer surface <NUM> of hub <NUM> where substantially equal to is ±<NUM>% of blade chamber distance <NUM><NUM>.

It should be noted that all blades <NUM> of impeller <NUM> are substantially identical and at least one of the described features has been labeled in the figures for representative purposes and convenience. Consequently, it should be understood that reference characters used to denote a feature in the figures for one blade have the same meaning for each blade <NUM> although not specifically labeled in the figures.

Fuel is drawn into each blade chamber <NUM> at a location between outer surface <NUM> of hub <NUM> and the midpoint of outer surface <NUM> of hub <NUM> and inner surface <NUM> of outer ring <NUM> and centrifugal force causes the fuel to be expelled from each blade chamber <NUM> at a location between the midpoint and inner surface <NUM> of outer ring <NUM> where the fuel continually recirculates in this way as the fuel travels through, and is pressurized within, outlet plate flow channel <NUM> before exiting through outlet plate outlet passage <NUM>. Due to the previously mentioned characteristics of draft angle 72n from the midpoint to the inner surface <NUM> of outer ring <NUM> and of blade chamber distance <NUM>n from the midpoint to the inner surface <NUM> of outer ring <NUM>, momentum transfer of fuel exiting blade chamber <NUM> and entering outlet plate flow channel <NUM> is promoted which increases pumping efficiency. It should be recognized that the draft angles at the entrance region of the blade length and the outlet region of the blade length can be independently adjusted to tune the flow path for efficient flow of the fluid entering the blade and efficient momentum transfer of fluid exiting the blade, i.e. adjust to the optimum spot for the operating point of the fuel pump. Computational Fluid Dynamics (CFD) analysis has indicated that this arrangement yields <NUM>% efficiency in comparison to <NUM>% efficiency for a fuel pump which did not include impeller <NUM> as describe herein but was otherwise equivalent in design. This results in increasing efficiency by <NUM>%.

Fuel pump <NUM> which in includes impeller <NUM> as described herein provides for increased pumping efficiency while maintaining manufacturability of impeller <NUM>.

Claim 1:
An impeller (<NUM>) for a fluid pump (<NUM>), said impeller (<NUM>) comprising:
a hub (<NUM>) configured to be rotationally coupled to a shaft (<NUM>) of said fluid pump (<NUM>) such that said shaft (<NUM>) provides rotational motion in a rotational direction (<NUM>) about an axis (<NUM>), said hub (<NUM>) having an outer surface (<NUM>);
an outer ring (<NUM>) which is concentric with said hub (<NUM>), said outer ring (<NUM>) having an inner surface (<NUM>); and
a plurality of blades (<NUM>) extending from a root (44a) at said outer surface (<NUM>) of said hub (<NUM>) to a tip (44b) at said inner surface (<NUM>) of said outer ring (<NUM>), each one of said plurality of blades (<NUM>) having a first leg (44c) and a second leg (44d) which meet at a vertex (44e), thereby forming a V-shape such that a concave side (44f) of said V-shape faces toward said rotational direction (<NUM>) and such that a convex side (<NUM>) of said V-shape faces away from said rotational direction (<NUM>);
wherein said first leg (44c), at said concave side (44f) of each one of said plurality of blades (<NUM>), forms a draft angle (<NUM>n) with said first leg (44c) at said convex side (<NUM>) of another one of said plurality of blades (<NUM>) which is immediately adjacent thereto in said rotational direction (<NUM>); and
the impeller is characterised in that
said draft angle (<NUM>n) at said inner surface (<NUM>) of said outer ring (<NUM>) is less than or equal to <NUM>% of said draft angle (<NUM>n) at said outer surface (<NUM>) of said hub (<NUM>).