CA2120977A1

CA2120977A1 - Impeller with alternating primary and secondary vanes of different geometries

Info

Publication number: CA2120977A1
Application number: CA002120977A
Authority: CA
Inventors: Baha Elsayed Abulnaga
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-04-11
Filing date: 1994-04-11
Publication date: 1995-10-12

Abstract

The present invention relates to the design of impellers for radial, centrifugal or mixed flow liquid and gas pumps and turbines. The impeller features a number of primary and secondary vanes (or blades). The primary vanes extend from the eye of the impeller to the tip diameter or periphery of the impeller. The secondary vanes extend from a higher radius outside the eye of the impeller to the tip diameter or periphery of the impeller. The primary and secondary vanes alternate. The role of the secondary vanes is to control the expansion and the slip between the primary vanes particularly towards the periphery of the impeller.

Description

2.0 DESCRIPTION OF THE CONCEPT 2 / 2 ~ ~ 7 7 Slip is a recognized phenomenon in impellers of turbomachines.
A slip factor is usualLy defined as the ratio as y = Ct,t. / C~t where C~t, is the actual peripheral velocity at the tip diameter of the impeller Cut is the design peripheral velocity (obtained from the tip speed and the relative velocity due to the vane tip angle~
The classical theory of slip was initially developed by Stodala who established a relationship between the slip factor Y and the number of vanes as Y = 1 - PI * sin ~tltZ
4~
where PI=3.1415 Bt = vane tip angle Z = Number of vanes Stodala's theory has been refined in the last few decades by various pump designers to take in account the vane profile, the relationship between casing and impeller. However ~ost theories concord that s~ip is minimized by increasing the number of the vanes up to a certain limit where flow could be 5~ blocked by an excessive number of vanes. Thus typically many water pumps have 6 to 8 vanes for their impellers.

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In the case of slurry pumps there are certain criteria of ~CE oF o acceptable particle size passage way and non clogging that restricts the number of vanes. Thus many small slurry pumps with a diameter of less than 14t' (350 mm) may have 2,3, or 4 S vanes. This tends to increase slip and decrease efficiency.
The inventor in an effort to design efficient impellers for for slurries examined the flow through an ~mpeller. In the case of a conventional impeller the distance between the vanes 1~ is found to be smaller at the eye of the impeller (2*PI*R -Z~t3~Z ~where t is the local thickness of the vane3 than at the periphery. As the flow moves through the impeller it experiences an expansion due to the increased spacing between vanes. If such an expansion is too acute some breakdown of the flow may occur causlng eddies and even reversal of the flow.
Thus every pump has a reg~on of best efficiency in which it is recommended for use.
In an effort to reach a compromise between sufficient number of vanes to minimlze slip without obstructing flow at the eye as to reduce particle size passage ways, the inventor decided to maintain a limited number of vanes as in conventionaL
impellers that the inventor called primary or principal vanes but to introduce a second set of secondary vanes between the primary vanes outside the eye of the impeller to extend to the tip diameter or periphery of the impeller. The primary and secondary vanes alternate. The role of the secondary vanes is to control the expansion and the slip between the primary vanes partlcularly towards the periphery of the impeller.
Thus referring to fig (13 which shows an embodiment of the invention. Starting at the eye (53 one notices a number of vanes ~2~ which extend from the eye of the lmpeller to the periphery of the impeller. These vanes are the type of vanes found in conventlonal pump impellers. They are equldistant and of identical geometry. they are referred to, in the present invention as primary vanes. As notlced ln fig ~13 the distance between the primary vanes increases with the radlus allowlng expansion of the flow.
At a certain dlameter ~4~ a set of secondary vanes ~33 are introduced. Their role ls to divlde the flow between the primary vanes. They reduce the expanslon angle and create additlonal channels. It is a fundamental princlple of fluid 4~ dynamics that if the expanslon angle is too large ~ larger than 15 degrees) that some pressure losses and additional flow reversal may occur. Thus the secondary vanes help to prevent an acute expansion angle particularly in solid handllng pumps by splltting lt lnto two smaller expansion angles (fig 2~.
The selection of the starting radlus for the secondary vanes is a matter of design. They could ~e started close to the eye or well far from it. The principle is to avoid blocking the flow at the eye. In solid handling pumps it is advised to start the secondary vanes at a radius to al~ow sufficient particle passage way.

The principle of the impeller with alternating vanes of different geometries may be applied to open, semi-open, semi-closed , closed ,single suction, double suction - Fig (21 shows an cross-sectional representation of an open, double suctlon impeller. Such an impeller was manufactured - Photos are included in the annex attached hereunder. In this particular design the primary vanes on the front shroud are out of phase with the vanes of the back shroud but they can be equally in phase back to back.
Fig (3) shows another embodiment of the invention by applying the principle to closed impel1er with shrouds on both sides of the vanes.

Claims

1- An impeller (1) for liquid , gas ,slurries and solid handling pumps as represented in Fig(1) featuring a number of primary vanes (2) and secondary vanes (3), wherein the primary vanes extend from the eye of the impeller (5) to the tip diameter or periphery of the impeller (6), while the secondary vanes extend from a radius (4) outside the eye of the impeller to the tip diameter or periphery of the impeller; or are of a different geometry, curvature or chord than the primary vanes and whereas the primary and secondary vanes alternate, with the secondary vanes playing a role of dividing the flow between the primary vanes to control the expansion and the slip between the primary vanes particularly towards the periphery of the impeller.

2- The principle of the impeller can be applied to the closed,semi-closed, open,semi-open, reverse vane impellers with single or double suction, for radial, mixed or centrifugal pumps and turbines.