The mixers referred to are used mainly to generate and maintain a motion within a volume of liquid, in order to prevent sedimentation or agglomeration of solid matter that is dispersed in the liquid, or for de-stratification of liquids having different densities, for homogenization or for the mixing of substances in liquid, etc. Typical implementations, for example, include waste water treatment, water purification, PH-neutralization, chlorine treatment processes, cooling applications, de-icing applications, manure treatment processes.
The typical mixer comprises a propeller that is driven by an electric motor. The motor is contained in a motor enclosure which protects the motor and electrical components from the surrounding liquid. A motor shaft extends from an end of the motor enclosure to mount the propeller's hub in axial relation to the motor and motor enclosure. The opposite end of the motor enclosure may be arranged with mountings by which the mixer can be supported from a wall of a liquid-holding container, albeit other mountings are also conceivable.
The propeller usually has at least two propeller vanes supported from a propeller hub to reach radially with respect to a propeller axis. Alternatively, a singular propeller vane could be arranged to run helically about a propeller hub. In rotation, the propeller causes a drop in pressure on a suction side thereof, and a corresponding raise in pressure on the pressure side. The pressure difference results in a liquid flow through the propeller, from the suction side to the pressure side thereof. Since the pressure side is typically facing from the motor and motor enclosure, the main flow is usually directed axially away from the mixer.
The propeller thus generates in rotation an axial thrust, the size of which is determined by the design of the hydraulic components of the mixer, propeller design, rotational speed, and motor capacity. The stirring result which is related to the capacity of the mixer to generate a circulating flow in a bulk of liquid is largely depending on the efficiency of the mixer to create a jet flow downstream of the propeller. The significance of an extended jet flow is readily appreciated in connection with the stirring of waste water containing solid matter such as fibrous material and heavy organic particles that consume the energy introduced by the mixer.
In the submerged mixers, open to surrounding liquid, the volume/time flow through the propeller is high resulting in a mainly axial flow. The propeller however also generates a rotational motion in the liquid. As the liquid passes through the propeller, the total energy is increased in terms of static pressure and kinetic energy. The static pressure provides the axial thrust, whereas the kinetic energy, which is usually not advantageous in the subject mixer applications, is the result of a rotational component of motion induced in the liquid as it passes the propeller. In order to achieve maximum static pressure/axial thrust, it would thus be desired to suppress the rotation of the liquid that exits from the mixer's propeller.
Propeller vane design in general is a well documented art. It is known (by the Equation of  Momentum) that axial thrust is proportional to the increase in axial velocity through the mixer. The magnitude and direction of the flow generated by propeller blades and vanes can be demonstrated by applying velocity triangles to a section of the propeller, as taught by e.g. Stepanoff (1948, reprint 1993): “Centrifugal and Axial Flow Pumps” (Chap. 3.1 and 3.5).
The propeller section considered here for the analysis is a stream surface defined by the rotation RD around the axis A of the “streamline” SL showed in FIG. 1. The streamline SL starts upstream the propeller, passes the propeller blade leading edge LE and ends downstream the trailing edge TE.
FIG. 1a shows velocity triangles for a stream surface example, diagrammatically illustrated. The absolute velocity C of liquid, the velocity U of the propeller in rotation and the velocity W of liquid relative to the propeller are related as C=U+W. This way, the absolute velocities C at the leading and trailing edges of the propeller section may be determined for a number of stream surfaces. At the leading edge of the propeller (denoted by index 1), the flow and absolute velocity vector is void of any circumferential component and is therefore parallel to the propeller axis. At the trailing edge of the propeller blade (denoted by index 2), the flow has been brought in rotation by the propeller and a circumferential component (denoted as Cu2) is added to the absolute velocity vector, which is no longer parallel with the propeller axis.
It is previously known from practise to provide a mixer with a ring-shaped envelope about the propeller, known as a jet ring. The purpose and operation of the jet ring is to ensure that liquid is drawn mainly axially into the propeller on the suction side. The ring is typically supported by struts reaching towards the propeller from the motor enclosure. Albeit the ring to some extent contributes to establish a jet flow, the ring and struts are however not contemplated and effective for control or neutralization of a rotational motion in the flow that exits the propeller.
In U.S. Pat. No. 4,566,801, Salzman discloses a submersible mixer comprising a propeller enveloped by a tubular section having baffles downstream of the propeller, and extending axially towards the propeller, i.e. contra the flow direction, from a cruciform arm base which is connectable to the exit end of the tubular envelope. These baffles are optionally used when prevention of a non-axial flow from the tube is occasionally asked for.
The mentioning herein of the Salzman structure is made also for purpose of illustration of another problem that needs to be addressed in the design of submersible mixers for some of the stated uses. Due to the straight leading edges of the cruciform arms and baffles crossing the flow at right angles, Salzman's mixer is susceptible of clogging from fibrous matter and is thus unsuitable for sewage and waste water applications, e.g.
Another problem related with the prior art mixers is air reaching the propeller in result of vortex formation caused as the circumferential flow component imparted by the propeller propagates towards the suction side of the propeller in rotation. The suction of air into the propeller results in a dramatically reduced thrust, i.e. a reduced flow in the axial direction.
Still another problem related with the prior art mixers is torsional stress and vibration resulting from the reactive forces acting on the mixer and its supporting structures.