As with any turbomachinery, pump design requires a lot of effort on finding the right blade profile for the specified application. As there is no right or wrong in the process, engineers have to make some general assumptions as a starting point. Generally, we can say that the focus of this task is to minimize losses. It is obvious that the selected blade shape will affect several important hydrodynamic parameters of the pump and especially the position of optimal flow rate and the shape of the overall pump performance curves. In addition to axial and radial pump design in recent years, we also have seen the development of mixed-flow pumps. A mixed flow pump is a centrifugal pump with a mixed flow impeller (also called diagonal impeller), and their application range covers the transition gap between radial flow pumps and axial flow pumps.
Let’s consider a dimensionless coefficient called “specific speed” in order to be able to compare different pumps with various configurations and features. The “specific speed” is obtained as the theoretical rotational speed at which a geometrically-similar impeller would run if it were of such a size as to produce 1 m of head at a 1l/s flow rate. In formulas:
where ns is the specific speed, n the rotational speed, Q is the volume flow rate, H is total head and g is gravity acceleration.
The specific speed for mixed flow pumps might go from ns=30 to ns=80 for low-speed mixed flow pumps and can increase between ns of 80 and 160 for higher speed mixed flow pumps, as shown in Fig 1 to the left.
As you can see, Mixed-flow pumps have specifics speeds right in-between radial and axial configurations. For mixed flow pumps we can mainly see two casing configurations. In case of low peed pumps, the most appropriate configuration is the volute casing, which shows big similarities with centrifugal pump designs. However, for higher specific speeds raising above the ns =~130/140 value, we see configurations combining the mixed flow impeller with a diffuser + tubular casing. This solution is chosen to avoid the unreasonably large volute outlet cross sections that would be required to maintain the flow due to the very low tangential component of the absolute velocity at the impeller outlet.
Mixed-flow pumps with tubular casing are often installed in a vertical arrangement, and can be found in multistage configuration for high flow applications, such as irrigation, urban water supply, thermal power plants etc.
In a nutshell, the advantage of mixed-flow pumps over the “competition” is to be a “jack of all trades”. They combine higher massflow of axial configurations with higher pressure achievable by centrifugal machines.