Brochure
8989
5.3.4 Recirculation loss
Recirculation zones in the hydraulic components typically occur at part
load when the flow is below the design flow. Figure 5.10 shows an example
of recirculation in the impeller. The recirculation zones reduce the eec-
tive cross-section area which the flow experiences. High velocity gradients
occurs in the flow between the main flow which has high velocity and
the eddies which have a velocity close to zero. The result is a considerable
mixing loss.
Recirculation zones can occur in inlet, impeller, return channel or volute
casing. The extent of the zones depends on geometry and operating point.
When designing hydraulic components, it is important to minimise the size
of the recirculation zones in the primary operating points.
Model
There are no simple models to describe if recirculation zones occur and if so to
which extent. Only by means of advanced laser based velocity measurements
or time consuming computer simulations, it is possible to map the recirculation
zones in details. Recirculation is therefore generally only identified indirectly
through a performance measurement which shows lower head and/or higher
power consumption at partial load than predicted.
When designing pumps, the starting point is usually the nominal operating
point. Normally reciculation does not occur here and the pump performance
can therefore be predicted fairly precisely. In cases where the flow is below
the nominal operating point, one often has to use rule of thumb to predict the
pump curves.
Figure 5.10: Example of recirculation in
impeller.
Recirculation zones