7 Dynamic Similitude
266
Even if energy considerations are of low relevance, as with home distribution
of rain and well water, pumps with variable rotational speed are applied. The control strategy is quite simple then, namely keeping discharge pressure constant and
switching off at zero flow. Variable rotational speed pumps have the advantage that
only a very small buffer reservoir is needed, which is just meant for detecting pressure. At present, both systems with variable rotational speed and very small reservoir and systems with fixed rotational speed and a larger reservoir are marketed,
with the variable rotational speed system getting increasingly popular. The larger
reservoir system indeed easily shows deficiencies. The buffer capacity comes from
water stored in a rubber bellows, enclosed in the air-filled reservoir. Air leaks from
the reservoir and bellows rupture are current flaws. Figure 7.12 shows the two types
of domestic pump groups.
7.4 Imperfect Similitude
7.4.1 Effect of Reynolds Number with the Same Fluid
With operation at low flow rate, the Reynolds number may become small. By the
mere application of the kinematic similitude condition, similitude is not entirely
correct. Especially, efficiency diminishes when the Reynolds number is reduced to
small values. For example, in the field of the characteristics of a centrifugal pump
for various rotational speeds, as sketched in Fig. 7.13, the efficiency contours only
follow the similitude parabolas in the centre of the diagram. At a low flow rate,
Fig. 7.12 Left domestic pump group with large reservoir and fixed speed; right small reservoir and
variable speed (Courtesy Wilo)
266
Even if energy considerations are of low relevance, as with home distribution
of rain and well water, pumps with variable rotational speed are applied. The control strategy is quite simple then, namely keeping discharge pressure constant and
switching off at zero flow. Variable rotational speed pumps have the advantage that
only a very small buffer reservoir is needed, which is just meant for detecting pressure. At present, both systems with variable rotational speed and very small reservoir and systems with fixed rotational speed and a larger reservoir are marketed,
with the variable rotational speed system getting increasingly popular. The larger
reservoir system indeed easily shows deficiencies. The buffer capacity comes from
water stored in a rubber bellows, enclosed in the air-filled reservoir. Air leaks from
the reservoir and bellows rupture are current flaws. Figure 7.12 shows the two types
of domestic pump groups.
7.4 Imperfect Similitude
7.4.1 Effect of Reynolds Number with the Same Fluid
With operation at low flow rate, the Reynolds number may become small. By the
mere application of the kinematic similitude condition, similitude is not entirely
correct. Especially, efficiency diminishes when the Reynolds number is reduced to
small values. For example, in the field of the characteristics of a centrifugal pump
for various rotational speeds, as sketched in Fig. 7.13, the efficiency contours only
follow the similitude parabolas in the centre of the diagram. At a low flow rate,
Fig. 7.12 Left domestic pump group with large reservoir and fixed speed; right small reservoir and
variable speed (Courtesy Wilo)