Performance Characteristic Curves

Velocity diagrams and ideal head-rise vary with flow rate Q as illustrated in Figure 6 for the typical case of constant rotative speed N or angular speed H. Flow patterns in Figure 6b correspond to points on the characteristic curves of Figure 6a. The inlet velocity diagrams (just upstream of the impeller) are shown there for high and low flow rate—with zero swirl being delivered by the inlet passageway to the impeller; that is, Vel = 0. The outlet velocity diagrams on Figure 6a are found one-dimensionally, the magnitude of the exit relative velocity vector W2 varying directly with Q and its direction being nearly tangent to the impeller blade. From these diagrams are found the absolute velocity vector V2 and its circumferential component VS, 2. Because blade speed U2 is constant, the resulting plot of the ideal head AHi = U3VJg (from Eq. 15b) is a straight line, rising to the point U22/g at zero-Q or "shut-off head." This is twice the impeller OD tip speed head U22/2g. The right-most velocity diagram in Figure 6a has zero VS2; however, the maximum or "runout" flow rate happens at lower Q than this. That is because the actual head H is less than AHi due to losses (as seen in Eq. 10), and AH = 0 at runout—where overall pump efficiency (Eq. 8) is also zero.

This one-dimensional analysis works well in the vicinity of the best efficiency point (b.e.p. or BEP) and at higher Q because the fluid flows smoothly through the impeller pas-

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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